Variation in the establishment of a non-native annual grass influences competitive interactions with Mojave Desert perennials

Abstract

Competition between native and nonnative species can change the composition and structure of plant communities, but in deserts, the highly variable timing of resource availability also influences non-native plant establishment, thus modulating their impacts on native species. In a field experiment, we varied densities of the nonnative annual grass Bromus madritensis ssp. rubens around individuals of three native Mojave Desert perennials-Larrea tridentata, Achnatherum hymenoides, and Pleuraphis rigida-in either winter or spring. For comparison, additional plots were prepared for the same perennial species and seasons, but with a mixture of native annual species as neighbors. Growth of perennials declined when Bromus was established in winter because Bromus stands had 2-3 months of growth and high water use before perennial growth began. However, water potentials for the perennials were not significantly reduced, suggesting that direct competition for water may not be the major mechanism driving reduced perennial growth. The impact of Bromus on Larrea was lower than for the two perennial grasses, likely because Larrea maintains low growth rates throughout the year, even after Bromus has completed its life cycle. This result contrasts with the perennial grasses, whose phenology completely overlaps with (Achnatherum) or closely follows (Pleuraphis) that of Bromus. In comparison, Bromus plants established in spring were smaller than those established in winter and thus did not effectively reduce growth of the perennials. Growth of perennials with mixed annuals as neighbors also did not differ from those with Bromus neighbors of equivalent biomass, but stands of these native annuals did not achieve the high biomass of Bromus stands that were necessary to reduce perennial growth. Seed dormancy and narrow requirements for seedling survivorship of native annuals produce densities and biomass lower than those achieved by Bromus; thus, impacts of native Mojave Desert annuals on perennials are expected to be lower than those of Bromus.

Short-term soil inorganic N pulse after experimental fire alters invasive and native annual plant production in a Mojave Desert shrubland

“…The late season precipitation patterns can afford advantages to different functional groups depending on the timing and the amount of resources available during the pulse versus inter-pulse periods (James and Richards 2006;James et al 2009). Invasive Mojave Desert annuals have the potential to use water and N more rapidly than native annual species (DeFalco et al 2003); however, invasive annuals established in response to later season rainfall have less competitive potential (James and Richards 2006;DeFalco et al 2007). While moisture availability may be more influential during other years, our data show that N availability was correlated with native and invasive annual plant production under seasonal drought.…”

“…Precipitation pulses influence N availability to plants, and the timing of these pulses can affect the survival and competitive ability of desert plants (James and Richards 2006;DeFalco et al 2007;Rao and Allen 2009). The timing of resource pulses and the consequent response by plants may be altered by wildfire but has not been documented for desert systems.…”

Native and invasive plant interactions in wetlands and the minimal role of invasiveness

“…The remaining question is whether the observed differences in native plant assemblages were caused by or merely correlated with the presence of L. salicaria or P. australis. L. salicaria and P. australis may have influenced the plant species composition and abundance by impacting nutrient cycling (Chambers et al 1999;Findlay and Groffman 1997), physical structure of the landscape, light access (Odum et al 1984;Gaudet and Keddy 1988;Edwards et al 1995;Minchinton and Bertness 2003), possibly flooding regimes, and availability of space (Hager and McCoy 1998). However, research conducted by Lenssen et al (1999) suggested that native species richness and plant assemblages in areas of low elevation may be merely correlated with the presence of L. salicaria or P. australis and caused by abiotic factors.…”

“…(common reed) invade freshwater wetlands. Both are considered to have negative effects on the native plant community they invade by ''overgrowing'' the native wetland flora and forming a monoculture (Odum et al 1984;Rawinski and Malecki 1984;Gaudet and Keddy 1988;Edwards et al 1995;Minchinton and Bertness 2003). Hager and McCoy (1998) noted the lack of data in support of the hypothesis that L. salicaria reduces plant diversity and species richness.…”

Short-term effects of experimental fires on a Mojave Desert seed bank

“…As a result, alien grasses increase and dominate communities while native perennial species decline due to fires (O' Leary and Minnich, 1981;McLaughlin and Bowers, 1982;Brown and Minnich, 1986;Esque et al, 2004;Brooks and Berry, 2006). Furthermore, the invasive Mediterranean annual grasses can outcompete native species for limited resources (Salo, 2004;DeFalco et al, 2007) and can outnumber native annuals after fires (Brooks, 1999a).…”