Decadal change of forest carbon stocks and tree demography in the Delaware river basin
Quantifying forest biomass carbon stock change is important to understand forest dynamics and its feedback with climate change. Northern hardwood forests are a representative ecosystem type in the northeastern U.S. and have been a net carbon sink in recent decades. However, carbon accumulation in some northern hardwood forests has been halted due to the impact of novel stresses such as invasive pests, land use change and climate change. The Delaware River Basin (DRB), sited in the southern edge of the northern hardwood forest, features diverse forest types and land-use histories. These forests are sensitive to the controlling factors defining forest dynamics. In 2001-2003, the DRB Monitoring and Research Initiative established 61 forest plots in three research areas along a climate gradient, using Forest Service inventory protocols and enhanced measurements. These plots were revisited and remeasured in 2012-2014. By comparing forest biomass in the two measurements, the major goals of this study are (1) to quantify the biomass carbon stock change in the DRB forest during the recent decade, (2) to investigate the controlling factors of forest biomass change at regional scale, and (3) to examine the impact of demographic dynamic on biomass change in different tree species.
Overall, the biomass of the DRB forest was a carbon sink in the past decade. The net biomass carbon stock change in the Neversink area in the north of the DRB was 1.34 Mg C ha-1 yr-1, greater than the biomass change in the French Creek area (0.95 Mg C ha-1 yr-1, southern DRB), and Delaware Water Gap Area (1.18 Mg C ha-1 yr-1, central DRB). Carbon change of dead biomass (including dead tree, dead sapling, coarse and fine woody debris) accounted for 25.3% of the total biomass change. The change of biomass carbon stocks did not correlate with any climatic factors within the research area. The change of biomass decrease with increasing stand age, but the contribution of large trees to total biomass increased in all the three research areas. In most of the major species, stem density was decreasing but the loss of biomass from mortality was offset by recruitment and growth. Demographically, the living biomass of chestnut oak, white oak and black oak decreased because of the large mortality rate. These species appear to be slowly replaced by other species such as white pine, American beech and sweet birch, which had a dramatic increase in both biomass and stem density.