Forest floor microbial biomass across a northern hardwood successional sequence

Microbial dynamics partially control forest productivity, yet are poorly un derstood in the northern hardwood forest. Following forest cutting, microbi al biomass may reflect changes in the forest floor environment. Using direc t counts, forest floor microbial biomass was studied in a sequence of north ern hardwood forest stands ranging in age since clearcutting from 3 to more than 120 yr. The organic horizon was collected 3 times in each of 1994 and 1995, in June, August and October. Samples were analyzed for active fungi, active bacteria, pH, forest floor moisture content and organic fraction. F orest floor temperature was measured in the field with each collection; for est floor mass was measured once in each stand. In 1995, forest floor sampl es were also analyzed for total N, nitrate and ammonium. Amounts of active microbial biomass (fungi plus bacteria) ranged from 38 to 103 mu g g(-1) or ganic matter; active microbial biomass per unit area ranged from 40 to 800 mg m(-2) forest floor organic matter. Microbial biomass varied significantl y among sampling periods in 1994, but not in 1995, and was greater in early - and late-successional stands than in mid-successional stands. Microbial b iomass was not very responsive to the environmental factors measured in thi s study. Moisture content was the environmental variable that most often co ntributed to variation in microbial biomass, and together with additional f actors explained only 23% of the variation in 1994, and only 27% in 1995. L ower microbial biomass in mid-successional stands suggests that microbial d ynamics in this forest are not controlled by factors directly related to fo rest harvesting. (C) 1999 Elsevier Science Ltd. All rights reserved.