Canopy Quantum Efficiency in a Northern Hardwood Forest: the contributing roles of direct and diffuse PAR

Recent work has shown that the proportion of diffuse and direct photosynthetically active radiation (PAR) has an effect on canopy quantum efficiency (α), and on net ecosystem CO₂ exchange. It is not well known, however, how α under direct (α_r) and diffuse (α_f) PAR respond to changing atmospheric conditions or how these parameters vary across ecosystems. We quantified α_r and α_f in a mixed hardwood forest with an LAI of 3.7 in northern lower Michigan (45° 35' N, 84° 42' W) during the 1999 and 2000 growing seasons from hourly CO₂ flux and global PAR measurements made at the Univ. of Michigan Biological Station Ameriflux facility. Direct and diffuse PAR were calculated using a model that related diffuse PAR to the solar zenith angle (θ ), the clearness index (k_t), and hourly global PAR measurements. Quantum efficiencies varied throughout the growing season with α_f being consistently higher than α_r for the two years. Mean α_f and α_r were 0.026/0.033 and 0.015/0.014 for the 1999/2000 seasons, respectively. Maximum α_f and α_r were attained early in the growing season (Julian Day <220) in both years with 2000 having ~10% higher α_f(max) and ~40% lower α_r(max) than observed in 1999. Our results also indicated that the responses of α_r and α_f to temperature and vpd were different and varied from year to year. This study stresses the importance of separating the direct and diffuse components of PAR in understanding biosphere and atmosphere carbon exchanges.