Fungi of a Long Island Oak-Birch Forest I. Community Organization and Seasonal Occurrence of the Opportunistic Decomposers of the A Horizon

Abstract

SUMMARY The soil microfungi of the A horizon of an oak-birch forest on Long Island were isolated by dilution plate from 630 soil samples collected over a period of 40 mo. Because these fungi co-occurred in time and space and showed similarities in their nutritional, reproductive, and survival strategies, they are considered a specific community designated the opportunistic decomposers. Dominance in this community was determined by numerical superiority. A discussion of the factors regulating propagule density in soil and a rationale for using propagule density as the quantitative measure of prominence when large numbers of samples are analyzed is presented. Community composition and structure were based upon analysis of 10,684 isolates from 80 samples collected during the spring, summer, fall, and winter of 1975. This assemblage, reduced to 89 taxa, contained four broad-amplitude species; Penicillium terlikowskii, P. daleae, Trichoderma pseudokoningii and Oidiodendron chlamydosporicum?. These species plus Torulomyces lagena, Acremonium diversisporum and Mortierella ramanniana are the characteristic fungi. Seventy-eight percent of the isolates were mycelial Deuteromycetes; the remainder included Zygomycetes and Ascomycetes (6% and 5% of all isolates, respectively) and asporogenous yeasts. Propagules of the opportunistic decomposers were abundant in the A horizon. They remained constant at approximately 340,000 during most of the year but increased to over one million per g dry soil in April. Species diversity was highest in the spring, lowest in the summer, and essentially constant at 16 species per 100 isolates during the other seasons. Stability characterized the community. Particular combinations of species persisted throughout the course of the study resulting in high coefficients of similarity when populations from different years were compared using a modified Sorensen's Index. Seasonal changes in propagule densities for 25 commonly encountered fungi were studied over a 40-mo period. The following patterns were revealed: (1) five species had essentially constant numbers throughout the year; (2) nine increased significantly in the spring; (3) the oidiodendrons showed a winter peak in density; (4) four species, including Beauzeria bassiana and Metarrhizium anisopliae, appeared only in the spring; and (5), five species had apparently random distributions. An attempt was made to correlate some of these patterns with other changes thought to be occurring in the ecosystem a the same time.