Intensive forest management (IFM) reduces the production of shrubs and herbaceous plants to decrease competition for resources for planted conifer seedlings. This may alter the quality and quantity of organic inputs to the forest floor, which may affect soil and conifer productivity since the litter layer mediates both soil carbon storage and nutrient cycling. IFM may also alter songbird and arthropod abundance, as well as cervid foraging, each of which may have direct or indirect effects on decomposition and mineralization processes. This study aims to determine how interactions between IFM and wildlife affect litter layer accumulation, decomposition, mineralization processes, and ultimately soil and Douglas-fir (Pseudotsuga menziessii) productivity.
Four herbicide treatment plots, consisting of a control, light, moderate, and intensive treatment, were established in harvested units across seven replicated blocks in the Northern Oregon Coast Range. Open herbivory and paired songbird/cervid exclosures were constructed within each plot. Five years after harvest, O-horizons, mineral soil cores (0-15cm and 15-30cm), and Douglas-fir needles were collected from all exclosures. O-horizons have been analyzed for bulk densities and depths and all samples will be analyzed for total C, N, 13C, and 15N.
Results/Conclusions
Preliminary results indicate a trend of decreasing O-horizon depth and increasing bulk density with increasing IFM intensity in open-access plots and cervid exclosures, with significant differences between controls and intensive herbicide treatments. These findings suggest that higher levels of IFM intensity lead to decreased accumulation of the litter layer. No significant differences between herbicide treatments for O-horizon bulk densities or depths were observed in songbird exclosures. However, 0-15cm mineral soils contained significantly more C in intensively treated songbird exclosures compared to the other herbicide treatments, as well as non-significantly higher C:N ratios. Perhaps detritivory by macroarthropods is responsible for this effect. Songbird exclusion may have allowed unmediated litter foraging by arthropods, diminishing relative herbicide effects on litter accumulation. Since O-horizon bulk densities and depths were not significantly different among the treatments for songbird exclosures, the increased C in mineral soils in the intensive treatment is likely the result of increased root decomposition, dissolved organic matter leaching, or translocation of particulate organic matter. The latter two possibilities may have been amplified if the relative proportion of litter fragmented by arthropods was greater in the intensive treatment. Future analysis of arthropod abundance and stable isotope analyses will aid in elucidating these effects.