Bigleaf maple (Acer macrophyllum Pursh) is a large deciduous tree that is abundant in western North America. This study addressed whether the predicted increase in abundance of bigleaf maple because of climate change could influence forest hydrology and site fertility due to species-specific effects on incident rainfall distribution and nutrient cycling. The study examined the spatial and temporal variation of forest floor, throughfall, and stemflow properties associated with bigleaf maple in a forest dominated by conifers. In bigleaf maple plots, the throughfall enrichment ratio of major chemicals was highest for NO3 during the leafed period and for P, K, Ca, and Mg during the leaf senescence period. The fluxes of DOC, total-N, DON, P, K, Ca, Mg, S, and SO4 in throughfall were higher in the leafless period than the leafed period. Compared to conifer trees, throughfall was larger for bigleaf maple and had a higher pH, and concentrations and fluxes of P and K. Similarly, stemflow of bigleaf maple had higher pH, and K concentration. The under-canopy and near trunk forest floor associated with bigleaf maple trees showed higher pH, total exchangeable bases, cation exchange capacity and concentrations of exchangeable Ca and Mg compared to Douglas-fir. The application of the PCNM method enabled us to segregate the broad and fine spatial patterns and to extract the main factors impacting forest floor pH and possibly other soil properties at multiple scales. It revealed that topography mainly acts broadly and canopy cover, canopy density and moisture content act at a finer scale. A random design has a higher accuracy in mapping the spatial distribution of forest floor pH and NH4 compared to stratified random and systematic cluster designs, and a sample size of 50 seems to be adequate for mapping the spatial structure of both variables. This study contributes to the understanding of species-specific impacts on soil properties in a mixed forest and suggests that bigleaf maple has a positive effect on site quality. This would enable it to have legacy effects on soil fertility, enhance overall ecosystem resilience, and promote conifer productivity later in succession following mortality of bigleaf maple.
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Thesis advisor: Schmidt, Margaret G.
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