Interspecific interactions in mixed stands of paper birch (Betula papyrifera) and interior Douglas-fir (Pseudotsuga mensiezii, var. glauca)

Growing mixed conifer-broadleaf forests instead of monoculture coniferous forests could reduce problems with seedling regeneration, disease and volume loss, all of which are expected to increase with warmer climates and more frequent droughts. Understanding mixed forest dynamics, as well as their quickly evolving mycorrhizal symbionts, could reveal key management strategies for adapting to climate change. This study is a long-term analysis of two field experiments established in 1992 in the southern interior of British Columbia, Canada, where I sought to gain insight into the outcomes and mechanisms of interspecific interactions in mixtures of broadleaves and conifers. The broader experiment examined interactions within mixed stands of Douglas-fir and paper birch in an extensive response surface design, while a second experiment isolated rooting areas of individual trees within two density pairings embedded in the larger experiment. The treatments were replicated across three geographically distinct sites within the same BEC subzone (ICHmw). Twenty-one years after the experiments were established, I found strong evidence of reduced Armillaria root disease and increased foliar nutrition in interior Douglas-fir with increasing density of paper birch neighbours, but no negative effect of paper birch competition on interior Douglas-fir growth. This last result may be due in part to the comparatively weak status of the planted paper birch, which never overcame early poor performance. The mechanisms by which the struggling paper birch interacted with interior Douglas-fir were revealed in the trenching experiment, where ability to form mycorrhizal networks resulted in cumulative benefits to paper birch over time, with significantly less growth loss in untrenched than trenched treatments. This benefit was consistent across densities and regardless of climatic stress, pointing to a pattern of constant benefits of belowground interactions for subordinate tree species. This finding points towards belowground interactions as a medium for balancing species inequalities and, by extension, of maintaining ecosystem diversity and stability. Taken as a whole, these results illustrate the possible benefits of maintaining broadleaves in commercially valuable conifer plantations, both in terms of direct health benefits to conifers, and in the broader sense of providing negative feedback mechanisms to species loss and ecosystem instability.Forestry, Faculty ofGraduat