Shrub Mounds Enhance Water Flow in a Shrub-Steppe Community in Southwestern Idaho, U.S.A.

In the Western United States, substantial areas of shrub-steppe have been replaced by annual exotic grasses during the past century. These structural changes have been accompanied by functional changes in the landscape in relation to the ability of the landscape to capture essential resources such as water and sediments. Surface waterflow was measured in three contrasting shrub-steppe communities (sagebrush, winterfat, and rabbitbrush) on mound and interspace surfaces using disc permeameters. Waterflow was substantially greater (2 to 6 times) under shrubs compared with the shrub-free, microphytic-dominated interspaces. The ratio of sorptivity under ponding to sorptivity under tension, which is a useful index of the macropore (pores > 0.75 mm in diameter) status of the soil, demonstrated that mounds were characterized by a significantly greater amount of macropores compared with interspaces. The dense cover of the moss Tortula ruralis, which grows below Artemisia canopies, intercepted approximately 14 L of water m and enhanced the infiltration capacity of shrub mound soils. The results reinforce the notion that these shrub-steppe communities are partitioned into moisture-rich mounds separated from moisture-deprived interspaces that are critical for the transfer of water to the shrub mounds. Disturbance of the interspaces ultimately leads to a reduction in water reaching the mounds and a decline in shrubland function. Introduction ____________________ Since the late nineteenth and early twentieth centuries, extensive and uncontrolled livestock grazing has resulted in substantial changes to the composition and structure of many native shrub-steppe communities. The combined impact of grazing and invasion of annual Eurasian weeds (Mack 1981), primarily cheatgrass (Bromus tectorum L.) and medusahead (Taeniatherum caput-medusae ssp. asperum L.), have increased the risk of wildfire in shrub communities (Wambolt and others 2001). Consequently, increased frequency of wildfire has resulted in conversion of substantial areas of Artemisia shrub-steppe to annual grasslands, which in turn burn at intervals of less than 10 years (Whisenant 1990). This increasing cycle of fire, conversion to annual grasslands, and more fire has increased unabated, and now large areas of grassland exist in former shrubsteppe communities. The dramatic change from shrublands to annual grasslands has resulted in substantial changes in landscape function at both local and regional scales. Changes in landscape structure have resulted in major impacts on biodiversity, including loss of overstory and understory flora, declines in many faunal taxa, and invasion of exotic species (Wambolt 2001, 2002; West 1993). Spatial heterogeneity in many shrub-steppe communities worldwide is characterized by a matrix of microphytic crust-dominated soils (the interspaces) on which are superimposed soil mounds colonized by perennial shrubs (shrub mounds). The microphytic crusts of the interspaces comprise cyanobacteria, bacteria, algae, mosses, and lichens (Hilty and others 2004; West 1990). Cyanobacterial polysaccharides bind together the surface of the interspace soils, giving it a tight microstructure that generally repels water (Eldridge and others 2000). In contrast, the surface soil of the shrub mound lacks the microphytic crust and is covered with loose soil particles. On functional shrub-steppe landscapes, these contrasting surfaces result in the concentration of water and nutrients within the nutrient-enriched shrub patches at the expense of the nutrientand water-deprived interspaces (Blark and Small 2003; Schlesinger and others 1990; Thiery and others 1995; Tongway and Ludwig 1994). This patchiness exists at a range of spatial scales from whole catchments and subcatchments to individual plants. Efficient, sustainable production of arid shrublands is dependent upon the movement of water, sediments, seed, and soil from the interspace to the shrub mounds. When these shrublands become degraded, resources are no longer trapped within the shrub mounds, and the landscape becomes dysfunctional (Ludwig and Tongway 1995). Healthy soils within shrub mounds are characterized by a high degree of macroporosity, that is, a large number of pores, generally biological in nature, greater than about 0.75 mm in diameter. These pores are important for capturing excess water generated from the interspaces. These macropores or biopores are destroyed by trampling, disturbance, and cultivation. Waterflow through soils over small spatial scales can