Natural climate variability is a prominent factor that affects many aspects of life, livelihoods, landscapes, and decision-making across the Southwestern U.S. (Arizona, California, Colorado, Nevada, New Mexico, and Utah; included are the adjacent United States-Mexico border and Southwest Native Nations land). These natural fluctuations have caused droughts, floods, heat waves, cold snaps, heavy snow falls, severe winds, intense storms, the battering of coastal areas, and acute air-quality conditions. And as a region that has experienced—within the relatively short time span of several decades—rapid increases in human population (Figure 1.1), significant alterations in land use and land cover, limits on the supplies of water, long-term drought, and other climatic changes, the Southwest can be considered to be one of the most “climate-challenged” regions of North America. This document summarizes current understanding of climate variability, climate change, climate impacts, and possible solution choices for the climate challenge, all issues that are covered in greater depth in Assessment of Climate Change in the Southwest United States.i

This chapter summarizes the scope of what is known and not known about climate in the Southwestern United States. There is now more evidence and more agreement among climate scientists about the physical climate and related impacts in the Southwest compared with that represented in the 2009 National Climate Assessment (Karl, Melillo and Peterson 2009). However, there remain uncertainties about the climate system, the complexities within climate models, the related impacts to the biophysical environment, and the use of climate information in decision making.

This report synthesizes the knowledge, opinions, and concerns of many Federal and State land managers, scientists, stakeholders, and partners from a workshop, held at the University of Nevada, Las Vegas, on April 20-22, 2010. Land managers, research scientists, and resource specialists identified common concerns regarding the potential effects of climate change on public lands and natural resources in the Great Basin and Mojave Desert and developed recommendations for mitigation, adaptation, and research needs. Water and, conversely, the effects of drought emerged as a common theme in all breakout sessions on terrestrial and aquatic species at risk, managing across boundaries, monitoring, and ecosystem services. Climate change models for the southwestern deserts predict general warming and drying with increasing precipitation variability year to year. Scientists noted that under these changing conditions the past may no longer be a guide to the future in which managers envision increasing conflicts between human water uses and sustaining ecosystems. Increasing environmental stress also is expected as a consequence of shifting ecosystem boundaries and species distributions, expansion of non-native species, and decoupling of biotic mutualisms, leading to increasingly unstable biologic communities. Managers uniformly expressed a desire to work across management and agency boundaries at a landscape scale but conceded that conflicting agency missions and budgetary constraints often impede collaboration. More and better science is needed to cope with the effects of climate change but, perhaps even more important is the application of science to management issues using the methods of adaptive management based on long-term monitoring to assess the merits of management actions. Access to data is essential for science-based land management. Basic inventories, spatial databases, baseline condition assessments, data quality assurance, and data sharing were identified as top information priorities by all participants at this workshop. Optimizing the utility of ecosystem monitoring data will require standardizing monitoring protocols across agencies. Better communication among researchers and managers and cooperation through partnerships to manage resources across boundaries were emphasized as necessary for adapting to changing climatic conditions. However, even these strategies may be insufficient unless policy mandates, agency missions, and funding are coordinated at a high level.

Michael L. Scott National Biologic(ll Service I n much of western North America, riparian (streamside) environments are the only part of the landscape moist enough to allow survival of trees (Fig. 1). Riparian landscapes are usually defined as ecotones or corridors between terres ­ trial and aquatic realms (Malanson 1993). In spite of their limited areal extent, riparian ecosystems are essential habitat for many verte­ brate species and provide critical physical and biological linkages between terrestrial and aquatic environments (Gregory et al. 1991). Because of their association with scarce sur­ face water resources, western riparian ecosys­ tems have long been influenced by human activ­ ities. Human-caused perturbations can alter energy and material flow in riverine ecosys­ tems, thus modifying riparian plant communi­ ties (Brinson 1990). Among the most serious impacts to riparian ecosystems are water impoundmept and diversion, groundwater pumping from alluvial aquifers, livestock graz­ ing, land clearing for agriculture or to increase water yield, mining, road development, heavy recreational demand, fire, the elimination of native organisms (e.g., beaver [Castor canaden­ sis]) or the introduction of exotics , and overall watershed degradation (Stromberg 1993).

The Aquatic and Riparian Effectiveness Monitoring Plan (AREMP) is intended to characterize the ecological condition of watersheds and aquatic ecosystems. It will determine present watershed condition, track trends in watershed condition over time, and report on the Northwest Forest Plan's effectiveness across the region. This Northwest Forest Plan presents options and guidelines for use in pilot testing and implementing an effectiveness monitoring program for aquatic and riparian systems. The base program is designed to evaluate status and trends of watershed, stream, and riparian conditions by using decision-support models. Although the focus of AREMP is on characterizing ecosystem status and trend, implementing the Northwest Forest Plan also will supply information that will be useful in determining causal relations to help explain those trends. Abstract Under the direction of the Regional Interagency Executive Committee, which oversees the implementation and management of the Northwest Forest Plan (Forest Plan), multi-agency federal teams have been developing monitoring programs to evaluate the effectiveness of the Forest Plan. Initial priorities assigned by the federal agencies for monitoring include the northern spotted owl (Strix occidentalis caurina), marbled murrelet (Brachyramphus marmoratus), late-successional and old-growth forests, and aquatic and riparian ecosystems (Mulder et al. 1999). The Aquatic and Riparian Effectiveness Monitoring Plan (AREMP) is intended to characterize the ecological condition of watersheds and aquatic ecosystems. It will determine present watershed condition, track trends in watershed condition over time, and report on the Forest Plan's effectiveness across the region. This Forest Plan presents options and guidelines for use in pilot testing and implementing an effectiveness monitoring program for aquatic and riparian systems. The base program is designed to evaluate status and trends of watershed, stream, and riparian conditions by using decision-support models (DSMs). Although the focus of AREMP is on characterizing ecosystem status and trend, implementing the Forest Plan also will supply information that will be useful in determining causal relations to help explain those trends. The effectiveness of the Forest Plan could be examined at smaller spatial scales; however , the direction for this effort is to describe status and trend of the condition of watersheds at the regional scale. Because the Forest Plan's Aquatic Conservation Strategy (ACS) provides a framework for managing aquatic ecosystems primarily at watershed and landscape (i.e., multiple watersheds) scales, the subwatershed (6 th-field hydrological unit) forms the basic geographic unit for monitoring. Sampling a minimum of 50 sub-watersheds annually in the Forest Plan area will …

Although large-scale transformation of Everglades landscapes has occurred during the past century, the patterns of association among hydrologic factors and southern Everglades freshwater marsh vegetation have not been well-defined. We used a 10-year data base on the aquatic biota of Shark Slough to classify vegetation and describe plant community change in intermediate- to long-hydroperiod Everglades marshes. Study area marsh vegetation was quantitatively grouped into associations dominated by 1) Cladium jamaicense, 2) a group of emergents including Eleocharis cellulosa, Sagittaria lancifolia, and Rhyncospora tracyi, 3) taxa associated with algal mats (Utricularia spp. and Bacopa caroliniana), and 4) the grasses Panicum hemitomon and Paspalidium geminatum. During the decade evaluated, the range of water depths that characterized our study sites approached both extremes depicted in the 40-year hydrologic record for the region. Water depths were near the long-term average during the mid-1980s, declined sharply during a late 1980s drought, and underwent a prolonged increase from 1991 through 1995. Overall macrophyte cover varied inversely with water depth, while the response of periphyton was more complex. An ordination analysis, based on plant species abundance, revealed that study area vegetation structure was associated with hydrologic patterns. Marsh plant community structure showed evidence of cyclic interannual variation corresponding to hydrologic change over the decade evaluated. Lower water depths, the occurrence of marl substrates, and high periphyton cover were correlated. These factors contributed to reduced macrophyte cover in portions of the study area from which water had been diverted.

Riparian plants have been classified as “drought avoiders” due to their access to an abundant subsurface water supply. Recent water-relations research that tracks water sources of riparian plants using the stable isotopes of water suggests that many plants of the riparian zone use ground water rather than stream water, and not all riparian plants are obligate phreatophytes (dependent on ground water as a moisture source) but may occasionally be dependent on unsaturated soil moisture sources. A more thorough understanding of riparian plant-water relations must include water-source dynamics and how those dynamics vary over both space and time. Many rivers in the desert, Southwest have been invaded by the exotic shrubTamarix ramosissima (saltcedar). Our studies ofTamarix invasion into habitats formerly dominated by native riparian forests of primarilyPopulus andSalix have shown thatTamarix successfully invades these habitats because of its (1) greater tolerance to water stress and salinity, (2) status, as a facultative, rather than obligate, phreatophyte and, therefore, its ability to recover from droughts and periods of ground-water drawdown, and (3) superior regrowth after fire. Analysis of water-loss rates indicate thatTamarix-dominated stands can have extremely high evapotranspiration rates when water tables are high but not necessarily when water tables are lower.Tamarix has leaf-level transpiration rates that are comparable to native species, whereas sap-flow rates per unit sapwood area are higher than in natives, suggesting thatTamarix maintains higher leaf area than can natives, probably due to its greater water stress tolerance.Tamarix desiccates and salinizes floodplains, due to its salt exudation and high transpiration rates, and may also accelerate fire cycles, thus predisposing these ecosystems to further loss of native taxa. Riparian species on regulated rivers can be exposed to seasonal water stress due to depression of floodplain water tables and elimination of annual floods. This can potentially result in a community shift toward more stress-tolerant taxa, such asTamarix, due to the inability of other riparian species to germinate and establish in the desiccated floodplain environment Management efforts aimed at maintaining native forests on regulated rivers and slowing the spread ofTamarix invasion must include at least partial reintroduction of historical flow, regimes, which favor the recruitment of native riparian species and reverse long-term desiccation of desert floodplain environments.

Water and salinity relations were evaluated in recovering burned individuals of the dominant woody taxa from low-elevation riparian plant communities of the southwestern U.S. Soil elemental analyses indicated that concentrations of most nutrients increased following fire, contributing to a potential nutrient abundance but also elevated alluvium salinity. Boron, to which naturalized Tamarix ramosissima is tolerant, was also elevated in soils following fire. Lower moisture in the upper 30 cm of burned site soil profiles was attributed to shifts in evapotranspiration following fire. Higher leaf stomatal conductance occurred in all taxa on burned sites. This is apparently due to higher photosynthetic photon flux density at the midcanopy level and may be partially mitigated by reduced unit growth in resprouting burned individuals. Predawn water potentials varied little among sites, as was expected for plants exhibiting largely phreatophytic water uptake. Midday water potentials in recovering Salix gooddingii growing in the Colorado River floodplain reached levels which are considered stressful. Decreased hydraulic efficiency was also indicated for this species by examining transpiration-water potential regressions. Recovering, burned Tamarix and Tessaria sericea had enriched leaf tissue δ13C relative to unburned controls. Higher water use efficiency following fire in these taxa may be attributed to halophytic adaptations, and to elevated foliar nitrogen in Tessaria. Consequently, mechanisms are proposed which would facilitate increased community dominance of Tamarix and Tessaria in association with fire. The theory that whole ecosystem processes are altered by invading species may thus be extended to include those processes related to disturbance.

Alluvial forest associations are often dominated by woody phreatophytes, plants that are tightly linked to aquifers for water uptake. Anthropogenic hydrological alterations (e.g., water impoundment or diversion) are of clear importance to riparian ecosystem function. Because decreased frequency of flooding and depression of water tables may, in effect, sever riparian plants from their natural water sources, research was undertaken to determine water uptake patterns for the dominant native and introduced woody taxa of riparian plant communities of the southwestern United States. At floodplain study sites along the Bill Williams and lower Colorado Rivers (Arizona, USA), naturally occurring D and 18 O were used to distinguish among potential water sources. Isotopic ratios from potential uptake locations were compared to water extracted from the dominant woody taxa of the study area (Populus fremontii, Salix gooddingii, and Tamarix ramosissima) to elucidate patterns of water absorption. Isotopic composition of water obtained from sapwood cores did not differ significantly from heartwood or branch water, suggesting that heartwood water exchange, stem capacitance, and phloem sap mixing may be inconsequential in actively transpiring Salix and Populus. There was evidence for close hydrologic linkage of river, ground, and soil water during the early part of the growing season. Surface soils exhibited D enrichment due to cumulative exposure to evaporation as the growing season progressed. Isotopic ratios of water extracted from Populus and Salix did not exhibit isotopic enrichment and were not significantly different from groundwater or saturated soil water sources, indicating a phreatophytic uptake pattern. Associations of isotopic ratios with water relations parameters indicated high levels of canopy evaporation and possible use of moisture from unsaturated alluvial soils in addition to groundwater in Tamarix.