Net Carbon exchange and biomass accumulation as a function of species composition and stand age at the Harvard Forest in central Massachusetts

Forests in New England include a mixture of deciduous and coniferous species and a range of disturbance history from intensive agriculture abandoned in the late 1800s to permanent woodlots that were managed for forest products but never cleared. Three flux tower sites established at the Harvard Forest Long-Term Ecological Research (LTER) site capture the range of forest types that typify central Massachusetts. The Environmental Measurement Site (EMS) is in a stand dominated by red oak and red maple with the oldest trees established prior to 1895 on abandoned pasture. Scattered pines and patches of hemlock make up as much as 44% of the basal area in the area northwest of the tower and are present as small saplings in the understory. The Hemlock (HEM) tower is sited adjacent to a nearly pure stand of eastern hemlock with some individuals up to 230 years old. The site has not been affected by any stand-clearing disturbance since European settlement. A third tower, Little Prospect Hill (LPH), is in a stand also dominated by red oak and red maple on abandoned pasture, but with fewer conifers and younger trees that have regenerated since a fire in 1957. Towers at each site are instrumented for eddy-covariance measurements of CO₂, H₂O, energy, and meteorological variables. Ancillary measurements near each tower track biomass and leaf area. The focus of this presentation will be on physiological differences between the three stands that affect their net CO₂ exchange and the patterns of carbon accumulation in woody biomass. Comparing the hourly CO₂ fluxes from the three towers highlights the similarities and differences in canopy physiology in the three stands (Figure 1). The peak CO₂ uptake rates (largest absolute value negative fluxes) in mid summer are greater for the two deciduous-dominated sites. The older EMS site slightly exceeds the younger site, LPH. The reduced magnitude of CO₂ uptake during summer months at HEM is offset by a much longer growing season that adds 1.5 – 2 month in the spring and fall compared to the active season for deciduous stands. The scattered conifer patches and subcanopy hemlocks near the EMS tower generate a modest CO₂ uptake during April before the deciduous leaves emerge. We don't see a similar shoulder, which might be attributed to light attenuation by the deciduous canopy that has turned brown, but not fully fallen from the trees. Biomass accumulation in above-ground wood has been remarkably similar for hemlock and deciduous-dominated stands over the last 2 decades (Figure 2). Even though the hemlock stands are older, they are still actively accumulating carbon. The future carbon uptake in these forest stands is threatened by invasion of Hemlock Wooly Adelgid (HWA), an exotic insect pest that kills hemlock trees and is expanding northward. The observations so far will be a unique baseline for comparison as the infestation proceeds. Figure 1 Observed rates of net CO₂ flux at the 3 Harvard Forest towers in 2010. Figure 2 Above-ground woody biomass derived from diameter measurements on the array of EMS biometry plots and a mapped hemlock plot near the Hemlock tower. The EMS plots were established in 1993 and resurveyed annually after 1998. The Hemlock plot was established in 1990 and resampled about every 10 years.