The classical environmental control model assumes that species distribution is determined by the spatial variation of underlying habitat conditions. This niche-based model has recently been ...challenged by the neutral theory of biodiversity which assumes that ecological drift is a key process regulating species coexistence. Understanding the mechanisms that maintain biodiversity in communities critically depends on our ability to decompose the variation of diversity into the contributions of different processes affecting it. Here we investigated the effects of pure habitat, pure spatial, and spatially structured habitat processes on the distributions of species richness and species composition in a recently established 24-ha stem-mapping plot in the subtropical evergreen broad-leaved forest of Gutianshan National Nature Reserve in East China. We used the new spatial analysis method of principal coordinates of neighbor matrices (PCNM) to disentangle the contributions of these processes. The results showed that (1) habitat and space jointly explained ~53% of the variation in richness and ~65% of the variation in species composition, depending on the scale (sampling unit size); (2) tree diversity (richness and composition) in the Gutianshan forest was dominantly controlled by spatially structured habitat (24%) and habitat-independent spatial component (29%); the spatially independent habitat contributed a negligible effect (6%); (3) distributions of richness and species composition were strongly affected by altitude and terrain convexity, while the effects of slope and aspect were weak; (4) the spatial distribution of diversity in the forest was dominated by broad-scaled spatial variation; (5) environmental control on the one hand and unexplained spatial variation on the other (unmeasured environmental variables and neutral processes) corresponded to spatial structures with different scales in the Gutianshan forest plot; and (6) five habitat types were recognized; a few species were statistically significant indicators of three of these habitats, whereas two habitats had no significant indicator species. The results suggest that the diversity of the forest is equally governed by environmental control (30%) and neutral processes (29%). In the fine-scale analysis (10 × 10 m cells), neutral processes dominated (43%) over environmental control (20%).
Question: Do tree and shrub species in an evergreen broadleaf forest show similar habitat associations across different life stages? Location: A 24-ha evergreen broadleaf forest plot in a ...heterogeneous landscape in Gutianshan National Nature Reserve, Zhejiang Province, Eastern China. Methods: Species having positive associations with four habitat types (low valley, low ridge, upper valley, and upper ridge) at three life stages (sapling, juvenile, and mature stages) were compared for 60 tree and shrub species using torus-translation tests. Results: A total of 117 significant positive associations with the four habitats were observed at the three life stages (43, 41, and 33 at the sapling, juvenile, and mature stages, respectively). For the 52 species significantly associated with habitats, only 16 were associated with the same habitat across all three stages. The majority of associated species at the juvenile stage (34 out of 40) were associated with the same habitat at their sapling stage, whereas half of species at the mature stage had consistent associations with the same habitat at their sapling stage. More species were associated with the upper ridge at the sapling and juvenile stages compared to the mature stage. Conversely, more species were associated with the low valley at the mature stage compared to the sapling and juvenile stages. Conclusions: Our results indicate that species ecological habitat associations can differ between developmental stages beyond 1 cm DBH, as most species habitat preferences were consistent from the sapling stage to the juvenile stage but changed at the mature stage.
Although negative conspecific density dependence among neighbours is widely studied, the general prevalence of the effects is still poorly understood due to a lack of studies from zonal forests other ...than the tropics. In addition, the detection of density dependence may be confounded by the influence of habitat heterogeneity. Here we examined the spatial distributions of 47 common tree species (diameter at breast height≥1 cm) using the pair‐correlation function g(r) in a fully mapped 24‐ha subtropical forest in China. We first investigated whether habitat heterogeneity influenced tree distributions, and then examined the conspecific tree patterns and density dependence after removing the effects of habitat heterogeneity. We found that the forest plot exhibited strong large‐scale heterogeneity in the distribution of both large adult trees of different growth forms and individual species. After the habitat heterogeneity was accounted for, 39 of the 47 species (83.0%) were found to exhibit density dependence predominantly at close distances among neighbors. Our findings highlight density dependence as a prevalent mechanism for regulating the population spatial structure of most tree species in the species‐rich subtropical forest studied here. Furthermore, the occurrence of density dependence is closely associated with species abundance and the strength of conspecific aggregation at local scales. Abundant species with high strength of conspecific aggregation tend to show density dependence.
Climate is widely recognised as an important determinant of the latitudinal diversity gradient. However, most existing studies make no distinction between direct and indirect effects of climate, ...which substantially hinders our understanding of how climate constrains biodiversity globally. Using data from 35 large forest plots, we test hypothesised relationships amongst climate, topography, forest structural attributes (stem abundance, tree size variation and stand basal area) and tree species richness to better understand drivers of latitudinal tree diversity patterns. Climate influences tree richness both directly, with more species in warm, moist, aseasonal climates and indirectly, with more species at higher stem abundance. These results imply direct limitation of species diversity by climatic stress and more rapid (co‐)evolution and narrower niche partitioning in warm climates. They also support the idea that increased numbers of individuals associated with high primary productivity are partitioned to support a greater number of species.
Summary
Tree size shapes forest carbon dynamics and determines how trees interact with their environment, including a changing climate. Here, we conduct the first global analysis of among‐site ...differences in how aboveground biomass stocks and fluxes are distributed with tree size.
We analyzed repeat tree censuses from 25 large‐scale (4–52 ha) forest plots spanning a broad climatic range over five continents to characterize how aboveground biomass, woody productivity, and woody mortality vary with tree diameter. We examined how the median, dispersion, and skewness of these size‐related distributions vary with mean annual temperature and precipitation.
In warmer forests, aboveground biomass, woody productivity, and woody mortality were more broadly distributed with respect to tree size. In warmer and wetter forests, aboveground biomass and woody productivity were more right skewed, with a long tail towards large trees. Small trees (1–10 cm diameter) contributed more to productivity and mortality than to biomass, highlighting the importance of including these trees in analyses of forest dynamics.
Our findings provide an improved characterization of climate‐driven forest differences in the size structure of aboveground biomass and dynamics of that biomass, as well as refined benchmarks for capturing climate influences in vegetation demographic models.
See also the Commentary on this article by Zuidema & van der Sleen, 234: 1544–1546.
Ecology Letters (2010) 13: 695-704 How extraordinary numbers of species can coexist in hyper-diverse communities remains unresolved. While numerous hypotheses have been proposed based on ...observational and theoretical investigations, little is known about which mechanisms are truly active in forest communities and less is known about their relative contributions to community assembly. In this study, generalized linear mixed models with crossed random effects were used to assess the relative contributions of density dependence and habitat association to community-level diversity maintenance. Species habitat associations were classified based on soil nutrients, topography and species composition. Local neighbourhood effects were also addressed with spatially explicit models of seedling survival. The results shown here reveal that local- and community-level seedling dynamics were consistent with density-dependent predictions, although habitat association played a more important role in shaping short-term seedling survival. We conclude that density dependence could promote species coexistence on the premise of habitat partitioning.
Arbuscular mycorrhizal (AM) and ectomycorrhizal (EcM) associations are critical for host-tree performance. However, how mycorrhizal associations correlate with the latitudinal tree beta-diversity ...remains untested. Using a global dataset of 45 forest plots representing 2,804,270 trees across 3840 species, we test how AM and EcM trees contribute to total beta-diversity and its components (turnover and nestedness) of all trees. We find AM rather than EcM trees predominantly contribute to decreasing total beta-diversity and turnover and increasing nestedness with increasing latitude, probably because wide distributions of EcM trees do not generate strong compositional differences among localities. Environmental variables, especially temperature and precipitation, are strongly correlated with beta-diversity patterns for both AM trees and all trees rather than EcM trees. Results support our hypotheses that latitudinal beta-diversity patterns and environmental effects on these patterns are highly dependent on mycorrhizal types. Our findings highlight the importance of AM-dominated forests for conserving global forest biodiversity.
The Janzen-Connell hypothesis is among the most important theories put forward to explain species coexistence in species-rich communities. However, the relative importance of Janzen-Connell effects ...with respect to other prominent mechanisms of community assembly, such as dispersal limitation, self-thinning due to competition, or habitat association, is largely unresolved. Here we use data from a 24-ha Gutianshan subtropical forest to address it. First we tested for significant associations of adults, juveniles, and saplings with environmental variables. Second we evaluated if aggregation decreased with life stage. In a third analysis we approximately factored out the effect of habitat association and comprehensively analyzed the spatial associations of intraspecific adults and offspring (saplings, juveniles) of 46 common species at continuous neighborhood distances. We found i) that, except for one, all species were associated with at least one environmental variable during at least one of their life stages, but the frequency of significant habitat associations declined with increasing life stage; ii) a decline in aggregation with increasing life stage that was strongest from juveniles to adults; and iii) intraspecific adult-offspring associations were dominated by positive relationships at neighborhood distances up to 10 m. Our results suggest that Janzen-Connell effects were not the dominant mechanisms in structuring the spatial patterns of established trees in the subtropical Gutianshan forest. The spatial patterns may rather reflect the joint effects of size-dependent self-thinning, dispersal limitation and habitat associations. Our findings contribute to a more comprehensive understanding of the relative importance of Janzen-Connell effects in influencing plant community structure under strong topographic heterogeneity.
Celotno besedilo
Dostopno za:
DOBA, IZUM, KILJ, NUK, PILJ, PNG, SAZU, SIK, UILJ, UKNU, UL, UM, UPUK
Aims: With the aim of understanding why some of the world's forests exhibit higher tree beta diversity values than others, we asked: (1) what is the contribution of environmentally related variation ...versus pure spatial and local stochastic variation to tree beta diversity assessed at the forest plot scale; (2) at what resolution are these beta-diversity components more apparent; and (3) what determines the variation in tree beta diversity observed across regions/continents? Location: World-wide. Methods: We compiled an unprecedented data set of 10 large-scale stem-mapping forest plots differing in latitude, tree species richness and topographic variability. We assessed the tree beta diversity found within each forest plot separately. The non-directional variation in tree species composition among cells of the plot was our measure of beta diversity. We compared the beta diversity of each plot with the value expected under a null model. We also apportioned the beta diversity into four components: pure topographic, spatially structured topographic, pure spatial and unexplained. We used linear mixed models to interpret the variation of beta diversity values across the plots. Results: Total tree beta diversity within a forest plot decreased with increasing cell size, and increased with tree species richness and the amount of topographic variability of the plot. The topography-related component of beta diversity was correlated with the amount of topographic variability but was unrelated to its species richness. The unexplained variation was correlated with the beta diversity expected under the null model and with species richness. Main conclusions: Because different components of beta diversity have different determinants, comparisons of tree beta diversity across regions should quantify not only overall variation in species composition but also its components. Global-scale patterns in tree beta diversity are largely coupled with changes in gamma richness due to the relationship between the latter and the variation generated by local stochastic assembly processes.
Extreme climatic events are predicted to increase in frequency and magnitude as global climate change continues. Extreme climatic events have profound impacts on community structure and dynamics, but ...their effects on the dominant species within a community remains unclear. To explore this issue, we analyzed changes in population dynamics and dead individuals’ spatial pattern for several dominant species (Castanopsis eyrei, Schima superba, Pinus massoniana, and Daphniphyllum oldhamii) among different habitats in a subtropical forest before and after a significant winter storm that occurred in February 2008. Using the Gutianshan 24-ha forest plot as a representative sample, we found that the plot-level DBH of P. massoniana and C. eyrei significantly increased after the winter storm, while the plot-level basal area of P. massoniana and S. superba decreased significantly. In addition, P. massoniana was most affected by the storm (mortality: 9.08%; population change rate: −8.93%), followed by C. eyrei (mortality: 6.93%; population change rate: −4.91%). Small-diameter individuals experienced higher mortality rates, but the diameter structure of the dominant species at the population level remained basically stable. The number of individuals, the density of the dominant population, the number of mortalities, and the mortality rate of the dominant species differed among habitats. The spatial point patterns of the dead individuals at each life stage were mainly aggregated in distribution, and the degree of aggregation tended to decrease with increasing scale. In conclusion, the population dynamics of dominant species were significantly altered following the winter storm, but the extent of the changes varied with species. Our study suggests that analyzing the dominant species of a community contributes to a better understanding of the biological response of forest ecosystems in the face of extreme climatic events.
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