Mediterranean annual grasses in western North America: kids in a candy store

Abstract

In this issue of Plant and Soil, Blank and Sforza (2007) contribute to understanding of how exotic annual grasses invade ecosystems in the western United States. Their findings, that medusahead wildrye (Taeniatherum caput-medusae [L.] Nevski) was most productive on non-invaded US soil and that plants from US-derived seeds may have evolved to utilize higher nutrient concentrations, parallel other results that suggest soils of vulnerable western US ecosystems are functionally different (i.e., more available-nutrient rich) than soils where these weeds are native and noninvasive (Blumenthal 2005; Davis et al. 2000). While it has been well established that exotic annual grasses can perpetuate their own environment by fostering increased available nutrients (e.g., Ehrenfeld and Scott 2001; Norton et al. 2003), we must remember, for the sake of ecological restoration, that initial invasiveness is caused by chronic disturbances that disrupt native nutrient and organic matter cycles and increase nutrient availability. The principal difference between western US ecosystems and those where invasive annual grasses are native is the amount of time during which chronic disturbances have occurred. In the Mediterranean region, intensive management of grass and shrubland ecosystems depleted nutrients long ago, so that native vegetation is adapted to a “here today, gone tomorrow” strategy. In ecosystems of the arid and semiarid western USA, perennial-plantdominated native nutrient cycles were disrupted by intensive land use much more recently. Chronic disturbances in the form of extensive grazing and altered fire regimes began to unlock nutrients stored in soil organic matter (SOM) for millennia. When seeds of exotic annual grasses arrived they found that the candy store was open. In this commentary, we review evidence for major shifts in ecosystem processes across the semiarid western USA and we call for more research that links results of ecological restoration efforts to the growing body of knowledge about interactions between invasive annuals and invaded environments. In western US ecosystems vulnerable to weed invasion, the consequence of almost any type of ecosystem disruption is a shift from immobilizing, Plant Soil (2007) 298:1–5 DOI 10.1007/s11104-007-9364-8