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Forage availability for mule deer (Odocoileus hemionus) in the Sonoran Desert depends on plant biomass, which is influenced by rainfall. We determined how rainfall, temperature and plant characteristics affected biomass of deer forage. We measured forage biomass, rainfall and temperature every 3 months from April 2000 to December 2002. Quarterly rainfall ranged 2 from < 1 to 57 mm, and forage biomass in desert washes fluctuated between 6 and 34 g m(-2). There was a positive relationship between forage biomass and rainfall the previous six months (p < 0.001), and a negative relationship between biomass and average temperature the previous 3 months (p < 0.001). Quarterly forage growth was positively influenced by rainfall (P < 0.001) and negatively influenced by forage biomass (p < 0.001). The relationships between deer forage and environmental factors established here will be useful in understanding population ecology of mule deer as part of an interactive model of plant-herbivore dynamics in and environments. (C) 2004 Elsevier Ltd. All rights reserved.