Case Studies of Red Alder and Sitka Alder in Douglas-fir Plantations - Nitrogen Fixation and Ecosystem Production

This PhD thesis investigates the influence of nitrogen-fixing alders on the productivity and nutrient dynamics of Douglas-fir plantations in the Pacific Northwest. Through seven case studies, the author compares the effects of red alder and Sitka alder, demonstrating that these species significantly enhance ecosystem net primary production and soil fertility on nutrient-poor sites. A critical finding is that the benefits of alder are site-dependent; while they accelerate tree growth on infertile sites, they can actually suppress conifer development and reduce biomass on fertile sites due to intense competition. This research positions Sitka alder as a low-competition alternative to red alder, suggesting both species are valuable biological tools for sustainable forest management on nitrogen-deficient land.