Background/Question/Methods Improving current understanding of the behavior of terrestrial productivity is essential in reconciling discrepancies in global carbon budgeting.  The amount of photosynthetically active radiation absorbed by the canopy (fPAR) has been shown to be a primary driver of biosphere-atmosphere carbon exchange, and therefore, it is important that this variable be accurately quantified when modeling terrestrial carbon fluxes. At present there are two major approaches for estimating fPAR within forest ecosystems. The more common of these two uses estimates of leaf area index (LAI) to model the absorption of radiation using radiative transfer theory.  Conversely, the second and more direct method involves the use of quantum sensors to measure transmitted PAR.    Although the first approach is easier to implement than the second, the former approach provides only an indirect measurement of fPAR, compounded by the fact that LAI values are themselves estimated from hemispherical sensors or from the normalized difference vegetation index.  This study focuses on evaluating fPAR using optical sensors for a mixed northern hardwood forest at the University of Michigan Biological Station in order to (i) verify satellite based measurements that are provided at relatively course spatial and temporal resolutions (i.e. 8 day and 1km x 1km), (ii) evaluate against estimates of fPAR derived from site-specific measurements of LAI and (iii) compare with gross primary production (GPP) estimates from meteorologically based net ecosystem exchange measurements. This comparison will provide insight as to the fPAR measurement technique that correlates best with terrestrial productivity for this ecosystem type.
Results/Conclusions Preliminary results show that fPAR from all methods were highly correlated with the timing of spring leaf development and subsequent increase in carbon uptake by the canopy.  The largest discrepancies in relationship between measurement type and carbon uptake were observed after the canopy was fully developed.  During this period, the approach that used estimates of LAI from site-specific measurements overestimated GPP as it did not account for changes in photosynthetic pigment quantity.  Measurements of sub-canopy PAR using spatially distributed quantum sensors were most highly correlated with GPP at all time scales. Results indicate that each method’s performance as a proxy for fPAR in modeling GPP was dependent on the specific time period and temporal resolution at which canopy carbon uptake was evaluated.