Scaling gross ecosystem production at Harvard Forest with remote sensing: a comparison of estimates from a constrained quantum‐use efficiency model and eddy correlation

Abstract

Two independent methods of estimating gross ecosystem production (GEP) were compared over a period of 2 years at monthly integrals for a mixed forest of conifers and deciduous hardwoods at Harvard Forest in central Massachusetts. Continuous eddy flux measurements of net ecosystem exchange (NEE) provided one estimate of GEP hy taking day to night temperature differences into account to estimate autotrophic and heterotrophic respiration. GEP was also estimated with a quantum efficiency model hased on measurements of maximum quantum efficiency (i2niax)' seasonal variation in canopy phenology and chlorophyll content, incident PAR, and the constraints of freezing temperatures and vapour pressure deficits on stomatal conductance. Quantum efficiency model estimates of GEP and those derived from eddy flux measurements compared well at monthly integrals over two consecutive years (R^ = 0*98).Remotely sensed data were acquired seasonally with an ultralight aircraft to provide a means of scaling the leaf area and leaf pigmentation changes that affected the light ahsorption of photosynthetically active radiation to larger areas. A linear correlation between chlorophyll concentrations in the upper canopy leaves of four hardwood species and their quantum efficiencies (R^ = 0-99) suggested that seasonal changes in quantum efficiency for the entire canopy can be quantised with remotely sensed indices of chlorophyll. Analysis of video data collected from the ultralight aircraft indicated that the fraction of conifer cover varied from < 7% near the instrument tower to about 25% for a larger sized area. At 25% conifer cover, the quantum efficiency model predicted an increase in the estimate of annual GEP of < 5% because unfavourable environmental conditions limited conifer photosynthesis in much of the non-growing season when hardwoods lacked leaves.

Soil warming, carbon–nitrogen interactions, and forest carbon budgets

“…Using a threshold number of 50% of the buds on the trees opening, we estimate that bud-break occurs between 4 and 7 d sooner in trees in the warmed area. This translates to a 3-4% increase in the mean growing season length of 161 d reported for deciduous stands at the Harvard Forest (34). A slightly longer growing season may be interacting with greater nitrogen availability to enhance plant productivity and carbon storage.…”

Estimating canopy structure of Douglas-fir forest stands from discrete-return LiDAR

“…In addition, the most commonly developed relationships (such as those between spectral band combinations and LAI, volume or total aboveground biomass) are nonlinear and saturate at approximately 100 Mg ha -1 (Cohen and Spies 1992) and an LAI greater than 3 . Likewise, synthetic aperture radar (SAR) has been found to be insensitive to differences in forest biomass above 150 Mg ha -1 (Waring et al 1995) which is generally well below that observed in temperate forests.…”

Improved global simulations of gross primary product based on a new definition of water stress factor and a separate treatment of C3 and C4 plants

“…TEC uses a universal e* with a value of 1.8 gC MJ À1 observed for C3 species in field (Waring et al, 1995) and already employed by 3-PG model (Landsberg and Waring, 1997). However, as leaf photosynthetic rates of C4 species are greater than those of C3 species (Jones, 1992;Pearcy and Ehleringer, 1984;Prince and Goward, 1995;Baldocchi, 1994).…”