Using hemispherical photography for estimating photosynthetic photon flux density under canopies and in gaps in Douglas-fir forests of the Pacific Northwest

Our primary objective in this study was to determine if hemispherical (fish-eye) photographs could be used to estimatephotosynthetic photon flux density (PPFD) in mature and old-growth Pseudotsuga—Tsuga forests in the Oregonand Washington Cascade Range. LI-COR quantum sensors sampled PPFD at 10-s intervals at six points in each of twoold-growth stands from January to December of 1991. Direct measures of PPFD were also made with photodiodes,which were calibrated against quantum sensors. They were used to sample PPFD for 6- to 14-day periods during thesummer at 15 points in each of four stands, two mature and two old growth. Hemispherical photographs were takenat these sample points and were digitized and analyzed using the program CANOPY. The software predicts overall siteopenness and openness along the sun path. Regression models were developed based upon those parameters. The modelspredicted the mean daily PPFD for each month of the year. Two major factors were found that influenced regressionmodels: stand age—structure (mature vs. old growth) and sky conditions (dry vs. wet seasons). Canopies in maturestands were more open than old growth stands, and their PPFD models were more heavily influenced by overall siteopenness. Conversely, PPFD models for old-growth canopies were more heavily influenced by openness along thesun path. Wet-season models were dominated by diffuse PPFD (overall site openness), and dry-season models weredominated by direct flux (openness along the sun path).,Combining data sets from stands of comparable age—structure and seasons considerably increased regression R--values.