Tree coexistence on a slope: An adaptive significance of trunk inclination

Under storey trees on slopes often incline their trunks downwards. The adaptive significance of this conspicuous phenomenon has, however, remained elusive. Here we present a theoretical model for the growth of under storey trees on a slope, which shows that the maximum rate of tree survival, and the optimal degree of trunk inclination, increase as the slope gets steeper, clearly indicating an adaptive significance of trunk inclination on slopes. Close examination of the results reveals that the advantage of trunk inclination on a slope is in shortening the distance from the canopy surface, and that this effect is enhanced the steeper the slope. Furthermore, the model predicts that the maximum tree survival rate increases with the slope angle more sharply under poorer light conditions. The predictions of the model are supported by an under storey species, Rhododendron tashiroi, which grows in evergreen forests on the Japanese island of Yakushima. R. tashiroi exhibits sharper trunk inclination and coexists more successfully on steeper slopes with the dominant canopy species, Distylium racemosum; and sustains itself even under poor light conditions where the slope is sufficiently steep. This also suggests that trunk inclination is a mechanism used by under storey species to coexist with the dominant canopy species.