There is an increasing demand for structural and compositional diversification of largely mono-specific Douglas-fir dominated forestlands in the Pacific Northwest. Similarly, many restoration efforts aim to broaden the spectrum of planted species to include many native trees which no longer dominate the landscape. Thus, achieving greater stand and landscape level diversity within relatively short time spans will require a thorough ecophysiological knowledgebase for a larger suite of native tree species. The proposed presentation features two studies evaluating the effect of available light, rhizosphere moisture, and the synergy between these two key factors on the survival and development of nine native tree species during the three critical years following transplant. Seedlings were planted into five characteristic stands across western Washington, each containing three geographically distinct but proximal plots replicating “open,” “partially shaded,” and “fully shaded” light regimes, respectively. A factorial outplanting simulation was established at a ‘control’ site using the same planting stock as the field sites and featured comparable [fixed] light and moisture levels. The in- and ex-situ trials were established – in parallel – to validate general findings and to reduce experimental confounding typically associated with stock quality.
Results/Conclusions
The trials were completed in 2017 and have yielded nuanced results for each species and corresponding environmental scenario. Across all species, mortality/survival was by in large driven by moisture availability during the first year and light was the significant (P<0.05) driver of above- and belowground seedling morphology. Partial shade conditions resulted in significantly (P<0.05) better survival and growth for many species facing extreme conditions of drought, browse, and other challenging edaphic conditions compared to the control. Our findings also challenge anecdote and insufficient past evidence suggesting fixed shade and drought tolerance thresholds for many species. This is likely a function of cultivation, but increasingly important for addressing planting practices for myriad revegetation efforts in current and predicted future climate conditions.