Historical records suggest that California valley grassland was originally widespread in both coastal areas and interior and mountain valleys of San Diego County, under a regime of frequent fire. Alternatively, some ecologists suggest that fire was infrequent and grasslands were very restricted in presettlement time, with present-day grasslands being a degradation product of chaparral subjected to increased frequency of fire. Mima-type earth mounds, formed by geomyid pocket gophers in grassland environments of western North America, are abundant in San Diego County, and may be a general marker of the original grassland-chaparral boundary. To evaluate this possibility, vegetation and habitat conditions were sampled along gradients from present-day mounded grassland to unmounded chaparral at foothill and coastal mesa sites. At the foothill site, where wildfires have been occasional in recent decades, chamise (Adenostoma fasciculatum) and other fire-adapted shrubs have advanced downslope 31-78 m. For chamise, which reproduces after fire both by seedlings and resprouts, the ratio of seedlings to older plants increases downslope, indicating active grassland invasion. Seedlings of coast blue lilac (Ceanothus tomentosus), which does not resprout after fire, parallel chamise seedings in actual abundance. On the coastal mesa, where fire has been rare in this century, chamise has colonized much of the mesa surface, but few other chaparral shrubs, particularly those with large animal-dispersed fruits, have followed. In both locations, mounds are foci of shrub establishment, and succession usually proceeds by the invasion of coastal sage scrub species and later replacement of these by larger chaparral shrubs. INTRODUCTION California valley grassland remains 'one of the least understood native plant communities of North America. This community, including areas with a savanna-like oak overstory, occupied perhaps 8.2 million ha of valley and coastal slope lands from the northern Sacramento Valley to the Mexican border in presettlement times (Heady, 1977). The principal dominant of the community was Stipa pulchra (Heady, 1977, but see Bartolome and Gemmill, 1981), with other perennial grasses and annual and perennial forbs locally common. Virtually all of this original grassland has been transformed by postsettlement influences of cultivation, water diversion, livestock grazing, introduction of alien plants and alteration of the fire regime. Most areas that remain in grassland are now categorized as California annual grassland, and are dominated by introduced annuals (Heady, 1977). Because of the early and extensive transformation of the valley grassland, its extent and composition prior to European settlement in much of southern California are uncertain. Dodge (1975) analyzed the early accounts of Pedro Fages, Fray Garces, members of the 1849 Mexican boundary survey party and others, concluding that in San Diego County, in the southwestern corner of the state, grasslands were widespread on the coastal terraces and low hills, as well as in the large coastal and mountain valleys. This view is also supported by Axelrod (1975) and Aschmann (1977). Others (Burcham, 1957; Oberbauer, 1978; Zedler et al., 1983) have doubted the widespread occurrence of grasslands prior to settlement of San Diego County. Oberbauer (1978) questioned the accuracy of the firsthand accounts of early travelers, while Zedler et al. (1983) suggested that much of the open grassland and coastal scrub habitat that now exists is the result of degradation and recession of chaparral.
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Jun 1, 1994
Nikolaus H Fischer + 3
Open Access