Changes in tree functional composition across topographic gradients and through time in a tropical montane forest

Abstract

Understanding variation in tree functional traits along topographic gradients and through time provides insights into the processes that will shape community composition and determine ecosystem functioning. In montane environments, complex topography is known to affect forest structure and composition, yet its role in determining trait composition, indices on community climatic tolerances, and responses to changing environmental conditions has not been fully explored. This study investigates how functional trait composition (characterized as community-weighted moments) and community climatic indices vary for the tree community as a whole and for its separate demographic components (i.e., dying, surviving, recruiting trees) over eight years in a topographically complex tropical Andean forest in southern Ecuador. We identified a strong influence of topography on functional composition and on species' climatic optima, such that communities at lower topographic positions were dominated by acquisitive species adapted to both warmer and wetter conditions compared to communities at upper topographic positions which were dominated by conservative cold adapted species, possibly due to differences in soil conditions and hydrology. Forest functional and climatic composition remained stable through time; and we found limited evidence for trait-based responses to environmental change among demographic groups. Our findings confirm that fine-scale environmental conditions are a critical factor structuring plant communities in tropical forests, and suggest that slow environmental warming and community-based processes may promote short-term community functional stability. This study highlights the need to explore how diverse aspects of community trait composition vary in tropical montane forests, and to further investigate thresholds of forest response to environmental change.

Fig 1. Non-metric dimensional scaling (NMDS) of tree species composition along a topographic gradient measured as Topographic Position Index (TPI).

A) Indicates variation in species composition during the first monitoring, B) indicates differences in floristic composition between dead and recruited summarized in a NMDS axis along the topographic gradient. Each point represents a forest monitoring plot.

Fig 2. Scaled Community Weighted Means (CWMs) of leaf and stem functional traits as a function of topographic variation Topographic Position Index (TPI).

All traits except for bark thickness and sapwood-specific conductivity varied significantly with TPI (S2 Table). The slopes of the linear regression lines represent the first (2008) and last (2016) sampling years were not significantly different for any trait. Each point represents a forest monitoring plot.

Fig 3. Summary of variation in community functional trait composition and climatic indices along a topographic gradient.

Fig 4. Community Temperature Index (A) and Community Precipitation Index (B) for the plant community along a topographic gradient Topographic Position Index (TPI).

Lines indicate statistically significant linear regressions (P≤0.05) for the first (2008) and last (2016) monitoring year. Each point represents a forest monitoring plot.

Fig 5. Community Temperature Index (A) and Community Precipitation Index (B) for the community’s demographic groups along a topographic gradient (Topographic Position Index = TPI).

Lines indicate statistically significant linear regressions (P≤0.05). Each point represents a forest monitoring plot.