The Distribution of Plant Water Stress and Vegetation Characteristics in Southern California Chaparral

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Abstract
Plant xylem pressure potentials and various shrub characteristics were measured in 7 chaparral stands located between the coastal and inland boundaries of the chaparral vegetation in southern California. Early summer xylem pressure potentials decreased progressively from the boundaries of chaparral to the center of the chaparral zone, with increasing stand age and with increasing foliage-area index. Pressure potentials increased with annual precipitation. Stem elongation, leaf size and leaf density were related more to stand age and transpiration per unit leaf-area index than to annual precipitation per se. The boundaries of the chaparral occur where transpiration is 200 mm/yr per unit leaf-area index, which occurs with an annual precipitation of about 400 mm/yr. With increasing precipitation, the leaf-area index can increase such that transpiration per unit leaf-area index remains at 200 mm/yr.