Stratification of Soil Ecological Processes: A Study of the Birch Forest Floor in the Alaskan Taiga

Abstract

In the forest floor of Alaskan taiga, annual layers of Equisetum (horsetail) litter demarcate cohorts of birch litter. We collected samples of the forest floor monthly during June-September 1993. Forest floor material was separated into each of the three most recent litter cohorts, plus the Oe layer, and the Oa layer. Overall, respiration potential decreased with depth of litter (litter age) and over the growing season. Nitrogen mineralization potential increased with depth, and fluctuated over time. Microbial biomass did not vary with depth, but did increase greatly in September in conjunction with increased litter moisture. Litter C:N ratio decreased with time and varied with depth according to the year-to-year variation in litter chemistry. We present a conceptual model of the forest floor describing microbial activity on a litter cohort as controlled primarily by litter chemistry, but modified by the vertical position in the forest floor and seasonal climatic variation. Litter quality was the main factor associated with changes in microbial activity with season and down the soil profile. Microbial activities occurred within an environment controlled by the climatic buffering of the decomposing leaves themselves. As the quality of litter as a substrate decreased with depth, the quality of the environment for microbial activity increased. Yearly precipitation cycles also played a role in controlling soil biomass and activity.