Near-term results of soil and wood CO2 flux from experimental manipulation of northern hardwood forest structure

Background/Question/Methods

The rate of surface CO₂ flux is known to be controlled by environmental conditions including soil temperature, moisture and substrate quality. An alteration to forest structure would be expected to change some or all of these environmental regulators of soil C cycling. We experimentally manipulated forest structure through canopy gap creation and woody debris additions in a second-growth northern hardwood forest in north-central Wisconsin. We quantified the above- and belowground consequences of these structural alterations and evaluated possible mechanisms. In the short-term, we hypothesized that canopy gaps would stimulate greater effects on surface CO₂ flux than woody debris additions. Gap sizes (200, and 380 m²) and the woody debris level (28.7 Mg/ha) were chosen to represent the typical range found in old-growth forests of the Great Lakes region. The treatments were carried out in late January 2007 under frozen ground conditions and snow cover. Soil and woody debris respiration was measured monthly in 2007 and 2008. Soil temperature and moisture were measured concurrently. We estimated dry biomass (Mg/ha) and ANPP (Mg/ha/yr) of the trees, coarse roots and saplings using regional allometric biomass equations, litterfall using 0.25 meters-squared traps, and the understory woody and herbaceous vegetation using sequential harvests and permanent vegetation monitoring quadrats for the same time period. Fine root biomass was estimated using sequential coring.

Results/Conclusions

We quantified significant changes due to the treatments in fine root biomass, ANPP, and for biomass of all aboveground vegetation components. Additionally, soil temperature, moisture and air temperature increased within canopy openings. However, preliminary results suggest that there is no net change in the surface CO₂ flux due to canopy gap addition. Soil surface flux was correlated most strongly with soil temperature at 10 cm, while woody debris flux was correlated with air temperature. Further analysis will evaluate the changes in C:N ratio of the litterfall and understory vegetation and how the substrate quality relates to surface flux from each treatment type. Identifying how these biological, chemical and physical properties interactively affect soil respiration within this site in the near-term will help us to understand and model the synergistic effects of canopy gaps and woody debris alterations on forest carbon budgets.