In recent decades, mature tropical forests across the globe have seen sustained net aboveground C accumulation however the future of this sink is highly uncertain. A large source of uncertainty stems from a lack of understanding in the role of tree diversity on the temporal stability of the tropical forest C sink. Diversity may play a key role in maintaining low variability of the long-term C sink but results from previous studies are inconclusive. The role of resource acquisition diversity among trees is particularly understudied. Here we examine a case where the landscape tree diversity axis is defined by the relative abundance of two legume species with distinct biogeochemical niches; one that fixes N at high rates and occupies diverse forests and one that does not fix N and forms monodominant stands. From this we ask: how does tree diversity influence temporal stability and thus the long-term C sink? How do tree N acquisition and cycling strategies affect overall long-term C dynamics? To answer these questions, we analyzed 30 years of demographic data from over 28,000 stems distributed across 90 permanent 1-ha stem inventory plots spanning monodominant to diverse lowland rainforests in Southern Trinidad.
Results/Conclusions
The forests of Trinidad provide a unique opportunity to examine how C sequestration overtime is influenced by tree diversity. Both diverse and monodominant forests display high and relatively stable above ground C stocks (~150-270 Mg C ha-1) over the past thirty years. Interestingly, however, while monodominant forests have maintained high C sequestration rates (~0.5-4 Mg C ha-1 yr-1), C sequestration in diverse forests (~0-2 Mg C ha-1 yr-1) has declined slightly in the last decade. Furthermore, despite these different trajectories, both high diversity and monodominant forests converge on similar interannual variance suggesting higher order control on dynamics. Finally, we report a unimodal relationship between the relative abundance of N-fixing trees and plot-level tree diversity wherein N-fixers are rare in the least diverse (monodominant) and most diverse plots but most abundant (~40% of total stems) in plots with intermediate levels of tree diversity. We discuss the implications of how long-term tree interactions and biogeochemical strategies shape the forest-carbon dynamics of these ecosystems.