| Summary: | Data on rates of atmospheric deposition is limited in many montane ecosystems, where high spatial variability in meteorological, topographic, and vegetation factors contributes to elevated atmospheric inputs and to the creation of deposition hotspots. Addressing the ecological consequences of increasing deposition in these areas will require a better understanding of surface controls influencing atmospheric deposition rates at both large and small-scales. The overarching objective of this thesis research was to understand the influence of vegetation structure and canopy exposure on small-scale patterns of atmospheric sulfate, nitrate, and chloride deposition inputs to a conifer forest in the Santa Cruz Mountains, California. Throughfall ion fluxes (i.e., ions delivered in water that pass from the forest canopy to the forest floor), bulk deposition (i.e., primarily wet deposition), and rainfall data were collected during the rainy period from October 2012 to May 2013. Throughfall SO42-, Cl-, and NO3- fluxes were measured beneath eight clusters of Douglas fir (Pseudotsuga menziesii) trees (three trees per cluster) differing in tree size (i.e., diameter at breast height; DBH) and canopy exposure. In each cluster, a throughfall collector was placed 1-meter from the bole of an individual tree, for a total of 24 individual collectors. The position of each throughfall collector was recorded with a Trimble® GPS. In addition, tree height, tree diameter, and leaf area index, were measured for all trees. LiDAR data were obtained from GeoEarthScope’s Northern California Airborne LiDAR project and used to model the elevation (DEM), canopy surface height (DSM), tree height (CHM), slope, and curvature of the canopy surface across the entire study area. Over the rainy season, total throughfall flux of SO42--S, a conservative tracer of total deposition (wet + dry + fog), to Douglas fir clusters ranged from 1.44 - 3.84 kg S ha-1 wet season-1, while dry and fog deposition ranged from 0.13 -2.37 kg S ha-1 wet season-1. Total deposition to exposed mature tree clusters was 1.7-2.7 times higher than other clusters. Patterns of total Cl- fluxes (17.10 – 54.14 kg Cl- ha-1 wet season-1) resembled patterns of total SO42--S inputs. Overall, net throughfall fluxes (throughfall – bulk deposition) to Douglas fir trees clusters were more variable than total throughfall fluxes. Net SO42--S and Cl- fluxes to individual collectors increased with tree DBH and the convexity of the canopy surface. Compared to SO42--S and Cl- in throughfall, total NO3--N fluxes (0.17 - 4.03 kg N ha-1 wet season-1) were low and appeared to vary with small-scale changes in elevation. Geospatial technologies and remote sensing tools, such as LiDAR, are promising in the study of relationships between atmospheric deposition and topography (including vegetation), and in scaling-up estimates of atmospheric deposition to larger spatial scales. Understanding small-scale surface controls on atmospheric deposition has implications for different areas of research within geography, including modeling the spread of emerging infectious disease and assessing the effects of nitrogen cycling on native and invasive plant species composition.
|
|---|
