Benchmark carbon stocks from old-growth forests in northern New England, USA

Abstract

Forests world-wide are recognized as important components of the global carbon cycle. Carbon sequestration has become a recognized forest management objective, but the full carbon storage potential of forests is not well understood. The premise of this study is that old-growth forests can be expected to provide a reasonable estimate of the upper limits of carbon storage for similar forest types in comparable site conditions. We sampled old-growth stands in Maine, Vermont, and New Hampshire to establish benchmark values for carbon storage in the forests of northern New England. Our specific objectives were: (1) develop estimates of carbon stocks in key live and dead biomass carbon pools of hardwood and softwood forests in northern New England, (2) compare these values to other estimates of carbon stocks in old-growth forests, and (3) compare data collected from mature second-growth forests to the old-growth benchmark values. Twelve sites in Vermont, New Hampshire, and Maine were sampled to estimate total carbon stocks in aboveground live and dead biomass, down dead wood, forest floor, and soil to 20 cm. Total carbon stocks averaged 216 t/ha for northern hardwoods and 267 t/ha in softwood sites, with 116 and 125 t/ha in the aboveground live tree biomass for hardwoods and softwoods, respectively. Our results showed old-growth softwood averaged about 25% more carbon than old-growth hardwood, primarily due to the higher carbon amounts in the thick forest floors characteristic of old-growth softwood. Old-growth hardwoods supported live biomass carbon stocks similar to those in mature hardwood stands (about 80–120 years old), although forest floor stocks in old-growth were about twice as high (a non-significant difference). Overall carbon stocks in mature second-growth hardwoods were 89% of those in old-growth stands; this difference was not statistically significant. Additional work is needed in mature second-growth softwoods; data were not available for comparison to the benchmarks.