The Mediterranean Environment of Greater California
Amalie Jo Orme
North America's Mediterranean-type environment occupies a long narrow portion of the continent, bordered on the west by the Pacific Ocean and extending eastward into mountainous terrain that reaches over 4000 m above sea level in the Sierra Nevada. Covering more than 350,000 km2 in area and 1600 km in latitude from southern Oregon (43°N) to Baja California Norte (30°N), the region, herein termed Greater California, never exceeds 300 km in width and in the south is little more than 100 km wide. Because of the rain shadow imposed by its high eastern mountains, the region abuts directly eastward against desert terrain of the Basin and Range Province, namely, the temperate Nevada Desert in the north and the subtropical Mojave and Sonoran Deserts farther south (fig. 20.1). To the north, the region merges into the temperate, mostly coniferous, forests of the Pacific coast; to the south, into the coastal desert of central Baja California.
Unlike the classic Mediterranean region of southern Europe and northern Africa, Greater California has no extensive east-west seaway, no large islands, and no internal peninsulas or enclosed seas. Instead, the dominant tectonic grain has imposed a series of northwest to southeasttrending mountain ranges and valleys that parallel the Pacific coast, modifying the inland penetration of oceanic influences. In this respect, it is more comparable to the Mediterranean region of central Chile, which also lies along an active mountainous plate margin. In other respects, however, Greater California is typically Mediterranean. It experiences warm, dry summers and cool, wet winters, with variability within the region reflecting latitude, elevation, aspect, and proximity to the ocean. In general, precipitation increases from south to north and from the coast to the mountains, whereas coastal fog is generated by advection across the cool California current. The adaptation and diversity of plant and animal life within this environment are also noteworthy, although, since European settlement in the late eighteenth century, many plant species have become rare or endangered and some are now extinct. Animal life has experienced a similar fate, with shrinkage of specific niches and expansion of others.
This chapter examines those features that give Greater California its distinctive physical geography, namely, the strongly seasonal climate and hydrologic regime, and the related ecological responses expressed in the vegetation cover and related ecosystem diversity. Greater California is the only region of North America characterized by winter rains and summer drought, where most streams flow vigorously in winter but atrophy in summer, and the only region where the vegetal response is adapted to such seasonality. Other features of this landscape are less unique and not discussed in detail. For example, the region's tectonic framework and rugged terrain, though spectacular, are more or less shared with neighboring parts of the continent's Pacific Rim. Certainly, the transfer of a narrow