Questions: Density dependence is thought to restrict exponential growth as well as give rise to size structure in populations. Size hierarchy in trees from tropical dry deciduous forests is studied to ask (1) whether nature of competition is symmetric or asymmetric and (2) what is the self thinning trajectory under a natural gradient of tree density.

Location: Western India.

Methods: Density was measured as the number of trees in 10-m radius circular plots (n = 96) and size was measured at DBH. Size variation was evaluated by the Gini coefficient (n = 1239 trees).

Results: Size inequality between neighbours decreased with density but in a non-linear manner. In the backdrop of existing theory this indirectly suggests that competitive interactions may be symmetric over a ‘depletive’ resource such as below-ground water (rather than a ‘pre-emptive’ resource such as light), which is very plausible in a semi-arid environment. The self thinning coefficient derived from the relationship between stem diameter and density (γ ≈ −1/4), is higher than expected from existing models of allometric plant growth (γ ≈ −1/3) which are based on above-ground interactions alone. Seen in conjunction, these results suggest that above-ground structures, such as stem size, do not adequately represent the outcome of competitive interactions when below-ground resources, such as water, may be more important under semi-arid conditions.

Conclusions: The non-linear relationship between size inequality and density indicates that there exists a density threshold beyond which investment in above-ground biomass becomes sluggish in semi-arid, deciduous forests. Since current allometric models do not incorporate below-ground biomass for trees, these data suggest that a more comprehensive allometric growth model may have higher predictive power and wider applicability.

Nomenclature: Champion & Seth (1968).