Grasslands in Transition: Emerging Issues and Challenges in the Western USA.

Grassland, shrub-steppe and savanna ecosystems (hereafter 'grasslands') are geographically extensive, represent a substantial fraction of terrestrial net primary productivity, contain a significant proportion of the world's human population and support the majority of the world's livestock production. As such, they play an important role in global carbon, water, and nitrogen cycles and human health. These ecosystems also provide habitat for game and non-game wildlife and provide a myriad of ecosystem goods and services, particularly in terms of hydrology and watershed-related phenomena important to rapidly growing settlements and cities that may be geographically distant. Historically, overgrazing and conversion to row-crop agriculture was the primary threat to these ecosystems. Encroachment by native tree and shrubs and non-native perennial and annual grasses are contemporary threats likely to be exacerbated by climate change. Reasons for woody plant encroachment, which are varied and the topic of active debate, will be reviewed. This phenomenon has long been recognized as a threat to livestock production; and managing the woody-herbaceous mixture to balance wildlife conservation and livestock production goals has been a source of controversy. More recently, woody plant proliferation has emerged as a threat to the conservation of grasslands and the plants and animals endemic to them; and may have significant impacts on biogeochemical cycles and land surface-atmosphere interactions. Policy and management issues related to grassland conservation thus extend well beyond the traditional concerns of livestock production and now include effects on stream flow and ground water, carbon sequestration, biological diversity and impacts on atmospheric chemistry and the climate system. Paradoxically, the introduction of non-native perennial forage grasses and other annual grasses now threatens to transform biologically diverse western savanna, shrub-steppe and grassland ecosystems in the western US into low-diversity disturbance-prone systems. This presentation will discuss challenges facing the research community in quantifying and monitoring these varied impacts; challenges facing the management community in achieving and maintaining woody-herbaceous vegetation mixtures that satisfy competing objectives; and pitfalls facing policy makers that fail to take a wide view.