Coarse Woody Debris and Ecosystem Carbon Dynamics in a North Temperate Forest

Many aspen-dominated forests in the upper Great Lakes region of North America are past maturity and beginning to decline. As trees senesce in these aging forests, coarse woody debris (CWD) is expected to be increasingly important to the ecosystem carbon (C) balance. We used a biometric approach to quantify C mass and the annual respiratory C loss from CWD and other major ecosystem components for a typical mature deciduous forest in northern lower Michigan, USA. Coarse woody debris mass (2.2 Mg C ha-1) was less than that of soils (104.1 Mg C ha-1) and boles (71.7 Mg C ha-1), but similar to that of leaves (1.8 Mg C ha-1). CWD respiration (RCWD) increased with increasing temperature and water content. Higher RCWD in more decayed wood was due to greater water absorption and, consequently, higher water content rather than to a greater temperature sensitivity of respiration ( Q10). The Q10 of RCWD ranged from 2.20 to 2.57 and varied inconsistently with decay status. Daily RCWD varied seasonally in response to temperature and water content, increasing rapidly following snow melt in early April and peaking at 0.17 g C m-2 d-1 in early June. Daily RCWD in the early growing season (day 130-200) was 13 % greater than during the late growing season (day 201-279) since CWD water content was 69 % higher even though temperature was 1.5°C cooler. Annual CWD respiration (FCWD, 0.21 Mg C ha-1 yr-1) was 12 % of bole respiration, 8 % of leaf respiration, and 2 % of soil respiration. Compared to the 1.53 Mg C ha-1 yr-1 average annual C storage by our forest, FCWD is a small, but substantial flux that is expected to increase over the next several decades. We show how concurrent increases in CWD production and temperature in the near future may temporarily reduce regional forest C storage.