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The partitioning of solar and net total irradiances between the canopy and soil was measured in mixed and chamise chaparral in southern California. The solar and net total irradiance absorbed within the canopy was a relatively constant fraction throughout the year. Variations can be explained by the changing path length of the solar beam as the solar altitude varies. The increased fraction of solar irradiance in winter, combined with the lower incident solar irradiance, produced a wide fluctuation of solar irradiance at the soil surface through the year. Although incoming solar irradiance on the bare soil surface is less on the north-facing slope than on the south-facing slope, the absorption of solar irradiance is greater in the canopy of the mixed chaparral on the north-facing slope because greater leaf and stem area has developed.

Plant xylem pressure potentials and leaf conductances to water loss were measured in the spring and summer at two sites in the western Cape Province, Republic of South Africa. The measurements were to test the hypothesis that the fynbos of South Africa was equivalent to the chaparral of California, therefore a period of plant water stress, similar to the period measured in chaparral species, should occur in flynbos species during the dry summer. The measurements indicated little or no plant water stress in most fynbos species at both sites. The sites were similar in plant water relations in spite of a more than twofold difference in annual precipitation. The minor differences in plant water relations can be explained in terms of different vegetation cover and different vapor pressure deficits during the measurements. The results support the conclusion that the arid fynbos may be equivalent to chaparral, but the majority of the fynbos is equivalent to coniferous and broad-leaved evergreen forest in California.

The seasonal course of water relations was measured in the field in Adenostoma fasciculatum, Quercus dumosa, Ceanothus greggii, and Arctostaphylos glauca, four prominent members of the southern California chaparral vegetation. Ceanothus greggii and A. glauca developed similar seasonal patterns of minimum leaf water potentials, as estimated by xylem pressure measurements, which were much less negative than A. fasciculatum and Q. dumosa growing in close proximity on the same pole-facing slope site. Adenostoma fasciculatum on an adjacent equator-facing slope developed more negative water potentials than did A. fasciculatum on the pole-facing slope.Leaf conductance differed between species, and by leaf age class and slope exposure within a species. The greatest differences were measured between leaf age classes in A. fasciculatum on the pole-facing slope, with new leaves showing the greatest conductances early in the season. The same trend was measured in A. fasciculatum on the equator-facing slope, but the differences were less between leaf age classes and diminished earlier in the season than in A. fasciculatum on the pole-facing slope. The analysis of daily hysteresis in the leaf conductance-water potential relation suggests that early in the season when water is available, stomatal behavior is simultaneously governed by a complex of environmental factors, while late in the season stomatal behavior becomes increasingly dominated by tissue water status.

A model to predict the daily courses of leaf resistance, leaf water potential, transpiration, leaf temperature and net photosynthesis based on soil-plant-atmosphere continuum and energy budget concepts is presented. The principle water relations parameters required by the model are the minimum leaf resistance, the response curves of leaf resistance to light, temperature, and leaf water potential, and the relationship between leaf water potential and water deficit. Predictions of the effects of changes in soil water potential on the daily patterns of leaf resistance, leaf water potential, leaf temperature, and net photosynthesis in an alpine climate are examined. The model was tested using data from two alphine species, Bistorta bistortoides and Caltha leptosepala, that exhibited different daily leaf resistance and leaf water potential patterns as water stress developed. Agreement was found between predicted and observed patterns. Differences in the daily courses between the species are shown to be due to differences in the physiological parameters. The relevance of the daily leaf resistance patterns is discussed in the context of drought adaptability.

The influence of annual precipitation and vegetation cover on soil moisture and on the length of the summer drought was estimated quantitatively using 9 years of soil moisture data collected at Echo Valley in southern California. The measurements support the conclusions that in the semi-arid mediterranean climate a soil drought will occur regardless of vegetation cover and annual precipitation, but the length of the drought is greatly dependent on soil depth and rockiness. Evergreen species which can survive this drought tend to accentuate the drought, especially in deep soil levels, by developing a canopy with a large transpiring surface.