Allometric projections of time-related growth trajectories of two coexisting dipterocarp canopy species in India

Background: TheWestern Ghats of India contain the westernmost dipterocarp forests of Asia. However, only a few dipterocarp tree species actually coexist in the forest canopy among which Vateria indica and Dipterocarpus indicus are the most common. The mechanisms contributing to the coexistence of these phylogenetically closely related species have not been identified. Aims: We investigated the time- related growth trajectories in diameter, height and crown size of these two species in the Uppangala Permanent Sample Plot to determine if trade-offs in their three-dimensional developmental strategies could contribute to their long-term coexistence. Methods: From annual diameter growth data of 692 trees > 9.55 cm in diameter at breast height over a 21-year period, we developed time-related diameter growth models for the two species, accounting for local density-dependent competition effects and topography. Combining the diameter growth models with static stem and crown allometries, we projected time-related tree growth trajectories in height and crown size. Results: While both species can reach similar dimensions, V. indica grows much faster, or at least as fast as D. indicus in diameter, height and crown size in all the observed situations. Both species respond similarly to topography, but V. indica appears to be more responsive to local density-dependent competition than D. indicus. Finally, V. indica shows higher mortality and recruitment rates and a greater basal area increase than D. indicus. Conclusions: These results refute our hypothesis that D. indicus coexists with the outperformer V. indica by a growth strategy allowing selected individuals in favourable conditions to reach the canopy more quickly than their competitors. The current coexistence of the two dipterocarp species at Uppangala appears not to be at a static equilibrium; V. indica probably being in a phase of canopy stand colonisation.