The interaction of precipitation with tree canopies is an important process that affects ecosystem-scale nutrient cycling, plant nutrition, and spatial patterns of deposition of pollutants. Precipitation that falls on a tree canopy, interacts with foliar and woody components of the canopy and passes through to the soil below is termed throughfall (TF). Throughfall is a pathway for nutrients from atmospheric deposition to enter the soil system, a pathway by which plants can take up or lose essential nutrients, and it is a process that can concentrate atmospheric pollutants scrubbed from the air by tree canopies. By investigating canopy leaching we can understand how throughfall processes influence forest nutrient cycling and nutrient use efficiency, as well as how trees intercept particulate matter and gaseous pollutants and funnel them to the soil below. For my dissertation, I investigated the transport, exchange, and deposition of nutrients and pollutants via throughfall across Michigan's urban, suburban, and rural forests. In Chapter 2, I studied the influence of adjacent land use, local point sources, and woodlot stand structure on subcanopy N transport and enrichment via throughfall in three woodlot fragments in southern Lower Michigan, USA. I found that one site had markedly higher TF N concentrations compared to the other two; however, my data indicate that elevated TF concentrations resulted from differences in tree species composition, rather than differences in surrounding land use. Specifically, I observed that the local abundance of basswood (Tilia americana) was positively associated, and the local abundance of northern red oak (Quercus rubra) was negatively associated with TF N concentrations. One site had markedly greater TF N fluxes than the other two, driven by a lack of understory vegetation. Together, the results of this study demonstrated that TF N concentrations and fluxes were more strongly influenced by the internal characteristics of fragmented woodlots, such as forest structure and species composition, than by the surrounding land use. In Chapter 3 I investigated the role of TF in driving the transfer of nutrients from trees to soil, and in determining the NUE across a range of forest stands in the Manistee National Forest in the northwestern lower peninsula of Michigan, USA. The results of Chapter 3, demonstrate that TF losses of nutrients and litterfall losses both increase from low to high-fertility soils; however, the relative contribution of TF to total losses is greater in high-fertility soils. TF losses varied across the nutrients studied, which include K > Mg > N > P > Ca. This reflects the fact that K occurs in readily soluble form in leaves and thus is leached at a comparatively high rate relative to the other elements studied, resulting in higher K concentrations in throughfall and litterfall. Contrarily, Ca is primarily associated with insoluble, structural compounds, resulting in lower concentrations in throughfall and litterfall. Accounting for TF losses reduced estimates of NUE for K by 20-70%, 0.6-7% for N, 0.5-17 % for P, 1-17% for Mg, and by less than 5 % for Ca. Microplastics are an anthropogenic contaminant of emerging concern due to their durability and persistence in the environment. Microplastics are particles with a size range of < 5 mm and have been detected in aquatic ecosystems, soil, and airborne particles. There are data gaps on the effect of rainfall and the fate and transport of microplastics in urbanized areas. In Chapter 4, I seek to understand the role of individual tree species in the removal of microplastics in a medium-to-low-density metropolitan area in East Lansing, Michigan, USA. The study focuses on the quantification of throughfall concentration and fluxes to determine the interspecific and temporal variability of throughfall microplastics under urban tree species during fully-leafed and partially-leafed periods. Throughfall samples were collected from four species: red maple (Acer rubrum), eastern white pine (Pinus strobus), honeylocust (Gleditsia triacanthos), London planetree (Platanus x acerifolia). A fluorescence microscope was used to identify and quantify microplastics in throughfall. While the morphology and origin of microplastic particles a scanning electron microscopy (SEM) combined with an energy-dispersive X-ray (EDS) was used. In my study I was able to characterize polystyrene (PS), polyethylene (PE), and fiberglass in throughfall samples. Throughfall microplastic concentration and fluxes trended higher under London planetree, when compared to other commonly occurring urban tree species, but the species effect was not statistically significant. Understanding the fate and transport of microplastics in urbanized areas can help us determine the role of tree species in removing microplastics or other airborne pollutants that could negatively impact ecosystems and human health.
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