Introduction: Exotic Annual Bromus in the Western USA

Abstract

The spread and impacts of exotic species are unambiguous, global threats to many ecosystems. A prominent example is the suite of annual grasses in the Bromus genus (Bromus hereafter) that originate from Europe and Eurasia but have invaded or are invading large areas of the Western USA. This book brings a diverse, multidisciplinary group of authors together to synthesize cur-rent knowledge, research needs, and management implications for Bromus . Exotic plant invasions are multifaceted problems, and understanding and man-aging them requires the biological, ecological, sociological, and economic per-spectives that are integrated in this book. Knowing how well information from one geographic or environmental setting can transfer to another is a key need for broadly distributed Bromus species especially given ongoing climate change. Thus, the chapters in the book compare and contrast invasibility of different ecoregions and invasiveness of different Bromus species. A universal theme is managing for ecosystems that are resilient to disturbance and resistant to inva-sion which will be essential for adaptation to the human-caused problem of Bromus in the Western USA. 1.1 The Spectrum of Exotic Annual Bromus Exotic annual grass invasions are transforming arid and semiarid landscapes world-wide. Mediterranean annual grasses have become the dominant species following European settlement in areas as widely separated as Chile, Australia (Rossiter et al. 2003 ; Gulmon 1977), and the Western USA (Mack 1981 ; D'Antonio and Vitousek 1992). The problem is particularly acute for arid and semiarid uplands of the Western USA, which are being invaded by several exotic annual species in the genus Bromus (Bromus hereafter) that originated from Europe or Eurasia. Settlement of these semiarid landscapes occurred over a century ago, and since then, urban, agronomic, and energy development have led to open, disturbed habitat, and land use practices such as overgrazing by livestock have depleted native perennial herbaceous spe-cies—which are the best competitors with Bromus (Mack 1981 ; Billings 1990). Not only are the Bromus species readily invading these disturbed areas, but because they are highly adapted to the relatively wet winters and drier summers that characterize much of the area, they also are expanding into native ecosystems with favorable climatic regimes . In many cases, increases in the abundance and continuity of fi ne fuels caused by these annual grasses are decreasing fi re return intervals and result-ing in the progressive conversion of native ecosystems to Bromus dominance. The life history and ecophysiological traits of Bromus , such as fall or early spring germination, rapid growth, and high reproduction, make them highly competitive with native species and greatly increase the diffi culty of management and restoration. Bromus now dominate millions of acres and are present as co-or subdominant species in many low-to mid-elevation ecosystems in the Western USA. Although traditional land uses facilitated initial invasion, expanding land uses, climate change, rising atmo-spheric carbon dioxide, and nitrogen (N) deposition have accelerated the process. In contemporary terminology, many of the affected ecosystems have lost or are losing " resistance " to invasive Bromus grasses and " resilience " to disturbances , such as altered fi re and climatic regimes (Chambers et al. 2014). Conversion of native ecosystems to annual grass dominance can change soil physical and chemical properties and alter ecosystem processes like nutrient cycling and soil water fl ux and storage (Wilcox et al. 2012). The progressive invasion of Bromus species and the global change process that facilitate invasion act at large spatial scales a nd cross conventional boundaries for land planning and management. The risks and problems associated with Bromus have been known in the USA for at least a century. There has been as much or more research and management atten-tion on Bromus species, such as B. tectorum L. (cheatgrass or downy brome), as on other exotic invasive plant species in the USA and elsewhere. A Google Scholar search of literature on the Bromus genus reveals more than 10,000 primary research publications in the last century. Moreover, many thousands of seeding, fencing, and rangeland " weed " treatments at scales ranging from a few to over 10,000 acres each and collectively covering millions of acres are on record for the decades leading up to 2015 in the Western USA (US Geological Survey's Land Treatment Digital Library). Despite this relatively high number of studies and publications on Bromus M.J. Germino et al. 3 and the ma ny attempts by managers to control them, their spread and impacts con-tinue. It is noteworthy that much of the ecological research on Bromus has focused on a limited number of explanatory variables and been conducted at local or site scales, while most of the management-oriented research has emphasized treatments targeting control of these species. A critical need exists for research, planning, and management that focuses on ecore-gional and larger scales. Scientists, policy specialists, and land managers would like to understand: • Effects of environmental factors on Bromus species distributions • Ecosystem attributes and processes that infl uence resistance to invasion • Traits of Bromus species that contribute to invasiveness • Impacts of Bromus invasions on ecosystems • Effects of pathogens on Bromus invasions and their potential for biocontrol • Effects of land uses on Bromus invasions • Management options for exotic annual Bromus and their application • Socioeconomic drivers and patterns of human response to Bromus invasion This book addresses these points for the Western USA. The book is timely because concern for the Bromus problem and efforts to address it at broad spatial scales have increased to unprecedented levels. For example, annual grasses and B. tectorum specifi cally are major aspects of the US Department of Interior (DOI) Secretarial Order #3336 issued in 2015, which mandates one of the largest conser-vation efforts in US history on behalf of Centrocercus urophasianus Bonaparte (greater sage-grouse). New efforts to evaluate Bromus and prioritize areas for pro-tection or restoration are now occurring at ecoregional and larger scales, such as in the DOI's Rapid Ecological Assessments (released in the last 2 years) and the Fire and Invasive Assessment Tool (2015;