A MODEL OF WOODY-HERBACEOUS BIOMASS RELATIONSHIPS IN EUCALYPT AND MESQUITE COMMUNITIES

A spatial simulation model was developed to examine the community-level relationships between woody overstory and herbaceous understory. The influences of individual trees on herbaceous understory were aggregated into stimulatory and competitive effects which were represented as indices. The net index at a particular point on the landscape was calculated by multiplying the indices of all trees having an effect at that point. Simulated sampling of computer-generated communities (calculating the net index at a number of randomly selected points) enabled the herbaceous production to be estimated for communities of defined tree density and size. The model was parameterized for eucalypt (Eucalyptus crebra F. Muell.) communities in northeastern Australia and for honey mesquite (Prosopis glandulosa var glandulosa Torr.)-mixed brush areas in southwestern U.S.A. When a net competitive effect exists around individual trees, a negative curvilinear relationship between herbaceous yield and tree density was observed in the simulations. If stimulatory effects dominate at the individual tree level, herbaceous yield at the community level was highest at intermediate tree densities. Thus the extremes of relationships seen in the literature can be reproduced simply by altering the relative strength of stimulation and competition in this model. The model can be used to examine the change in herbaceous yield at the community level following manipulation of woody cover.