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A debate whether natural selection plays an important role in reinforcing reproductive isolation during the earliest stages of speculation is presented. It is shown that this has occurred atleast once in populations of the grass Anthoxanthum odoratum, growing in the Part Grass Experiment where flowering time has shifted at the boundaries between plots.

Summary Savanna ecosystems show codominance of trees and grasses, and the mechanisms involved in their coexistence remain unresolved. We investigated the possible influence of small-scale heterogeneities and disturbances in determining tree spacing and tree-grass coexistence in semi-arid savannas, using a spatially explicit, grid-based simulation model. We added factors such as seed clumping, locally improved moisture conditions, vegetation clearing and combinations of all three of these variables to a previously published model. We also examined the influence of changing the size and spatiotemporal correlation among individual heterogeneities. Increased seed availability in localized clumps, especially in combination with the other heterogeneity types, had the strongest impact on the long-term tree-grass coexistence in the model. Localized deposition of tree seeds in herbivore dung and underground seed caches built by seed-collecting rodents may act as a determinant of the distribution of trees in savannas. Spatially autocorrelated small-scale heterogeneities led to a more stable coexistence of trees and grasses than uncorrelated heterogeneities did, and at high levels of autocorrelation runs with all three variables led to long-term coexistence in up to 60% of the rainfall scenarios tested. The size of individual heterogeneities, also played a significant role in determining whether trees would establish as lone individuals or in clumps. In simulations that included small-scale heterogeneities, the number of isolated trees scattered throughout the landscape increased, whereas large heterogeneity patches and high spatiotemporal autocorrelation tended to promote tree clustering. Simulations based on rainfall data from the southern Kalahari produced a realistic density and spatial distribution for trees in this semi-arid savanna for a realistic range of small-scale heterogeneities. Small-scale disturbances and heterogeneities, at least those that furnish better establishment conditions for tree seedlings, therefore act to increase the range of environmental conditions under which trees and grasses can coexist as savanna. [ABSTRACT FROM AUTHOR]

Summary We studied how annual plant species colonize a set of patches (natural and humanmade) in a shrubland landscape in the Negev desert. We asked what patch properties and species' attributes affect colonization during 4 years after experimental formation of eight sets of pits and mounds. The ‘sink function’ of patches was measured as the numbers of colonizing species, the proportion of patches colonized during each year, and the abundance of the colonizers. The effects of species' dispersal mode and propagule size were examined. In the first growing season, pits and mounds were colonized by large numbers of species. The total number of colonizing species was similar for the two patch types, but incidence was higher in pits than in mounds. The early colonizers were mainly wind-dispersed and showed both high abundance and incidence. Dispersal mode did not affect colonization in the second year and later, nor did propagule size througout the experiment. Most early colonizers became resident in the patches, but pits and mounds showed some further colonization during the second year. In the third and fourth year both the numbers of colonizing species and their incidence decreased. Pits were mainly colonized from outside the experimental units to which they belonged, whereas mounds received species mostly from the adjacent pits within the same units. From the second year on, patches of undisturbed crusted soil were also colonized, mainly from the adjacent pits and mounds. The number of colonizing species in undisturbed patches increased during the last, high-rainfall, year both per patch and for the whole set of patches. We conclude that the sink function of patches for colonization increases with removal of vegetation, soil crust and seed bank, with patch capacity to capture resource and plant propagules, and with amount of rainfall. Sink function decreased with time due to decreasing availability of new species, but increased with the presence of stronger sink patches in the vicinity. We suggest that the concept of sink function, in terms of the numbers, incidence and abundance of species of colonizing sets of patches, can be a powerful tool for linking community and landscape processes. [ABSTRACT FROM AUTHOR]

Summary Indirect seed dispersal in the system Lycium intricatum (Solanaceae)–lizards (Gallotia atlantica) –shrikes (Lanius excubitor) was studied in Alegranza, a xerophytic small island of the Canarian archipelago. A total of 835 seeds (224 obtained from lizard droppings and 611 from shrike pellets) was found by analysing 123 droppings and 146 pellets. Lycium fruit remains (including seeds) were observed in 31.7% of the lizard droppings and 50% of the shrike pellets. As would be expected for legitimate seed dispersers, external seed damage produced by each species was negligible. Lycium seeds were significantly matched with the presence of lizard remains in shrike pellets. Seeds in shrike pellets have come from fruit consumed by lizards that have been predated by the shrikes, and not directly from bird frugivory. Seeds from shrike pellets showed significantly higher germination rates than those from uneaten fruits and lizard droppings. It seems probable that different treatments in the guts of each species and retention time are two main factors influencing the germination process. While gut pass time in G. atlantica has been estimated to be 2.42 days on average, the retention time of a seed inside a shrike gizzard is much shorter (45–55 min). This is a new case of the scarcely known phenomenon of indirect seed dispersal, which seems to acquire a relatively important role in small islands where the animal–plant interaction is very intense and all the elements of the system are native. [ABSTRACT FROM AUTHOR]

Summary Population differentiation in variable environments is often related to the different local selection pressures that plants may experience. We examine the relation between reproductive success and morphological and phenological traits in woodland and open habitat populations of the tetraploid grass Arrhenatherum elatius. The reproductive correlates of morphological and phenological variation were estimated in woodland and open habitats, and on plants from each of three populations from woodland and three from open habitats grown in a garden environment at two light intensity levels. Genetic variation in these populations was estimated in uniform conditions. Temporal and spatial differences in phenotypic selection on morphology and phenology were observed within and between the two sites. Plants from the two sites showed significant phenotypic and genetic differences, and there is a dramatic reduction in reproductive success in the woodland habitat and in plants from this habitat grown in the two garden environments. In controlled conditions, selection patterns were similar to those observed in natural habitats, suggesting that light availability may be an important ecological factor involved in population differentiation in A. elatius. The presence of significant genetic variability for the studied traits indicates the presence of a genetic potential to respond to the detected selection pressures. We argue that a clear understanding of patterns of local adaptation and phenotypic selection in this species requires that they are examined in the light of regional processes related to seed dispersal and relative abundance in open and woodland habitats. [ABSTRACT FROM AUTHOR]

Summary Fire and vegetation dynamics in a boreal Pinus banksiana and Picea mariana forest are reconstructed using a 580 year fossil pollen and charcoal record obtained from 128 contiguous samples, each representing c. 4.55 years of sediment. The general post-fire vegetation changes, as indicated by cross-correlation analyses, are an increase in ground layer taxa and shrubs, followed by an increase in coniferous trees and Betula papyrifera. The size and exact timing of the peak abundance in each taxon differs after individual fires. Sixteen fires are identified using pollen and charcoal records. The mean fire interval is 34 years, similar to fire cycle estimates of 38 and 39 years from dendrochronological studies. On two occasions, Picea mariana appears to replace Pinus banksiana as the local dominant, each time taking less than 70 years. An unusual decline in Pinus pollen between c. AD 1816 and 1861 corresponds with a similar decline in a pollen record from 50 km WSW of this site. This may have been caused by Little Ice Age cooling. [ABSTRACT FROM AUTHOR]

Summary The advantages of guerrilla and phalanx growth for the guerrilla Elymus lanceolatus ssp. lanceolatus and phalanx E. l. ssp. wawawaiensis were evaluated over 2 years in two taxon mixtures with a range of densities of each subspecies and under two levels of watering. Ramet numbers and biomass of the guerrilla subspecies were higher than those of the phalanx grass in the first year but in the second year declined greatly, while the phalanx grass showed no change in biomass and an increase in ramet numbers. High neighbour densities affected the phalanx subspecies more strongly than the guerrilla subspecies in the first year, but in the second year there were few differences between subspecies. Biomass of the guerrilla grass remained greater than that of the phalanx grass but ramet numbers were similar in the second year. For both subspecies in both years, probability of flowering decreased at higher neighbour densities, indicating adaptation for competitive ability. In the first year, biomass was more strongly reduced by densities than flowering was, but in the second year, when crowding was apparently greater, flowering was more severely affected. Genet survival was high and similar for both subspecies. The presumed advantage of guerrilla subspecies in exploiting open space was supported. The guerrilla grass exploited resources more quickly in the first year by faster growth and greater ramet production, but its biomass, ramet numbers and rhizome growth, and thus its advantage, were reduced in the second year. The phalanx subspecies had slower growth, produced more ramets in later years, and delayed flowering until later years. Although less able to exploit open resources, it appeared adapted to more stressful conditions, and may be able to exploit temporal resource pulses more effectively. [ABSTRACT FROM AUTHOR]

Summary Evolution of a microbial nitrogen-uptake strategy that removes nitrogen at less than the maximum potential rate creates soil environments favourable for increased plant access to nitrogen. Plant productivity is thus increased, and achievement of maximum microbial biomass is facilitated through increased microbial access to plant-derived carbon. Simulations show that micro-organisms exhibiting such a ‘strategic’ nitrogen uptake will be competitively excluded by more voracious micro-organisms in a homogeneous system, but can persist in a spatially explicit system. This persistence of less voracious micro-organisms contrasts with the frequently held view that micro-organisms in soil systems take up nitrogen at the maximum possible rate. The selection of strategic or voracious microbial types in a spatially explicit environment is sensitive to the frequency of plant and microbial disturbance, and to the rate of diffusion of inorganic nitrogen in the soil system. In particular, the strategic (less voracious) microbial types are favoured under conditions of low plant disturbance, low to moderate diffusion of inorganic nitrogen, and moderate microbial disturbance. When selection favours strategic microbial types, plant access to nitrogen is increased. Plants may also evolve strategies in a spatially explicit environment that will increase population densities of favourable, strategic microbial types. [ABSTRACT FROM AUTHOR]

Summary We examined the impact of nitrogen addition on early successional dynamics and plant species diversity in replicate plots of an old-field succession treatment in south-western Michigan, USA, for 7 years following abandonment from agriculture. Nitrogen addition significantly increased above-ground plant biomass, but had no effect on the displacement or persistence of different life-history groups. Annual species made up a similar proportion of community biomass and decreased at equivalent rates in control and nitrogen-addition microplots. Perennial species biomass increased at similar rates in the control and nitrogen-addition microplots. Annual species dominated initially, but declined in abundance over the first 4 years and were replaced by biennials and then herbaceous perennials. The resulting high numbers of species losses and gains were not affected by nitrogen addition. Nitrogen addition did not affect mean species density or the Shannon index of diversity. There was also no temporal change in the similarity of species composition between control and nitrogen-addition microplots, indicating that nitrogen addition did not cause divergence over time in community composition. In year 7, similar species dominated the nitrogen-addition and control microplots, but there were differences in species composition and abundance of the subordinate species in each community. Only in year 6 of this study was there a significant negative relationship between community biomass and the Shannon diversity index. There was no significant relationship between community biomass and species density in any of the first 7 years following abandonment. Thus, the community response to increased biomass due to nitrogen addition was expressed primarily through changes in community structure, not richness. The effects of nitrogen addition on the early successional dynamics, species composition and consequently diversity of a plant community may depend on initial site conditions. The response to nitrogen enrichment may be slower in sites such as ours with fertile soils that become dominated by herbaceous perennial dicots. [ABSTRACT FROM AUTHOR]

Summary Two series of competition experiments involving the four annual plant species Bromus hordeaceus, Calycadenia multiglandulosa, Plantago erecta and Microseris douglasii were conducted in an experimental array of pots located at the Jasper Ridge Biological Preserve, San Mateo County, California, USA. One experiment was conducted during the 1988-89 growing season, the other during the 1989-90 growing season. A four-species yield-density model was constructed using seed yield data from the 1988-89 experiment. Model predictions were tested against seed yield data from the 1989-90 experiment. A systematic bias in predicted seed yields was found. As a consequence, the original model was reparameterized to provide a better fit to the observed yields for the 1989-90 experiment. Analyses were conducted to determine the importance of potential sources of bias in the original model, including (i) mis-specification of density effects at low densities due to the parameterization protocol; (ii) interaction effects among competing species; and (iii) year-to-year variability in competitive interactions. All three sources had a significant effect on at least one of the species in the experiment, although significant sources differed between species. Using two forms of sensitivity analysis, a comparison was made between the original and modified versions of the yield–density model to determine how they differed in predictions of long-term plant community dynamics. Rank order of average densities among species did not differ between models; however, there were several important differences in their predictions with respect to overall community structure. Implications of these results with respect to the use of yield–density models in studying plant community dynamics are discussed. The present study indicates that the effectiveness of the generally employed yield–density model at characterizing competitive interactions under different environmental conditions depends upon the criteria employed in making an assessment. One possible explanation is that competitive interactions within the serpentine community are highly sensitive to changes in environmental conditions and we found some evidence for this in Calycadenia and Plantago. [ABSTRACT FROM AUTHOR]

Summary Dicranopteris linearis (Gleicheniaceae), a native fern common throughout the Old World tropics and Polynesia, forms dense thickets > 3m deep over large areas of open-canopy, oligotrophic, wet Hawaiian rainforests. Our objectives were to identify leaf- and whole plant-level traits that are key to its success and to determine its community-and ecosystem-level consequences in primary successional sites. Along an elevational gradient from 90 to 1660m, mean maximum net assimilation rates of Dicranopteris ranged from 2.9 to 5.0 μmol m-2 s-1 , compared with 3.6–9.5 μmol m-1 s-1 in the codominant tree Metrosideros polymorpha. Gas-exchange characteristics did not explain Dicranopteris' success, nor its trends in production. However, indeterminate, clonal growth form, shallow rhizomes, marcescent leaves with low decomposability, and a mat-forming capacity enabled Dicranopteris to colonize sites and to maintain dominance via high effective leaf area, despte its low biomass. Phosphorus use efficiency, which reached 24 kg g-1 , was exceptionally high, allowing colonization of phosphorus-poor sites. Dicranopteris contributed up to 74% of above-ground net primary productivity in a site where it contained only 14% of live biomass. It accounted for up to 57% and 47% of total nitrogen and phosphorus uptake by plants, respectively, where it contained only 24% and 30% of plant nitrogen and phosphorus. Dicranopteris leaves are short-lived and slow to decompose; thus, fixed carbon is transferred quickly to soil detrital pools where it contributes to aggrading soil organic matter pools and may exacerbate oligotrophic conditions, thereby strongly influencing soil genesis and ecosystem development. The fern therefore influences forest-floor light regimes and directs later community development. An exclusion experiment demonstrated that Dicranopteris competed with Metrosideros, but lack of revegetation in 40% of the exclusion area after 39 months showed that Dicranopteris also colonized microenvironments unavailable to its endemic codominants. Dicranopteris may play an important role in resisting invasions of exotic species into Hawaiian rainforests. [ABSTRACT FROM AUTHOR]

Summary Population dynamics of an annual legume, Kummerowia stipulacea , were followed in permanent plots in a mown field for 4 years. There was a large reduction in the number of plants within seasons. Laboratory studies showed that biotic factors were responsible for seed death in the soil. Using soil from the field plot location, we isolated fungi pathogenic to K. stipulacea seeds and seedlings. Two pathogenic fungi (Rhizoctonia solani and Pythium irregulare) were used in a glasshouse study of the effects of pathogens and intraspecific competition on plant mortality and growth. Our null hypothesis was that death of individual plants due to disease would be balanced by increased growth of survivors. A factorial experiment was established with five soil treatments (three controls, P. irregulare and R. solani ) and seven seed densities. Inoculation by both R. Solani and P. irregulare led to a reduction in final numbers of plants, with the reduction greatest for populations inoculated with R. solani. Average biomass per plant was reduced with increased sowing density in all treatments except the R. solani inoculation, where surviving plants in high-density pots may be larger because of the reduced density resulting from pathogen-induced death. No treatment differences in size hierarchies were evident. Surviving plants in both inoculation treatments could not take full advantage of the reduced density, and thus total biomass and number of trifoliate leaves per population (a surrogate for seed production) were reduced compared to controls, leading us to reject the null hypothesis. The reduction was more severe at high sowing densities. These results suggest that disease has the potential to reduce seed production and thus plant population numbers, if populations are seed limited. Shapes of yield-sowing density relationships were asymptotic for all treatments; thus, disease-induced ‘overcompensation’ leading to fluctuating population dynamics was not suggested. [ABSTRACT FROM AUTHOR]

Summary The effects of increased atmospheric nitrogen inputs, from both NOy and NHx, on diversity in various semi-natural and natural ecosystems are reviewed. The severity of these impacts depends on abiotic conditions (e.g. buffering capacity, soil nutrient status and soil factors that influence the nitrification potential and nitrogen immobilization rate) in the particular system. The sensitivity of fresh water ecosystems, wetlands and bogs, species-rich grasslands, heathlands and field layer of forests, all of which have conservational value, are discussed in detail. The most important effects of nitrogen deposition are: (i) accumulation of nitrogenous compounds resulting in enhanced availability of nitrate or ammonium; (ii) soil-mediated effects of acidification; and (iii) increased susceptibility to secondary stress factors. Long-term nitrogen enrichment has gradually increased the availability of nitrogen in several vegetation types, leading to competitive exclusion of characteristic species by more nitrophilic plants, especially under oligo- to mesotrophic soil conditions. Soil acidification (with losses of buffering capacity and increased concentrations of toxic metals) is especially important after nitrification of ammonium in weakly buffered environments: acid-resistant plant species then become dominant at the expense of the often rare plants typical of intermediate pH. The related change in the balance between ammonium and nitrate may also affect the performance of several species. The susceptibility of plant species to secondary stress factors (pathogens; frost and drought) may be affected by air-borne nitrogen but data are only available for a few communities (e.g. dry heathlands). Most global biodiversity is contained within natural and semi-natural vegetation. It is thus crucial to control emissions of nitrogenous compounds to the atmosphere, in order to reduce or prevent effects on diversity in these systems. Most research has focused on acidification in forestry stands and lakes and on the effects on trees. We highlight serious gaps in knowledge of other ecosystems. [ABSTRACT FROM AUTHOR]

Primula vulgaris is a woodland herb often associated with canopy gaps. The canopy regeneration cycle results in a permanently changing environment which confers a dynamic character to the overall system of P. vulgaris populations inhabiting a forest. In order to describe thedynamics of this spatially structured population, we built a matrix model that incorporates both the spatio-temporal variation in its population dynamics and the dynamics of the forest habitat given by the opening and closure of canopy gaps. To build the model, we combined empirical data on 1) the demography of populations in various forest patches and 2) canopy closure rate. We then used the model to test theeffect of disturbance rate and long-distance (i.e., off-patch) seed dispersal on overall population dynamics. Simulation results suggest that overall population growth rate of P. vulgaris would be enhanced by high gap opening rates, as this would increase the relative frequency of clearings, where population growth rate is maximal. Our analysis also predicts a negative effect of long-distance seed dispersal onoverall population dynamics, since seeds dispersing out of occupied patches would have a high probability of landing in closed canopy patches. These results are an example of the costs of long-distance seeddispersal on overall population dynamics. Finally, the potential role of long-distance seed dispersal in the colonization of newly openedgaps is discussed. [ABSTRACT FROM AUTHOR]