The Imprint of Land-use History: Patterns of Carbon and Nitrogen in Downed Woody Debris at the Harvard Forest

Abstract

Few data sets have characterized carbon (C) and nitrogen (N) pools in woody debris at sites where other aspects of C and N cycling are studied and histories of land use and disturbance are well documented. We quantified pools of mass, C, and N in fine and coarse woody debris (CWD) in two contrasting stands: a 73-year-old red pine plantation on abandoned agricultural land and a naturally regenerated deciduous forest that has experienced several disturbances in the past 150 years. Masses of downed woody debris amounted to 40.0 Mg ha⁻¹ in the coniferous stand and 26.9 Mg ha⁻¹ in the deciduous forest (20.4 and 13.8 Mg C ha⁻¹, respectively). Concentrations of N were higher and C:N ratios were lower in the deciduous forest compared to the coniferous. Pools of N amounted to 146 kg N ha⁻¹ in the coniferous stand and 155 kg N ha⁻¹ in the deciduous forest; both are larger than previously published pools of N in woody debris of temperate forests. Woody detritus buried in O horizons was minimal in these forests, contrary to previous findings in forests of New England. Differences in the patterns of mass, C, and N in size and decay classes of woody debris were related to stand histories. In the naturally regenerated deciduous forest, detritus was distributed across all size categories, and most CWD mass and N was present in the most advanced decay stages. In the coniferous plantation, nearly all of the CWD mass was present in the smallest size class (less than 25 cm diameter), and a recognizable cohort of decayed stems was evident from the stem-exclusion phase of this even-aged stand. These results indicate that heterogeneities in site histories should be explicitly included when biogeochemical process models are used to scale C and N stocks in woody debris to landscapes and regions.