The Effects of Biomass Removal and Vegetation Control on Douglas-fir Foliar Nitrogen and Phosphorus in the Pacific Northwest, USA.
Root, Amelia M.
MetadataShow full item record
As the demand for forest products increases, there is concern about the long-term impacts on site productivity. This study examines the foliar nitrogen (N) and phosphorus (P) of Douglas-fir (Pseudotsuga menziesii) trees in three sites – Fall River, Matlock, and Molalla – in the Pacific Northwest to determine the impacts of competing vegetation control and increased biomass removal. Needles samples were collected from randomly selected Douglas-fir trees in the winter of 2016. We analyzed the samples for nitrogen and phosphorus. Standard mixed-model ANOVA tests were run on the final linear models, followed by Tukey’s multiple comparison test to determine whether the treatments had an impact on the foliar nutrients. The impacts of increased biomass removal and vegetation control on foliar N and the N:P ratio were only significant in Matlock, the least productive site. There was no impact of increased biomass removal and vegetation control on foliar P within any of the sites. However, for each foliar nutrient, site was the most significant factor, indicating that the determination of the impact of each treatment must be site specific. Foliar nutrients were correlated with soil nutrient pools. The sites with larger soil N and P pools had higher foliar N and P concentrations, respectively. Fall River was the most productive site, and had the highest foliar N and P concentrations. Molalla had a smaller soil P concentrations, and had a slight foliar P deficiency (although not below critical deficiency levels). Matlock had a smaller N pool, and had the lowest foliar N values (although not below critical deficiency levels). Because the foliar N and P concentrations were not below critical deficiency levels at any of the sites, there was no immediate concern about the impacts of increased biomass removal. However, smaller soil and foliar nutrient pools at Matlock and Molalla suggested that these sites should be monitored to assure that there are no long-term impacts on soil productivity following intensive biomass removal. Our analysis of foliar nutrients at Matlock was complicated by the presence of scotch broom (Cytisus scoparius), an aggressive invasive species. Scotch broom is the dominant woody competitor at Matlock and out-competed the Douglas-fir saplings. Scotch broom fixes N, which enables it to outcompete Douglas-fir on N limited sites. Our data showed that scotch broom cover had a significant impact on foliar N. The plots with increased biomass removal had significantly higher scotch broom cover. Increased scotch broom cover led to increased foliar N, and although higher foliar N typically indicates growth, increased foliar N was associated with lower diameter at base height (DBH). The negative correlation indicates that although the trees were getting more N, they were also struggling to compete against the scotch broom. The Douglas-fir trees could not grow tall enough to shade out the scotch broom. The data suggested that productivity and commercial viability of the Matlock site requires the removal of invasive scotch broom.
- Forestry