Effects of irradiance and photosynthetic downregulation on the photochemical reflectance index in Douglas-fir and ponderosa pine

Using the “photochemical reflectance index” (PRI) as a measure of xanthophyll pigment activity and photosynthetic light-use efficiency, we examined physiological responses to diurnal illumination in mature forest stands. In a Douglas-fir forest in Corvallis, Oregon, PRI varied primarily with illumination, which was strongly influenced by canopy aspect and time of day. Once normalized for illumination, PRI exhibited a pattern of midday depression similar to that of leaf photosynthesis and stomatal conductance. Comparable optical responses to illumination were detected at canopy and leaf scales, demonstrating that remote spectroradiometry could be applied to monitor photosynthetic downregulation in uniform, closed stands. In similar measurements at a ponderosa pine forest in Black Butte, Oregon, an old tree exhibited more suppressed midday PRI values than a young tree, once values were normalized for illumination. Unlike the PRI response in Douglas-fir, variation in the diurnal PRI response between individual ponderosa pine trees was a predominant source of PRI variation. This contrasting age effect was consistent with other studies at this site showing reduced midday photosynthesis and stomatal conductance in old trees due to hydraulic limitations. These results indicate that diurnal and spatial patterns of photosynthetic activity in structurally complex evergreen forest stands can be characterized with narrow-band spectral reflectance, provided measurements are properly normalized by illumination. These findings also support recent studies using field and satellite remote sensing that report strong effects of illumination on the PRI signal from forest stands, and provide additional evidence that individual canopy responses can reveal contrasting degrees of photosynthetic downregulation due to varying stress effects within a single forest stand. Together, these results support the hypothesis that photosynthesis is coordinately regulated, allowing PRI to detect changing levels of stomatal activity and carboxylation. While illumination patterns and photosynthetic downregulation both influenced PRI, pigment pool sizes and enhanced PRI under prolonged low light provided additional sources of PRI variation in the canopy signal. Further understanding of these multiple PRI responses could help realize the goal of remote sensing of photosynthetic activity using PRI.