Positive effects of native shrubs on Bromus tectorum demography

Abstract

There is increasing recognition that overall interactions among plant species are often the net result of both positive and negative effects. However, the positive influence of other plants has rarely been examined using detailed demographic methods, which are useful for partitioning net effects at the population level into positive and/or negative effects on individual vital rates. This study examines the influence of microhabitats created by the native shrubs Artemisia tridentata and Purshia tridentata on the demography of the invasive annual grass Bromus tectorum in the Great Basin Desert, California, USA. Shrub understory environments differed significantly from intershrub space and were characterized by higher soil fertility and less extreme microclimates. There existed a strong spatial association between B. tectorum and the shrubs across four years, with more than double the density of B. tectorum in shrub microhabitats compared to intershrub space. Periodic matrix models were used to calculate population growth (lamda) and reproductive potential (RP, expected lifetime fecundity of seedlings) of B. tectorum in different microhabitats over two years. Modeled population growth was significantly increased in shrub microhabitats in the first of two years. This was primarily due to increased seedling establishment in Artemisia microhabitats, rather than effects during the growing season. In the following year, B. tectorum individuals in shrub microhabitats had a significantly greater reproductive potential than those in intershrub microhabitats, indicating shrub facilitation during the growing season. Loop analysis revealed an interacting effect of year and microhabitat on B. tectorum life history pathway elasticity values, demonstrating a fundamental influence of spatiotemporal factors on which life history pathways are important and/or possible. Life table response experiment (LTRE) analysis showed that increased survival and growth rates positively contributed to population growth in both years under Purshia, but only in the second year under Artemisia. This research provides evidence that the positive effects of native shrubs on B. tectorum can be strong enough to produce net positive effects at the population level, although positive effects were variable. In this study, a rigorous demographic approach was particularly useful in partitioning overall interactions into positive and negative components.

Effects of precipitation change and neighboring plants on population dynamics of Bromus tectorum

“…We found little evidence of a seedbank that lasts for >1 year. Studies in other invaded areas have found a similar lack of dormancy over multiple years in B. tectorum seeds (Hulbert 1955;Griffith 2010), although some studies have found Table 2 Results of linear mixed-effects models of demographic measurements of Bromus tectorum and aboveground biomass of neighboring plants in response to precipitation treatments a a Only predictor variables that accounted for significant variation in the response variables were included in final analyses. All treatment estimates are compared to the control treatment.…”

“…Competition from resident species can affect resource availability and contribute to the invasion resistance of ecosystems (e.g., Tilman 1997;Naeem et al 2000;Corbin and D'Antonio 2004). Competition from established species may limit the ability of new species to invade (Kennedy et al 2002) or, alternatively, neighboring plant species may have no effect, or even facilitate the establishment of new species (Bulleri et al 2008;Griffith 2010). The outcomes of interactions between species may also shift under a changing climate (Dormann 2007;Concilio et al 2013).…”

Variation in phenotypic plasticity for native and invasive populations of Bromus tectorum

“…Population growth of B. tectorum is strongly sensitive to rates of seedling germination and establishment, more so than to any other life history transition (Griffith 2010). Thus, the timing of emergence for this annual grass may be absolutely critical for invasive success, and can represent a strong mortality filter.…”

“…particular functional traits), but this contributes to an overall 'jack-of-alltrades' result with less variability across environments in terms of population-level consequences (Richards et al 2006). For an annual species like B. tectorum, biomass is tightly linked to lifetime seed production (Griffith 2010), and plasticity in function that helps to maintain biomass accumulation is likely to be important.…”

Facilitation and interference of seedling establishment by a native legume before and after wildfire

“…In contrast, L. argenteus had a weak, non-significant inhibitory effect on Bromus emergence. Other field studies investigating the effects of mature herbaceous vegetation on Bromus emergence have found both positive and negative effects (Chambers et al 2007;Griffith 2010). Emergence differed among species with Bromus having the highest overall percentage of seeds emerge (59%) followed by Elymus (54%) and Eriogonum (32%).…”