Effects of Climate, Site, and Soil Characteristics On Soil Moisture In Douglas-Fir Plantations.

Douglas-fir grows on many different soil types with distinct climates over the coastal Pacific Northwest.   The objectives of this study were to determine the best predictors of soil water-holding capacity using climate, site, and soil properties for sixty Douglas-fir installations from northern Vancouver Island to southern Oregon.  Lowest soil moisture is defined as the lowest summer volumetric soil water concentration from soil moisture sensors measured at 50-cm on each installation in 2010.  A boosted regression tree (BRT) model was used to describe differences in lowest soil moisture according to a combination of mapped and measured data sources.  Results from the BRT model were spatially interpolated with inverse distance weighting over the 60 installations to identify regions with similar predictor values.  The boosted regression tree model explained 81% of the deviance in lowest soil moisture.  Soil water-holding capacity was highest in deeper soils with larger A horizons, higher clay contents, and lower sand contents.  March temperature was positively related to soil water-holding capacity.  High under-canopy growing degree days, which led to lower soil water-holding capacity, were related to climate, but were also higher on installations with sparse canopies.  Low, medium, and high model criteria polygons designated areas with significantly different lowest soil moisture.  Installations in British Columbia and northern Washington were found to have the lowest water-holding capacity, while installations in southern Washington and coastal Oregon contained the highest water-holding capacity.  These results demonstrate regions of the Pacific Northwest that may experience droughts in dry summers.