Changes in Autumnal Leaf Reflectance Measurements of Deciduous Trees in Relation to Nitrogen Resorption Efficiencies

During autumn, leaves play a significant and influential role in a forested ecosystem's hydrologic and biogeochemical cycles. Starting at pre-senescence, trees resorb essential nutrients, such as nitrogen and phosphorus, in order to maintain nutrient levels throughout the winter and into spring growth. Initial spring growth is almost exclusively dependent on nutrients resorbed before abscission. Climate change is expected to impact nutrient resorption and thus the need exists to be able to monitor changes in nutrient resorption. Spectrophotometry has been successfully used to identify pigment concentrations in senescing leaves, but current nitrogen metrics have not been reproducible. These metrics focus on reflectance values at key wavelengths and relationships between wavelengths in order to attempt to deduce the nitrogen concentration. We are interested, though, if the patterns of change in the leaf reflectance signatures throughout the fall can be used to compare resorption efficiencies. During 2015 and 2016, chlorophyll concentrations and light reflectance signatures were routinely measured for leaves of Fagus grandifolia (American beech), Liriodendron tulipifera (yellow poplar), and Betula lenta (sweet birch) trees throughout senescence. Leaves from each tree were collected both in August and post-abscission and analyzed for nitrogen concentrations using a CNHS Elementar Cube. Patterns in the light reflectance data were analyzed using Kohonen's self-organizing maps. Preliminary data suggests that the temporal leaf reflectances for each tree maps closer to those of trees with similar resorption efficiencies, thus potentially providing a method to non-destructively monitor nitrogen resorption efficiencies.