Katharine I. Predick

Climate Change and Ecosystems of the Southwestern United States

Arid desert ecosystems of the western United States are particularly susceptible to climate change and climate variability. Plants and animals in this region live near their physiological limits for water and temperature stress. Slight changes in temperature or precipitation regimes or a change in the frequency and magnitude of extreme climatic events could therefore substantially alter the composition, distribution, and abundance of species, as well as the products and services that arid lands provide. In the United States, arid lands are located in the subtropical hot deserts of the Southwest, comprised of the Mojave, Sonoran, and Chihuahuan deserts, and the temperate cold deserts of the Intermountain West. Annual precipitation is low (<400 mm), but the seasonality of precipitation differs substantially among hot deserts (Fig. 1). The Mojave Desert is dominated by winter precipitation; thus biological activity is greatest during the cool season. The Chihuahuan Desert is dominated by summer precipitation with biological activity during hotter conditions. The hottest of the three deserts, the Sonoran, is intermediate, receiving both winter and summer precipitation. Each of these deserts is characterized by low productivity and slow plant growth, both of which are primarily water-limited. Vegetation communities are typically desert scrub, shrub– steppe, or desert grassland/savanna and are home to charismatic plants, including saguaro cacti, organ pipe cacti, and Joshua trees. The Chihuahuan Desert is the largest desert in North America, stretching from the southwestern United States deep into the Central Mexican Highlands. It has been classifi ed by the World Wildlife Fund as a Global 200 Ecoregion, a science-based global ranking of the Earth’s most biologically outstanding habitats. Arid lands currently provide a variety of products and services, including a large ranching industry, wildlife habitat, plant and animal diversity, regulation of water fl ow and quality, opportunities for outdoor recreation, and open spaces for expanding urban environments.

An ecosystem services perspective on brush management: research priorities for competing land-use objectives

Summary The vegetation of semi-arid and arid landscapes is often comprised of mixtures of herbaceous and woody vegetation. Since the early 1900s, shifts from herbaceous to woody plant dominance, termed woody plant encroachment and widely regarded as a state change, have occurred world-wide. This shift presents challenges to the conservation of grassland and savanna ecosystems and to animal production in commercial ranching systems and pastoral societies. Dryland management focused on cattle and sheep grazing has historically attempted to reduce the abundance of encroaching woody vegetation (hereafter, ‘brush management’) with the intent of reversing declines in forage production, stream flow or groundwater recharge. Here, we assess the known and potential consequences of brush management actions, both positive and negative, on a broader suite of ecosystem services, the scientific challenges to quantifying these services and the trade-offs among them. Our synthesis suggests that despite considerable investments accompanying the application of brush management practices, the recovery of key ecosystem services may be short-lived or absent. However, in the absence of such interventions, those and other ecosystem services may be compromised, and the persistence of grassland and savanna ecosystem types and their endemic plants and animals threatened. Addressing the challenges posed by woody plant encroachment will require integrated management systems using diverse theoretical principles to design the type, timing and spatial arrangement of initial management actions and follow-up treatments. These management activities will need to balance cultural traditions and preferences, socio-economic constraints and potentially competing land-use objectives. Synthesis. Our ability to predict ecosystem responses to management aimed at recovering ecosystem services where grasslands and savannas have been invaded by native or exotic woody plants is limited for many attributes (e.g. primary production, land surface–atmosphere interactions, biodiversity conservation) and inconsistent for others (e.g. forage production, herbaceous diversity, water quality/quantity, soil erosion, carbon sequestration). The ecological community is challenged with generating robust information about the response of ecosystem services and their interactions if we are to position land managers and policymakers to make objective, science-based decisions regarding the many trade-offs and competing objectives for the conservation and dynamic management of grasslands and savannas.

Woody Plant Encroachment: Causes and Consequences

Woody vegetation in grasslands and savannas has increased worldwide over the past 100–200 years. This phenomenon of “woody plant encroachment” (WPE) has been documented to occur at different times but at comparable rates in rangelands of the Americas, Australia, and southern Africa. The objectives of this chapter are to review (1) the process of WPE and its causes, (2) consequences for ecosystem function and the provision of services, and (3) the effectiveness of management interventions aimed at reducing woody cover. Explanations for WPE require consideration of multiple interacting drivers and constraints and their variation through time at a given site. Mean annual precipitation sets an upper limit to woody plant cover, but local patterns of disturbance (fire, browsing) and soil properties (texture, depth) prevent the realization of this potential. In the absence of these constraints, seasonality, interannual variation, and intensity of precipitation events determine the rate and extent of woody plant expansion. Although probably not a triggering factor, rising atmospheric CO2 levels may have favored C3 woody plant growth. WPE coincided with the global intensification of livestock grazing that by reducing fine fuels, hence fire frequency and intensity, facilitated WPE. From a conservation perspective, WPE threatens the maintenance of grassland and savanna ecosystems and its endemic biodiversity. Traditional management goals aimed at restoring forage and livestock production after WPE have broadened to support a more diverse portfolio of ecosystem services. Accordingly, we focus on how WPE and management actions aimed at reducing woody plant cover influence carbon sequestration, water yield, and biodiversity, and discuss the trade-offs involved when balancing competing management objectives.

Influence of vegetation and seasonal flow patterns on parafluvial nitrogen retention in a 7th-order river.

Abstract We examined whether presence of vegetation and seasonal changes in flow affected N chemistry and denitrification rates within sandbars of a 7th-order sandy alluvial river (Wisconsin River, USA). We addressed these questions with a broad-scale approach of measuring parafluvial water chemistry and denitrification rates in multiple sandbars distributed along a 15-km river reach during summer 2004 and 2005. After recession of spring flooding, parafluvial chemistry in unvegetated bars was characterized by moderate dissolved O2 (DO) and elevated NO3−-N concentrations (>3.5 mg N/L), whereas vegetated bars tended to be hypoxic (<2 mg/L DO) and depleted in NO3−-N (0.2 mg N/L) relative to unvegetated bars and surface water (0.47 mg N/L). As flow declined over the summer, NO3−-N also declined in both bar types, whereas SO42− was relatively constant in unvegetated bars but decreased in vegetated bars. Amendment experiments demonstrated that denitrification was limited primarily by NO3−-N and secondarily by organic C in both bar types, but the strength of this limitation varied over time and was greater in vegetated bars, a result suggesting loss of denitrification capacity. Thus, spatial and temporal patterns of water chemistry and denitrification activity among multiple sandbars indicated that unvegetated bars shifted from N transformers/NO3−-N sources early in the summer to N retainers/NO3−-N sinks as discharge declined, whereas vegetated bars always supported anaerobic processes and probably shifted from NO3−-N to SO42− sinks. We hypothesize that the contribution of vegetated islands to overall riverine N retention is small because establishment of vegetation reduces hydrologic linkages between bars and surface water. Modern changes in the flow regime of the Wisconsin River have increased establishment of riparian vegetation on exposed bars, a pattern suggesting that parafluvial N retention is being reduced while riverine N loading is increasing.