Tree-mediated water-nutrient fluxes from the microbial to regional scale: insights from mixed-deciduous forests in the northeastern United States

Date
2019
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University of Delaware
Abstract
Most people consider trees in passing. Trees provide necessary ecosystem services, chief among them are the production of oxygen and sequestration of carbon dioxide through photosynthesis, the creation of stable and fertile soils, and reduction of storm run-off. Additionally, humans utilize tree biomass as fuel, consumable products, building material, and foodstuffs. However, trees are rarely thought of as agents of biogeochemical influence and change. In the Northeast region of the United States, knowledge of the ways in which trees influence and change watersheds are key, as the surface waters held within these watersheds are essential for human activities and often the primary source of drinking water. Not only do trees experience changes in their environment at different temporal scales (e.g., individual storm events, periodic disturbances, seasons), they also create a changing environment for chemical and microbial processes within those conditions. Further, trees not only exist in but are themselves, a habitat. This dissertation explores some ways in which trees can influence a watershed. ☐ The decomposition of broadleaved tree leaves can contribute a substantial amount of energy to forested watersheds via dissolved organic matter, nutrients, and biological activity. Less is known about how these inputs may vary within a single tree species that is known to have two genetically distinct and geographically separate populations, or how these inputs may change throughout autumn senescence and abscission. We analyzed the morphological and chemical leaf traits, and leachates from Fagus grandifolia (American beech) leaves during three phenophases: fresh green leaves, senescing leaves, and fallen leaves collected from four sites along a geographic transect stretching from Vermont to North Carolina (over 1400 km). Leachates were analyzed for routine solutes and nutrients, as well as fluorescent and UV-visible absorbance indices. Results suggest significant differences in leached nutrients among sites and optical properties and nutrients among phenophases. These results also suggest that geographically (or genetically) separate populations of the same species do not experience senescence in the same way and that implicit assumptions of intraspecific uniformity of leaf-litter leachate chemistry for a given tree species may be invalid. ☐ Precipitation incident on a forest canopy is partitioned into throughfall, stemflow, and interception loss. Throughfall is the dominant subcanopy water flux and is responsible for numerous nutrient cycles between the atmosphere, canopy, and soil. Events impacting the eastern United States (Maryland, Rhode Island, and Vermont) were sampled, and throughfall chemistry was explored for each site in the lens of large-scale storm events. Season was found to be the strongest driver of base cation flux differences among the study sites. Additionally, strong regional deposition played a key role in the discrimination of throughfall chemistry in the region, overriding the influence of the synoptic scale storm events. ☐ The second subcanopy water flux, stemflow, differs from throughfall in nutrient enrichment and residence time. Stemflow coupled with bark texture and hypothesized bark microclimates could provide a refuge for bark surface bacteria. Additionally, it is expected that microbial communities will differ along an urban to rural gradient. This study found that there were indeed multiple phyla of microbial life on the bark surface, and while their biological functions are not understood at this time, there is a difference in composition and location along the tree bole between the urban and rural sites. Stemflow also acts to translocate microbial biomass during precipitation events creating sources and sinks of microbial diversity on the bark.
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