RELATIONSHIPS BETWEEN POSTFIRE SAGUARO SHRUB SPECIES, SOIL DEVELOPMENT AND LANDSCAPE MORPHOLOGY ON A DISSECTED ALLUVIAL FAN SURFACE, TONTO NATIONAL FOREST, ARIZONA

Linking species occurrence to specific landscape positions or soil types may help in managing the saguaro shrub resource after fire. We examined the influence of soil development and geomorphic landscape position on the saguaro shrub in burned and unburned areas west of the Mazatzal Mountains on the Tonto National Forest, Arizona. Soil horizons were mapped using 2-dimensional (D) ground penetrating radar (GPR) to a depth of 1.5 m along two 350 m point quarter transects (including 4 quadrants associated with eight pre-designated points). For each transect, two 3-D GPR scans were taken--GPR readings were calibrated using descriptions of five soil pits located at five landscape positions: toeslope, footslope, backslope, shoulder, and crest. Additionally, soil moisture and auger depth to calcic horizons was measured. Vegetation data was collected along both transects and evaluated in relation to soil sampling. Plant cover, density and diversity were measured in a circle (10 m from pit center) and 5 m around identified saguaro. Plants closest to soil samples were identified and measured. Preliminary results identified dense, buried calcic pedogenic horizons to about 1 m. Soil development on interfluve crests had multiple buried calcic horizons acting as aquitards for underlying roots. Pits in alluvial fan channels contained thick strata of cemented clay and gravel (< 40% of each). Soil moisture was low throughout: <5%, however, amassing of fine roots in infilled animal burrows was indicative of higher moisture. Plant species differed similarly from toeslope to crest between burned and unburned areas. Plant diversity was similar in both areas, but cover and abundance was less in the burn. Preferential flow between calcic horizons was dominated by bioturbations (e.g. root channels and burrows) channeling water to lower horizons. Increased soil moisture observed in soil accumulations around bushes and burrows may improve plant survival. Landscape position reflected both changes in soil and plant species. Further investigation into bioturbations would be instructive when assessing saguaro shrub habitat.