Directing ecological succession: The role of competition in restoring semi-arid grasslands dominated by invasive plants

Successful ecosystem restoration requires an understanding of the ecological processes directing succession. One of the challenges in the semi-arid grasslands of western United States is replacement of native species by invasive annual grasses. Solutions to this problem require identifying and manipulating ecological processes that direct succession to favor desired vegetation. The overall objective of this research was to identify and understand processes or factors directing restoration of semi-arid grassland ecosystems dominated with invasive annual grasses. Two invasive annual (Bromus tectorum L. and Taeniatherum caput-medusae L. Nevski) and two native perennial ( Pseudoroegneria spicata (Pursh) A) and Poa secunda J. Presl) grass species were used to provide a model system of semi-arid grasslands of western United States.;Plant competition is considered to be the primary ecological process limiting the success of grassland restoration. Successful restoration requires knowing the relative strength and magnitude of competition during the early stages of plant growth and how this might be impacted by nitrogen (N) availability. My research involved three experiments designed to compare competition and growth rates of native and invasive species. First, in order to understand the degree to which intra- versus inter-specific competition controls invasive and native plant growth during the early phase of establishment, I performed a diallel competition experiment with species grown either alone or in 1:1 binary combinations in a greenhouse. I hypothesized that the type and intensity of competition for invasive and native species would vary among harvest times and competitive intensity for invasive species will be higher than native species with higher N availability. My results indicated that invasive and native species are subject to both intra- and inter-specific competition; however, the dominant type differed among harvests. Invasive species also became more competitive than native species with increasing N. I suggest that opportunities to improve restoration success exist by determining the optimum combination of density, species proportion, and their spatial arrangement in various ecosystems and environments.;Second, I performed an addition series competition experiment in the field for two years to determine the intensity and importance of competition in an arid, resource poor production system. My results indicated that in resource poor environments, the intensity of competition did not significantly influence plant dominance during the first two years of plant establishment, and thus, competition was not important. I suggest that land managers may be most successful at restoration of resource poor ecosystems by overcoming the barriers associated with plant establishment other than plant-plant interactions, such as abiotic factors.;Third, I studied growth rate and growth patterns of medusahead with bluebunch wheatgrass and cheatgrass in the field for two years. I hypothesized that medusahead would have a higher RGR, a longer period of growth, and as a consequence, more total biomass at the end of the growing season than bluebunch wheatgrass and cheatgrass. Medusahead had a longer period of growth, more total biomass and higher RGR than cheatgrass. However, bluebunch wheatgrass had more biomass and higher RGR than medusahead in 2008, but the relationship was reversed in 2009. Weather data identified that precipitation in 2008 was well below average, and this level of drought was very infrequent. Collectively, my results suggest that the continued invasion and dominance of medusahead onto native and cheatgrass dominated grasslands will continue to increase in severity because of its higher RGR and extended period of growth.;The inability to identify key ecological processes important in directing invasion and succession has limited the adoption and implementation of ecologically based invasive plant management (EBIPM). A framework that allows ecologist to identify and prioritize ecological processes most in need of repair would help overcome this barrier. I developed an initial framework that allows ecologists to prioritize the ecological processes that appear to play a dominate role in vegetation dynamics. This was accomplished by using sensitivity analysis to identify the most important transitions in the life cycle of associated species and linking those transitions with key ecological processes and their modifying factors. This method could increase land manager's ability to implement EBIPM by allowing identification and prioritization of those ecological processes that appear to play a dominating role in vegetation dynamics.