Lack of Vesicular-Arbuscular Mycorrhizal Inoculum in a Ponderosa Pine Forest

Comparison of live and beetle-killed ponderosa pine stands revealed that understory biomass was higher under beetle-killed pines. Carex heliophila, a nonmycorrhizal plant, was virtually the only species present beneath live ponderosa pines. We examined the extent to which vesicular- arbuscular mycorrhizae (VAM) colonized understory plants on both live and beetle-killed pine sites; the availability of live- and killed-pine soil VAM inoculum in situ and in the laboratory; the abundance of VAM spores in field soils; and the possible inhibition of VAM formation by forest floor decom- position products in live-pine soils. Roots of field species from live and beetle-killed pine soils, as well as bioassay plants (species not native to the study site) grown in situ and in the laboratory were cleared, stained, and examined for presence or absence of VAM colonization. Sixty percent of the understory of live pine was composed of nonmycorrhizal species, 26% of mycorrhizal species, and 14% of species not examined for VAM. Twenty-nine percent of the understory of killed pine was composed of nonmycorrhizal species, 51% of mycorrhizal species, and 20% of species not examined for VAM. All laboratory bioassay plants were nonmycorrhizal in live-pine soils and mycorrhizal in killed-pine soils. No in situ bioassay plants became mycorrhizal beneath live pines. Counts of 50-250 Am VAM spores in May 1981 and August 1982 were 0.013 and 0.098 spores/g, respectively, in live-pine soils and 0.36 and 4.97 spores/g, respectively, in killed-pine soils. We found no evidence that VAM colonization in corn plants was inhibited by forest floor decomposition products. Mycorrhizal colonization of field species was greater in the killed-pine understory than in the live- pine understory. Laboratory and field studies showed that VAM inoculum was absent in the understory of live ponderosa pines. Allelopathic-type suppression, either of host plants or of the mycorrhizal colonization process, might explain the lack of inoculum. Direct inhibition of the colonization process, however, was not observed in greenhouse experiments.