Ginormous trees have big effects: The influence of giant sequoia (Sequoiadendron giganteum) on soils
Several previous studies have examined the influence of tree species on soils, either in natural stands or in experimental ‘common gardens.’ However, no study to date has assessed the impact of the largest trees on earth, giant sequoia (Sequoiadendron giganteum), on soil nutrient pools and processes. Considering their sheer size, corresponding high litter production, and their longevity (up to a few thousand years), we expected giant sequoia trees to have large effects on soil properties. To evaluate the relative impact of these organisms on soils, we sampled forest floor and upper mineral soils (0-5 cm) under the crown of individual mature giant sequoia trees and co-dominant, old-growth sugar pine (Pinus lambertiana) trees in two groves within Yosemite National Park. These two groves (Mariposa and Merced) occur on similar soils (both coarse-loamy, isotic, frigid Ultic Haploxeralfs) that are derived from contrasting parent materials (metavolcanic and metasedimentary, respectively). We analyzed forest floor and soil samples for a suite of soil chemical properties known to affect soil fertility. We also assessed soil biological activity by measuring in-situ rates of trace gas emissions (CO2, CH4, and N2O) from soil under the crowns of these two tree species during the growing season within each grove.
Despite over a 3.5-fold greater aboveground biomass of giant sequoia trees, there was no difference in forest floor mass under giant sequoia and sugar pine trees in either grove. Trace gas emissions from soil and soil inorganic nitrogen concentrations were also similar under these tree species in both groves. In contrast, anaerobically mineralizable nitrogen (an index of the size of the microbial nitrogen pool) and extractable calcium, magnesium, and potassium concentrations were generally 30-90% greater under giant sequoia than under sugar pine trees within a given soil horizon. Soil pH was about 0.4-0.9 pH unit higher and extractable aluminum was 60-75% lower under giant sequoia than under sugar pine trees. Differences in soil pools and processes between species were generally greater than those between groves, and the magnitude of the putative influence of giant sequoia compared to sugar pine trees on underlying soils was typically greater on the metasedimentary than the metavolcanic parent material. Our results suggest that individual giant sequoia trees can strongly influence the soil properties underneath their crowns, resulting in improved soil fertility. Furthermore, these tree effects are mediated secondarily by the nature of the geological substrate from which the soil was derived.