JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.Wiley is collaborating with JSTOR to digitize, preserve and extend access to Ecology This content downloaded from 128.233.210.97 on Tue, Abstract. A preliminary simulation model of primary productivity and carbon allocation in creosotebush (Larrea tridentata [DC] Cov.) is described. The model utilizes a systems approach in which movement of assimilate within the plant is in response to changes in source-sink strengths of leaves, stems, roots, early reproductive buds, maturing reproductive buds, flowers, and fruits. Two distinct compartments are defined per organ or developmental stage to separate assimilate into a pool fraction (labile or translocatable) and a structural fraction (nonlabile). The changes in magnitude (within upper and lower limits) of a pool compartment during the course of a simulation (i.e., growth and development of the plant) are a function of the rates of maintenance respiration and growth as well as a priority scheme governing allocation of assimilates; the increases and decreases in dry weight of a structural compartment are a function of aging and the magnitude of its associated pool (which determines structural growth and physiological death).A 1-yr simulation of a hypothetical Larrea plant shows that the model exhibits a reasonable behavior, although no validation is attempted at this stage in its development. The heuristic value of the model is illustrated in the sensitivity analysis, which shows the need for detailed knowledge of "priority" carbon movement during both vegetative and reproductive growth periods, the importance of substrate controlled respiration rates, and the need for further studies of the dynamics of labile pools in the plant.The model has been an excellent tool in our initial attempt to integrate the voluminous information on Larrea into a complete functional description of the autecology of the species. Further refinement of this model as data from our current research and that of other investigators become available should lead us to a better understanding of the ecological role of Larrea in desert ecosystems.