| Grassland areas in California represent one of the most dramatic examples of large-scale species replacement due to plant invasions. Today, an area of approximately 7,000,000 ha is dominated by non-native annual grasses, altering the original vegetation to such an extent that the original floristic composition is unknown. However, it is probable that, at least in some areas, perennial bunchgrasses (including Nassella pulchra and Nassella cernua) were the dominant members of grassland communities. Using a multi-scale approach that included long-term field observations, field experiments, controlled environment studies, and historical research, I sought to determine how present-day biotic and abiotic factors are affecting the distribution of Nassella pulchra.; Although N. pulchra did form perennial grasslands in some areas that are today dominated by non-native annual grasses, I found that the evidence also suggests that many of the current grasslands were originally dominated by other vegetation types. Therefore, absence of N. pulchra in any given location could simply be because it was not there in the first place.; In the longest term study to date, I found that, despite the ubiquitous presence of non-native grasses, Nassella populations have generally increased in at least one unmanaged area in central California over the past 52 years. However, population trends for any given stand are site specific and depend on pre-European vegetation type and land-use history. Field experiments in southern California confirmed that non-natives do negatively affect growth and water status of established N. pulchra individuals, and these effects are evident on large as well as small plants.; Interactions between N. pulchra and non-native annual grasses are different in different environments. For example, drought affected seedlings of non-native grasses more than N. pulchra. Also, N. pulchra responded differently to temperature than non-native grasses. Nassella pulchra, which had the lowest relative growth rate at near-optimal temperatures, compared to two non-native grasses, performed relatively better at sub- and super-optimal temperatures. Thus, seasonal and year-to-year variability in climate could aid in the continued persistence of California native perennial grasses faced with interference from non-native annual grasses. |
