Invasion of Mediterranean Annual Grasses in Western North America Ecosystems: Kids in a Candy Store

Authors: NORTON, JAY - UNIVERSITY OF WYOMING; Monaco, Thomas; NORTON, URSZULA - UNIV. OF CALIFORNIA

Submitted to: Plant and Soil
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/20/2007
Publication Date: N/A
Citation: N/A

Interpretive Summary: In this commentary, we review evidence for major shifts in ecosystem processes across the semiarid western US, and we call for more research that links results of ecological restoration effort to the growing body of ecological knowledge about interactions between invasive annuals and invaded environments.

Technical Abstract: In this issue of Plant and Soil, Blank and Sforza (2007) contribute to a growing body of work focused on describing mechanisms by which 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 have apparently 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 (Blumentahl 2005; Davis et al. 2000). The propensity for invasiveness demonstrated by exotic annual grasses suggests some major ecological shift in western ecosystems. In this commentary, we review evidence for major shifts in ecosystem processes across the semiarid western US; and we call for more research that links results of ecological restoration efforts to the growing body of ecological knowledge about interactions between invasive annuals and invaded environments. In western US ecosystems vulnerable to invasion, the consequence of almost any type of ecosystem disruption is ultimately a shift from more conservative, immobilizing nutrient cycling to more leaky, mineralizing cycles with excess nutrients that are uncaptured by soil and plant communities (Haynes and Williams 1993; Schimel 1986). We believe the evidence shows that myriad disturbances across arid and semiarid western landscapes have the ultimate, synergistic effect of shifting nutrient cycling from conservative cycles dominated by uptake, immobilization, and storage in protected organic matter pools, to more open, "leaky" cycles where carbon and nitrogen are rapidly mineralized and excesses are potentially lost via leaching or greenhouse gas emissions (Norton et al. in press). Smith et al. (1994) found that undisturbed sagebrush-steppe plant communities, with diverse herbaceous, woody, and microbial components, have very conservative nutrient cycles with very little net nitrogen mineralization. It follows that relatively high concentrations of mineral nutrients observed by Blank and Sforza (2007) and others signify a shift from conservative to leaky nutrient cycling. Recent research by Davies et al. (2007) shows that maintaining plant functional diversity decreases soil nutrient concentrations in semiarid bunchgrass-dominated plant communities, and that removal of individual functional groups increases mineral nitrogen concentrations.