Gap dynamics help to maintain functional tradeoffs in Neotropical forest succession

Damla Cinoglu, Robin R. Decker and Caroline Farrior, Department of Integrative Biology, University of Texas at Austin, Austin, TX, Nadja Rüger, German Centre for Integrative Biodiversity Research (iDiv) Halle-Jena-Leipzig, 04103 Leipzig, Germany, Nadja Rüger, Department of Economics, University of Leipzig, 04109 Leipzig, Germany

Background/Question/Methods
Five demographic strategies (fast, slow, intermediate, short-lived breeders, long-lived pioneers) spanning the growth-survival and stature-recruitment tradeoffs capture the successional dynamics of secondary and old-growth forests in central Panama. However, the mechanisms that allow for the maintenance of these strategies are still a mystery. One possible mechanism that may promote demographic diversity along tropical forest regeneration is gap dynamics. Here, we aim to answer the following question: Can the demographic strategies we observe in the Neotropics be maintained solely through gap dynamics and height-structured competition for light in the canopy?
We parameterize a Perfect Plasticity Approximation forest patch model with gaps (Farrior et al. 2016) and annual growth, mortality, and fecundity parameters for five strategies and four canopy layers from BCI, Panama (Rüger et al. 2020). Patches are connected by stochastic seed rain, and strategies differ in how much recruitment is favored in gaps. We investigate whether competition for light following gaps generates coexistence of the five strategies. To quantify the coexistence of strategies in these stochastic simulations, we report the average time of the first extinction (average coexistence time) and the necessary strength of conspecific negative density dependence (CNDD) needed to maintain coexistence for 10,000 years in 90% of the simulation runs.
Results/Conclusions
Our preliminary results show that gap dynamics promote coexistence among two strategies across the growth-survival and stature-recruitment tradeoffs independently, and significantly enhance the average coexistence time of the five strategy simulations with stochastic seed rain. Given the metacommunity structure, we estimate that 80% more imposed CNDD is needed for five strategies to coexist for 10,000 years when gaps are not included than when they are. When we eliminate species differences, gaps hinder the coexistence of neutral models. However, five strategy neutral models with gaps coexist longer than niche models with gaps.
Our analysis shows that a simple model of competition for light following patch-level disturbances can promote empirically observed demographic diversity. However, the findings that neutral species coexist better than those with different strategies highlight the need for still other mechanisms to explain observed tropical forest demographic diversity.