Evaluation of irradiance metrics with respect to predicting sapling growth

Abstract

Accurate and efficient measurement of photosynthetically active radiation (PAR) is critical in ecological studies. We evaluated 25 metrics of PAR with respect to predicting growth of sugar maple ( Acer saccharum Marsh.) and white ash ( Fraxinus americana L.) saplings from understory to large gap conditions. PAR metrics were derived from gallium arsenide photodiodes, hemispherical canopy photographs (film and digital), and a LI-COR LAI-2000 plant canopy analyzer. In general, percent canopy openness, estimated with film photographs or LAI-2000, best predicted growth. Mean daily photosynthetic photon flux density (PPFD) from photodiodes ranked intermediate; direct beam radiation (from digital and film photographs) was among the poorest growth predictors. Metrics that integrate direct radiation may be relatively poor predictors of growth, because sunflecks were above PPFD levels at which photosynthesis saturates but fully contributed to the calculation of mean daily PPFD. Mean daily PPFD based on truncated sunflecks (to PPFD levels at which photosynthesis saturates) improved predictions of white ash radial growth. Film canopy photographs and the LAI-2000 had relatively low measurement error (indicated by repeatability). High contrast in film photographs, compared to digital, reduced ambiguity in manual thresholding. From a plant-centered perspective, percent canopy openness measured with either the LAI-2000 or film hemispherical photographs provided the best growth predictions.