Jay E. Anderson

Physiological ecology of North American Desert Plants

This book begins with the physical and biological characterization of the four North American deserts and a description of the primary adaptations of plants to environmental stress. In the following chapters the authors present case studies of key species representing dominant growth forms of the North American deserts, and provide an up-to-date and comprehensive review of the major patterns of adaptations in desert plants. One chapter is devoted to several important exotic plants that have invaded North American deserts. The book ends with a synthesis of the adaptations and resource requirements of North American desert plants. Further, it addresses how desert plants may respond to global climate change.

LANDSCAPE-SCALE CHANGES IN PLANT SPECIES ABUNDANCE AND BIODIVERSITY OF A SAGEBRUSH STEPPE OVER 45 YEARS

Increasing demands on arid and semiarid ecosystems, which comprise one-third of Earths terrestrial environment, create an urgent need to understand their biodiversity, function, and mechanisms of change. Sagebrush (Artemisia) steppe, the largest semiarid vegetation type in North America, is endangered because of losses to agriculture, excessive grazing, and invasive species. Establishment in 1950 of what is now designated as the Idaho National Engineering and Environmental Laboratory (southeastern Idaho, USA) created the largest existing reserve of this extensive vegetation type. We used cover, density, and frequency data for vascular plants sampled on 79 permanent plots nine times during 45 years to (1) assess long-term changes in abundance and distribution of major species and life forms, (2) assess changes in species richness and plot similarity, and (3) test the hypotheses that plant cover and stability of cover are positively associated with species richness and that invasibility is inversely relate...

The biogeochemistry of a north-temperate grassland with native ungulates: Nitrogen dynamics in Yellowstone National Park

Nutrient dynamics of large grassland ecosystems possessing abundant migratory grazers are poorly understood. We examined N cycling on the northern winter range of Yellowstone National Park, home for large herds of free-roaming elk (Cervus elaphus) and bison (Bison bison). Plant and soil N, net N mineralization, and the deposition of ungulate fecal-N were measured at five sites, a ridgetop, mid-slope bench, steep slope, valley-bottom bench, and riparian area, within a watershed from May, 1991 to April, 1992.Results indicated similarities between biogeochemical properties of Yellowstone grassland and other grassland ecosystems: (1) landscape position and soil water affected nutrient dynamics, (2) annual mineralization was positively related to soil N content, and (3) the proportion of soil N mineralized during the year was negatively related to soil C/N.Grazers were a particularly important component of the N budget of this grassland. Estimated rates of N flow from ungulates to the soil ranged from 8.1 to 45.6 kg/ha/yr at the sites (average = 27.0 kg/ha/yr), approximately 4.5 times the amount of N in senescent plants. Rates of nitrogen mineralization for Yellowstone northern range grassland were higher than those measured in other temperate grassland ecosystems, possibly due to grazers promoting N cycling in Yellowstone.

Water use efficiency and carbon isotope composition of plants in a cold desert environment

SummaryThe effects of the availabilities of water and nitrogen on water use efficiency (WUE) of plants were investigated in a sagebrush steppe. The four species studied wereArtemisia tridentata (shrub),Ceratoides lanata (suffrutescent shrub),Elymus lanceolatus (rhizomatous grass), andElymus elymoides (tussock grass). Water and nitrogen levels were manipulated in a two-by-two factorial design resulting in four treatments: control (no additions), added water, added nitrogen, and added water and nitrogen. One instantaneous and two long-term indicators of WUE were used to testa priori predictions of the ranking of WUE among treatments. The short-term indicator was the instantaneous ratio of assimilation to transpiration (A/E). The long-term measures were 1) the slope of the relationship between conductance to water vapor and maximum assimilation and 2) the carbon isotope composition (δ13C) of plant material. Additional water decreased WUE, whereas additional nitrogen increased WUE. For both A/E and δ13C, the mean for added nitrogen alone was significantly greater than the mean for added water alone, and means for the control and added water and nitrogen fell in between. This ranking of WUE supported the hypothesis that both water and nitrogen limit plant gas exchange in this semiarid environment. The short- and long-term indicators were in agreement, providing evidence in support of theoretical models concerning the water cost of carbon assimilation.

Factors Controlling Transpiration and Photosynthesis in Tamarax Chinensis Lour.

Photosynthetic and stomatal responses of Tamarix chinensis to temperature, light, and humidity were investigated in the field in New Mexico and in the laboratory. Transpiration rates for T. chinensis were similar to those of several herbaceous plants and co—occurring phreatophytes. Net photosynthetic rates and water use efficiency of T. chinesis were lower than for other species. Photosynthesis was light saturated at a photon flux density equal to 44% of full sunlight. Carbon dioxide assimiliation was tightly coupled to irradiance below light saturation. Leaf resistances remained low at photon flux densities above one—third of full sunlight, but increased linearly with decreasing photon flux density below that level. Shading for 5 min resulted in a doubling of leaf resistance. The rapid response of stomata to changing light conditions is probably an adaptation to conserve moisture when light is limiting to photosynthesis. Optimal leaf temperatures for photosynthesis were 23°—28° C, which correspond roughly to ambient temperatures during the early part of the day when evaporative demand was relatively low. T. chinensis stomata appeared to respond directly to changes in the leaf—to—air absolute humidity gradient. At constant temperature, leaf resistance increased linearly with increases in the leaf—air humidity gradient. Midday depressions of gas exchange invariably occurred in the field, despite the fact that the plants had an abundant water supply. These depressions resulted from increases in leaf resistance in response to increasing evaporative demand of the air. This response results in improved water use efficiency during the hottest portion of the day. Plant water potential decreased from pre—dawn values of about —0.9 MPa to minimal values of about —2.6 MPa by midmorning. Improvements in bulk water status were often observed during the afternoon when leaf resistances were higher. Diurnal patterns suggested that leaf resistance was largely a function of temperature, light, and humidity, rather than plant water status.

Control of the soil water balance by sagebrush and three perennial grasses in a cold‐desert environment

Abstract This study compared the capacity of monocultural stands of crested wheatgrass, giant wildrye, streambank wheatgrass, and Wyoming big sagebrush to deplete water from a clay‐loam soil. Soil water balance of a natural sagebrush steppe was used as a reference point. Transplanted crested wheatgrass plants extracted 243 mm of water from the soil during the first growing season, using most of the plant‐available water. The other species extracted less water, but evapotranspiration from all stands was well over the average annual precipitation for the study area (224 mm). In the second year, all four species extracted water from the entire 2.2‐m profile and used most of the available water. The native community removed essentially all of the available soil moisture in each of the three years. Despite phenological and morphological differences among species, patterns of water depletion were generally similar. Seasonal evapotranspiration varied little among species but was correlated with the soil water av...

Vegetal recovery following wildfire in seeded and unseeded sagebrush steppe.

Following an August wildfire, sagebrush (Artemisia L.)/grass benchlands adjacent to Pocatello, Ida., were seeded with a mixture of exotic wheatgrasses and forbs by rangeland drill in November 1987. The effects of seeding on vegetation development in the immediate postfire years were evaluated by comparing plant density, vegetal cover, species composition, species diversity, and standing crop in seeded areas to that in unseeded control plots in 1988 and 1989. We also examined cover of bare ground, litter, and growth form between treatments and between sampling periods. Twenty paired 10-m transects were established in seeded and unseeded areas on each of 3 plots on the burned benches. Plant density, vegetal cover, and species diversity were lower in the seeded areas than in the unseeded areas in 1988 and 1989. Species composition, species richness, and standing crop were similar between treatments. Establishment of seeded species was poor, probably as a result of drought conditions in 1987 and 1988. Most plants observed in seeded and unseeded areas in the spring of 1988 sprouted from established perennials. Even though the first postfire season was a drought year, plant cover in the unseeded areas (18.3%) approached that estimated by a U.S. interagency task force as needed to stabilize soils on that site. In the following year, which had average precipitation, plant cover in both treatments exceeded the task forces estimate of prefire cover. Because the indigenous plant species recovered rapidly, seeding of this burn was unnecessary to establish plant cover and counterproductive in terms of erosion potential. These results serve to emphasize that objective criteria should be established for evaluating the necessity of postfire seeding.

Diets of black-tailed jack rabbits in relation to population density and vegetation.

Diets of black-tailed jack rabbits (Lepus cutYfornicus) and composition of plant communities were compared among habitats that supported different densities of jack rabbits in sagebrush (Arlemisia tridentata)/perenniaI grass communities on the Idaho National Engineering Laboratory (INEL) in southeastern Idaho. Diets were more similar than vegetation among areas, indicating that jack rabbits feed selectively; winterfat (Ceratoides hata) and perennial grasses were staple foods, comprising about 80% of the diet in all areas. Jack rabbit densities were higher in areas having higher proportions of grass cover. Similarity between diet and vegetation was positively correlated with jack rabbit density and with the amount of grass cover in the habitat. Jack rabbits selected grassdominated areas for feeding at night and then fed as generalists on the grass species present.

Salinity affects development, growth, and photosynthesis in cheatgrass.

The effects of salt stress on growth and development of cheatgrass (Bromus tectorum L.) were investigated in 2 greenhouse studies. The first study assessed developmental and physiological responses of this grass to 4 salinity levels. Salinity stunted growth through reduced leaf initiation and expansion, and reduced photosynthetic rates. Reduction of photosynthetic rates appeared to be primarily due to stomatal limitation. Salinity also reduced carbon isotope discrimination, indicating long-term effects on conductance and carbon gain. Root growth was severely inhibited by high salinity, resulting in a shift in the root to shoot allocation pattern. The second study investigated growth patterns of cheatgrass in relation to intraspecific variation in salt tolerance using plants grown from seeds collected at non-saline and saline sites. Salinity reduced growth of plants from both environments. However, plants from the saline site accumulated leaf and root area at nearly twice the rate as those from the non-saline site, even in the control group. Because plants were grown in a common environment, growth differences between populations were genetically based. Thus, the potential for rapid growth may enable plants from the saline site to rely on shallow, less saline moisture reserves available early in the growing season.

AGE AND POPULATION STRUCTURE OF JOSHUA TREES (YUCCA BREVIFOLIA) IN THE NORTHWESTERN MOJAVE DESERT

Abstract Many desert perennials are long-lived, but there are few data on ages or population structures of desert plants. We used 2 methods to estimate ages of a population of Yucca brevifolia (Joshua tree) in southwestern Utah from a 14-year census of plant sizes. Plant height at the 1st census ranged from 0.08 m to 6.0 m, and trees grew in height at a mean rate of 3.75 cm · yr−1. Plants also increased slowly in basal diameter (0.142 cm · yr−1) and branch length (0.024 m · yr−1), but basal diameter varied greatly from year to year, with many plants decreasing in diameter between sequential censuses. Forms of a nonlinear growth equation (the Richards function) did not fit the data on growth in height, but a simple linear relationship fit the data well. Using height as a linear predictor of age, we estimated that ages of a sample of 69 Joshua trees ranged from <20 to >300 years old, with 75% of the trees between 20 and 100 years and 11% over 150 years old. These data support the assumption that Joshua trees are indeed long-lived.