Growth dynamics of three tropical savanna grass species: an individual-module model

Abstract

We model the dynamics of grass plant growth as a collection of the individual dynamic behavior of shoots and leaves. The model is inspired in data for plants of three species ( Elyonurus adustus, Leptocoryphium lanatum and Andropogon semiberbis) of common grasses in the Venezuelan savannas that were sampled monthly for 1 year. These species represent different architecture and regeneration response to fire. Modules (shoots and leaves) were counted in each cell of a square grid in each one of several vertical levels. Module density per cell provides the horizontal distribution within a level and is aggregated by level to obtain vertical distribution. Both distributions are simulated by a dynamical model based on shoot emergence and mortality, elongation of shoots and leaves given by Richards’ equation, plus a few simple geometric considerations. For quantitative comparisons of model results to data, the transient and final values for vertical distribution plus two metrics of horizontal distribution at each level, were calculated for the simulation results and the field data. Proportion of occupied cells and maximum distance to the center of growth were the two metrics selected to capture the dispersion and range of the horizontal distribution. The model results indicate predictable final vertical profiles (of proportional density plus the two metrics) similar to the profiles of measured distributions for each species. A reasonable prediction of the transient behavior was also obtained but with larger deviations as evaluated by the root mean square error between model and data. Differences in vertical and horizontal patterns of module density among species are explained by changing a set of parameter values related to growth form and phenology. Thus, the model could be applied to generate plant functional types for analysis of savanna dynamics subject to fire. With modifications, the model is potentially applicable to other grass plants and other grassland ecosystems.