Bromus tectorum L. (cheatgrass) is an Eurasian annual grass that has invaded ecosystems throughout the Intermountain west of the United States. Our purpose was to examine mechanisms by which established perennial grasses suppress the growth of B. tectorum. Using rhizotrons, the experiment was conducted over 5 growth cycles: (1) B. tectorum planted between perennial grasses; (2) perennials clipped and B. tectorum planted; (3) perennials clipped and B. tectorum planted into soil mixed with activated carbon; (4) perennials clipped, B. tectorum planted, and top-dressed with fertilizer, and; (5) perennial grasses killed and B. tectorum planted. Water was not limiting in this study. Response variables measured at the end of each growth cycle included above-ground mass and tissue nutrient concentrations. Relative to controls (B. tectorum without competition), established perennial grasses significantly hindered the growth of B. tectorum. Overall, biomass of B. tectorum, grown between established perennials, increased considerably after fertilizer addition and dramatically upon death of the perennials. Potential mechanisms involved in the suppression of B. tectorum include reduced nitrogen (possibly phosphorus) availability and coopting of biological soil space by perennial roots. Our data cannot confirm or reject allelopathic suppression. Understanding the mechanisms involved with suppression may lead to novel control strategies against B. tectorum. 1. Introduction The Eurasian annual grass B. tectorum L. (cheatgrass, downy brome) has come to dominate many ecosystems in the Intermountain Region of the Western United States [1]. Pathways by which B. tectorum facilitate its expansion are myriad and include phenotypic plasticity to new host environments [2], it increases the rates and sizes of wildfires, which fosters more invasion [3], rapid above-and below-ground growth rates [4, 5], prolific seed production [6], landscape disturbance [7–9], and elevated atmospheric CO2 [10, 11]. Given the invasion success of B. tectorum, one might conclude that it is competitive. In the seedling stage, B. tectorum is quite competitive against native and introduced perennial grasses [12–14]. Particular ecosystems, however, are resistant to invasion by B. tectorum [15]. A common thread in ecosystem resistance to B. tectorum invasion is healthy, well-established, perennial grass communities such as the natives Pesudoroegneria spicata (bluebunch wheatgrass), Elymus elymoides (bottlebrush squirreltail), Poa secunda (bluegrass), and Festuca idahoensis (Idaho fescue), and the
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