Allometry of Constitutive Defense: A Model and a Comparative Test with Tree Bark and Fire Regime

In a simulation model, the relative advantage of negative versus positive allometry of defense, that is, early versus delayed defensive investment, depends on the forms of the relationships between body size and growth, body size and mortality deterrence, and defense size and mortality deterrence. Positive allometry is favored by strongly sigmoidal growth, a strongly convex body-size/deterrence relationship, and a concave defense-size/deterrence relationship. Rapid early growth, a weakly convex body-size/deterrence relationship, and a convex defense-size/deterrence relationship favor negative allometry. The model is tested by comparing, among Pinus and Quercus species, measured intraspecific bark allometry with a predicted ranking of bark allometry along the negative-positive continuum. The ranking is based on the forms of body-size/deterrence and defense-size/deterrence relationships expected in each species' typical habitat given the fire regime and canopy type. This predicted ranking of bark allometry is significantly correlated with measured bark allometry. Negative bark allometry and thick sapling bark are associated with habitats characterized by frequent low-intensity fire and relatively open canopies. Fire-resilient Pinus spp. and species of infrequently burned, closed-canopy forests tend to have positive bark allometry and thin sapling bark.