Understory variability of photosynthetically active radiation in a Mid-Atlantic deciduous forest and its effects on Lindera benzoin L. Blume (northern spicebush)

Levia, Delphis F.Photosynthetically active radiation (PAR, 400-700 nm) is of critical importance to the physiological ecology of plants, partially governing photosynthesis and the carbon balance of forest ecosystems. The spatiotemporal variability of PAR is particularly critical for understory shrub species which rely on uneven PAR inputs for their survival. Employing a combination of field and laboratory techniques over the seven phenoseasons of deciduous forests, this research sought to better understand the spatial and temporal dynamics of subcanopy PAR and link differences in the length and intensity of PAR to the physiological ecology of Lindera benzoin L. Blume (northern spicebush). Nearly 4,600 individual observations of PAR were made under a Mid-Atlantic deciduous forest canopy to quantify the effect of phenoseason on the spatial and temporal distribution of light reaching the subcanopy and how this distribution of light impacted L. benzoin within the plot. Additionally, laboratory experiments were conducted to assist in the quantification of the effect of various photointensities on the physiological ecology of L. benzoin. Health was determined by monitoring physical growth and biomass, as well as the amount of chlorophyll a, chlorophyll b, and carotenoid by way of UV-vis spectrophotometry. The leafless subcanopy PAR values were almost 10 times higher than leafed season PAR. Sunflecks and sun patches could be three orders of magnitude higher than the subcanopy mean during the leafed season. Phenoseason (i.e. the combination of canopy state and celestial geometry) is responsible for nearly three-quarters of the variation between levels in this mid-Atlantic deciduous forest. Understory PAR is typically less than 40% of open PAR, values of PAR in the shrub canopy are often ~5% lower than the subcanopy. Growth of spicebush in the field is significantly clustered. Locations with spicebush growth are in the top 36% of annual PAR receipt. UV-vis spectrophotometry showed significant differences in Root to shoot ratios, biomass, initial stomatal conductance, and all chemical attributes. Changes in photointensity resulted in significant changes in biomass. Spicebush under lab conditions do significantly alter their individual pigments and pigment ratios in response to high intensity light conditions. Temporal light sequences in the field may be a very important factor in the health of northern spicebush. This work represents a novel approach to measuring photosynthetic photon flux density. By utilizing instruments capable of providing the user with a 15 second spatially integrated one meter linear average of the PPFD, obtaining measurements at multiple locations and elevations in the subcanopy over a full year for all cloud conditions, and coupling our research with detailed laboratory experiments, we are able to synthesize our understanding of PAR and its impact on the ecology and physiology of a common and significant regional understory plant.University of Delaware, Department of GeographyM.S