Showing papers by "Thorsten Wiegand published in 2023"

Spatial facilitation and competition regulate tree species assembly in a tropical dry forest

Abstract: Analyzing the spatial association pattern among species can help to better understanding the mechanisms that drive forest dynamics and assembly. We applied techniques of spatial point pattern analysis to data from a fully mapped plot of tropical dry forest (TDF) in Colombia to assess the spatial association network among the eight most abundant species and we tested the hypothesis that species traits related to the ability to cope with drought stress could explain the observed spatial association patterns. We conducted three analyses, first we classified the types of spatial association patterns of species pairs against a null model of spatial independence, second, we used a heterogeneous Poisson (HP) null-model to assess competitive and facilitative interactions, and finally, we integrated the spatial association network with a traits space spanned by hydraulic functional traits. Overall, the proportion of significant negative and positive associations were low and we found at smaller spatial scales (5 m) prevalence of positive association patterns (11%) and at intermediate scales (16 m) negative interactions (13%). The dominant, evergreen and bird-dispersed species Trichilia oligofoliata, which followed a hydraulically save strategy, was involved in most positive associations at small scales, whereas the evergreen large statured species Aspidosperma polyneuron, which also follows a conservative resource-use strategy, was involved in most negative interactions. In TDFs where water stress is prevalent, tree community assembly and spatial patterns formation are regulated by environmental heterogeneity (e.g., topography), and both facilitative and competitive processes act simultaneously, but at different spatial scales and involving different species. Our findings highlight the potential importance of the examined association patterns, not only for our understanding of community assembly, but also to provide restoration directions.

Latitudinal scaling of aggregation with abundance and its consequences for coexistence in species rich forests

Abstract: The complex spatial structure and dynamics of ecological communities continue to defy explanation by simple principles despite decades of attention from ecologists and theoreticians. For example, the relationship between plant spatial distributions and species coexistence is often challenging to resolve in empirical settings. By analysing the spatial patterns of trees in 21 large forest plots we find a general and strong latitudinal gradient in the relationship between conspecific aggregation and abundance of tree species, with stronger negative abundance-dependency as latitude increases. To derive theoretical expectations for how interactions between multiple spatial pattern and processes can impact species coexistence, we incorporate the observed spatial patterns together with neighbourhood crowding competition into a mathematical model to estimate the local extinction risk of species. Strikingly, we find simple relationships that predict species local extinction risk from their demography and spatial distribution. Compared to a corresponding non-spatial analysis, accounting for spatial patterns reduces the 1000-year extinction risk on average by 52% when species invade from low abundances of 50 individuals. Additionally, based on their current abundances, only 8% of the species had an extinction risk greater than 5%. Our approach opens up new avenues for integrating observed spatial patterns with multiple ecological processes into mathematical theory. Our results demonstrate that emerging spatial patterns can contribute substantially to coexistence in species-rich forests, emphasizing the need to understand the interacting multiple processes underpinning spatial patterns in greater detail than has previously been appreciated.