Studies of ecosystem processes on the Jornada Experimental Range in southern New Mexico suggest that longterm grazing of semiarid grasslands leads to an increase in the spatial and temporal heterogeneity of water, nitrogen, and other soil resources. Heterogeneity of soil resources promotes invasion by desert shrubs, which leads to a further localization of soil resources under shrub canopies. In the barren area between shrubs, soil fertility is lost by erosion and gaseous emissions. This positive feedback leads to the desertification of formerly productive land in southern New Mexico and in other regions, such as the Sahel. Future desertification is likely to be exacerbated by global climate warming and to cause significant changes in global biogeochemical cycles.

A field experiment udng 2 patterns of irrigation and 1 level of nitrogen fertilizer (10 g-N m-2) was conducted in order to discern water and nitrogen interactions that may control production of creosotebush, (Laweu tridentatu (D.C.) Cov. The 2 patterns of irrigation simulated precipitation from small, frequent events (6 mm water added weekly) or large, infrequent events (25 mm water added monthly). Understanding the factors controlling the production of this rangeland shrub may aid in the development of strategies for its management. Vegetative growth occurred mostly during March-May (spring) and August-October (summer-fall). Fruit production occurred mainly in the spring and root growth occurred mainly in the summer-fall. Irrigation increased vegetative growth and decreased fruit production. Responses to irrigation were greater during summer-fall than in the spring. Small, frequent water additions caused larger increases in vegetative plus fruit growth than did large, infrequent water additions. Nitrogen fertilization increased the growth of both vegetation and fruit in irrigated and unirrigated plots. Stem mortality and root growth were not significantly affected by irrigation or nitrogen fertilizer. Tbeae results suggest that creosotebush production is limited by both soil moisture and nitrogen availability and that temporal patterns of ralnfail may be as important as total amounts.

The close correlation of plant communities to landforms and geomorphic surfaces resulted from differences in the redistribution of water and organic matter between landforms in the northern Chihuahuan Desert. Biotic processes are limited by water and nitrogen, and the interactions between landforms, geomorphic processes, soils, and plant communities control the redistribution of these limiting resources within internally drained catchments. Geomorphic processes are regulated by the geologic structure and gross topographic relief of internally drained catchments over geological time scales. Land forming processes can be viewed as static at time scales of 10's to 100's of years, with individual landforms regulating geomorphic processes, namely erosion and deposition resulting from the horizontal redistribution of water within the catchment. The vegetation composition is a critical feedback, reinforcing the erosional or depositional geomorphic processes that dominate each landform.The Jornada Long-Term Ecological Research site may be one of the simplest cases in which to decipher the relationship between landforms, geomorphic processes and plant communities. However, these geomorphic processes are common to all internally drained catchments throughout the Basin and Range Province, and result in the development of characteristic landforms and associated vegetation communities. Although the patterns may be modified by differences in parent material, watershed size, and land use history -erosional, depositional, and transportational landforms can still be identified.The sharpness of ecotones between plant communities on individual landforms is related to the degree to which landforms are linked through the flow of water and sediment. Sharp ecotones occurred at the transition from depositional to erosional landforms where little material was transferred and steep environmental gradients are maintained. Gradual ecotones occurred at the transition from erosional to depositional landforms where large quantities of material were transferred leading to the development of a gradual environmental gradient.The relationships between geomorphic processes and vegetation communities that we describe have important implications for understanding the desertification of grasslands throughout semi-arid regions of North America. Disturbances such as grazing and climate change alter the composition of plant communities, thereby affecting the feedbacks to geomorphic processes, eventually changing drainage patterns and the spatial patterns of plant communities supported within the landscape.

Encelia farinosa is one of a number of species of desert perennial shrubs in which individuals exhibit considerable seasonal variation in the amount and structure of leaf tissue. The function of seasonal leaf variability in adapting this species to the desert environment was investigated. Field observations demonstrated that leaf quantity and structure are controlled by the moisture status of the environment. The field observations also established the influence which leaf quantity and structure have on the CO2 exchange capacity and water status of the shrub. These observations were corroborated by laboratory experiments. The laboratory investigations also indicated that the influences of leaf structure are brought about by alterations in the resistances to CO2 and water vapor diffusion.

The effects of light, temperature, and salinity on growth, net CO2 exchange and leaf anatomy of Distichlis spicata were investigated in controlled environment chambers. When plants were grown at low light, growth rates were significantly reduced by high substrate salinity or low temperature. However, when plants were grown at high light, growth rates were not significantly affected by temperature or salinity. The capacity for high light to overcome depressed growth at high salinity cannot be explained completely by rates of net photosynthesis, since high salinity caused decreases in net photosynthesis at all environmental conditions. This salinity‐induced decrease in net photosynthesis was caused largely by stomatal closure, although plants grown at low temperature and low light showed significant increases in internal leaf resistance to CO2 exchange. Increased salinity resulted in generally thicker leaves with lower stomatal density but no significant differences in the ratio of mesophyll cell surface area to leaf area. Salinity and light during growth did not significantly affect rates of dark respiration. The mechanisms by which Distichlis spicata tolerates salt appear to be closely coulpled to the utilization of light energy. Salt‐induced leaf succulence is of questionable importance to gas exchange at high salinity in this C4 species.

Data from the US/IBP Desert Biome validation studies indicate that above-ground production and biomass allocated to reproduction in Larrea tridentata vary from one year to another depending upon the timing and extent of soil-moisture availability. In an attempt to verify these observations and determine to what extent water availability can affect total aboveground production and reproductive allocation in this widely distributed warm desert shrub, a series of soil-moisture augmentation experiments were conducted. High levels of soil moisture had a greater effect on reproductive allocation than on total above-ground production. Enhanced soil moisture during the period of active growth increased total above-ground production and reduced the percentage of biomass allocated to reproduction. Enhanced soil moisture during the normal periods of little or no growth did not increase total above-ground production.

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Diurnal stem xylem H2O potentials of Larrea tridentata (DC.) Cov. were measured with a pressure chamber during the summers of 1973, 1974. Plants growing in dry soils had minimum stem xylem H2O potentials at night and maximum values during the daylight hours. Vertical H2O vapor movements in the soil profile in response to temperature gradients may be implicated although further data are needed to establish their role conclusively. Water vapor apparently moves up out of the rooting One at night and back down into the rooting zone during the day. The occurrence of this phenomenon probably enhances the ability of Larrea shrubs to maintain photosynthetic activity when soil H2O potentials are low and makes questionable the use of predawn stem xylem H2O potential measurements to assess seasonal trends in plant H2O status.