A model of multiple-element limitation for acclimating vegetation

In this paper we present a simple model of multiple-element limitation of plant production and biomass accumulation. The primary aim of this models is to develop a theoretical framework for examining multiple-element limitation vs. single-element limitation and for examining the relationship between short-term and long-term responses to changes in element availability. In the model we assume that there is an @'optimal@' ratio of mineral elements in vegetation biomass, and that the vegetation continually adjusts its relative element uptake capacities to compensate for shifts away from this optimum. We examine the responses of this model to changes in element availability in the plant environment, where @'availability@' is defined either as fixed concentrations of non-depletable elements or as fixed replenishment rates of depletable elements. The model results suggest that the nature of the controls on element availability has a major impact on whether single- or multiple-element limitation prevails, even when plants can acclimate so as to maintain an @'optimal@' nutritional balance. Single-element limitation occurs when the replenishment rate of an essential element to the available pool is limited and sustainable plant uptake of that element equals the replenishment rate. Furthermore, when single-element limitation prevails, there is little or no correlation between short-term responses to a change in element availability and long-term, equilibrium responses. In general, previous experimental studies and models of plant growth in response to changes in relative availability of multiple, essential elements have either not specified how those resources are controlled, or have examined only one type of control. Our results help to explain the diversity of results of past studies of multiple-element limitation, suggest some improvements in experimental design for future studies, and have important implications for the extrapolation of the results of controlled experiments to field situations.