Scott, Neal (Woods Hole Research Center, PO Box 296, Woods Hole, MA, 02540;
Phone:  508 548 9375 ext 145; Fax: 508 540 9900; Email: nscott@whrc.org)

Factors controlling carbon sequestration at Howland Forest, Maine: long-term trends, interannual variability, and forest management impacts

Neal A. Scott *, David Y. Hollinger, Eric A. Davidson, D. Bryan Dail, Holly Hughes, John B. Lee, Charles A. Rodrigues

Several factors interact to control rates of carbon (C) sequestration in forests, and these factors operate over temporal scales of minutes to decades. Measurements of C sequestration are further complicated by the fact that storage occurs in various pools (e.g. wood, soil, dead wood). The eddy covariance technique integrates the various fluxes of C over time scales ranging from minutes to decades, providing critical information for development and testing of models that predict future climate and management effects on C sequestration.

We are examining the factors controlling C sequestration at Howland Forest in central Maine. Howland Forest is a commercial forest managed by International Paper, located about 35 miles north of Bangor, Maine. The forest is dominated by red spruce (Picea rubens) and eastern hemlock (Tsuga canadensis), with small contributions from red maple (Acer rubrum), white pine (Pinus strobus), and northern white cedar (Thuja occidentalis). Net annual C sequestration in an undisturbed stand (~140 years old) averaged 1.7 Mg C ha^-1 y^-1 over the last decade (Figure 1). The majority of the C is stored in growing trees. Interannual variations in C sequestration are driven by climatic variations; warmer than average spring and fall conditions lead to enhanced C uptake and warmer than average summer temperatures lead to reduced C uptake. Spatial variability, demonstrated using two independent towers, is lower than temporal variability (Figure 1). This tower serves as a control for ongoing experimental manipulations of the C cycle at Howland Forest (e.g. shelterwood harvest).

Forest management practices also have a large impact on C sequestration rates. We evaluated the C consequences of a commercial shelterwood harvest (2001-2) at Howland Forest. This management system (three harvests over ~60 y) has become increasingly popular over the past decade. Shelterwood harvest removed about 15 Mg C ha^-1 (~30% of live biomass), and created 5.3 and 5.2 Mg C ha^-1 of detritus (aboveground and belowground, respectively). Respiration (decay) from aboveground slash correlated positively with temperature and moisture. Net C storage measured by eddy covariance in August 2001 (pre-harvest) was similar to that of the control tower, then reduced by about 25% the first year after harvest. In 2004 (three years post-harvest), net C storage in August had almost returned to pre-harvest levels, suggesting that C uptake by the vegetation during the growing season recovers in about three years after a shelterwood harvest.