We measured component and whole-system respiration fluxes in northern hardwood (Acer saccharumMarsh., Tilia americana L., Fraxinus pennsylvanica Marsh.) and aspen (Populus tremuloides Michx.) forest stands in Price County, northern Wisconsin from 1999 through 2002. Measurements of soil, leaf and stem respiration, stem biomass, leaf area and biomass, and vertical profiles of leaf area were combined with biometric measurements to create site-specific respiration models and to estimate component and whole-system respiration fluxes. Hourly estimates of component respiration were based on site measurements of air, soil and stem temperature, leaf mass, sapwood volume and species composition. We also measured whole-system respiration from an above-canopy eddy flux tower.
Measured soil respiration rates varied significantly among sites, but not consistently among dominant species (P < 0.05 and P > 0.1). Annual soil respiration ranged from 8.09 to 11.94 Mg C ha−1 year−1. Soil respiration varied linearly with temperature (P < 0.05), but not with soil water content (P > 0.1). Stem respiration rates per unit volume and per unit area differed significantly among species (P < 0.05). Stem respiration per unit volume of sapwood was highest in F. pennsylvanica (up to 300μmol m3 s−1) and lowest in T. americana (22 μmol m3 s−1) when measured at peak summer temperatures (27 to 29 °C). In northern hardwood stands, south-side stem temperatures were higher and more variable than north-side temperatures during leaf-off periods, but were not different statistically during leaf-on periods. Cumulative annual stem respiration varied by year and species (P < 0.05) and averaged 1.59 Mg C ha−1year−1. Leaf respiration rates varied significantly among species (P < 0.05). Respiration rates per unit leaf mass measured at 30 °C were highest for P. tremuloides (38.8 nmol g−1 s−1), lowest for Ulmus rubraMuhlenb. (13.1 nmol g−1 s−1) and intermediate and similar (30.2 nmol g−1 s−1) for T. americana, F. pennsylvanica and Q. rubra. During the growing season, component respiration estimates were dominated by soil respiration, followed by leaf and then stem respiration. Summed component respiration averaged 11.86 Mg C ha−1 year−1. We found strong covariance between whole-ecosystem and summed component respiration measurements, but absolute rates and annual sums differed greatly.