Contingency in California grassland restoration: Biogeographic history, inter-annual variation, and priority effects.

摘要

Ecological restoration in the grasslands of California's Central Valley is a formidable prospect. First, this system represents one of the largest and most complete invasions of any ecosystem on Earth. Second, the highly competitive annual grassland species that have invaded California, while considered ruderal within their native ranges, have formed a remarkably stable novel system in California. Third, interannual rainfall in the seasonally xeric Central Valley is highly variable, and both late onset of winter rains and mid-winter (growing season) droughts are common.;Despite these issues, restoration in the grasslands of California's Central Valley is proceeding rapidly. In an effort to increase my understanding of the first two issues I ask whether physiological differences between native and exotic grass guilds can help explain the invasion and current community composition of this system? I found that California perennials are more drought tolerant than Mediterranean annuals but less drought tolerant than Mediterranean perennials, despite the fact that California's Central Valley has a more intense summer drought than the Mediterranean Basin. These patterns may help explain why Mediterranean annuals, but not Mediterranean perennials, have been more successful invaders of interior California grasslands.;Driven by my observations of the high interannual variability in the Central Valley, and the resultant variability in success of grassland restoration plantings in this system, I then ask how important is interannual variability in ecological experimentation? I found that the results of many ecological field experiments are likely to be contingent on the year in which they are implemented and argue for more deliberate investigation of temporal contingency in ecological experimentation, especially in the field of restoration ecology, which may be particularly sensitive to treatment by initiation-year interactions.;Finally, I revisit the issue of why is this novel annual grassland system so stable. Does earlier germination in exotic annual species may help explain their unprecedented invasion and continued dominance of California grasslands? Does an early germination advantage relative to exotic annual grasses increase the establishment of native grasses? I show that a two week germination advantage significantly increased the establishment success of native perennial grasses, with population-level priority effects showing greater persistence over time than individual-level priority effects.