Where the Scrub Meets the Desert: Plant Ecology of the Transition Zone at the Southern Boundary of the California Floristic Province

Background/Question/Methods

Worldwide, Mediterranean ecosystems transition equator-wards into coastal fog deserts and occur along ocean coasts with cold-water upwelling, winter rain, and dry summers. In North America this transition occurs in north-western Baja California at the southern extent of the California Floristic Province (CFP), where it meets the northern edge of the Vizcaino Desert. Although temperatures north of the 30^th parallel are fairly constant, rainfall decreases rapidly as you move south of the international border, providing a broad precipitation gradient. The area has a predominant contingent of California plants and is a hotspot of plant endemism with ca. 30% of the native flora being locally endemic. Weather patterns in the region are controlled by ocean currents, which in turn dictate the distribution of vegetation types. Several physiognomic adaptations to drought tolerance are observed, each with a broadly corresponding habitat. Years of anomalously high sea-surface temperatures (El Niño) typically favor higher winter precipitation, but many years are very dry. These oscillations in moisture favor a flora that is largely plastic, and responds rapidly to local weather patterns. Timing and seasonality of plant phenological events in turn affects all higher trophic levels, causing pulses in the availability of resources.

Results/Conclusions

The frequent and heavy fogs caused by the cool upwelling of the California Current provide significant additional moisture, allowing many CFP species to survive through the drier seasons. Moisture availability from fog has led to the development of two highly restricted local vegetation types, which are globally scarce. Chaparral is typically a fire-adapted evergreen vegetation type that occurs inland, but along the coast, the fog eliminates fire, resulting in a coastal species assemblage of ‘Maritime Chaparral’. The drought-deciduous species of the Coastal Sage Scrub slowly give way to an increasing number of succulent rosette-forming taxa that harvest moisture from fog, forming a specialized ‘Succulent Maritime Scrub’. The high endemism in the flora of the region appears to be the result of recent speciation and radiation events, as well as the survival of paleo-taxa. Seasonality of rainfall is more predictable either side of the 30^th parallel, which may indicate greater climatic change has historically occurred to the north and south, but fog has buffered climatic variation in the transition zone. Pleistocene and Holocene climates varied considerably, but the transition zone of Baja California appears to have been a refuge for taxa that are now extirpated elsewhere and have no close relatives in the modern flora.