Plant and soil water relationships in a typical nebraska Sandhills prairie were examined to 1) explain the observed distribution patterns of several dominant grasses along a topographic gradient, and 2) show how spatial and temporal variations in soil moisture are critical to community organization on a sandy substrate. An experimental transect encompassing the major community and soil types along a steep, west-facing vegetated dune was established. Maximum available water was shown to be significantly higher in the fine textured surface soils of the lowland sites than the coarse textured sands of the dune sites. Seasonal (1979) patterns of available soil moisture of the sampling sites on the transect showed that in the upper elevation dune sands, moisture was available in the entire profile with surface depletions not occurring until mid to late summer. In contrast, moisture in the surface 60-80 cm in the fine textured lowland soils was exhausted by early to mid-summer with the entire profile nearly dry by late summer. Deep-rooted, C species, Andropogon hallii and Calamovilfa longifolia which are common on upper, coarser sandy soils showed significantly greater water stress on fine textured soils than on dune sands. C, shallowrooted species, Agropyron smithii, Stipa comata, and Koeleria cristata always experienced lower mid-day and predawn leaf water potentials than the C species. The C species, with the exception of Koeleria are most abundant on finer textured soils that provide substantial moisture during their peak activity in the spring. It appears that the C species show more conservative water use patterns than the C species as significantly lower leaf conductances in the C's were measured when soil water was abundant. The C species appear to be opportunistic with available water and rapidly deplete surface soil moisture as a result of high transpiration rates. These data suggest that the temporal and spatial distribution of available water along this gradient controls species distribution according to rooting morphology, photosynthetic physiology, and water deficits, incurred by transpirational losses. Competitive interactions between species that utilize soil moisture differently may be an important factor in community organization.

The summer drought—deciduous coastal sage in Southern California near Los Angeles has a bimodal, coastal and interior, distribution at low elevations (sea level to 300 m), whereas the evergreen sclerophyllous chaparral is developed at higher elevations. Winter rainfall is 300—350 mm at the low elevations and 500—600 mm at the higher altitudes. Photosynthesis and transpiration in response to artificial drying were examined in field—collected leaves of species from the two vegetation types. The fully hydrated mesomorphic leaves of the coastal sage species had initially higher photosynthesis and transpiration rates which declined rapidly during the first 20—30 min of drying, presumably because of hydroactive stomatal closure. At zero net photosynthesis cuticular loss of water was high, nearly 25% of the maximum rate for these species. In the chaparral species photosynthesis and transpiration rates were one—fourth to one—third those of the coastal sage species. Decline of rates with drying time was not nearly as rapid. Net photosynthesis continued at 20—30% of maximum after 70—80 min with lower cuticular losses of water vapor. During cool spring periods with sufficient soil moisture, high transpiration and assimilation rates allow for rapid growth, flowering, and fruiting in plants of the coastal sage community. They are drought—evading by virtue of their summer deciduous habit and are better adapted to prolonged summer drought in areas of low rainfall. Chaparral species, at slightly higher altitudes with higher precipitation and with deeper and more extensive root systems and xeromorphic leaf characteristics, are less sensitive to conditions of high evaporative demand and can maintain net assimilation, although low at times, throughout the long summer drought period.

Based on the physiological characteristics and responses of C, C, and CAM plants to environmental factors, it is generally predicted that C and CAM plants will become more abundant with increasing temperature and decreasing precipitation. To test this prediction, the relative contribution of each photosynthetic type to total plant community biomass was examined at seven study areas along an altitudinal transect in southeastern Wyoming grassland. In going from high (2,652 m) to low (1,405 m) elevation along this transect, mean annual temperature increased and annual precipitation decreased.The percentage of C biomass composing each study area decreased with increasing elevation, while the percentage of C biomass increased. All elevations had a significantly higher percentage of C biomass in August than in June, reflecting the warm season growth characteristic of C plants. Regressions of relative abundance of photosynthetic types on climatic variables showed that both mean annual temperature and annual precipitation were equally reliable as predictors of C-C biomass, although we feel that temperature is of primary importance in explaining our observations. CAM species were present at all elevations, but showed no trends in biomass distribution with respect to elevation.

No abstract

Food habits of Arphia conspersa Scudder and Arphia pseudonietana (Thomas) were studied along an altitudinal transect in southeastern Wyoming shortgrass mixed prairie. Stable carbon isotope ratios indicated that diets were significantly different between study sites, between species, and between sexes. These differences were found to be primarily related to the availability of different food plants along the transect, although species with the C pathway of photosynthesis were consumed in greater proportion than their availability in the habitat. The preference for C species is presumably related to their higher nutritional value and digestibility, in spite of the fact that more time and energy must be spent to locate these food plants in some of the habitats studied. This study demonstrates the utility of the carbon isotope method in studying plant-animal interactions in habitats containing both C and C plants.