Facilitation of an invasive plant by native shrubs

Background/Question/Methods
There is increasing recognition that positive species interactions – or facilitation – can be important drivers of ecological patterns. Most models of plant invasions focus on the importance of negative interactions, but facilitation has been observed to benefit several invasive plant species. However, the general role of facilitation in plant invasions has received relatively little attention. This study demographically examines plant-plant facilitation of the invasive annual grass Bromus tectorum by the native shrubs Artemisia tridentata and Purshia tridentata in the Great Basin Desert. Periodic matrix models were used to calculate population growth (λ) and reproductive potential (R_p = expected lifetime fecundity of seedlings) of B. tectorum in shrub canopy and intershrub microhabitats.

Results/Conclusions
Modeled population growth (λ) was significantly increased in shrub microhabitats in the first of two years. This was primarily due to increased seedling establishment in A. tridentata microhabitats, rather than effects during the post-establishment growing season. In the following year, B. tectorum individuals in shrub microhabitats had a significantly greater reproductive potential (R_p) than those in intershrub microhabitats, indicating facilitation during the growing season. Loop analysis of pathway elasticities revealed an interacting effect of year and microhabitat on B. tectorum life history strategy, such that increased growth was more important in shrub microhabitats only in the second year. Life table response experiment (LTRE) analysis showed that increased survival and growth rates positively contributed to population growth in both years under P. tridentata, but only in the second year under A. tridentata. This research provides evidence that native shrubs can facilitate B. tectorum at the population level. However, positive effects varied between years and shrub species, and were not always consistent across different measures (e.g. λ and R_p). These results also contribute to the growing recognition that overall species interactions are a balance between both positive and negative interactions. In this study, a rigorous demographic approach was particularly useful in partitioning the overall interaction into its positive and negative components.