Abstract:
Community trait assembly in highly diverse tropical rainforests is still poorly understood.
Based on more than a decade of field measurements in a biodiversity hotspot of southern
Ecuador, we implemented plant trait variation and improved soil organic matter dynamics in a
widely used dynamic vegetation model (the Lund-Potsdam-Jena General Ecosystem Simulator,
LPJ-GUESS) to explore the main drivers of community assembly along an elevational gradient.
In the model used here (LPJ-GUESS-NTD, where NTD stands for nutrient-trait dynamics),
each plant individual can possess different trait combinations, and the community trait composition
emerges via ecological sorting. Further model developments include plant growth
limitation by phosphorous (P) and mycorrhizal nutrient uptake.
The new model version reproduced the main observed community trait shift and related
vegetation processes along the elevational gradient, but only if nutrient limitations to plant
growth were activated. In turn, when traits were fixed, low productivity communities
emerged due to reduced nutrient-use efficiency. Mycorrhizal nutrient uptake, when deactivated,
reduced net primary production (NPP) by 61–72% along the gradient.
Our results strongly suggest that the elevational temperature gradient drives community
assembly and ecosystem functioning indirectly through its effect on soil nutrient dynamics
and vegetation traits. This illustrates the importance of considering these processes to yield
realistic model predictions.
