Stress Related Changes In Photosynthetic Activity In The Amazon Rainforest

Abstract:

Tropical forests are critical for global biodiversity, carbon and energy cycling. The Amazon basin accounts for about 15% of terrestrial gross primary productivity (GPP); climate related changes of tropical GPP would therefore affect atmospheric CO₂ levels. GPP is a function of the photosynthetically active radiation (PAR), the fraction of this radiation absorbed by vegetation (fPAR) and the efficiency with which this radiation can be utilized to produce sugars and fixate carbon (light use efficiency, ε). PAR and fPAR can be determined from remote sensing measurements, but ε is dependent on complex limitations. The Photochemical Reflectance Index (PRI) has been used to derive ε as a function of the status of the xanthophyll cycle, a biophysical mechanism that controls the down-regulation of photosynthesis but PRI alone depends on extraneous effects and the sun observer geometry. As a result, this technique is not applicable over larger areas or seasonally. Recent findings have shown that ε may be inferred from multi-angle observations by relating ε to the slope of PRI with respect to shadow fractions. This technique minimizes background effects and provides an approach that is based on radiative transfer theory. The lack of multi-angle data has been the main limitation of this multi-angle technique to be applied, however, over fairly homogeneous tropical regions, multi-angle observations may be obtained from a few days of MODIS overpasses, under the assumption that ε remains relatively constant during these overpasses. To obtain multi-angle PRI from MODIS, we took advantage of a new multi-angle implementation of atmospheric correction (MAIAC) that provides surface reflectance without assuming a Lambertian reflectance model, thereby preserving the multi-angle character of MODIS observations. We hypothesize that vegetation stresses will result in down regulation of photosynthesis prior to more obvious changes such as litter fall or dieback. Our finding show clear seasonality of light use efficiency over tropical forests that are related to dry and wet season cycles and correspond well to flux tower related measurements of photosynthesis. Multi-angle MODIS observations, while not optimal for measuring short term changes in ε, may provide realistic estimates of photosynthesis over tropical regions.