Praveena Krishnan

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

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.

Dynamics of spectral bio-indicators and their correlations with light use efficiency using directional observations at a Douglas-fir forest

The carbon science community must rely on satellite remote sensing to obtain global estimates of photosynthetic activity, typically expressed as net primary production (NPP), gross primary production (GPP) or light use efficiency (LUE). The photochemical reflectance index (PRI), calculated as a normalized difference reflectance index using a physiologically active green band (~531 nm) and another physiologically insensitive green reference band (~570 nm), denoted as PRI(570), has been confirmed in many studies as being strongly related to LUE. Here, we examined the potential of utilizing PRI(570) observations under different illumination conditions for canopy LUE estimation of a forest. In order to evaluate this, directional hyperspectral reflectance measurements were collected continuously throughout the daytime periods using an automated spectroradiometer in conjunction with tower-based eddy covariance fluxes and environmental measurements at a coastal conifer forest in British Columbia, Canada throughout the 2006 growing season. A parameter calculated as the PRI(570) difference (dPRI(570)) between shaded versus sunlit canopy foliage sectors showed a strong correlation to tower-based LUE. The seasonal pattern for this correlation produced a dramatic change from high negative (r ~ ?0.80) values in the springtime and early fall to high positive values (r ~ 0.80) during the summer months, which could represent the seasonality of physiological characteristics and environmental factors. Although the PRI(570) successfully tracked canopy LUE, one or both of its green bands (~531 and 570 nm) used to calculate the PRI are unavailable on most existing and planned near-term satellites. Therefore, we examined the potential to use 24 other spectral indexes for LUE monitoring that might be correlated to PRI, and thereby a substitute for it. We also continued our previous investigations into the influence of illumination conditions on the observed PRI(570) and other indexes. Among the 24 indexes examined, three PRI indexes using different reference bands (488, 551 and 705 nm) showed high correlations to the traditional PRI(570), especially PRI(551) and PRI(705). This indicates three additional PRI variations for LUE monitoring if the traditional reference band at 570 nm is not available but the 531 nm band is available. Five other indexes also yielded high correlations to PRI(570): Dmax and DM705, two indexes calculated from derivative reflectance spectra; a simple ratio of reflectance values at 685 nm and 655 nm (SR685_655); and a double-peak optical index (DPI). The diurnal and seasonal dynamics of these eight indexes and PRI(570) were explored. All of these indexes except DPI expressed linear dependence on available sunlight and more strongly expressed diurnal dynamics in April than in August during summer drought. The differences for shaded versus sunlit canopy foliage sectors were also calculated for the eight indexes, and their correlations to canopy LUE across the season were examined. The performances were similar for the most successful and seasonally stable indexes: dPRI(551), dPRI(705) and dPRI(570). The other five indexes showed good correlation to LUE in some but not all the months, and the months with high correlations varied among them.

Comparing evapotranspiration from eddy covariance measurements, water budgets, remote sensing, and land surface models over Canada

AbstractThis study compares six evapotranspiration ET products for Canada’s landmass, namely, eddy covariance EC measurements; surface water budget ET; remote sensing ET from MODIS; and land surface model (LSM) ET from the Community Land Model (CLM), the Ecological Assimilation of Land and Climate Observations (EALCO) model, and the Variable Infiltration Capacity model (VIC). The ET climatology over the Canadian landmass is characterized and the advantages and limitations of the datasets are discussed. The EC measurements have limited spatial coverage, making it difficult for model validations at the national scale. Water budget ET has the largest uncertainty because of data quality issues with precipitation in mountainous regions and in the north. MODIS ET shows relatively large uncertainty in cold seasons and sparsely vegetated regions. The LSM products cover the entire landmass and exhibit small differences in ET among them. Annual ET from the LSMs ranges from small negative values to over 600 mm acros...

Relating a Spectral Index from MODIS and Tower-Based Measurements to Ecosystem Light Use Efficiency for a Fluxnet-Canada Coniferous Forest

Hyperspectral reflectance data collected diurnally from an instrumented tower were examined in conjunction with the eddy correlation fluxes and meteorological measurements made throughout a growing season at a mature Douglas fir forest in British Columbia, Canada (DF49). Here we present 2006 in situ results relating the Photochemical Reflectance Index (PRI551) to photosynthetic light use efficiency (LUE). Canopy structure information was used to partition the forest canopy into sunlit and shaded fractions. At each observation period, the PRI551 was examined for the sunlit, shaded, and mixed sunlit/shaded canopy segments as defined by their instantaneous position relative to the solar principal plane (SPP). We show that the PRI551 clearly captures the differences in leaf groups on sunny days. We also examined PRI551 from MODIS ocean band imagery acquired over DF49 during a five year period (2001-2006) from both Terra (late morning) and Aqua (early afternoon) platforms. The MODIS observations from Terra and Aqua were acquired in different viewing planes above the landscape over a range of view zenith angles, and sampled the backscatter (sunlit) and forward scatter (shaded) sectors of the forests bidirectional reflectance distribution function. When tower-based bulk canopy LUE from 2006 was recalculated to estimate foliage-based values for the three foliage groups under their incident light environments, a strong linear relationship with PRI551 was demonstrated (r2~0.80). A similar relationship between the MODIS PRI551 and tower-based bulk LUE was obtained from satellite observations (r2~0.62), but only for the backscatter observations obtained at high light levels (APAR, >1500 mumol m-2 s-1). The MODIS observations, while not ideal in terms of spatial resolution (> 1 km2) or optimal viewing configuration, nevertheless provided a means to monitor forest under stress using narrow spectral band indices and off-nadir observations.