Distribution and ecophysiological adaptations of annual and perennial grasses in Mediterranean climate zones of California and the Iberian Peninsula

Grasses play important but differing roles in vegetative communities of the Mediterranean climate zones of California and the Iberian Peninsula. In California, native perennial grasslands are one of the most threatened plant community types; reversing the historic transformation of these systems to dominance by non-native annual grasses now presents one of the great challenges to restoration practitioners in the region. On the Iberian Peninsula, grassland communities exist on human-impacted "degraded" sites where forests long ago dominated. Both regions have a very similar palette of annual grass species, since nearly all were introduced to California via Spain. These annuals tend to be grassland dominants in California, while they only sometimes dominate on the Iberian Peninsula. The first two chapters of this thesis describe current distributions---with respect to measurable environmental parameters---of annual versus perennial grasses in natural vegetation of both regions. Perennial grass distribution in both regions appears particularly dependent on the intensity of summer water stress, though the environmental parameters behind this stress are quite different: on the Iberian Peninsula, average warm-season rainfall is closely correlated with perennial grass cover, while in California, where summer rainfall is consistently absent, proximity to the cooling (and fog-producing) effects of the Pacific coast favors perennials. At the landscape scale on both continents, disturbance and microsite characteristics appear to play lesser roles in determining annual versus perennial dominance. The final two chapters examine physiological mechanisms of responses to drought in grass species from Mediterranean climate zones. Chapter three experimentally examines water relations strategies of three important species in Iberian Peninsula successional grasslands. Chapter four compares ecophysiological parameters related to water capture, conservation, and utilization within a suite of perennial grass species from California and Mediterranean-climate Spain. California perennials showed fewer drought tolerance traits than Spanish species, suggesting a greater reliance on drought avoidance---rather than tolerance---mechanisms in California species. Some Spanish species showed clear drought tolerance characteristics, while others showed none, indicative of a greater diversity of drought-tolerance strategies in that setting.