Soil Microfungi in Relation to the Hardwood Forest Continuum in Southern Wisconsin

Abstract

SUMMARY A survey study was made of the microfungal flora of the soils in a series of upland hardwood forests in southern Wisconsin. These forests constituted a vegetational continuum which was composed of a continuously shifting series of combinations of tree species in a definite sequence that ranged from groupings of more pioneer species at one extremity of the series to more climax assemblages at the other. The analytical measures, frequency and relative density, commonly used for analyzing higher vegetation, were applied to the measurement of the microfungal flora. The size of the accompanying bacterial and actinomycete populations was also determined. When the average frequencies of the microfungal species were plotted along the gradient of the forest continuum, the range of the continuum over which a species was distributed and the point of its optimum development were revealed. Characterized according to the position in the continuum in which they reached their crest of frequency, Spicaria violacea and Penicillium nigricans were typical “climax” species; Penicillium janthinellum, Mucor ramannianus, and Oospora sulphurea were “pioneer” species, and Penicillium granulatum was an “intermediate” type. The restricted distributional ranges of P. janthinellum, P. nigricans, Mucor angulisporus, and Spicaria sp. (14–13) have rendered these species valuable as “indicators” of certain soil environments. Relative density, when compared with frequency, either more sharply defined the optimum range of development of a species or confirmed the range shown by frequency. The number of species of microfungi was found to increase from the pioneer toward the climax end of the continuum; as a result of the overlapping of ranges of both pioneer and climax forms, the greatest number of species occurred in stands somewhat preceding the most climax in the continuum. There was a steady increase in the percentage of penicillia in the soil from the pioneer to the climax forests, while the reverse of this trend was true for the Mucorales. The aspergilli were poorly represented in all forest soils studied. The size of the soil microfloral populations correlated well with the moisture and organic matter content of the soil. The bacterial and fungal constituents were not markedly affected by soil temperature, since they were higher in winter than in summer; the actinomycetes, however, showed an opposite trend. During each season the numbers of bacteria and actinomycetes were consistently higher in the soil of a maple-basswood stand than in the soil of an oak stand; however, during these same periods the microfungal population was greater in the oak woods soil. Although there was little seasonal change in the species of microfungi present in the soils, there were seasonal density and frequency maxima shown for some species. In an exploratory study of the vertical distribution of microorganisms in the soils of two forest stands at opposite ends of the continuum, the species of microfungi most commonly encountered in the surface layers were also found throughout most of the profile; however, at the lowest depths sampled they were replaced in part by a few undetermined species characterized by slow growth. It is concluded that, analogous with the situation existing in the higher plant cover in southern Wisconsin, the forest soil microfungi do not form discrete communities in which particular forms are consistently associated; instead, a series of progressively changing species combinations is found along the gradient of the forest continuum.