PARENT SESSION
Poster Session #19: Nutrient Cycling.
Tuesday, August 7, 2001. Presentation from 3:30 PM to 5:00 PM. Exhibition Hall

Carbon dynamics of belowground systems in Pacific Northwest coniferous forests.

KLOPATEK, JEFFREY¹, ¹

ABSTRACT- Following cutting or other disturbances, the temporal carbon source-sink relationships of Pacific Northwest coniferous forests are unresolved. Forest floor material and soil organic matter may act as both a carbon source and a sink in global CO₂ cycles. Thus, the ecosystem processes controlling these pools are central to understanding the transfers of C between the atmosphere and terrestrial systems. Three Douglas-fir (Pseudotsuga menziesii) and western hemlock (Tsuga heterophylla) forest stands were examined in the Cacade Mountains of southcentral Washington State. Belowground C and N storage and soil respiration were measured in stands of a 20 and a 40 yr old successional forest and an old-growth forest. Data show that at present both the 20 and 40 yr old sites have greater C stores in the soil than does the old-growth site, that has greater C storage in living biomass and coarse woody debris. The 40 yr old site had a significant mix of red alder within the Douglas-fir and consequently had significantly greater (almost two-fold) concentrations and pools of N in the forest floor and soil than either the 20 year old or old-growth sites. The N rich 40 year old site was characterized by low levels of soil CO₂ efflux during the growing season as compared to the other two stands. Peak rates of soil CO₂ efflux at the 40 year old stand were 50% less than the 20 year old and old-growth stands. Estimated annual soil C efflux was 1464, 927, and 1494 g m^-2 for the 20, 40, and old-growth stands, respectively. Totals were even higher in a warmer and dryer year. The rate and amount of soil CO₂ efflux was closely related to soil temperature at the old-growth site, but not at the younger stands. It is hypothesized that the differences are a function of two major components of soil respiration- root and microbial respiration and the different proportions at the three sites. Fine root growth and soil respiration data support the hypothesis that the belowground allocation of C decreases in a more productive/fertile environment. Soil bag data also show major losses of soil C from the young stands but increases in the old-growth stand. Both belowground systems of the young stands appear to be sources of C.

KEY WORDS: CO₂, soil carbon, soil respiration, forest floor