Mima Mounds as an Indicator of the Presettlement Grassland-Chaparral Boundary in San Diego County, California

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Abstract
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, wiht present-day grasslands being a degration 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 with 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 actural 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 invation of coastal sage shrub species and later replacement of these by larger chaparral shrubs.