Remotely sensing vegetation photosynthetic rate: the role of US and Canadian field campaigns

Eddy covariance measurements of net ecosystem exchange (NEE), combined with methods for estimating daytime respiration (R), have provided nearly continuous information of the plant photosynthetic rate or Gross Primary Production, GPP, for over 20 years. In this paper we recount the role that these important measurements have played in the development of methods for direct measurements of GPP using satellite data. Remote sensing of GPP is based on the Monteith formulation GPP = ε PAR Fapar, where PAR is the photosynthetically active radiation incident on the vegetated canopy (megajoules), Fapar is the fraction of that radiation absorbed by the canopy's photosynthetically active elements and ε is the efficiency with which the canopy fixes carbon (grams carbon/MegaJoule – gC/MJ). The measurements of tower PAR and Fapar have been instrumental in developing a quite mature satellite capability to measure both PAR and Fapar using top of the atmosphere satellite measurements of radiation in several narrow bands. However, the Monteith formulation of GPP is deceptively simple. The efficiency term, ε, as inferred from tower measures of APAR and GPP, is variable from one period to the next and one vegetation type to the next, and is an extremely complex function of biophysical processes that are only poorly understood and difficult to simulate in models. The photochemical reflectance index, PRI, defined by the reflectance in two narrow bands centered at 531 and 570 nm, PRI = (ρ531 -ρ570)/ (ρ531 +ρ570), has been used empirically since the early 90's to measure ε at the leaf level. But the high sensitivity of PRI to extraneous effects such as canopy structure, non-photosynthetic canopy elements and the view-observer geometry has hampered its use at canopy, landscape and global scales because the relationship between PRI and ε measured at one illumination and view angle is profoundly affected by shadow fraction and the ratio of photosynthetically to non-photosynthetically active canopy fractions in the sensor field of view. Recent research however has shown that multi-angular observations of PRI provide a powerful tool for global estimation of ε. Using continuous, tower-based observations acquired from an automated multi-angle spectroradiometer platform (AMSPEC II) installed at a coniferous (Douglas-fir (DF) dominated) and a deciduous (Aspen) stand, it has been demonstrated in just the past three years that multi-angle observations of PRI can be used to measure ɛ at a forest stand scale throughout the vegetation season (r2=0.91 and 0.88, for DF and Aspen, respectively, p<0.00). This result was achieved over two very different forest types, in multiple years, in two climatically different biomes. It has also been shown that PRI can be observed by MODIS at varying solar illumination and viewing angles using an atmospheric correction (MAIAC) applied to MODIS bands 11 and 12 (r2=0.54 and 0.63, respectively, p<0.00). Importantly, the tower measurements using eddy correlation and multi-angle spectrometers have shown that the PRI:ε relationship observed at the coniferous and the deciduous site may be described by a single functional relationship between ε and ∂PRI/∂(αs). Thus, this relationship could be used with nearly simultaneous, multi-angle satellite PRI measurements to quantify ε in two very climatically and geographically different vegetation biomes over multiple years using remote sensing data. In conclusion, eddy correlation tower measurements, together with tower spectrometer measurements are defining a pathway to the development of a spaceborne sensor that can simultaneously measure ε, Fapar and PAR, hence vegetation photosynthetic rate at landscape and global scales.