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Decreases in abundances and declines in growth of eastern white pine over the past century due mainly to human activities have resulted in few large intact old-growth white pine forests in Ontario. These stands may be vulnerable to replacement by deciduous species from temperate forests further south, where recruitment in canopy gap disturbances can greatly define the regeneration process. We investigated recruitment dynamics in canopy gaps of an old-growth white pine forest of Temagami, northern Ontario, Canada, the northern limit of the temperate-boreal ecotone. White pine, red pine, black spruce and eastern white cedar represented 85 % of the mature canopy abundance, where trees and saplings established equally in gaps and the closed canopy. Balsam fir and paper birch were more abundant in gaps, showing increases of abundance and basal area with increases in gap size representing canopy self-replacement (balsam fir) and autogenic succession (paper birch). Red maple, at its northernmost range limit, was the only species to show linear increases of abundance and basal area with increases in gap size and gap age. This result, along with adult red maples present in gaps but absent from the closed canopy, identifies the establishment of a northward migrating species in gaps as hypothesized for pine forests at the northern limit of this broad ecotone. We discuss how migration pressures, coupled with pine recruitment limitation through reduced fire frequency by regional fire suppression and predicted future increased warming of 2-4 °C over the next century, threatens replacement of old-growth white pine forests at this latitude with northward migrating tree species found further south. [ABSTRACT FROM AUTHOR]

This paper deals with the dynamics of postfire regeneration of Pinus brutia Ten. in an artificial forest ecosystem of North Greece, after a fire in 1982. The following issues are studied: the natural development of P. brutia stands 20 years after the fire, the current stand structure, and the influence of thinning treatment on stand population dynamics and tree growth patterns. The present work summarises and updates data taken during the years 1987–2002. The results show that the postfire regeneration was successful and contributed to the re-establishment of the pre-fire forest not only at the sites of good quality but at the medium quality sites as well. Regarding the postfire development, it is observed that an abundant P. brutia re-establishment is followed by a natural and gradual reduction of tree population caused by the influence of the physical environment during the early postfire years and caused by self-thinning later. The stands have entered the stem exclusion stage and they are growing at a narrow spacing in all cases. The evolution pattern and the stand structure were affected by thinning, which resulted in the improvement of tree quality and growth and accelerated their early fruition, thus contributing to higher ecosystem resilience. [ABSTRACT FROM AUTHOR]

Ecosystems are influenced by multiple drivers, which shape ecosystem state and biodiversity. In some ecosystems, interactions and feedbacks among drivers can produce traps that confine an ecosystem to a particular state or condition and influence processes like succession. A range of traps has been recognized, with one of these – "a landscape trap" first proposed a decade ago for the tall, wet Mountain Ash and Alpine Ash forests of Victoria, south-eastern Australia. Under such a trap, young flammable forest is at high risk of reburning at high severity, thereby precluding stand maturation, and potentially leading to ecosystem collapse. These young forests are more common because recurrent wildfire and widespread clearcutting have transformed historical patterns of forest cover from widespread old-growth with small patches of regrowth embedded within it, to the reverse. Indeed, approximately 99% of the montane ash ecosystem is now relatively young forest. Based on new empirical insights, we argue that at least three key inter-related pre-conditions underpin the development of a landscape trap in montane ash forests. A landscape trap has been sprung in these forests because the pre-conditions for its development have been met. We show how inter-relationships among these pre-conditions, leading to frequent high-severity fire, interacts with life history attributes (e.g., time to viable seed production) to make montane ash forests (e.g., which have been highly disturbed through logging and frequent fire) vulnerable to ecosystem collapse. We conclude with the ecological and resource management implications of this landscape trap and discuss how the problems created might be rectified. [ABSTRACT FROM AUTHOR]

The influence of animal-mediated processes on the purported cyclical succession model of vegetation dynamics in arid subtropical thicket of southern South Africa was examined. We used spatially explicit sampling methods to evaluate the influence of dispersal and decomposition on plant community structure. The harvester ant Messor capensis was the only ant species recorded harvesting Portulacaria afra fruit and the elaiosome bearing seed of the tree Gymnosporia polyacantha from experimental piles. Propagules were dispersed to nest mounds positioned beneath woody shrubs. Vertebrates dispersed fleshy-fruited tree species to regeneration sites beneath P. afra clumps. The minute seeds of succulent shrubs were secondarily dispersed from beneath shrubs to ant nest sites located in the open. The results from two experiments indicate that soil nutrients and decomposition activity are spatially localised. The rate of below-ground decomposition, and the incidence of an invertebrate detritivore assemblage, was high beneath P. afra clumps compared with exposed nutrient-enriched and grassy sites. Animal dispersal vectors are integral to the regeneration of major plant groups in arid subtropical thicket by moving seeds from competitively harsh environments to safe sites beneath nurse plants. Decomposition and nutrient release are localised beneath P. afra clumps due to the unique conditions that support a suite of detritivore taxa recorded nowhere else at the study site. [ABSTRACT FROM AUTHOR]

Vascular epiphytes represent almost 10% of all terrestrial plant diversity. Despite the extensive research on the functional ecology and challenges of epiphytic growth, there is still very little known on how exposure to mechanically induced stress affects the growth and development of epiphytes. Therefore, this study investigated the effect of such mechanical stress on the growth and biomass allocation of epiphytic bromeliads. Juvenile plants of two species were subjected to two types of mechanical stress in the greenhouse—permanent displacement and temporary, recurring mechanical flexing. ANOVAs were used to test possible treatment effects on growth, root–shoot ratio, root diameter, and root area distribution ratio. Contrary to previous studies on herbaceous plants, these bromeliads showed little to no change in root and shoot properties in either species. The root–shoot ratio increased in disturbed Guzmania lingulata plants, but not in Vriesea sp. Treatment effects on growth were inconsistent: a stress effect on growth was significant only in the first 2 months of the experiment in G. lingulata, whilst none of the stress treatments negatively affected growth in Vriesea sp. All disturbed plants showed some degree of curvature on their stems and leaves against the area of stress to obtain an upright position. This was probably related to the maintenance of a functional tank. This study provides quantitative and qualitative data to understand thigmomorphogenic responses of bromeliads to mechanical stress. Future studies could include field surveys to quantify on-site mechanical stresses and the corresponding morphological changes in vascular epiphytes. [ABSTRACT FROM AUTHOR]

Changes in community diversity and dynamics after fires in Mediterranean ecosystems are rarely investigated more than a few years after the fire even though pronounced changes can be expected in the longer term due to substitution of canopy species. Pinus halepensis is strongly promoted by wildfire and should therefore be gradually substituted by Quercus species as the time since the last fire increases. We hypothesized that this tree substitution would cause changes in understorey plant diversity by changing resource availability and the abundance and properties of woody debris, leading to changes in biogeochemical processes. To test this hypothesis, we investigated the effect of the time since last fire on vascular plant composition and diversity by studying a 130 years post-fire chronosequence in mixed Mediterranean forests. The canopy composition went from domination by Pinus halepensis to domination by Quercus 70 years after the most recent fire. This transformation was associated with a change in the understorey involving a rarefaction of species present during the first decades after the fire. The plant density or cover also changed with time since the last fire, indicating a succession driven by species rarefaction rather than substitution. The mean richness and Shannon diversity per quadrat were highest shortly after the fire, and were significantly lower 70 or more years after the last fire. Fires are important for supporting highly diversified fire-dependent plant communities, and total plant richness decreases monotonically over time after fires, suggesting that fire suppression may reduce diversity in Mediterranean forests. [ABSTRACT FROM AUTHOR]

Anthropogenic alteration of fire regimes is implicated in the extinction or decline of species across the globe. Active management of fire regimes using specified guidelines can help sustain diversity and counter this loss. However, some species occur across multiple communities with differing fire regime requirements or may exploit the dynamics of associated ecotones. These complexities are not easily understood via a simple consideration of life history traits or unravelled by a simple experimental framework. Population modelling, however, may provide valuable insights in these instances, but to date this remains unexplored. A population model based on detailed demographic, habitat and fire regime data was developed for the species Prostanthera askania. The species occurs across rainforest/eucalypt forest ecotones subject to long-term alteration, fragmentation and invasion by exotic species. Modelling revealed that current recommended fire guidelines will not sustain this species under these novel conditions. Fire regimes that minimised extinction risk in some scenarios had double or five times the fire frequencies of recommended regimes. Managing fire was also more important in specific habitat (gully habitat rather than on slopes or ridges). Currently applied management actions for Prostanthera askania do not include any fire management and the regimes that apply to most habitat in which the species occurs (no fire or fire at up to 50-year intervals in gully habitat) is not optimal for the species. Management will be substantially more effective if specific fire treatments revealed by this study are employed (e.g. fire intervals of 20-30 years in gully habitat). The study demonstrates that a conceptualisation and consideration of communities as both temporally and spatially dynamic can substantially contribute to better fire management outcomes. The approach used herein can be readily adapted and applied to a plethora of species via a range of software packages and codes. [ABSTRACT FROM AUTHOR]

The Japanese wingnut Pterocarya rhoifolia is a riparian canopy tree species that grows under a variety of climate conditions, including heavy snowfall, despite the difficulties posed to canopy trees in such environments. This suggests that P. rhoifolia might adapt its life history strategy to different snowfall conditions. This study compared several life history traits of this tree species in a cool temperate mountainous area in central Japan along a gradient of maximum snow depth. The following trends were observed with increasing maximum snow depth: (1) diameter at breast height decreased, maximum stem length and tree height shortened, and trees tended toward a 'dwarf shrub' form; (2) the number of sprout stems increased significantly, and these sprouts contributed to maintaining the population; and (3) seed production decreased. Our results suggest trade-offs between clonal growth (sprouting) and sexual reproduction (seed production), and between sprouting and height growth. We concluded that the life history strategy P. rhoifolia demonstrated adaptive plasticity in response to a gradient of maximum snow depth. [ABSTRACT FROM AUTHOR]

Plants able to resprout from roots have a potential bud bank that gets initiated after injury to overcome meristem limitation after loss of all stem parts and to facilitate regeneration. Knautia arvensis is reportedly able to sprout from its roots on arable land, but information is missing regarding such ability in serpentine populations or how it might differ between diploids and tetraploids. We hypothesized that (1) 'ruderal' non-serpentine populations better tolerate severe disturbance than relic, serpentine ones; (2) tetraploid populations resprout more readily than diploids due to enhanced growth of higher ploidy levels; and (3) plants of different ploidy levels from serpentine soils are, for evolutionary reasons, more similar in their response to disturbances than plants from non-serpentine soils. To test these hypotheses, we conducted a pot experiment. Its results do not support our hypothesis that the ability to sprout from roots is a factor driving the spread of new weedy taxa into central Europe or the hypothesis that it is related to polyploidization in the genus Knautia. Both tetraploids and plants from non-serpentine populations regenerated less vigorously than diploids and plants from serpentine populations. However, the genetically closer populations of serpentine origin were more similar in their response to experimental manipulations than their genetically distinct non-serpentine counterparts. The success of non-serpentine taxa in disturbed habitats of central Europe might be related to traits other than the ability to resprout. [ABSTRACT FROM AUTHOR]

The distribution of sample units in multivariate species space typically departs strongly from the multivariate normal distribution. Instead of forming a hyperellipse in species space, the sample points tend to lie along high-dimensional edges of the space. This dust bunny distribution is seen in most ecological community datasets. The practical consequences of the distribution to the analysis of community data are well known and severe, but no one has demonstrated how population processes generate these problems. We evaluate potential causes of dust bunny distributions by simulating a large number of non-equilibrial communities under varying conditions, verifying that they resemble real data, then analyzing the relationship between the intensity of the dust bunny distribution in these datasets and the population and environmental parameters that gave rise to them. All community datasets, both simulated and real, departed strongly from multivariate normal and lognormal distributions. Four parameters influenced intensity of dust bunnies: time since community-replacing disturbance, number of environmental factors, dispersal limitation, and niche width. Samples measured soon after community-replacing disturbance had strong dust bunny distributions. Near-equilibrial communities sampled from a narrow range in environments lead to only weak dust bunnies. Community samples taken across multiple simultaneous strong environmental gradients are likely to show strong dust bunnies, regardless of the successional state, niche width of the component species, and degree of dispersal limitation. Dust bunny intensity depends not only on population processes and disturbance, but also on the properties of the sample, such as sample unit area or volume. [ABSTRACT FROM AUTHOR]

Species reintroduction programmes should include consideration of potential impacts on key species in the recipient community. Wild boar ( Sus scrofa) have been reintroduced into Britain after a 700-year absence. There is an urgent need to understand how this ecosystem engineer will affect plant communities in the habitats that it invades. We investigated the impact of rooting by wild boar on bluebells ( Hyacinthoides non- scripta), a species that is highly valued for its impressive floral displays and is an important and legally protected component of the UK forest flora. We monitored bluebell performance over three growing seasons in woodland habitats that are routinely rooted by boar in southern England. H. non- scripta cover and reproductive performance were monitored in small-scale experimental exclosures to exclude boar, compared to open control plots, set up on areas that either had or had not been previously rooted. Immediate effects were that rooting significantly reduced the percentage cover and density of H. non- scripta plants, by up to 95 and 60 %, respectively, and also adversely affected the number of flowering stems. However, there was evidence that cessation of rooting brought about by excluding the boar enabled substantial recovery in percentage cover and the density of flowering stems within 2 years. A positive effect of rooting on germination may have assisted this recovery. Thus, the impact of wild boar rooting on bluebell populations is locally severe, but there is potential for rapid recovery if plants are protected. Long-term effects of sustained or frequently repeated rooting still need to be investigated. [ABSTRACT FROM AUTHOR]

Variation in seasonal sprouting pattern from roots and rhizomes of perennial herbaceous plants influence the success of plant proliferation ability, invasiveness and escape from weed control measures. The latter often rely on methods, which repeatedly fragment the underground system, thereby trigger adventitious and axillary buds to sprout, and consequently reduce the amount of stored energy. If carried out at times when no re-growth occurs, treatments will have little effect on weed populations, but cost much in terms of labour and energy. The purpose of this experiment was to determine the seasonal variation in bud sprouting capacity after fragmentation. Five troublesome perennial weed species, collected in northern and southern Sweden, were grown outdoors in Uppsala, Sweden (N 59°49′, E 17°39′), from May 2009 to January 2010. Cut root and rhizome fragments, taken at two weeks intervals from July to January, were used to evaluate bud sprouting capacity, which was statistically analyzed using generalized additive models. In Elytrigia repens from southern Sweden and Sonchus arvensis sprouting capacity was significantly impaired during a period from September to November. In Equisetum arvense and Tussilago farfara sprouting was low between July and November where after it increased. In contrast, Cirsium arvense and E. repens from northern Sweden sprouted readily throughout the period. Except for E. repens, a model by populations was significantly better than one based on latitudinal origin. The result suggests a species-specific timing of treatments in weed management, avoiding the non-effective autumn period for E. arvense, S. arvensis and T. farfara, and in some cases in E. repens. [ABSTRACT FROM AUTHOR]

Knowledge on the limitation of plant species' distributions is important for preserving alpine biodiversity, particularly when the loss of alpine habitats because of global warming or land use changes is faster than colonization of new habitats. We investigated the potential of the rare alpine plant Campanula thyrsoides L. to colonize grassland sites of different suitability on a small mountain plateau in the Swiss Alps. A total of 15 experimental sites were selected according to their differences in habitat suitability for adult C. thyrsoides, which was measured by the Beals index. At each site we applied a disturbance treatment, added seeds at different densities and monitored the survival of seedlings over two consecutive years. The number of surviving seedlings was not positively related to habitat suitability for adult C. thyrsoides. Furthermore, C. thyrsoides appears to be strongly dispersal limited at the regional scale because seed addition to unoccupied habitats resulted in successful germination and survival of seedlings. Since an increase of seed density in already occupied sites did not affect the number of seedlings, we suggest that C. thyrsoides is microsite limited at the local scale. Microsite limitation is supported by the result that seedling survival of the species was enhanced in vegetation gaps created by disturbance. We conclude that C. thyrsoides may become endangered in the future if environmental changes cause local extinction of populations. An appropriate management, such as a disturbance regime for enhancing recruitment in existing populations, may ensure the long-term survival of this rare alpine plant species. [ABSTRACT FROM AUTHOR]

Riparian forests are highly valued for maintaining water quality through the retention of sediments and nutrients. They also provide some of the most diverse and species-rich habitats in the world. What is largely unknown, however, is how sediment deposition affects plant community composition in these forests. The objective of this study was to examine changes in plant community composition across a gradient of increasing rates of sedimentation in riparian forests in the southeastern Coastal Plain, USA. Seventeen plots were established within riparian forests receiving between 0 and 5.5 cm year of sediment deposits. Species density and biomass estimates were collected annually from 2002 to 2006 for overstory and mid-story plant species within each plot. Percent cover and nested frequency of understory plant species were determined annually during 2004–2006. Measures of community composition in the understory, mid-story, and overstory layers of forests were compared to changes in environmental factors associated with increased sedimentation. In the understory, annual, exotic, and upland species had higher importance values in plots receiving high sediment deposition. The densities of shade-intolerant and N-fixing species in the mid-story also increased with increasing sedimentation rates. Increased overstory mortality was associated with high sedimentation rates, though increases in understory light levels in these gaps were not the main driver of understory species changes. Edaphic factors, such as soil texture, moisture, and temperature, were significantly correlated to species composition in all three forest layers, suggesting that changes in soil physical structure due to sedimentation may drive community-level changes in these forests. [ABSTRACT FROM AUTHOR]

We modelled the effect of habitat heterogeneity on the abundance of the submediterranean Saponaria bellidifolia, a red list species in Romania. The study was designed at two scales: 100 and 0.5 m2. At larger scale, generalized additive models and canonical correspondence analysis were used to model the density of ramets, whereas at microscale, binomial logistic regression was employed to model the species’ occurrence. S. bellidifolia abundance responded sensitively to habitat type (classified as “grassy”, “rocky” and “scree”), rather than to microclimatic variables. At both scales, habitat type was the best predictor of ramet abundance, followed by slope and vegetation cover. At 0.5 m2, soil depth was also a good predictor of species occurrence. The data revealed that screes are the most suitable habitats for hosting relatively large populations of this rare species, because of occasional natural disturbances and presumably lower interspecific competition. [ABSTRACT FROM AUTHOR]

Additive influences of the invasion ability of species (invasiveness) and the characteristics of the habitat which make it invasible (invasibility) cannot fully explain grassland invasion patterns. We tested the hypothesis that different species assemblages of grassland communities may partly result from interactive influences between the relative invasiveness of available species and community invasibility. During 10 months, we evaluated seed and microsite limitation of seedling emergence, survival, and recruitment of plants belonging to species with different seed size (large-seeded species vs. small-seeded species), with in a two-phase community mosaic typical of semi-natural grasslands in the southern flooding Pampa of Argentina. Seeds of large versus small-seeded species were sown either in species-poor patches dominated by a tall tussock grass (“pajonal”) or in species-rich patches dominated by short grasses (“matrix”), subjected to different levels of canopy disturbance (cut vs. uncut). Seed addition promoted seedling emergence for 7 out of the 10 species sown, and this effect was higher for large than for small-seeded species. After seed limitations were removed, interactive effects among seed size, community state and canopy disturbance reflected a strong positive influence of seed size on plant recruitment only in cut pajonal patches. Therefore, according to the stage of invasion process, relative species success may depend on non-interactive (seed-size effect on seed limitation to seedling emergence) or interactive influences among species invasibility and community invasiveness (from seedling emergence to plant recruitment). As a general conclusion, different assemblages of species are expected to successfully colonize spatially close grassland patches, according to both the available invasible species (seed size) and the community state and stage (species composition and canopy disturbance). [ABSTRACT FROM AUTHOR]

Disturbance regimes in many temperate, old growth forests are characterized by gap-scale events. However, prior to a complex stage of development, canopy gaps may still serve as mechanisms for canopy tree replacement and stand structural changes associated with older forests. We investigated 40 canopy gaps in secondary hardwood stands on the Cumberland Plateau in Tennessee to analyze gap-scale disturbance processes in developing forests. Gap origin, age, land fraction, size, shape, orientation, and gap maker characteristics were documented to investigate gap formation mechanisms and physical gap attributes. We also quantified density and diversity within gaps, gap closure, and gap-phase replacement to examine the influence of localized disturbances on forest development. The majority of canopy gaps were single-treefall events caused by uprooted or snapped stems. The fraction of the forest in canopy gaps was within the range reported from old growth remnants throughout the region. However, gap size was smaller in the developing stands, indicating that secondary forests contain a higher density of smaller gaps. The majority of canopy gaps were projected to close by lateral crown expansion rather than height growth of subcanopy individuals. However, canopy gaps still provided a means for understory trees to recruit to larger size classes. This process may allow overtopped trees to reach intermediate positions, and eventually the canopy, after future disturbance events. Over half of the trees located in true gaps with intermediate crown classifications were Acer saccharum, A. rubrum, or Liriodendron tulipifera. Because the gaps were relatively small and close by lateral branch growth of perimeter trees, the most shade-tolerant A. saccharum has the greatest probability of becoming dominant in the canopy under the current disturbance regime. Half of the gap maker trees removed from the canopy were Quercus; however, Acer species are the most probable replacement trees... [ABSTRACT FROM AUTHOR]

We investigated the importance of clonal growth to the recovery of a common eastern North American sub-shrub, Gaultheria procumbens L. (Ericacea), after clearcut logging . Changes in vegetative growth and development of G. procumbens clones and clonal populations were examined in a chronosequence of logged stands representing different stages of successional development after clearcutting (open habitat, young regenerating forest, closed regenerating forest) and in neighboring undisturbed late-successional forests representative of presettlement conditions. We specifically quantified seedling presence and above-ground ramet production, demographic condition (e.g., sexual vs. vegetative stems), belowground rhizome growth and spread, and assessed the degree of intraspecific variation in clonal morphology and biomass allocation in stands differing in their disturbance history and degree of successional development. Recovery in G. procumbens was largely driven by the “release growth” of pre-existing clonal bud-banks in response to canopy removal. Release growth was expressed as greater ramet initiation, rhizome branching and clonal spread. Conversely, we found no evidence of sexual establishment in the species, although production of reproductive biomass (e.g., inflorescence mass, number of flowering shoots) was significant. These findings support a deterministic model of vascular resistance and resilience to catastrophic disturbance, in which recovery of forest plant communities derives from the life-history characteristics of constituent species. [ABSTRACT FROM AUTHOR]

Plant litter may play an important role in herbaceous plant communities by limiting primary production and influencing plant species richness. However, it is not known how the effect of litter interacts with fertilization. We tested for the role of litter and fertilization in a large-scale experiment to investigate effects on diversity and biomass of plant species, growth forms, native vs. non-native groups, and abiotic ecosystem components (e.g., soil moisture, PAR). We manipulated plant litter (removed vs. left in situ) and nutrient availability (NPK-fertilized vs. unfertilized) for 4 years in 314-m2 plots, replicated six times, in an old-field grassland. While many of our species-level results supported previously published studies and theory, our plant group results generally did not. Specifically, grass species richness and forb biomass was not affected by either fertilization or plant litter. Moreover, plant litter removal significantly increased non-native plant species richness. Relative to native plant species, all of our experimental manipulations significantly increased both the biomass and the species richness of non-native plant species. Thus, this grassland system was sensitive to management treatments through the facilitation of non-native plant species. We coupled biotic and abiotic components within a nonmetric multidimensional scaling (NMS) analysis to investigate treatment effects, which revealed that specific treatments altered ecosystem development. These results suggest that fertilization and plant litter may have larger impacts on plant communities and on ecosystem properties than previously understood, underscoring the need for larger-scale and longer-term experiments. [ABSTRACT FROM AUTHOR]

A broad range of herbaceous plant comunities in a Mediterranean Landscape in central Italy, running from heavily urbanized areas to semi-natural pastures, has been studied. These communities can be easily arranged along a gradient of ruderality. We inspected which of a series of soil parameters could better explain this gradient (pH, CaCO3, granulometry, N, P, C/N, P/N). We show that (1) the single most important explanatory variable is P/N ratio of soil; (2) nitrogen and carbon pools in soil are related, in the set of communities studied, with another gradient of decreasingly frequent, predictable, moderate disturbance such as trampling. We discuss the meaning of these results. [ABSTRACT FROM AUTHOR]

Since most studies of ecosystem dynamics after disturbance require longer durations of study than the life span of most research careers, many studies rely on chronosequence approaches to substitute space for time. We tested the chronosequence approach for assessing the change in plant functional type cover and leaf area index ( L) using three replicated mountain big sagebrush ( Artemesia tridentata var. vaseyana (Rydb.) Boivin) dominated ecosystems in southern Wyoming. We further tested our broader inferences of mountain big sagebrush ecosystem chronosequences by assessing whether dynamics in spatial patterning of plant functional type cover and leaf area index would compromise the chronosequence approach. We hypothesized that (1) L and total cover increase with age at similar rates across replicated chronosequences, (2) spatial autocorrelation is greatest with shrub cover, and (3) spatial autocorrelation increases with age. We failed to reject all three hypotheses. Our analyses showed that mean shrub cover, total cover, and L all increased linearly with time since disturbance across all three replicated chronosequences. While neither graminoid nor forb cover was correlated with time since disturbance, graminoid cover did show an inverse relationship with shrub cover and L. Semivariogram analysis showed that spatial patterning increased with shrub cover and time since disturbance. Thus, while we cannot yet provide a process to fit the spatial patterns, the chronosequence approach for sagebrush ecosystems recovering from disturbance has survived a rigorous test because the mean changes in shrub cover, total cover, and L were replicable across three different sites. [ABSTRACT FROM AUTHOR]

High species richness, resource availability and disturbance are community characteristics associated with forest invasibility. We categorized commonly measured community variables, including species composition, topography, and landscape features, within both mature and 15-year-old clearcuts in West Virginia, USA. We evaluated the importance of each variable for predicting the degree of forest invasion by early-establishing exotic invasive plants. Biotic variables, including overall richness (excluding exotic invasive species) and mutually exclusive native and exotic non-invasive species richness, were the strongest indicators of invasibility. Sites that were located on northeast-facing slopes, more mesic conditions, or in clearcuts were more likely to be invaded by exotic invasive plants. Invasion of clearcut sites was more dependent on available microsites (e.g., lower solar radiation, northeast-facing slopes, and lower elevations) within each site than on the condition of the surrounding landscape, whereas invasion into the mature forests was dependent more on the surrounding landscape (e.g., proximity to paved roads). Our results indicate that exotic invasive plant species in our study area respond similarly as other plant species to resource availability and that competitive interactions are relatively unimportant. Current invasion into this landscape is more likely to be a passive reaction to site conditions instead of a driver of change. [ABSTRACT FROM AUTHOR]

We used clearcut logging in establishing four replicated sizes of canopy openings (0.016, 0.08, 0.4, and 2.0 ha) in a southern Appalachian hardwood forest in 1981 to examine the long-term effects of disturbance size on plant community structure, biomass accumulation, aboveground net primary productivity (NPP), and mode of recovery. The reestablishment of NPP and biomass following logging was 6–7-fold greater in large than small openings by 17 years. Total biomass in the 2.0 ha openings (127.3 Mg ha−1) recovered 59.5% as NPP (19.7 Mg ha−1 yr−1) reached 225% of precut forest levels. Biomass accumulation was 2.6–3.6-fold greater in interior than edge locations of all but the 0.016 ha gaps. The absence of significant patch size or edge vs. interior differences in tree densities suggests that growth rates of individual trees were enhanced in more insolated microenvironments. Sprouting (86–95% of tree NPP) was much more important than advance regeneration (4–10%) or seedling germination (<2%) during early recovery in all opening sizes. Canopy dominant Quercus and Carya trees exhibited limited sprouting following disturbance. Instead, shade-intolerant Robinia pseudoacacia and Liriodendron tulipifera were major sprouters that used N-fixation ( Robinia) and rapid growth ( Liriodendron) in attaining 7.4 and 5.9 fold greater biomass accumulation, respectively in 2.0 ha than 0.016 ha opening sizes. Seedling germination and understory production were extensive in all openings following logging, but declined rapidly as the young tree canopy began closing by 4–6 years. The relative importance of shade-intolerant tree biomass approximately doubled over 17 years as shade-tolerant tree seedlings, herbs, and shrubs gradually regained importance under the emerging canopy. Sprouting caused the persistence of a tree species composition in all openings that remained relatively similar to the precut forest. Large disturbances on mountain slopes of the southern Appalachians generally promote sprouting and rapid recovery, whereas small disturbances in low-elevation cove forests lead to a gradual recovery through seedling germination and/or advance regeneration. Continued logging in the southern Appalachians will increase the relative size and frequency of large disturbances, further the importance of sprouting of shade-intolerant species, and lead to more even-aged forest stands throughout the region. [ABSTRACT FROM AUTHOR]

Landslides are a frequent disturbance in montane tropical rainforests that result in heterogeneous environments for plant and soil development. Natural inputs of organic matter and associated nutrients contribute to soil fertility patchiness within landslides. To test the importance of organic matter and nutrient addition to landslide soil fertility and plant growth, we mixed three types of organic matter substrates that are common to landslides ( Cecropia leaves, Cyathea fronds, and forest soil) and commercial fertilizer into recently eroded soil on five landslides in Puerto Rico. In addition, we sowed seeds of two common landslide colonists ( Paspalum and Phytolacca) into the soil treatment plots in order to test treatment effects on seed germination and seedling growth. Soils, seed germination, and seedling growth were monitored for one year and the field experiment was replicated in a one-year screen-house experiment. Despite highly variable initial landslide conditions, responses to soil treatments were similar across all five landslides. The forest soil addition increased total soil nitrogen and soil organic matter on landslides within 60 days, whereas Cecropia leaves provided increased soil organic matter only after 210 days. Commercial fertilizer increased plant-available soil nitrogen and phosphorus within 60 days, and also increased seed germination of Paspalum seeds when compared to soils treated with Cecropia leaves. Despite these positive effects of treatments on soils and germination, there were no treatment effects on seedling growth in the field, perhaps due to leaching or other losses of soil nutrients evident in the lack of significant treatment differences in soil resources at 370 days. In the screen-house, forest soil and commercial fertilizer treatments significantly increased soil fertility and seedling growth of both Paspalum and Phytolacca compared to control treatments. These different responses to three common types of organic matter inputs create patchy soil conditions with important implications for plant colonization and landslide succession. [ABSTRACT FROM AUTHOR]

Dormancy in the hard seed coats of Mediterranean species is considered a strategy that enables persistent seed banks to be formed in the soil. An important factor related to seed coat fracture and dormancy breakage in Mediterranean ecosystems is heat. Nevertheless, the effect of factors other than heat on dormancy breakage in these species has hardly been studied. To investigate the different ecological factors involved in germination, in the laboratory we applied several scarification treatments to seeds with chromatic polymorphism. We evaluated the effect of soil seed depth during experimental burns by sowing seeds at −1, −3 and −5 cm in the soil profile, and we also studied the effect of seed origin on the posterior germination of seeds from 4 and 10 year-old shrubs as well as from the soil seed bank. U. parviflorus shows clear chromatic polymorphism: its brown seeds present higher dormancy levels than its yellow seeds. The different techniques of dormancy breakage result in different degrees of germination; the highest degree of germination is generated by the mechanical treatment, followed by the acid and the heat treatments, in that order. The depth of the seeds in the soil determines the temperature thresholds and the residence times of these temperatures and whether they stimulate a massive germination at the −1 cm soil profile or only a slight germination at the −5 cm depth. Seeds recently produced by the plant show higher dormancy levels than seeds extracted from soil seed banks. Dormancy levels also depend on the shrubland age used for extracting the soil samples (3>9 years old). In effect, from the point of view of dormancy, the germination behaviour of U. parviflorus seeds seems to follow a multiresponse strategy based on different seed populations and involving both biological and abiotic processes to break dormancy. [ABSTRACT FROM AUTHOR]

Bray and Curtis ordination was used to explore which environmental variables explained importance values and the presence–absence of tropical tree seedlings, saplings and adults in La Escondida-La Cabaña, Sierra de Manantlán, Jalisco, Mexico. The diameters of trees ≥2.5 cm DBH and the presence and height of seedlings and saplings were measured in nine 0.1 ha sites. Four matrices including presence–absence data and importance value indices for trees and seedlings and saplings were analyzed through Bray and Curtis ordination. The matrices were based on density, frequency, and dominance of adult trees as well as seedlings and saplings. The environmental matrix consisted of 18 variables, including elevation, slope, canopy gaps, disturbance, and soil variables. We recorded 63 tree species and 38 seedling and sapling species in the nine sites. The ordination explained 70.9% of the variation in importance value data for trees and 62.6% for seedlings and saplings. The variation explained in presence–absence data for trees was 67.1 and 77.4% for seedlings and saplings. The variance in the ordination axes of seedlings and sapling presence–absence data was poorly explained by the number of gaps in the tree, shrub, or herb layer, suggesting little light specialization by seedlings and saplings. Habitat specialization for soil nutrients appears to be important in explaining the presence–absence of seedlings and saplings. Seedling and sapling specialization along different soil microsites could promote species coexistence in this forest, while heterogeneity in light conditions may instead determine differences in growth and, thus, importance value of trees. We hypothesize that in tropical dry forest in Jalisco, Mexico, a habitat specialization for soil resources is likely more importance at early stages in tree life histories than in later life history. [ABSTRACT FROM AUTHOR]

Persistent soil seed banks are presumed to play a significant role in bridging and exploiting the effects of major flood disturbances in riparian systems. However, presently there is little empirical data on the correlation between flooding and seed bank dynamics. The objective of this study was to assess the impact of inter-annual flooding variation on soil seed bank dynamics in flood-meadows. We analysed seed bank composition at 46 plots along the northern Upper Rhine River, Germany, after two periods with contrasting flooding conditions (1996-1997: low and rare flooding; 1998-1999 high and very frequent flooding). Between both sampling occasions the total number of seedlings emerging from the seed bank decreased by 50% and average species-richness per sample by 30%, irrespective of the sampling depth. Similar decline occurred in mesophytes and wetland species as well as over different functional groups, with the exception of legumes. Considerable compositional changes in the seed bank were also indicated by shifts in DCA ordination and by the comparison of similarity matrices from both years with the Mantel test. The Mantel test also confirmed that compositional changes were more pronounced in the seed bank than in aboveground vegetation. There was poor correlation between the decline in total seed densities and species in the soil seed bank and environmental variables such as flooding duration and ecological groupings such as floodplain compartment and seed bank community types. Further distinct patterns of decline and persistence were evident only at the species level. While 21 species displayed a significant decline, only two species increased. Annual arable weeds and perennial ruderals with high temperature or nutrient requirements to break dark dormancy were amongst the most significantly declining species. In contrast, there was no decline in typical winter annuals and certain perennials that preferably germinate in autumn and fall into dormancy at the beginning of the warm period. Generally, differences in germination requirements and dormancy cycles of species gave the best explanation for the patterns of decline and persistence after intense flooding. Given these findings, the heavy depletion of the persistent soil seed bank we observed is most likely attributed to a post-flood germination flush due to very favourable recruitment conditions prevailing during the draw-down period after early summer flooding in 1999. The results of our study suggest that persistent soil seed banks are of outstanding importance to bridge and exploit the highly variable hydrological conditions that are typical of flood-meadows. Thus, they play a significant role in maintaining the floristic diversity of this habitat type over time and space. [ABSTRACT FROM AUTHOR]

Large, infrequent natural disturbances have been proposed as a key component in determining the distribution and abundance of Nothofagus in southern Andean temperate forests. In this study, a comparison of influences of small-to-intermediate-sized natural disturbances and effects of selective logging on the establishment and growth of Nothofagus forests in south-central Chile are synthesized with the results of other forest studies to develop general hypotheses on the regeneration dynamics of southern Chilean Nothofagus alpina forests. A synthesis is given for regeneration pattern in the abundance of 18 temperate rain forests in the Andean Range, Chile. The study aimed to determine the role of life history differences in promoting coexistence of the five main tree species (N. alpina, N. dombeyi, N. pumilio, N. macrocarpa and Laurelia philippiana) in N. alpina-dominated forests. Age data reported for N. alpina, a shade mid-tolerant emergent tree in the temperate rain forests of southern Chile, indicate maximum lifespans > 650 years, figures unprecedented for N. alpina. In low elevation stands, N. alpina coexists with broad-leaved evergreen tree species, such as L. philippiana, L. sempervirens, Persea lingue, and in these stands an intermittent establishment of Nothofagus occurred and appeared to be most dependent on small-to-intermediate disturbances. In high-elevation stands, in contrast, Nothofagus establishment was less dependent on disturbance, regeneration being much more continuous even in the absence of canopy openings. The forests studied provide another example of the general pattern of increasing dependence of Nothofagus on disturbance towards the more productive end of the environmental gradients. As a long-lived pioneer and despite its dependence on disturbance in lowland sites, N. alpina has been subject to selection for complementary growth, adult survivorship, and mid-tolerance to shade. Thus, interspecific differences in juvenile and adult life history characteristics of N. alpina and its competitors may be sufficient to maintain its persistence in the landscape. In conclusion, this study of population structures and replacement patterns provides a comprehensive picture on our understanding of regeneration dynamics and trait differentiation in southern temperate forests by recognizing both the influences of environmental gradients (i.e., altitude) on competing species and the disturbance regimes. [ABSTRACT FROM AUTHOR]

The objective of this study was to evaluate the effects of prolonged recreational disturbance on vegetation structure. Therefore, plant species variation along trails was determined in some common vegetation types of Flanders (Belgium). The study sites included two forest types, one grassland and one heathland. Inventory was conducted in transects perpendicular to trail centre. Field data on frequency, percent cover and multiple habitat variables were collected in the path, transition and undisturbed zones of the respective transects. In order to determine plant response groups in relation to recreational disturbances, distribution of plant species across paths was linked with their respective life form and plant strategy. Generally, species diversity increased towards the trail centre. The intensity of soil compaction was highly correlated with species cover and composition. Since soil compaction is interrelated with other aspects of the biotic and abiotic environment, it is clear that trampling exerts multiple stress on vegetation. [ABSTRACT FROM AUTHOR]

In Mediterranean ecosystems, as well as in other fire prone ones, seedling establishment by some species is particularly favorable right after fire. It has been well established that many plants from these ecosystems have enhanced germination after exposing their seeds to heat or to chemicals related to the passage of fire. Less known it is how variable is this germination response among seeds from different sources (populations, individuals within a population, or parts of the plant within an individual) and whether such differences persist after exposing the seeds to germination-triggering, fire-related cues. In this work we studied the germination response to several fire-related factors of the Mediterranean, lignotuberous shrub Erica australis. This species produces a very combustible fuel, and resprouts vigorously after fire, but conditions for seedling establishment are poorly known. The objective of this study was to evaluate how variable was the germination response of this species among seeds from different sources, and whether this variability changed after exposing the seeds to fire-related factors. Seeds from three different sites, from different individuals at each site, and from different branches within each individual were set to germinate under control conditions or after exposing them to heat or to additions of nitrogenous compounds. Germination was highly variable among populations, individuals within populations or branches within individuals. Exposure to moderate heat promoted germination, but high temperatures were lethal. Differences at the various provenance levels were, however, also observed after heat exposure. Germination of seeds exposed to nitrogenous compounds increased, but differences among populations were always significant. These results indicate that the germination of Erica australis may be linked to disturbances, but it does not seem to be particularly related to passage of fire. On the contrary, this type of response appears more common in plants from heterogeneously disturbed environments. [ABSTRACT FROM AUTHOR]

In the Ninole ridges of Hawai'i we investigated how landslides influence ecosystem development and modify land cover and the distribution of biomass. We estimated above and below-ground biomass, and N and P concentration in leaves (Metrosideros polymorpha) and very fine roots (all species), for vegetation developing on landslides of three age classes (young, < 18 yr; intermediate, ≤ 42 yr; and old ca. 124 yr) and on undisturbed soils (ca. 430 yr). The undisturbed soils were derived from ash underlain by basalt. To quantify changes in land cover and the distribution of biomass we combined our estimates of biomass with estimates of the area covered by each vegetation class. The latter estimates were obtained from the analysis and classification of color-infrared aerial photographs. Average above- and below-ground biomass for the herbaceous vegetation (young landslides) was 10.4 and 3.2 t/ha, whereas for the ohia-non ash forest (intermediate and old landslides) was 37.5 and 5.2 t/ha, respectively. For the ohia-ash forest (undisturbed sites), average above and below-ground biomass was 354.6 and 9.5 t/ha, respectively. Average foliar N for the herbaceous and ohia-non ash forest ranged between 0.80–0.84%, whereas root P between 0.056–0.040%, respectively. For the ohia-ash forest, average foliar and root P was 0.918% and 0.036%, respectively. Based on changes in vegetation cover during the last 430 yr, we estimated rate of disturbance at 15% per century or equivalently that 53 t/ha biomass per century exited through the system. The removal of ash-derived soils by landslides significantly alters successional trajectories and by doing so may be transforming the Ninole ecosystems in irreversible ways. [ABSTRACT FROM AUTHOR]

We examined the importance of the dwarf mistletoe Arceuthobium americanum in determining the size-class structure of Pinus contorta var. murrayana forests in the central Oregon pumice zone. We considered the interrelationships among A. americanum abundance, P. contorta population structure, soil type, site topography, the density of competing tree and shrub species, and understory woody litter. We found that A. americanum was associated with reduced live basal area (P < 0.001) and increased bole diameter variability (P < 0.001) of P. contorta, and also with an increase in the density of saplings and seedlings (P < 0.001) at the expense of dominant and tall-intermediate trees. Saplings and seedlings were over 15 times as dense in heavily infested stands as in uninfested stands. Reflecting this shift toward smaller size classes, each unit increase in stand-level dwarf mistletoe rating (DMR) reduced live basal area by 1.32 m2/ha and reduced the density of dominant-sized trees by 45 trees/ha. The significant reductions in live basal area and the density of dominant trees contrast with other studies conducted in P. contorta stands containing A. americanum. However, DMR uniquely explained less than 25% of the total site-level variance in all size class variables except sapling density (36%), and varied in importance relative to interspecific competition between Pinus ponderosa and P. contorta, the effects of abiotic factors, and intercohort dynamics in determining the structure of P. contorta stands across the study site. DMR uniquely explained less of the variance in live basal area (12%) than abiotic factors (28%) and P. ponderosa density (16%), and only 15% of variability in the density of the dominant P. contorta size class. Furthermore, the relative effects of DMR on specific P. contorta size classes varied across patch- and stand-level spatial scales. We conclude that the relative importance of the effects of A. americanum on specific P. contorta size classes depends on spatial scale, and should be interpreted in relation to patch dynamics, site-level topoedaphic variability, and interspecific competition among the dominant tree species present. [ABSTRACT FROM AUTHOR]

We studied soil and forest floor conditions, regeneration patterns oftrees, and forest structure and floristic composition along a gradient ofdominance by Pinus spp. in disturbed stands in thehighlands of Chiapas, southern Mexico. Seedlings, saplings, and adults of treespecies were counted and measured in 2-3 circular plots (1000m2 each) in 36 forest stands (a total of 38 treespecies). Dominance of broadleaved trees other than Quercusspp. was negatively correlated with basal area of Pinusspp. ( P < 0.001). Soils of pine-dominated stands weremore compacted, less acidic, and less fertile (lower C.E.C., nitrogen, andorganic matter content). Numerous broadleaved trees may depend on anoak-dominated canopy to regenerate, and changes associated to pinelandsexpansion may compromise their long-term persistence. However, seedlings andsaplings of Quercus spp. were abundant in stands with bothan oak- or pine-dominated canopy. The results suggest that a plan forsustainable forest utilization in the study region, and in other similarpopulated tropical highlands, could take advantage of the abundant oakregeneration and coexistence with pines in the canopy. [ABSTRACT FROM AUTHOR]

This study presents a quantitative partitioning of the variance infloristic data from grazed semi-natural vegetation of summer farms inRøldal, western Norway. The data consist of 189 taxa recorded in 1074-m2 sample plots within 10 summer farms with differentland-use histories. Thirty-five environmental variables were recorded,includingaltitude, slope, radiation, geology, soil chemistry, and past and presentland-use. A series of (partial) canonical correspondence analyses (CCAs) wereused to partition the total variation into within-farm and between-farmcomponents, and to investigate the explanatory power of different groups ofenvironmental and land-use variables at the two scales. The results show that:(1) although local gradients are of overriding importance for floristiccomposition, landscape-scale processes also contribute significantly to theobserved patterns; (2) the measured land-use and environmental factors accountfor comparable amounts of compositional variance at the two scales; and (3)evenif the relative contributions of the two classes of explanatory variables arecomparable, details differ, showing that broad-scale environmental and land-usepatterns are not just scaled-up versions of the fine-scale patterns or viceversa. These results support a multi-process view of vegetation patterns. [ABSTRACT FROM AUTHOR]

The production of plant macroremains was studied in a conifer forest twomonths after it burned in 1996 in northern Québec. The proportions ofvarious types of charred and uncharred pieces (needles, cones andwood) produced by black spruce ( Piceamariana) and jack pine ( Pinusbanksiana) were determined by sampling around individual trees.Both species produced equivalent masses of charred material, but pieces of woodcharcoal from jack pine are generally larger that those of black spruce. Theproportion of charred versus uncharred needles is the bestindicator of the species dominance in the forest. Although the fall of uncharredneedles is delayed from the time of a fire, they contribute to more than half ofall remains produced. Jack pine cones remain on the tree for a long time after afire, while charred cones of black spruce are dehiscent (cones come off thebranches easily). Trees are poor wood charcoal producers compared toundergrowth shrubs. As a result, the macroremains assemblage associated with afire event is made up of large amount of uncharred material from trees and alarge proportion of charred pieces produced by undergrowth vegetation. Modernassemblages of plant macroremains indicate that in order to reconstruct pastvegetation associated with fire disturbance, it is important to distinguishbetween the various types of remains, because wood charcoal is mainly producedby material that is already dead. [ABSTRACT FROM AUTHOR]

A special type of clonal growth, spread by lateral roots, ishypothesized to be a favourable trait of invasive, opportunistic plant speciesof disturbed habitats. We tested this hypothesis for the invasive forb Rorippa austriaca (Brassicaceae). Regenerationfrom root fragments, subsequent vegetative spread and allocation patterns inrelation to varied nutrient supply and intensity and pattern of interspecificcompetition were analyzed in container experiments. Regeneration success fromroot fragments was 100% and clonal spread was rapid but vegetativeperformance was strongly reduced under unfertilized conditions and,particularly, when interspecific competition was present. While the ratio ofabove- to belowground bio-mass did not differ considerably betweentreatments, R. austriaca allocated a high amount ofresources to belowground growth resulting in low aboveground but highbelowground biomass at harvest time relative to the matrix vegetation.Differences in shoot number or biomass between simulated gaps and denselyvegetated quadrants in the containers were (relatively) weak.Reproductive effort was less reduced under low resource levels, and the clonesdid not set seed at all, irrespective of the treatment. Our results show thatclonal growth by lateral roots and plasticity in clonal growth patterns in R. austriaca promote both exploitation of gaps andnutrient-rich microsites and resistance to competitors. Such plasticity,combined with its ability to regenerate from widely-dispersed rootfragments, contribute to the ability of the species to invade and persistwithindisturbed and spatially heterogeneous habitats. [ABSTRACT FROM AUTHOR]

Shadetolerance and regeneration of some heavily exploited timber species( Uvariopsis congensis, Antiaris toxicaria, Funtumia africana,Prunus africana and Aningeria altissima)were studied in two adjacent sites in Kakamega tropical rain forest. The twosites have been subjected to different logging intensities. Within one of thesites, relatively intact areas were compared with adjoining more disturbedones.Seedling presence or absence in mature forest phase, sapling proportions in gapand the mature phase and diameter distributions were used to classify thespecies into shade tolerance groups. Results indicate that except for Uvariopsis, the species are non-pioneer lightdemanders and could fit into three shade tolerance groups: A groupresembling pioneers but with seedling in shade, a mid-tolerant group andone showing a high shade tolerance. Amount of regeneration and disturbance wererelated: regeneration being higher in the more disturbed site except for Uvariopsis. The intersite differences were mirrored whendisturbed and adjoining more intact areas within one of the sites werecompared.Species more abundant in the disturbed site also generally scored low in shadetolerance. However, the mid-tolerant Funtumia andnot Prunus or Aningeria that hadlowershade tolerance scores, dominated regeneration in the disturbed forest parts.The shade tolerance data suggest that Uvariopsis, Funtumiaand Antiaris could be amenable to natural forestmanagement. For Prunus and Aningeria,artificial regeneration in buffer plantations around the natural forest may beabetter option. [ABSTRACT FROM AUTHOR]

Theeffects of light competition and litter on seedling recruitment and theperformance of established individuals were examined in Violapalustris. This polycarpic perennial plant was a common component ofspecies-rich fen meadows in northwestern Germany until the middle of thiscentury, but today is considered to be regionally endangered. From summer 1996until summer 1998 a bi-factorial field experiment combining three standing croptreatments (mowing, thinning, control) with two litter layer treatments (litterremoval, no litter removal) was carried out in an abandoned fen meadow toinvestigate the effects of these factors both on V.palustris and on aboveground species composition and diversity.MANOVAs (multivariate analysis of variance) revealed significant treatmenteffects for the performance of V. palustris related toexperimental manipulation. Mowing increased vital attributes including thenumber of rosette leaves, the percentage of individuals with chasmogamous andcleistogamous flowers and the number of rhizomes. In contrast, the length of theleafstalk, mean leaf area and the length of rhizomes were negatively associatedwith mowing. Litter removal significantly increased the number of V.palustris seedlings. The removal of the litter layer already resultedin an increase in aboveground species richness in the first year of theexperiment. After three years of experimental manipulation, both abovegroundspecies richness and diversity were positively associated with mowing.Redundancy Analysis showed that typical wet meadow species ( Cardaminepratensis, Lychnis flos-cuculi, Lotuspedunculatus) and species of mesotrophic fens ( Violapalustris, Agrostis canina, Potentillapalustris) were positively influenced by mowing. It can be concludedthat V. palustris recruitment and growth is promoted by aregular disturbance regime on fen meadows. The conservation of many typicalspecies of wet meadows and fens in northwestern Germany depends on moderategrazing or cutting. These management types prevent successional changes whichlead to an increase in standing crop and the development of a litter layer, bothof which can cause local extinction. [ABSTRACT FROM AUTHOR]

Maps of physiognomic classes of vegetation in the northeastern UnitedStates before European human impact and for the late 20th century, based onpollen data, closely resemble each other, indicating a robust pattern in theface of the novel and extensive disturbances of the last 500 years. On the otherhand, species abundances have changed considerably, with today's vegetationhaving less Fagus L. and TsugaCarrière and more Betula L., Picea A. Dietr., and Abies Miller. Picea and Castanea Miller increasedfrom 1700 to 1900, but have decreased in this century. The difference betweenthe main physiognomic classes has remained strong, however, with continuouslymore Picea, Abies and Betula in the north and more QuercusL. and Carya Nutt. in the south, indicating the dominanceof climate in regulating the relative abundances of these genera. Thevegetational patterning within these broad classes has changed in some placesand remained stable in others, suggesting differences in local factors thatdetermine relative species abundances. The influence of slight altitudinalgradients, for example, may influence forest composition only after severalgenerations of trees, while substrate differences may be apparent in the firstforest that regenerates after agricultural abandonment or logging. Forests oftoday, therefore, reflect complex interactions of disturbance and environment,and may be in equilibrium with some but not all features of their currentenvironments. [ABSTRACT FROM AUTHOR]

Species distribution and abundance patterns in the southern Cascades are influenced by both environmental gradients and fire regimes. Little is known about fire regimes and variation in fire regimes may not be independent of environmental gradients or vegetation patterns. In this study, we analyze variation in fire regime parameters (i.e., return interval, season, size, severity, and rotation period) with respect to forest composition, elevation, and potential soil moisture in a 2042 ha area of montane forest in the southern Cascades in the Thousand Lakes Wilderness (TLW). Fire regime parameters varied with forest composition, elevation, and potential soil moisture. Median composite and point fire return intervals were shorter (4-9 yr, 14-24 yr) in low elevation and more xeric white fir (Abies concolor)-sugar pine (Pinus lambertiana) and white fir-Jeffrey pine (P. jeffreyi) and longest (20-37 yr, 20-47 yr) in mesic high elevation lodgepole pine (Pinus contorta) and red fir (Abies magnifica)-mountain hemlock (Tsuga mertensiana) forests. Values for mid-elevation red fir-white fir forests were intermediate. The pattern for fire rotation lengths across gradients was the same as for fire return intervals. The percentage of fires that occurred during the growing season was inversely related to elevation and potential soil moisture. Mean fire sizes were larger in lodgepole pine forests (405 ha) than in other forest groups (103-151 ha). In contrast to other parameters, fire severity did not vary across environmental and compositional gradients and >50% of all forests burned at high severity with most of the remainder burning at moderate severity. Since 1905, fire regimes have become similar at all gradient positions because of a policy of suppressing fire and fire regime modification will lead to shifts in landscape scale vegetation patterns. [ABSTRACT FROM AUTHOR]

Inventories of trees and lianas were performed at a 13-year interval (1979–1992) in an evergreen forest in north-east Gabon. The number of trees decreased while their basal area and species richness increased. In contrast, both the richness and density of lianas decreased. Lianas with active climbing mechanisms were the main types to disappear. [ABSTRACT FROM AUTHOR]

This study explored the determinants of spread of four alien Pinus species and the ability of models to predict invasion dynamics in a complex fragmented landscape. The role of environmental factors, natural and anthropogenic disturbance in relation to invasion history was assessed for different stages in the invasion process using a Geographic Information System. Pines escaped from plantations over the past 30 years and spread into the natural semi-arid shrubland (renosterveld). The pattern of spread was compared with a simulated random distribution using two different techniques, a standard logistic regression, and a new recursive modelling approach (Formal Inference-based Recursive Modelling; FIRM). FIRM analysis improved the accuracy of predictions and revealed interactive effects of variables hidden by the logistic regression analysis. More than 80% of isolated pine individuals were found in 20% of the habitat classified as suitable by the models. Soil pH was the most important predictor for the distribution of isolated trees, whereas the establishment of dense pine stands was largely determined by fire history. Differences in invasive behaviour could be explained by species attributes such as limited dispersal for P. canariensis, and better drought-tolerance for P. halepensis. Sixty-five percent of the current pine distribution was accurately predicted by the spatial distribution of the first trees to have invaded. Such models could be used to predict potential spread of invasive plants and gain a better understanding of the main factors driving the invasion process. However, the spread of invasive species in fragmented landscapes, strongly modified by human activities, is very complicated, and the spread remains difficult to predict in the long term. The dynamics of invasion are discussed in relation to changes in land use and disturbance regime. [ABSTRACT FROM AUTHOR]

In southern African savannas, bush encroachment is a major problem for range managers. However, little is understood of the actual regeneration processes leading to it, and in particular the role of soil seed banks. The horizontal (between microsites) and vertical (with depth in litter and soil) distribution of soil seed banks of the microphyllous woody species, Acacia tortilis, A. nilotica and Dichrostachys cinerea (all legumes of the Mimosoideae), were quantified in an area with low intensity grazing (reserve), and a bordering cattle farm with high intensity grazing (farm). Species differed in seed bank densities between microsites and sites. Seed densities for all species were highest below parent tree canopies and decreased with distance beyond the canopy, and with soil depth. D. cinerea had the smallest seed bank associated with parent trees, particularly on the farm (8 vs. 1643 seeds/tree on the reserve), A. tortilis had the largest (6357, 31910), with A. nilotica intermediate (1789, 1906). The proportion of current (recently fallen) versus old (≥1 year old) seeds differed between species and sites. These species form at least short-term persistent seed banks with the old seeds largely representing the persistent seed bank. Seed densities in the open (inter-canopy) and those dispersed under either of the other two (non-parental) study species were much lower than those associated with parent trees. The latter were mostly found under the acacias (single-stemmed) rather than D. cinerea (multistemmed). Total seed store per parent plant increased with plant size (best fits were mostly power curves of canopy area). A large proportion of intact seeds were viable, namely 81–84% for A. tortilis, 68–77% for A. nilotica and 63–78% for D. cinerea, with no differences between sites. Viability tended to increase with depth of burial, except for A. nilotica seeds at the 3–5 cm depths on the farm. At the landscape scale there were 1.5 million and 140 000 A. tortilis seeds/ha on the reserve and farm respectively, with corresponding values of 2000 and 31 000 for D. cinerea, and 23 000 and 86 000 for A. nilotica. [ABSTRACT FROM AUTHOR]

In many temporary wetlands such as those on the Northern Tablelands of New South Wales Australia, the development of plant communities is largely the result of germination and establishment from a long-lived, dormant seed bank, and vegetative propagules that survive drought. In these wetlands the pattern of plant zonation can differ from year to year and season to season, and depth is not always a good indicator of the plant community composition in different zones. In order to determine which aspects of water regime (depth, duration or frequency of flooding) were important in the development of plant communities an experiment using seed bank material from two wetlands was undertaken over a 16 week period in late spring–early summer 1995–1996. Seed bank samples were exposed to 17 different water-level treatments with different depths, durations and frequencies of flooding. Species richness and biomass of the communities that established from the seed bank were assessed at the end of the experiment and the data were examined to determine which aspects of water regime were important in the development of the different communities. It was found that depth, duration and frequency of inundation influenced plant community composition, but depth was least important, and also that the duration of individual flooding events was important in segregating the plant communities. Species were grouped according to their ability to tolerate or respond to fluctuations in flooding and drying. The highest biomass and species richness developed in pots that were never flooded. Least biomass and species richness developed in pots that were continuously flooded. Short frequent floods promoted high species richness and biomass especially of Amphibious fluctuation-tolerator species and Amphibious fluctuation-responder species that have heterophylly. Terrestrial species were able to establish during dry phases between short floods. Depth was important in determining whether Amphibious fluctuation-tolerator or Amphibious fluctuation-responder species had greater biomass. Longer durations of flooding lowered species richness and the biomass of terrestrial species. Experiments of this kind can assist in predicting vegetation response to water-level variation in natural and modified wetlands. [ABSTRACT FROM AUTHOR]

The Chilean matorral is characterized by multispecific shrub clumps in dry areas but has a continuous canopy in wetter sites. It has been hypothesized that this difference is due to easier recolonization of open patches by shrub seedlings under more mesic conditions. Within the mesic range of the matorral we designed a field experiment to compare shrub seedling emergence, growth, and survival under the closed canopy of a secondary forest versus three types of open patches: burned, cleared of shrubs but with a herbaceous layer present, and clear without a herbaceous layer. After the first summer, survival of Quillaja saponaria seedlings was 75% in the burned site, 30% in the cleared patch without herbaceous vegetation, and 15% with herbaceous vegetation present, whereas there was 0% survival under the secondary forest canopy. After eight years, the percentages had dropped to: 22%, 12% and 3%, respectively. These results contrast strongly with the seedling establishment patterns in drier areas of the matorral where early seedling survival is higher under the shade of large shrubs. In the experimental mesic sites, seedlings did best on the burned site, not only in terms of survival, but also in terms of growth. After one year, seedling mean height was 10.2 cm in the burned site, whereas 3.8 cm and 5.3 cm in the cleared patches without and with herbaceous respectively. After eight years, mean height differences between treatments had increased further: 147.7 cm in the burned site, 40.3 cm in the cleared patch without herbaceous cover and 13 cm in the cleared patch with herbs. Our results indicate that the facilitative effect of nurse shrubs on seedling establishment found in dry ranges of the matorral is less important in more mesic sites. This difference may explain the continuous shrub cover in relatively mesic areas as opposed to the characteristic patchy structure of the matorral in its drier range. [ABSTRACT FROM AUTHOR]

Tree regeneration outcomes are challenging to generalize and difficult to predict. Many tree species can establish new propagules in a variety of post-disturbance environments and many different reproductive mechanisms may be used. In order to develop conceptual models that accurately reflect reproductive potential, we need a better understanding of the similarities in regeneration ecology among species. We used information from the forest ecology literature to evaluate the reproductive attributes of sixty-two tree species in the central hardwood region of the eastern United States. Each species was classified categorically for features such as flowering, seed production and dispersal, seed dormancy, germination requirements, seedling characteristics, and vegetative reproduction. Cluster analysis (Jaccard's similarity coefficient, complete linkage method) and ordination (homogeneity analysis) were used to separate nine groups (guilds) of species that had similar reproductive attributes. Individual attributes that had high variance in the first and second dimensions included: seed banking, seed dispersal, seedling shade tolerance, and seedbed requirements. Members of each guild had similar levels of reproductive specialization and guilds were either pioneer-like, opportunistic, or persistent. Pioneer guilds included: short-lived or fugitive species that colonize sites rapidly and are too shade intolerant to replace themselves; shade-tolerant species that colonize frequently disturbed sites; and stress-tolerant pioneers that survive on dry or nutrient-poor sites. Opportunistic guilds contained species that are remarkably versatile in their reproductive effort. The most flexible opportunists can colonize new sites, maintain seed in a seed bank, sprout from existing stems and persist as a seedling or sapling bank. Persistent guilds contain species that develop and maintain advance regeneration. These include: species with moderate understory tolerance that regenerate via cycles of dieback and resprouting; and more tolerant species that maintain seedling or sapling banks. Our regeneration guilds may provide a useful approach for more realistically representing large and diverse sets of tree species in forest ecosystem models. [ABSTRACT FROM AUTHOR]

The sources of seed for seedbank build-up in an acidic grassland were identified from analysis of differences in seedbank build-up over one year between plots where the input of rabbit pellets to ‘seed-free’ soil had either been left or removed. In parallel, the flux of seed arriving in rabbit pellets was monitored. Pellet seed content and total seed input were highest in late summer/early autumn and again in the spring. The seed content of the pellets was dominated by a small number of species: Sagina apetala, Senecio jacobaea, Urtica dioica and Veronica arvensis. Smaller seeded species were more likely to be present as germinable seed in the pellets. Seedbank build-up as a result of wind, splash or adhesive dispersal totalled 547 seedsm2. The additional effect of allowing seed input in pellets was 267 seedsm2, though this increase was not significant. The total increase in seedbank over one year was equivalent to 15–20% of the seedbank present in undisturbed soil. Eight species showed a significant build-up of seed in the seedbank over one year as a result of all means of dispersal, but only Myosotis discolor showed a significantly higher soil germinable seed content in the plots where pellets had been allowed to remain in situ. The build-up of seed in the seedbank is contrasted with the build-up of vegetation on disturbed areas within the same system. [ABSTRACT FROM AUTHOR]

We report tree community diversity, guild composition, and forest structure from three 1.2 ha (100 m × 120 m) permanent forest research plots on Tutuila, American Samoa, an isolated volcanic island in the South Pacific Ocean. Plots were established in three habitat types of lowland hill forest: two in mature tracts that differed in substrate type (talus vs non-talus), and a third in a 30–40 year-old abandoned plantation on non-talus soil. We encountered a total of 57 tree species ≥10 cm dbh. Richness was similar across sites, but composition differed substantially. We were able to classify with confidence 24 tree species into four distinct guilds based on forest-type preference: early successional or persistent successional (disturbed forest), generalist (no preference), or mature-phase. Sample size limitations or interactions between site age and substrate precluded categorization of all species. Thirty-eight percent (9/24) of the tree species were successional, a result which contrasts sharply with data from (formerly) continental forest in Panama. Spatial distributions of 33 species revealed 17 species exhibiting clumping or hyperdispersion (i.e., regular spacing) in at least one site. Possible non-anthropogenic mechanisms promoting clumping in the plots were (a) topography (edaphic), (b) gap affiliation, (c) inefficient or altered patterns of propagule dispersal, and (d) lack of natural seedling predators. Forest structure differed across site type, with stem densities highest in regenerating forest; conversely, regenerating forest had the lowest basal areas. Steep talus forest sequestered the most carbon (344.3 Mg ha−1), and secondary forest sustained only 42% of levels found in talus forest (145.5 Mg ha−1). Mature forest on non-talus soil sequestered the majority of carbon in mid-sized trees (30–50 cm dbh). Future assessments of land-use cover and biomass will provide for a complete estimate of the carbon budget of Tutuila. Finally, the results of this study suggest that conservation of the native fauna is essential in retaining the potential for regeneration of native forest after large-scale disturbance. [ABSTRACT FROM AUTHOR]

The arrangement of trees within a stand by location and age (stand structure) is in part determined by the life history strategy of the species and the disturbance history of the stand. In western North America such disturbances are often the product of wildfires and human management activities. The current study uses spatial analysis to characterize three stands of Pinus torreyana with known disturbance histories. Two stands are located at Torrey Pines State Reserve (TPSR). Of these, one stand has burned twice since 1972. Fire has been successfully excluded from the other stand since the early part of this century. A third stand, on Santa Rosa Island, Channel Islands National Park (SRI), has been grazed heavily since the mid-19th century and has not experienced fire since that time. One-hectare study plots were established in the interior of each stand. Considering the known life history attributes of P. torreyana, and the disturbance histories of the stands, predictions are made concerning the spatial characteristics of the respective stands. All trees within each study plot were sized by diameter breast height (DBH) and mapped. Three techniques of spatial analysis are applied to the resulting unweighted point pattern distributions and the distributions weighted by the square root of DBH as a surrogate for age. The results are consistent with predictions and confirm the following generalizations concerning patterns of aggregation in Torrey pine stands. Young trees tend to be more aggregated than old trees within the same stands. Young stands tend to be more aggregated than old stands on otherwise ecologically similar sites. On a periodically disturbed site there are clusters of trees that represent cohorts of post-disturbance recruitment. [ABSTRACT FROM AUTHOR]

Gap models have been applied to a wide range of ecosystems, mainly temperate and boreal forests, but rarely have such models been applied to Mediterranean ecosystems. In the present review we address some problems of gap models for predicting the long-term dynamics of Mediterranean ecosystems, and we suggest plant functional types suitable for modelling based on responses to disturbance. Most gap models do not take into account different life forms, interactions with fire (e.g., resprouting and stimulation of the germination), and underground structures. Long term human impact on Mediterranean ecosystems has made a significant impact on the current landscapes. That intense land use, involving long-lived slow-growing species, has had long-term consequences. It is not possible to understand Mediterranean vegetation and to validate any model without considering these factors. The lack of data for Mediterranean species may be overcome by taking into account correlations of traits and trade-offs between different functional types. A simple disturbance-based functional group system is discussed. [ABSTRACT FROM AUTHOR]