The interplay between size-symmetric competition for nitrogen and size-asymmetric competition for light is critically important to the structure of forests across fertility and disturbance gradients. However, the difficulty in mathematically describing height-structured competition for light has led to two classes of models with complementary weaknesses. Simple “green slime” models yield basic ecological insights but cannot make quantitative predictions about forest structure because they lack mechanistic rigor and are parameterized in ways that correspond only loosely to variables that foresters measure. At the other extreme, complicated simulation models often yield accurate predictions of forest structure but, because of their complexity, are opaque to basic ecological insights about underlying causes. Here, we present a model that bridges this gap. We build a link between nitrogen competition and light competition using the Perfect Plasticity Approximation, an innovative modeling approach that maintains quantitative height structure while removing the need to track spatially-explicit tree locations. Although the model is capable of mapping any number of meaningful physiological and morphological traits of species to stand structure, we focus for simplicity on allocation to fine root biomass, a characteristic that varies considerably among species and plays an important and well-understood role in nitrogen competition.
Results/Conclusions
Our model predicts that many of the salient features of closed-canopy forests should change in predictable and quantitative ways along gradients of soil fertility and canopy disturbance. These features include LAI; biomass; NPP; basal area; canopy height; mean tree height; and the nitrogen-, light-, or co-limitation of either understory or canopy stages. We predict that the fine root biomasses of species that are uninvadible (i.e. ESS strategies) should be greater in areas of either low soil fertility or high canopy disturbance. For resident monocultures of non-ESS species, invaders that are sufficiently similar to the resident but closer to the ESS will invade and replace the resident, providing a path to the ESS from any non-ESS species (i.e. the ESS is convergence stable). However, when the fine root biomasses of any two species are sufficiently different, they can exhibit founder control or stable coexistence depending on assumptions of plasticity or allometry between understory and canopy stages.
