WOODY DEBRIS ALONG AN UPLAND CHRONOSEQUENCE IN BOREAL MANITOBA AND ITS IMPACT ON LONG-TERM CARBON STORAGE

Authors: MANIES, KRISTEN - U S GEOLOGICAL SURVEY; HARDEN, JENNIFER - U S GEOLOGICAL SURVEY; BOND-LAMBERTY, BEN - UNIV OF WISCONSIN MADISON; O Neill, Katherine

Submitted to: Canadian Journal of Forest Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/1/2005
Publication Date: 3/10/2005
Citation: Manies, K., Harden, J., Bond-Lamberty, B., O Neill, K.P. 2005. Woody debris along an upland chronosequence in boreal manitoba and its impact on long-term carbon storage. Canadian Journal of Forest Research 35:472-482, doi: 10.1139/X04-179.

Interpretive Summary: Boreal forests contain more than one-third of the world's terrestrial carbon (C) and are currently believed to function as a long-term net carbon sink of atmospheric CO2. Coarse woody debris represents a major reservoir for C in these systems over long time scales. However, the role of woody debris has not been fully accounted for in regional C models, resulting in either over or underestimation of soil C sequestration. To address this problem, we modified a long-term mass-balance model of ecosystem carbon storage to explicitly address potential contributions from woody debris. We then measured the abundance, distribution, and decay rate of woody debris along a fire chronosequence of black spruce (Picea mariana) stands in northern Canada and used these data to model the impact of wood-based carbon inputs on long-term carbon storage. The model suggests that more than half of the carbon stored in black spruce systems over long time scales may be derived from woody debris. The model was most sensitive to assumptions for fire return interval, carbon inputs from net primary production, and decay rates of wood char. Landscape variations in these parameters could account for large variations the amount of C stored in organic-rich soils over millennial time scales. This study highlights the importance of down woody debris as a regulator of C storage in boreal ecosystems and suggests that models of regional C cycling that do not include a coarse and buried wood component may overestimate rates of wood decay by as much as a third.

Technical Abstract: This study investigated the role of fire-killed woody debris as a source of soil carbon in black spruce (Picea mariana (Mill.) BSP) stands in Manitoba, Canada. We measured the amount of standing dead and woody debris along an upland chronosequence, including woody debris partially and completely covered by moss growth. Such woody debris is rarely included in measurement protocols and comprised up to 26% of the total amount of woody debris in older stands, suggesting that it is important to measure all types of woody debris in ecosystems where burial by organic matter is possible. Based on these data and existing net primary production values, we modeled the potential impact of fire-killed wood on the long-term carbon storage of this site. We used the amount of carbon stored in deeper soil organic layers, which persists over millennia, to represent this long-term carbon. We estimate that between 10 to 60% of the deep soil carbon is likely derived from wood products. Sensitivity analyses suggest that this estimate is most affected by the fire return interval, decay rate of wood, amount of net primary production, and decay rate of the char (a post-fire) carbon pool. Therefore, landscape variations in these terms could account for large variations in deep soil carbon. The model was less sensitive to fire consumption rates and to rates at which standing dead becomes woody debris. All model runs, however, suggest that woody debris dynamics play an important role in long-term carbon storage.