Effect of soil nitrogen stress on the relative growth rate of annual and perennial grasses in the Intermountain West
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A high relative growth rate (RGR) is thought to be an important trait allowing invasive annual grasses to exploit brief increases in nitrogen (N) supply following disturbance in the Intermountain West. Managing soils for low N availability has been suggested as a strategy that may reduce this growth advantage of annual grasses and facilitate establishment of desirable perennials grasses. The objective of this study was to examine the degree to which soil N availability affects RGR and RGR components of invasive annual and desirable perennial grasses. It was hypothesized that (1) invasive annual grasses would demonstrate a proportionately greater reduction in RGR than perennial grasses as soil N stress increased, and (2) the mechanism by which low N availability decreases RGR of annual and perennial grasses would depend on the severity of N stress, with moderate N stress primarily affecting leaf mass ratio (LMR) and severe N stress primarily affecting net assimilation rate (NAR). Three annual and three perennial grasses were exposed to three levels of N availability. RGR and components of RGR were quantified over four harvests. Moderate N stress reduced RGR by decreasing LMR and severe N stress lowered RGR further by decreasing NAR. However, reduction in RGR components was similar between invasive and natives, and as a consequence, annual grasses did not demonstrate a proportionately greater reduction in RGR than perennials under low N conditions. These results suggest managing soil N will do little to reduce the initial growth advantage of annual grasses. Once perennials establish, traits not captured in this short-term study, such as high tissue longevity and efficient nutrient recycling, may allow them to compete effectively with annuals under low N availability. Nevertheless, if soil N management does not facilitate the initial establishment of perennials in annual grass infested communities, then there is little likelihood that such techniques will provide a long-term benefit to restoration projects in these systems.