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Aims: Primary biomass production is a fundamental process for ecosystem functioning. Yet, little is known on the mechanisms driving temporal stability of biomass production in annual plant communities, particularly when subjected to highly variable environments and undergoing temporal changes in species composition. We aimed to disentangle the relative importance of biomass production, species diversity, dominance and asynchrony of species fluctuations as drivers of biomass stability in mediterranean and semi-arid annual plant communities. Location: Mediterranean (31°42′ N; 35°03′ E) and semi-arid (31°23′ N; 34°54′ E) sites, Israel. Methods: Above-ground biomass and species abundance were monitored in 15 plots of 250 m² per site during eight consecutive years. Relationships between stability drivers and community stability were studied at the regional (between sites) and local (within sites) spatial scales. Results: Community biomass stability (mean biomass/SD) increased from the semiarid to the mediterranean site concomitantly with higher biomass production, richness, and evenness. Differences in stability between sites were due to opposite effects of site conditions on the mean and SD of community biomass, leading to higher stability in the mediterranean site. Within sites, species asynchrony was the key driver of stability at the local spatial scale. Richness and biomass production affected stability indirectly through asynchrony, but in different ways at each site. At the mediterranean site, these factors had indirect negative effects on stability by reducing asynchrony, but did not rescind a positive effect of asynchrony on community stability. At the semi-arid site, biomass production had indirect positive effects on stability through asynchrony, while richness had no effect on asynchrony and stability. Stability was not driven by species evenness in either site. Conclusions: Our study provides new insights into the complex control of biomass stability in the dynamics of mediterranean and semi-arid annual plant communities, with different mechanisms driving stability across the regional vs local spatial scales. [ABSTRACT FROM AUTHOR]

Aims Management of silvo-pastoral systems in planted and natural forests in semi-arid Mediterranean regions often employs seasonal night corrals for animal protection. This management system changes the spatial distribution of animal excreta, resulting in a net transfer of soil mineral resources and their accumulation in the corrals. After abandonment, corrals are colonized by ruderal species, becoming focal sources for their spread in the forest. We aimed to implement a rational management of seasonal sheep corrals based on a better understanding of the vegetation processes occurring in abandoned corrals, in order to alleviate their negative impact in the forest. Methods Relationships between temporal changes in the vegetation, the soil seed-bank and levels of soil nutrients were studied in a chronosequence of abandoned sheep corrals and compared with nearby reference plots in planted Eucalyptus forests grazed by sheep in the semi-arid North-Western Negev, Israel. The region has a bi-seasonal Mediterranean climate, with high dominance of annual species in the grazing range. Important Findings Abandoned sheep corrals were colonized by seeds of ruderals originating in older abandoned corrals. Subsequent successional changes occur at a slow rate, driven by the depletion of soil resources in the abandoned corrals, and were still in progress 20 years after abandonment. Ruderals were gradually replaced, first by taller grasses and followed by short grasses, but most forbs and particularly geophytes did not recover during this period. Recovery of the original herbaceous vegetation in the corrals was through seed dispersal from the surrounding vegetation, not from the original soil seed-bank remaining in the corrals after abandonment. Ruderal species in the grazed, planted forests behave as patch-tracking metapopulations. Their persistency depends on constant creation of new corrals compensating for the gradually dwindling populations in older abandoned corrals, and on the availability of dispersal vectors. [ABSTRACT FROM AUTHOR]

A synthesis of a long-term (19 years) study assessing the effects of cattle grazing on the structure and composition of a Mediterranean grassland in north-eastern Israel is presented, with new insights into the response of the vegetation to grazing management and rainfall. We hypothesized that the plant community studied would be resistant to high grazing intensities and rainfall variability considering the combined long history of land-use and unpredictable climatic conditions where this community evolved. Treatments included manipulations of stocking densities (moderate, heavy, and very heavy) and of grazing regimes (continuous vs. seasonal), in a factorial design. The effect of interannual rainfall variation on the expression of grazing impacts on the plant community was minor. The main effects of grazing on relative cover of plant functional groups were related to early vs. late seasonal grazing. Species diversity and equitability were remarkably stable across all grazing treatments. A reduction in tall grass cover at higher stocking densities was correlated with increased cover of less palatable groups such as annual and perennial thistles, as well as shorter and prostrate groups such as short annual grasses. This long-term study shows that interannual fluctuations in plant functional group composition could be partly accounted for by grazing pressure and timing, but not by the measured rainfall variables. Grazing affected the dominance of tall annual grasses. However, the persistence of tall grasses and more palatable species over time, despite large differences in grazing pressure and timing, supports the idea that Mediterranean grasslands are highly resistant to prolonged grazing. Indeed, even under the most extreme grazing conditions applied, there were no signs of deterioration or collapse of the ecosystem. This high resistance to grazing intensity and interannual fluctuation in climatic conditions should favor the persistence of the plant community under forecasted increasing unpredictability due to climate change. [ABSTRACT FROM AUTHOR]

Question: Large variation in the similarity between seed bank and standing vegetation generally occurs along environmental gradients. We asked: 1. How is seed bank-vegetation similarity in Mediterranean semi-arid annual plant communities related to variation in primary productivity; 2. How is this productivity-similarity relationship affected by exclusion from grazing. Location: Mediterranean, semi-arid rangeland in the Northern Negev desert, Israel. Methods: Density of seeds and plants, and species composition of the seed bank and vegetation were compared in grazed and ungrazed subplots, in four neighbouring topographic sites differing in productivity. Seed bank samples were collected in autumn, just before the rainy season, and vegetation samples in the following spring, at peak above-ground biomass, when net primary productivity was assessed. Results: Quantitative and qualitative similarities between seed bank and the vegetation (Sørensen index values) varied between 0.14-0.61 and 0.40-0.68. Quantitative similarity and productivity were positively and linearly related within the low productivity range (up to 160 g.m-2), in both grazed and ungrazed subplots. In contrast, at higher productivity levels (up to 500 g.m-2), similarity decreased slightly with productivity in the ungrazed subplots, but no trend occurred in the grazed subplots. At low productivity, grazing did not affect similarity, while at higher productivity grazing prevented the reduction in similarity with increasing productivity. Conclusion: seed bank - vegetation similarity, with and without grazing, is positively related to productivity up to a threshold range, above which soil resource availability is no longer the factor limiting plant density. Above this range grazing prevents a reduction in similarity with increasing productivity, by diminishing vegetation cover and litter accumulation that constrain germination, seedling emergence and plant survival. [ABSTRACT FROM AUTHOR]

1. Contrasting patterns of change in the seed bank of natural grasslands are frequently found in response to grazing by domestic herbivores. Here, we studied the hypotheses that a) patterns of change in seed bank density and composition in response to grazing depend on spatial variation in resource availability and productivity, and b) that variation among species in patterns of seed bank response to grazing is linked to differences in species size traits (i.e. size of plant, dispersal unit and seed). 2. Effects of sheep grazing exclusion on the seed bank were followed during five years in a semiarid Mediterranean annual plant community in Israel. Seed bank density and composition were measured in autumn, before the rainy season, inside and outside fenced exclosures in four neighboring topographic sites differing in vegetation characteristics, soil resources and primary productivity: Wadi (dry stream terraces, high productive site), Hilltop, South- and North-facing slopes (less productive sites). 3. Topographic sites differed in seed density (range ca 2500–18000 seed m−2) and in seed bank response to grazing exclusion. Fencing increased seed density by 78, 51 and 18% in the Wadi, South- and North-facing slopes, respectively, but had no effect in the Hilltop. At the species level, grazing exclusion interacted with site conditions in determining species seed bank density, with larger or opposite changes in the high productive Wadi compared to the other less productive sites. 4. Changes in seed bank structure after grazing exclusion were strongly related to species size traits. Grazing exclusion favored species with large size traits in all sites, while seed density of tiny species decreased strongly in the high productive Wadi. Species with medium and small size traits showed lesser or no responses. 5. The size of plants, dispersal units and seeds were strongly correlated to each other, thus confounding the evaluation of the relative importance of each trait in the response of species to grazing and site conditions. We propose that the relative importance of plant size vs seed size in the response to grazing changes with productivity level. [ABSTRACT FROM AUTHOR]

Questions: Do invasions of P. halepensis (Aleppo pine) from plantations into adjacent natural communities occur in the Mediterranean region, where the species is native? What are the spatio-temporal processes involved in pine invasions in contrasting Mediterranean and semi-arid climatic regions? Location: Mediterranean and semi-arid regions of Israel. Methods: The density of invading Pinus was measured in relation to the distance from the plantation edge. Plants were categorized by age, height, basal stem girth and developmental stage, their spatial distribution was also recorded. Results: Analysis of plant age distribution indicates that the invasion process started when the plantations were 20-25 years old. Most invading plants were found within 20 m from the plantation edge, but a few individuals reached distances up to 100 m and became new invasion foci. Plant density declined sharply with distance from adult trees, data showing a better fit to a power model than to a negative exponential model. Invading Pinus began to produce cones earlier in the semi-arid than in the Mediterranean region (9 vs 12 years to 50% reproductive plants). In both regions, higher densities of invading plants were found on the west side of the plantation, the opposite direction to the hot winds that prevail during seed release. Conclusion: The frontal advance of P. halepensis from plantations is relatively slow, but the populations also expand by a saltation process, creating spreading 'islands' of pine trees in the natural vegetation. Spatial pattern of recruits with distance from the source population was remarkably similar to the pattern of seed dispersal in the same region (Nathan et al. 1999). This implies that the probability of a dispersed seed developing into a plant is independent of the distance from the forest edge. [ABSTRACT FROM AUTHOR]

Variation in the onset of summer dormancy and flowering capacity of 16 populations of Poa bulbosa, collected along a steep north–south aridity gradient in Israel (810–110 mm rain year–1), was studied under controlled conditions in a phytotron (16 h daylength, 22/16 °C day/night) and under natural conditions in a garden experiment in a net‐house. Plant age at the onset of dormancy varied markedly amongst populations (7–16 weeks under controlled conditions) and was positively correlated with mean annual precipitation at the site of origin of the population, i.e. dormancy was earlier as aridity increased. Flowering capacity in the different populations was negatively correlated with rainfall in the original habitat and, consequently, also with the age at onset of dormancy, i.e. the lower the mean annual precipitation, the earlier the onset of dormancy and the higher the proportion of flowering plants and panicles per plant. Differences in xeromorphic leaf traits were also observed among populations from locations differing in aridity. Plants from the more arid sites (110–310 mm year–1) generally had greyish and curved leaves, whereas plants from more humid sites (500–810 mm year–1) tended to have green and straight leaves. Thus, plants with curved and/or greyish leaves generally had a higher flowering capacity and entered dormancy earlier than plants with straight and/or green leaves. The significance of the association among these traits for the adaptation of P. bulbosa to increasing aridity is discussed. [ABSTRACT FROM PUBLISHER]

• 316 plant species were recorded in a planted East-Mediterranean P. halepensis forest. • Overstory cover determines plant diversity in East-Mediterranean pine forests. • Overstory cover-understory diversity relationships are spatial scale dependent. • At the stand level, overstory cover dictates plant diversity through "environmental filtering". • At inter-stand level, overstory cover shapes plant composition through spatial heterogeneity. • Variable thinning strategy will enhance biodiversity in East-Mediterranean pine forests. Mediterranean forest ecosystems are of high conservation value and therefore increasingly managed to restore and promote their natural biodiversity and complexity. The present study aimed to assess plant species diversity of East-Mediterranean conifer forests as affected by overstory cover manipulation at multiple spatial scales. Understanding this interaction in water-limited forest ecosystems is particularly challenging as overstory-understory relations involve both water and light constrains. The experiment was conducted in a mature (40y) planted Pinus halepensis forest in the Jerusalem Mountains of Israel. Understory plant community structure was monitored for five years in sixteen experimental plots (0.5 ha) representing a range of overstory cover levels (leaf area index-LAI = 0–4 m2m−2) created by tree thinning. Light and water availability in the forest understory were monitored using fisheye camera (irradiance), predawn leaf water potential (PLWP) of dominant species and soil water content (SWC) measurements. Plant diversity was measured at increasing spatial scales (grain sizes, 0.01–400 m2), using hierarchical nested sampling. Irradiance and SWC increased with decreasing LAI while PLWP was not affected. Stand-level understory richness (α diversity) increased linearly with decreasing LAI, indicating a consistent, positive resource availability-diversity relationship. The results pointed towards strong dependency of species richness on light availability as compared to its dependency on water availability that was less definite. Understory species composition varied across LAI levels (β diversity at the forest level) indicating species turnover, mainly between annual herbs benefiting from reduced overstory cover vs. woody species benefiting from higher cover. The relationships between overstory cover and understory richness and composition were rank-invariant across grain sizes. These relationships were stronger at the larger (10, 100, 400 m2) compared to the smaller (0.01, 0.1, 1 m2) grain sizes. The 10-m2 grain-size, corresponding to the crown projection area of individual trees, showed the strongest relationship. Observed patterns of overstory cover–understory richness suggest that resource availability, rather than spatial resource heterogeneity, was the main factor shaping understory diversity. Furthermore, grain-size effects on this relationship support "environmental filtering" rather than "resource density" as the main mechanism through which reduced overstory cover promoted understory diversity. 'Synthesis and applications': Our findings demonstrate the importance of overstory cover as a key factor determining plant diversity in water-limited East-Mediterranean forests. We highlight the important role of spatial heterogeneity of overstory cover at both the stand (intra-plot) and forest (inter-plot) scales and support dynamic variable thinning as a management strategy to enhance forest biodiversity and complexity. [ABSTRACT FROM AUTHOR]

Summary 1. A 4-year study was conducted in a Mediterranean herbaceous community in north-eastern Israel to investigate the effects of cattle grazing management on the structure and composition of the community. Understanding the effects of grazing on the dynamics of Mediterranean herbaceous communities is important in formulating rational management plans for both conservation and sustainable animal production. 2. The relationships among plant functional groups were studied in the context of inter-annual variation in rainfall. Treatments included manipulations of stocking rates (moderate, heavy and very heavy) and grazing regimes (continuous vs. seasonal), in a factorial design. 3. The herbaceous community was rich in species, with 166 species recorded at the site, of which 74% were annuals. Plant cover was dominated by 10 species that accounted for 75% of the total cover. 4. Inter-seasonal rainfall variation was a dominant factor in the expression of different grazing treatments on the structure of the plant community. Grazing effects were stronger in wet years than in dry years. 5. Paddocks under continuous grazing were higher in number of species compared with paddocks subjected to seasonal grazing, independently of grazing intensity. 6. Functional group analyses showed that reduction in cover of tall grasses was correlated with an increase in cover of prostrate annual legumes and less palatable groups such as annual and perennial thistles, crucifers and forbs. 7. Cover of functional groups composed of hemicryptophytic species was less variable (lower coefficient of variation) in response to grazing treatments and inter-annual variation in climatic conditions compared with functional groups with annual species. 8. The persistence of the dominant species and the relatively small amplitude of change in plant cover of the functional groups suggest that the community was rather stable in spite of wide variation in grazing regimes and climatic conditions. East-Mediterranean grasslands appear to be adapted to grazing due to their long history of human association. [ABSTRACT FROM AUTHOR]

Background and Aims: Versatility in the reproductive development of pseudoviviparous grasses in response to growth conditions is an intriguing reproduction strategy. To better understand this strategy, this study examined variation in flowering and pseudovivipary among populations, co-occurring clones within populations, and among tillers in individual clones of Poa bulbosa, a summer-dormant geophytic grass that reproduces sexually by seed, and asexually by basal tiller bulbs and bulbils formed in proliferated panicles., Methods: Clones were collected from 17 populations across a rainfall gradient. Patterns of reproduction were monitored for 11 years in a common garden experiment and related to interannual differences in climatic conditions. Intraclonal variation in flowering and pseudovivipary was studied in a phytotron, under daylengths marginal for flowering induction., Key Results: Clones showed large temporal variability in their reproductive behaviour. They flowered in some years but not in others, produced normal or proliferated panicles in different years, or became dormant without flowering. Proliferating clones did not show a distinct time sequence of flowering and proliferation across years. Populations differed in incidence of flowering and proliferation. The proportion of flowering clones increased with decreasing rainfall at the site of population origin, but no consistent relationship was found between flowering and precipitation in the common garden experiment across years. In contrast, flowering decreased at higher temperatures during early growth stages after bulb sprouting. Pulses of soil fertilization greatly increased the proportion of flowering clones and panicle production. High intraclonal tiller heterogeneity was observed, as shown by the divergent developmental fates of daughter plants arising from bulbs from the same parent clone and grown under similar conditions. Panicle proliferation was enhanced by non-inductive 8 h short days, while marginally inductive 12 h days promoted normal panicles., Conclusions: Interannual variation in flowering and proliferation in P. bulbosa clones was attributed to differences in the onset of the rainy season, resulting in different daylength and temperature conditions during the early stages of growth, during which induction of flowering and dormancy occurs., (© The Author 2014. Published by Oxford University Press on behalf of the Annals of Botany Company. All rights reserved. For Permissions, please email: journals.permissions@oup.com.)

Afforestation efforts have resulted in extensive plantations of either native or non-native conifers, which in many regions has led to the spread of those conifers into surrounding natural vegetation. This process of species colonization can trigger profound changes in both community dynamics and ecosystem processes. Our study disentangled the complexity of a process of colonization in a heterogeneous landscape into a simple set of rules. We analyzed the factors that control the colonization of natural woodland ecosystems by Pinus halepensis dispersing from plantations in the Mediterranean region of Israel. We developed maximum-likelihood models to explain the densities of P. halepensis colonizing natural woodlands. Our models unravel how P. halepensis colonization is controlled by factors that determine colonization pressure by dispersing seeds and by factors that control resistance to colonization of the natural ecosystems. Our models show that the combination of different seed arrival processes from local, landscape, and regional scales determine pine establishment potential, but the relative importance of each component varied according to seed source distribution. Habitat resistance, determined by abiotic and biotic conditions, was as important as propagule input in determining the density of pine colonization. Thus, despite the fact that pine propagules disperse throughout the landscape, habitat heterogeneity within the natural ecosystems generates significant variation in the actual densities of colonized pine. Our approach provides quantitative measures of how processes at different spatial scales affect the distribution and densities of colonizing species, and a basis for projection of expected distributions. Variation in colonization rates, due to landscape-scale heterogeneity in both colonization pressure and resistance to colonization, can be expected to produce a diversity of new ecosystems. This work provides a template for understanding species colonization processes, especially in light of anthropogenic impacts, and predicting future transformation of natural ecosystems by species invasion.

Plant colonization studies usually address density-dependent processes in the narrow sense of recruitment constraints due to negative density-dependent seed and seedling mortality. However, complex density-dependent effects may be involved in additional stages of the recruitment process. We hypothesized that seed arrival and seedling establishment are influenced by density dependence acting at small scales at the site of colonization, and at larger scales as a function of the colonizing species' landscape abundance. These hypotheses were tested in a study of colonization of pine forests by oaks in a heterogeneous Mediterranean landscape. Maximum-likelihood models show that density effects switch from positive to negative along the range of landscape-scale oak seed source abundance. Contrary to expectations, high seed source densities limited oak recruitment, suggesting a landscape-scale Janzen-Connell effect. We propose a range of mechanisms that generate positive or negative density dependence during colonization, resulting in nonlinear density-dependent feedbacks that can generate unexpected colonization patterns.

Land use and climate changes induce shifts in plant functional diversity and community structure, thereby modifying ecosystem processes. This is particularly true for litter decomposition, an essential process in the biogeochemical cycles of carbon and nutrients. In this study, we asked whether changes in functional traits of living leaves in response to changes in land use and climate were related to rates of litter potential decomposition, hereafter denoted litter decomposability, across a range of 10 contrasting sites. To disentangle the different control factors on litter decomposition, we conducted a microcosm experiment to determine the decomposability under standard conditions of litters collected in herbaceous communities from Europe and Israel. We tested how environmental factors (disturbance and climate) affected functional traits of living leaves and how these traits then modified litter quality and subsequent litter decomposability. Litter decomposability appeared proximately linked to initial litter quality, with particularly clear negative correlations with lignin-dependent indices (litter lignin concentr tion, lignin:nitrogen ratio, and fiber component). Litter quality was directly related to community-weighted mean traits. Lignin-dependent indices of litter quality were positively correlated with community-weighted mean leaf dry matter content (LDMC), and negatively correlated with community-weighted mean leaf nitrogen concentration (LNC). Consequently, litter decomposability was correlated negatively with community-weighted mean LDMC, and positively with community-weighted mean LNC. Environmental factors (disturbance and climate) influenced community-weighted mean traits. Plant communities experiencing less frequent or less intense disturbance exhibited higher community-weighted mean LDMC, and therefore higher litter lignin content and slower litter decomposability. LDMC therefore appears as a powerful marker of both changes in land use and of the pace of nutrient cycling across 10 contrasting sites.