Climate projections suggest increased droughts for much of the Pacific Northwest region in the near-future, which is expected to lead to tree growth decline and forest die-offs. Forest thinning is often proposed to reduce competition and mitigate drought stress, but that effect has yet to be tested in long-term experiments. To test these effects, we examined tree- and stand-level sensitivity to climate in Douglas-fir forests. To this end, we relied on three watersheds with differing styles of forest management at the HJ Andrews Experimental Forest near Blue River, Oregon. We asked to what degree has management affected tree sensitivity to climate variability and to what extent can that effect be predicted from forest characteristics. Specifically, we sought to answer two questions: First, what are the effects of forest thinning and clear cuts on the climate sensitivity of Douglas-fir trees and how is that effect related to differences in structure and function relative to old-growth forests? Second, how is tree-level variation in growth related to stand-level climate sensitivity under different management regimes? To answer these questions we developed basal area increment (BAI) records from 45 dominant trees combined with streamflow data and remotely sensed canopy productivity (NDVI) and water use (NDWI).
Results/Conclusions
Preliminary results show that the average BAI of young Douglas-fir trees within managed stands was markedly higher than the BAI observed during early growth (trees of the same age) in old-growth stands. However, growth rates converged across managed and unmanaged watersheds after trees reached canopy dominance. Notably, a marked growth decline beginning after an unusually dry year, 2010, for trees in managed stands, a trend not displayed by trees in old-growth stands. Based on correlation coefficients generated for the relationship of climate and hydrologic variables, and standardized tree-ring widths, we conclude that old-growth stands buffered the effects of climate variability on tree growth. We hypothesize that the decline in managed watersheds has been induced by competition for water and productivity of the entire watershed. To test this hypothesis, we will analyze the relationship between evapotranspiration, NDVI, and NDWI as well as soil data (27 soil profiles; 9 per watershed; 0-60 cm depth) to differentiate water stress from other factors (e.g. progressive N limitation). Our ultimate goal is to link this research to metrics of drought resistance and carbon-water budgets to better understand how interactions between climate and forest management affects the overall health, productivity, and water yields of PNW forests.