Influence of structural heterogeneity on ecosystem processes in northern hardwood forests

Background/Question/Methods

Ecological forestry practices include managing for older forest with its more diverse structure of canopy opening sizes and abundance and more downed coarse woody debris. To better understand the effects of this structural heterogeneity on ecosystem function and services, we conducted a large, replicated field experiment to add old-growth structural elements in a second-growth northern hardwood forest in northern Wisconsin. Our objective here was to determine the near and mid-term effects of environmental and biological drivers on decomposition rates. We used litter-bag and log decomposition studies to test the hypothesis that in the near-term both fine litter and downed coarse woody debris decomposition rates will be greater in gaps than intact canopies, in proportion to increases in soil moisture and temperature. Following gap creation, 12 replicate mesh bags containing leaf litter were placed atop the forest floor at six locations across a north-south gradient through variable-sized gaps. Acer saccharum and Fraxinus americana logs (2 m in length x 20 cm diameter) were placed adjacent to the litterbag locations. Mass loss of fine litter was measured for 3 years and downed wood was measured years 1, 2, 3 and 5 following gap creation.

Results/Conclusions

Assessment of fine litter mass loss patterns indicated that while canopy gap treatments had a large effect on decomposition rates the presence of woody debris moderated the effect. Fine litter decomposition was significantly greater in gap addition plots (8.0 g yr^-1), than in gap and woody debris addition plots (7.0 g yr^-1) or woody debris addition or untreated plots (6.2 g yr^-1). Rates were similar among gap sizes and locations across the gap; all were significantly greater than rates in plots where the canopy was not manipulated. Soil temperature and moisture followed a similar pattern. Log decomposition measured by mass loss in the intial years following canopy gap additions, did not differ among treatments or species. It is expected these patterns will change with time, as a concurrent study of respiration patterns of coarse woody debris indicates higher rates of CO₂ flux of logs in canopy openings.  These results demonstrate the importance of structural heterogeneity to sustainability of site productivity.