Drought and Grazing Disturbances and Resistance to Invasion by Warm- and Cool-Season Perennial Grassland Communities

Healthy and robust grasslands are better able to compete with invasive plants for available resources. Disturbances can reduce the competitive ability of perennial grass communities by creating niches that allow invasive plants to establish. In established warm-season and cool-season perennial grass communities, a study was conducted to determine the effects of repeated grazing and extreme drought on the establishment of introduced musk thistle (Carduus nutans). For each community, the following treatments were applied: over seeding of musk thistle or no musk thistle in the first year (2011), and simulated grazing or no grazing in both years (2011 and 2012). Annual precipitation was extremely low in the second year. In warm-season, grazed communities with musk thistle, grass cover declined to less than 10% as musk thistle became dominant by the second year. The grasses in all warm season, non grazed communities had \u3e 65% cover and heights of \u3e 100 cm from June to October, which prevented establishment of musk thistle. In cool-season grass communities, biomass was low in 2012, yet early emergence of grasses from dormancy and high amount of cover (\u3e 50% in May) most likely contributed to the suppression of musk thistle seedlings the previous year (2011) and further restricted their development in combination with the drought conditions the second year. Our results support the practice of properly grazing warm-season and cool-season grasses to create sustainable perennial grass communities, especially in areas subjected to continuous or heavy grazing, extreme drought, and the threat of invasive plant species