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Traditionally, species absence in a community is ascribed either to dispersal limitation (i.e., the inability of propagules of a species to reach a site) or to habitat limitation (abiotic or biotic conditions of a site prevent species from forming a viable population); sowing experiments can then distinguish between these two mechanisms. In our view, the situation is even more complicated. To demonstrate the complexity of the problem, we designed and applied simulations based on an extension of matrix models covering effects of propagule pressure and habitat limitation, and reflecting various characteristics of a species and of a habitat. These included life history, fecundity, seed bank viability of a species, habitat carrying capacity and disturbances. All the investigated factors affected proportion of occupied habitats. Whereas they can, to a large extent, compensate for each other, simultaneous decrease of habitat suitability and propagule input can be detrimental to the survival of a population. Our model demonstrated that in many cases, the absence of a species in a community is of stochastic nature, and result of interaction of species life history and various external conditions, and thus cannot be simply attributed to a single cause. The model results are supported with examples of case studies. The results also explain some well-known ecological phenomena, as decrease of niche breadth from the center to the margins of area of distribution. Finally, the model also suggests some caveats in interpretation of the results of sowing experiments.

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Recruitment limitation has been hypothesized to promote the maintenance of high species diversity in forests by slowing down competitive exclusion. However, the difference of recruitment limitation for tree species with varying seed masses, which is a common phenomenon in tropical or subtropical forests, is largely unknown. In this study we conducted a seed sowing experiment for five dominant tree species with varying seed mass (a proxy of dispersal ability) in a subtropical evergreen broad-leaved forest at different successional stages to test the hypothesis that the determinants of species recruitment vary with their seed masses in Heishiding Nature Reserve (Guangdong Province, China). The effects of seed predators, soil pathogens, light conditions, plant litter, seed additions, and the presence of adult conspecific trees on the performance of seeds and seedlings for the five species were examined. We particularly investigated the effects of habitat hazards and seed size on the relative importance of dispersal limitation and establishment limitation. The results show that all five sowing species experienced recruitment limitation at the microsite level, although the causes of the limitation of these species varied between pathogen infection, animal predation, litter covering and shading. Seedling recruitment of the wind-dispersed, small-seeded species was mostly limited by microsite condition, while large-seeded species were mostly limited by dispersal ability.

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The vegetation in the southern Brazilian highlands is characterized by Araucaria forest and Campos grassland. Evidences indicate that Araucaria forest is currently expanding over grassland and that this expansion may occur by nucleation or edge dynamics. Nucleation mechanisms of Araucaria forest expansion are well documented, whereas mechanisms of expansion starting from the forest edge are not. In this study, we aimed (1) to assess how prominent is Araucaria forest expansion over Campos grassland starting from the forest edge, and (2) to discuss about the possible mechanisms underlying sapling community colonization in grassland. We conducted our sampling in 11 transects disposed from the forest edge towards the grassland. The transects were distributed in four study sites. Each transect was 50 m long and was divided into 25 plots with area of 8 m2. All forest woody sapling individuals were identified and registered. We considered distance from forest edge, presence of dead shrubs, Baccharis uncinella cover, nurse tree cover and rock cover as predictors of forest plant abundance and composition. Niche and distance contribution on the explanation of sapling composition and abundance were partitioned using, respectively, canonical correspondence analysis and multiple regressions. Cover of nurse trees explained almost 31% of total variation in sapling abundance, followed by the distance from the forest edge that explained 6.9%. Site explained 7.6% of total variation in sapling species composition, followed by distance from the forest edge, which explained 2.3%, whereas niche had a minor (2.1%) and non-significant proportion of variation explanation. Our findings show that Araucaria forest expansion over native grasslands in southern Brazil relies deeply on the nurse plant effect. Previous studies have demonstrated that Araucaria forest forms nuclei scattered in the grassland where nurse plants or nurse rocks are established. Here, we bring evidences that nurse plants are important also to the tree encroachment starting closer to the forest edge. This study provides new information on the mechanisms involved in the forest expansion over native grasslands.

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The importance of space as an ecological factor is an emerging paradigm in community ecology, particularly as a driving force of biodiversity patterns. We analysed β-diversity linked to spatial structure in four communities (tropical dry forest, savanna, xerophytic vegetation, subdeciduous forest) that occur in a tropical complex landscape of southern Mexico. The landscape was described through an object-oriented classification of a Quickbird satellite image. The classification revealed a highly heterogeneous spatial arrangement of the four communities. Global (landscape-level) β-diversity was 0.12 (mean Sørensen index), a value smaller than those observed for the individual communities (0.20–0.41). By using multivariate classic and partial Mantel tests, and Mantel correlograms based on two distance classes, we analysed β-diversity spatial variation related to landscape configuration. The Mantel statistic values for the four communities combined were negative and very similar both for the classic Mantel (r M = −0.23) and the partial Mantel test (r M = −0.19). Correlograms proved significant spatial autocorrelation across most of the analysed distance classes, except in the case of riparian subdeciduous forest. Tropical dry forest and savanna occupy large, highly connected areas in the landscape. Correlograms for these two communities showed decreasing trends, starting at positive, significant autocorrelation values at short distances. The loss of floristic autocorrelation beyond a 5000 m distance for the tropical dry forest may reflect spatially autocorrelated dispersal, and thus a dispersal-limited community. Xerophytic vegetation displayed a distinctly different correlogram, with a wavy shape showing an alternation of positive and negative values, in agreement with the insular configuration of this community in the landscape. We suggest that the routinely incorporation of a spatial approach in community ecology research will help in furthering our understanding of the elusive issues of species turnover across space.

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Battaglia, L.L., D.W. Pritchett and P.R. Minchin. 2008. Evaluating dispersal limitation in passive bottomland forest restoration. Restoration Ecol. 16:417–424. Minchin PR Evaluating

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