Graduate Thesis Or Dissertation
 

Top-Down Effects of Wildlife and Bottom-Up Drivers of Soils and Productivity in Intensively Managed Forest Plantations in the Northern Oregon Coast Range

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  • In intensively managed forest plantations in the northern Oregon Coast Range, herbicides are often applied during site preparation and early stand regeneration to reduce competition for resources for planted conifer seedlings. In addition to reducing competition for crop trees, herbicide applications may affect soil processes including decomposition and nutrient cycling, which may feed back to influence conifer productivity. Since various organisms including arthropods, cervids, and birds may also affect soil processes and conifer productivity, we investigated how wildlife and a range of vegetation control intensities influence decomposition, soil productivity, resource availability to Douglas-fir (Pseudotsuga menziesii) trees, and Douglas-fir productivity in early stand development. We hypothesized that increased vegetation control intensities would result in reduced litter layer accumulation and quality by decreasing vegetative cover and altering plant communities. We anticipated that decreased litter layer accumulation, increased mineral soil moisture, and decreased biotic nutrient retention with increasing IFM intensity would decrease soil productivity by reducing fresh detrital inputs, increasing decomposition rates of soil organic matter, and increasing nutrient losses, respectively. However, we expected that more intensive herbicide applications would result in increased conifer productivity by reducing competition for resources for Douglas-firs. We also predicted that selective herbivory by cervids would decrease litter layer inputs and quality, and that the magnitude of cervid effects on the litter layer would depend on forage availability, and thus, on vegetation control intensity. We anticipated that cervids would exert positive effects on conifer productivity in less intensively treated stands by reducing vegetative competition through selective foraging. In more intensively treated stands, we expected that direct browsing on Douglas-firs would reduce conifer productivity. We also posited that birds influence litter layer decomposition rates and nutrient availability to conifers by exerting top-down control on detritivorous arthropods. To test these hypotheses, we utilized a landscape-scale experimental framework consisting of 28 stands that were clearcut in 2009, replanted with Douglas-fir seedlings in 2011, and grouped into seven distinct blocks. Blocks were situated across a 100 km latitudinal by 45 km longitudinal area in the northern Oregon Coast Range. Within each block, one stand was randomly designated as an unsprayed control and three stands were randomly assigned one of three herbicide treatments each, creating a range of vegetation control treatments in each block. Within each stand, cervid and cervid+bird exclosures were paired with a complete wildlife access plot, allowing us to investigate the roles that wildlife play in soil processes and conifer productivity across a gradient of vegetation control intensities. We found little evidence that herbicide applications or wildlife exclusion influenced decomposition or soil productivity. There were no differences between herbicide or exclosure treatments in carbon stocks, nitrogen stocks, or C:N ratios in the litter layer or mineral soils, although median litter layer masses tended to be lower in the most intensively treated stands. Despite the lack of treatment effects on soil C:N ratios, C stocks, or N stocks, our results provide suggestive evidence that complex interactions between wildlife and vegetation control may influence litter quality in the O-horizon and nitrogen dynamics in the surface mineral soil, but mechanisms driving these effects were unclear. While herbicide applications and wildlife exerted limited detectable effects on decomposition and soil productivity, effects on conifer growth were marked. More intensive herbicide applications generally resulted in increased conifer productivity compared to unsprayed and lightly treated stands, and cervid access exerted differential effects on conifer productivity depending on the level of vegetation control. In unsprayed and lightly treated stands, cervid access tended to inhibit conifer growth. In moderately treated stands, cervids did not influence conifer growth, and in the most intensively treated stands, cervid access resulted in large increases in conifer productivity. We did not find evidence for treatment effects on availability of a single resource that consistently resulted in increased conifer productivity, although calcium was the resource most often associated with increased conifer productivity. Our findings indicate that vegetation control may serve to increase access to a variety of resources, and that growth responses in Douglas-fir trees likely depend on site-specific factors. Additionally, our results suggest that the influence of cervids on conifer productivity early in stand development may depend on the level of competing vegetation.
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