The Adaptive Significance of Trunk Inclination on Slopes: A Commentary

A recent study explored the adaptive significance of trunk inclination for trees growing on steep slopes. The authors used an optimality argument to predict how much a tree should respond to sidelight. Their calculations of the costs of leaning are flawed, because: (i) leaf mass has an allometric relationship with total volume, rather than being a fixed proportion of wood mass; (ii) the cost of support wood tissue is mainly a growth rate cost, not a maintenance respiration cost; (iii) small trees are sufficiently elastic to need very little support tissue, and thus they have a different risk structure; and (iv) most crown gaps are ephemeral rather than permanent, which also changes the risk equation. The argument I proposed in a previous study, addressing exactly the same question, is that canopy species are under selective pressure to maintain a strong central trunk that will reach the canopy and thus should not respond to sidelight. A reproductive value model is presented to illustrate this evolutionary question. Small, short-lived species or those from habitats with permanent openings (such as river margins) should be responsive. Both my previous data and the data of the other study support my model, but the model in the other study leaves many facts unexplained.