EFFECTS OF QUERCUS-EMORYI TREES ON PRECIPITATION DISTRIBUTION AND MICROCLIMATE IN A SEMIARID SAVANNA

We evaluated the effects of individual Quercus emoryi trees on precipitation distribution and microenvironment in a semi-arid savanna in south-eastern Arizona, U.S.A. Tree size was explicitly considered, with trees varying in crown area from 5 to 70 m(2). Throughfall (the proportion of precipitation falling through the tree crown) was influenced by tree size and precipitation event size for precipitation events less than 30 mm. Non-linear regression indicated an exponential relationship between precipitation event size and stemflow, with precipitation event size explaining 66% of the variability in stemflow. Tree size was linearly related to stemflow, and accounted for an additional 6% of the variability. Soil moisture and temperature varied between sampling locations and between trees, although tree size had no apparent effect. Soil temperatures were lower beneath tree canopies in summer and higher in winter, compared to grassland zones. Regardless of tree size, trees had no measureable influence on soil bulk density or relative soil surface elevation. (C) 1995 Academic Press Limited