Intraspecific Variation in Tree Growth Rates Promotes Species Coexistence in Tropical Forests

Baruah, Gaurav Kumar (2015) Intraspecific Variation in Tree Growth Rates Promotes Species Coexistence in Tropical Forests. Masters thesis, Indian Institute of Science Education and Research Kolkata.

PDF (MS dissertation of Gaurav Kumar Baruah (10MS30)) Gaurav_Kumar_Baruah-10MS30.pdf - Submitted Version Restricted to Repository staff only Download (3MB)

Abstract

The ecological mechanisms that promote coexistence of tree species in diverse tropical forests are not well understood. Niche-based mechanisms invoke inter-specific differences in life histories, but the importance of intra-specific differences plant life histories and performance for species coexistence has remained largely unexplored. Classical theory would predict that species with higher growth rates would exclude species with inferior growth rates in the absence of other factors that stabilize differences in fitness. Here we study a species-rich tropical rainforest in Panama, Central America and test whether intra-specific variation in tree growth rates can promote coexistence of tree species that belong to the same light guild. We use the Kolmogorov forward diffusion equation as a mathematical model to study the evolution of size density distribution dynamics of tree species. The model was parameterized from 50 ha plot data from Panama, Central America. We chose 39 species that are shade-tolerant (can regenerate under shade) canopy trees for parameterizing the demographic rates in our model. The size density dynamics is basically governed by growth, recruitment and mortality. All these demographic functions were parameterized by linear regression and optimization methods. We found that light competition has a significant positive relationship with intraspecific variation in growth rates. We choose combinations of two to seven tree species that belong to the same light guild that incorporates important demographic parameters and competitive effects in determining the evolution of tree size distributions over time. Our model simulations show that variation in mean growth rates increase significantly as competitive interaction increases. We also found that a species with lower mean growth rates can coexist on ecological time scales with superior species provided its variation in mean growth rates is substantially higher. We demonstrate this for two-species, three-species, and five-species communities. We also found out the parameter space of intraspecific variation till which it can equalize fitness differences manifested in growth rates due to competition for light. Short term coexistence will be promoted only in this specific range of parameter space of intraspecifc variation above and below which competitive exclusion will occur. Our results are consistent with certain theoretical models and empirical data on species coexistence and suggest that individual variation in tree rates can promote species coexistence in species rich communities.

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