which generally have little or no vegetation, produce runoff; sink areas, located downslope of the source ar-In many semiarid regions, runoff and erosion differ according to eas, receive and store the runoff and thereby become vegetation patch type. These differences, although hypothesized to fundamentally affect ecological processes, have been poorly quanti-enriched and relatively productive. The results of sevfied. In a semiarid piñ on-juniper woodland [Pinus edulis Engelm.eral theoretical studies suggest that the transfer of water and Juniperus monosperma (Engelm.) Sarg.] in northern New Mexico, and nutrients through this process is important both we measured runoff and erosion from the three patch types that ecologically and hydrologically (Mauchamp et al., 1994; compose these woodlands: Canopy patches (those beneath woody

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shrublands (Elkins et al., 1986;Lyford and Qashu, 1969;Wainwright et al., 2000), mesquite rangelands (Wood In semiarid environments, vegetation affects surface runoff either and Blackburn, 1981), and piñ on-juniper rangelands in by altering surface characteristics (e.g., surface roughness, litter absorption) or subsurface characteristics (e.g., hydraulic conductivity).the USA (Roundy et al., 1978). Similar findings have Previous observations of runoff within a piñ on-juniper [Pinus edulis been reported from other parts of the world. Examples Englem. and Juniperus monosperma (Englem.) Sarg.] woodland led are Australia, where studies were performed in both us to hypothesize that hydraulic conductivity differs between vegetamulga woodlands (Greene, 1992) and arid shrublands tion types. Using ponded and tension infiltrometers, we measured (Dunkerley, 2000a); Niger, in tiger bush (Bromley et saturated (K s ) and unsaturated [K(h )] hydraulic conductivity at three al., 1997); and Spain, in semiarid shrublands (Cerda et levels of a nested hierarchy: the patch (canopy and intercanopy), the al., 1998). In other studies, differences in infiltrability unit (juniper canopy, piñ on canopy, vegetated intercanopy, and bare have been found within the intercanopy, between areas intercanopy), and the intercanopy locus (grass, biological soil crust, exhibiting differing degrees of herbaceous cover (Wilbare spot). Differences were smaller than expected and generally not cox et al., 1988). Similarly, Wood and Blackburn (1981) significant. Canopy and intercanopy K s values were comparable with the exception of a small number of exceedingly high readings under found higher infiltration rates for mid-grass than for the juniper canopy-a difference we attribute to higher surface macro-short-grass areas. And in Spain, Cerda (1997) reported porosity beneath juniper canopies. The unsaturated hydraulic conducthat infiltration rates under the grass species Stipa tenativity, K(h ), values were higher for canopy soils than for intercanopy cissima were almost double those for adjacent bare soils, although differences were small. At the unit level, the only ground. significant differences were for K(h ) between juniper or piñ on cano-Enhanced infiltrability under vegetation canopies pies vs. bare interspaces. Median K values for vegetated intercanopy may be due to a number of factors, including textural areas were intermediate between but not significantly different from differences resulting from rain splash or trapping of those for canopies and bare areas. There were no significant differeolian sands by vegetation (Parsons et al., 1992); higher ences between grass, biological soil crust, and bare spots within the organic-matter content of the soil under vegetation; proherbaceous intercanopy area. Overall, the observed differences in K between canopy and intercanopy patches do not account for differ-tection of the soil surface by leaf litter; enhanced aggreences in runoff observed previously.

Piñon‐juniper woodlands in the semiarid western USA have expanded as much as fivefold during the last 150 yr, often accompanied by losses of understory vegetation and increasing soil erosion. We conducted this study to determine the differences in soil morphology between canopy and intercanopy locations within a piñon (Pinus edulis Engelm.)‐juniper [Juniperus monosperma (Engelm.) Sarg.] woodland with uniform parent material, topography, and climate. The woodland studied, located near Los Alamos, NM, has a mean tree age of 135 yr. We examined soil morphology by augering 135 profiles in a square grid pattern and comparing soils under piñon and juniper canopies with intercanopy soils. Only two of the 17 morphological properties compared showed significant differences. The B horizons make up a slightly greater proportion of total profile thickness in intercanopy soils, and there are higher percentages of coarse fragments in the lower portions of canopy soil profiles. Canopy soils have lower mean pH and higher mean organic C than intercanopy soils. Regression analysis showed that most soil properties did not closely correspond with tree size, but total soil thickness and B horizon thickness are significantly greater under the largest piñon trees, and soil reaction is lower under the largest juniper trees. Our findings suggest that during the period in which piñon‐juniper woodlands have been expanding, the trees have had only minor effects on soil morphology.

Soil water dynamics reflect the integrated effects of climate conditions, soil hydrological properties and vegetation at a site. Consequently, changes in tree density can have important ecohydrological implications. Notably, stand density in many semiarid forests has increased greatly because of fire suppression, such as that in the extensive ponderosa pine (Pinus ponderosa Laws.) forests that span much of western USA. Few studies have quantified how soil water content varies in low-versus highdensity stands both by depth and years, or the inter-relationships between water content, stand density, and ecohydrological processes. Over a 4-year period, we measured the soil water content throughout the soil profiles in both low-density (250 trees/ha) and high-density (2710 trees/ha) ponderosa pine stands. Our results document significantly greater soil water contents in the low-density stands over a wide range of conditions (wet, dry, winter, summer). We observed substantial differences in water contents at depths greater than are typically measured. Our results also show that differences in monthly average soil water contents between the low-and high-density stands fluctuated between 0Ð02 and 0Ð08 m 3 m 3 depending on the time of year, and reflect a dynamic coupling between infiltration and stand evapotranspiration processes. The difference in soil water availability between low-and high-density stands is substantially amplified when expressed as plant-available water on a per tree, per biomass or per leaf area basis. Our findings highlight important ecohydrological couplings and suggest that restoration and monitoring plans for semi-arid forests could benefit from adopting a more ecohydrological focus that explicitly considers soil water content as a determinant of the ecosystem process.

Rangelands have undergone-and continue to undergo-rapid change in response to changing land use and climate. A research priority in the emerging science of ecohydrology is an improved understanding of the implications of vegetation change for the water cycle. This paper describes some of the interactions between vegetation and water on rangelands and poses 3 questions that represent high-priority, emerging issues: 1) How do changes in woody plants affect water yield? 2) What are the ecohydrological consequences of invasion by exotic plants? 3) What ecohydrological feedbacks play a role in rangeland degradation processes? To effectively address these questions, we must expand our knowledge of hydrological connectivity and how it changes with scale, accurately identify ''hydrologically sensitive'' areas on the landscape, carry out detailed studies to learn where plants are accessing water, and investigate feedback loops between vegetation and the water cycle.

This chapter is organized around the concept of ecohydrological processes that are explicitly tied to ecosystem services. Ecosystem services are benefits that people receive from ecosystems. We focus on (1) the regulating services of water distribution, water purification, and climate regulation; (2) the supporting services of water and nutrient cycling and soil protection and restoration; and (3) the provisioning services of water supply and biomass production. Regulating services are determined at the first critical juncture of the water cycle-on the soil surface, where water either infiltrates or becomes overland flow. Soil infiltrability is influenced by vegetation, grazing intensity, brush management, fire patterns, condition of biological soil crusts, and activity by fauna. At larger scales, water-regulating services are influenced by other factors, such as the nature and structure of riparian zones and the presence of shallow groundwater aquifers. Provisioning services are those goods or products that are directly produced from ecosystems, such as water, food, and fiber. Work over the last several decades has largely overturned the notion

Wetlands In Northern Plains Prairies: Offer Societal Values Too Unlike wildlife habitat and livestock forage, societal services provided by prairie wetlands are not easily observed, measured or valued.