Linking foliage spectral responses to canopy-level ecosystem photosynthetic light-use efficiency at a Douglas-fir forest in Canada

Abstract

The light-use efficiency (LUE) of a mature Canadian Douglas-fir forest (DF49) was studied using high-resolution in situ temporal, spatial, and spectral measurements in conjunction with fluxes acquired from an instrumented tower. We examined the photochemical reflectance index (PRI), a spectral index responsive to high light conditions that alters reflectance at 531 nm, in combination with several alternative reference bands at 551, 570, and 488 nm. These indices were derived from directional reflectance spectra acquired by a hyperspectral radiometer system mounted on the DF49 tower, viewing the canopy through almost 360° rotations multiple times an hour daily throughout the 2006 growing season. From canopy structure information, three foliage sectors within the canopy were delineated according to instantaneous illumination conditions (sunlit, shaded, and mixed shaded–sunlit). On sunny days, the PRI indices for the sunlit foliage sector captured high light-induced stress responses, expressed as significantly different PRI values for sunlit versus shaded foliage. This difference was not observed on highly diffuse or overcast days. PRIs on sunny days tracked the diurnal photoregulation responses throughout the growing season in concert with illumination intensity. We computed the effective instantaneous LUE for the three foliage groups (LUEfoliage) using modeled and measured information. We provide convincing evidence that LUEfoliage can be well described and strongly related to all variations of the PRI within this coniferous forest under relatively clear skies (0.59 > r² > 0.80, P < 0.0001). LUEfoliage varied through the growing season between 0.015 and 0.075 µmol C µmol⁻¹ absorbed photosynthetically active radiation (APAR), and the lowest daily values were associated with the sunlit foliage group. The mixed sunlit-shaded foliage was the only group to exhibit monthly averages close to the maximum ecosystem LUE parameter (εmax) used in LUE models for evergreen needle forests (0.0196 µmol C µmol⁻¹ APAR). Implications for remote sensing of carbon uptake dynamics and the interaction of canopy structure and physiology are discussed.