INFLUENCES OF NON-NATIVE PLANTS ON THE GEOCHEMICAL PROPERTIES OF A SERPENTINITE SOIL-- PRESIDIO, SAN FRANCISCO, CALIFORNIA

One of California's rarest plant communities - serpentinite prairie - exists in a remnant stand at Inspiration Point in the Presidio of San Francisco, California. This unique soil, laden with heavy metals, supports a variety of perennial native bunchgrasses and annual forbs, including the federally endangered Presidio clarkia and the federally threatened Marin dwarf flax. Previous land management practices of planting non-native trees and grasses have changed the ecology of the serpentinite prairie. These practices have created stands of exotic (Eucalyptus and Monterey pine) forest canopies, which in turn have altered the original serpentinite soil properties. As a result, the Presidio Natural Resource Division has initiated the Inspiration Point Viewshed Restoration project to 1) increase serpentinite prairie habitat acreage by removing exotic forest canopies, 2) restore populations of federally endangered plant species, 3) delineate the spatial distribution of serpentinite soils, and 4) quantify geochemical soil alteration resulting from non-native plant species. Accordingly, this study will assess the influences of non-native trees and grasses on serpentinite prairie soil ecology.

Two Inspiration Point field sites (serpentine prairie and an adjacent forest canopy) were analyzed using standard soil protocols for their Ca/Mg ratio, exchangeable cations, pH, available nutrients, and total heavy metal content. Preliminary results for the two sites are as follows: Ca/Mg ratios (0.09-1.94), exchangeable cations-- Cr (0.0-.47 mg/kg), Ni (0.0-1.6 mg/kg), Mn (1.0-32.4 mg/kg), pH (4.87-6.89), NO3 (1.4–4.0 mg/kg), and total heavy metal concentrations Cr (320-2,540 mg/kg), Ni (109-1,965 mg/kg), Pb (18-100 mg/kg), and Zn (66-118 mg/kg). An analysis of variance reveales significant differences (P < 0.05) between the Ca/Mg ratios, pH, extractable Cr, and total heavy metal concentrations at the two sites. As a result of past land management practices substantial changes in serpentinite soil characteristics exist which may inhibit future habitat restoration and endangered plant revegetation projects at the site.