Temporal dynamics in optical properties of tree-derived dissolved organic matter among four co-occuring tree species

Abstract

Stemflow transports canopy derived dissolved organic matter (DOM) to the forest floor and its utility as a proxy for lignin in tree biomass is tested in this study. The amount and character of stemflow DOM from four major tree species in the eastern United States, Betula lenta L. (sweet birch), Fagus grandifolia Ehrh. (American beech), Liriodendron tulipifera L. (yellow poplar), and Pinus rigida Mill. (pitch pine) was analyzed over a 12-month period to examine the effect of phenophase on tree-derived DOM composition. This study found that DOM concentration and composition is species-specific and changes as a function of phenophase. Specific phenophase differences in specific UV absorbance (SUVA254) implied a phenophase dependent change in DOM aromaticity. In contrast, other spectroscopic properties of stemflow DOM, the absorbance at 250:365 (E2:E3) and spectral slope (SR) ratios, did not imply a change in DOM aromaticity. The humification index (HIX) decreased in all species during emergence, which may indicate these tree species are utilizing humic substances for regrowth during emergence. This is the first known study to report fluorescence characterization values for these four particular species. A four-component parallel factor (PARAFAC) model was created and validated which showed component variations within overall and species-specific phenophase comparisons. The model indicates both interspecific and temporal changes impact the composition of humic and protein-like substances. Lignin content of the biomass for all species and phenophases was compared with all four components and a relationship with fluorescence and %Lignin was determined. Engineers can use this PARAFAC model as a proxy of lignin for renewable polymer production.