RESUMEN
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, C4 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. C3, shallowrooted species, Agropyron smithii, Stipa comata, and Koeleria cristata always experienced lower mid-day and predawn leaf water potentials than the C4 species. The C3 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 C4 species show more conservative water use patterns than the C3 species as significantly lower leaf conductances in the C4's were measured when soil water was abundant. The C3 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.
RESUMEN
For a dioecious plant species in which males are associated with more xeric habitats and females with more mesic ones, (a) the xeric-mesic habitat difference was confirmed by measuring plant water potential, (b) and males and females had similar water balances and seemed to have no different adaptations to drought. There are slight differences in water potential between the sexes of dioecious plant species, but water balance can be more favorable in either the male or the female. On this account, we reject the "disruptive selection" hypothesis of Freeman et al. (1975) as an explanation for habitat assortment of sexes in dioecious plants. Alternative explanations, based upon parental determination of offspring sex ratios, or environmentally determined sex change, seem more likely.
RESUMEN
Based on the physiological characteristics and responses of C3, C4, and CAM plants to environmental factors, it is generally predicted that C4 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 C4 biomass composing each study area decreased with increasing elevation, while the percentage of C3 biomass increased. All elevations had a significantly higher percentage of C4 biomass in August than in June, reflecting the warm season growth characteristic of C4 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 C3-C4 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.
RESUMEN
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 C3 pathway of photosynthesis were consumed in greater proportion than their availability in the habitat. The preference for C3 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 C3 and C4 plants.
