Tropical tree species assemblages in topographical habitats change in time and with life stage

Abstract

ummary 1. Recent studies have documented shifts in habitat associations of single tropical tree species from one life stage to the next. However, the community-level consequences of such shifts have not been investigated, and it is not clear whether they would amplify, neutralize or completely alter habitat structuring during the transitions to the adult community. 2. We compared habitat-driven species assemblages at three life stages (i.e. recruitment, juvenile and reproductive stages) and six censuses for tree and shrub species in a fully censused 50-ha plot of Panamanian lowland forest. Habitat types were determined using multivariate regression trees that group areas with similar species composition (i.e. species assemblages) according to their topographical characteristics. 3. Three topographical variables (a topographical wetness index, slope and elevation) were major determinants of species assemblages. When analysing individuals of all life stages together, we found a distinct and temporally consistent structuring of the plot into four dominant habitat types (low and high plateaus, slope and swamp) which was consistent with previous classifications. Basically, the same habitat structuring emerged for the juvenile communities of individual censuses. However, recruits showed a weak and temporally inconsistent habitat structuring. 4. A notable homogenization in species assemblages occurred during the transition from juvenile to reproductive, through both a reduction in the number of species assemblages (in 3 censuses, one large reproductive assemblage covered 93% of the plot, and in others, an additional slope habitat emerged) and a reduction in the classification error. Overall, habitat structuring became noisier and weaker over the 25 years of the study. 5. Synthesis. Our results suggest that mortality processes during the transition from recruits to juveniles must enhance the signal of habitat structuring. However, during the transition to the reproductive stage, species may have lost the advantage of being in the habitat with which they had become associated, or the quality of habitat changed during their life span because of larger climatic changes. The homogeneous assemblages of the reproductive stage could be interpreted as support for neutral theories, but further research is required to unravel the mechanisms behind these intriguing observations.

Habitat filtering across tree life stages in tropical forest communities

“…It is likely relevant that MRT models with different groups were compared (the number of groups and the distribution of quadrats among groups was an important component of the interpretation) and that the CVRE obtained from an MRT analysis has a high degree of variability (figure 2b), even when using the same census and environmental data. However, the discrepancy between our results and those of Kanagaraj et al [15] is certainly affected by the fact that they did not control for differences in the number of stems among life-history stages. Most often, changes in the effect of environmental variables across tree size classes were small, with the notable exception of Korup, where there was a large increase in the effect of habitat heterogeneity over the transition from saplings to juveniles determined using rigid dbh cutoffs.…”

“…The differences in the variation explained between life stages were most often less than 1 per cent and represented less than a 20 per cent relative increase or decrease. This contrasts with the greater than twofold differences in the variation explained by the environment between trees of different size classes or developmental stages in previous analyses [15,16]. Overall, the amount of habitat structuring observed for the adult communities is similar to the amount that would be expected based on random survival of individuals at earlier life stages.…”

Seedling recruitment factors in low‐diversity Hawaiian wet forest: towards global comparisons among tropical forests

“…However, all species were positively associated with more v www.esajournals.org than one habitat category, and species' habitat associations overlapped substantially. Our results are consistent with studies in high-diversity forests in which new recruits had weak habitat specificity (Kanagaraj et al 2011) or in which a number of tree species shared preferred habitats (Webb and Peart 2000). The co-occurrence of strong habitat associations and high niche overlap found here for seedling species may be a typical pattern and should be further examined across forests and life-stages.…”

Multispecies coexistence of trees in tropical forests: spatial signals of topographic niche differentiation increase with environmental heterogeneity

“…Additional processes, whether neutral or related to other effects (such as the JanzenConnell effect, which our findings do not take account of), may be occurring in the plots, as may niche differentiation in ways that we cannot detect with current data (e.g. temporally, at smaller scales, or at early life stages) [46,54,55]. A parsimonious interpretation of our observation of changes in spatial structure with environmental heterogeneity over a wide sample of biogeographic conditions within the range of tropical forests is that the increasing variability in spatial overlaps with increasing heterogeneity stems from niche differentiation of the species.…”