Population and biomass dynamics of trees in a northern hardwood forest at Hubbard Brook

We quantified the dynamics of the tree stratum at Hubbard Brook Experimental Forest (HBEF), New Hampshire, to examine why live biomass reached a plateau in about 1980. Total aboveground biomass increased from 209 Mg·ha^-1 in 1981 to 216 Mg·ha^-1 in 2001. From 1991 to 2001, in-growth of ≥10 cm diameter at breast height (DBH) trees averaged 4.7 trees·ha^-1·year^-1 with a corresponding in-growth biomass of 0.29 Mg·ha^-1-year ^-1. Mortality of trees ≥10 cm DBH averaged 5.3 trees·ha^-1·year^-1 (1.12% of trees·year^-1). Dying trees represented 2.24 Mg·ha ^-1·year^-1 of aboveground biomass from 1991 to 2001. The biomass pools of standing dead, snags, and coarse woody debris in this forest currently are near steady state with residence times of 7.5, 15, and 6.2 years, respectively. The plateau in live biomass was mostly associated with lower wood production. Aboveground net primary productivity was estimated at 6.53 Mg·ha^-1·year^-1 (3.28 Mg·ha ^-1·year^-1 for aboveground woody tissues and 3.25 Mg·ha^-1·year^-1 for leaf production), considerably lower than published estimates for the 1956-1965 period at the HBEF. Net ecosystem productivity in this young, second-growth forest is near zero, indicating that it may not be a sink for carbon.