Effects of Geoclimatic Factors on Soil Water, Nitrogen, and Foliar Properties of Douglas-Fir Plantations in the Pacific Northwest

摘要

Soil water and nitrogen (N) availability drive Douglas-fir (Pseudotsuga menziesii [Mirb.] Franco) growth and productivity in the Pacific Northwest, but the methods for accurate assessment and modeling of soil water and N availability are not well developed. The objectives of this study were to identify geoclimatic factors that effectively predict soil water content, N availability, and foliar properties and correlate these factors with productivity of 71 Douglas-fir plantations in Oregon, Washington, and British Columbia. Boosted regression tree models were used to compare the performance of both mapped and field-measured variables, as well as various combinations of variables from these two general sources, while allowing for nonlinear relationships. Linear relationships between predictor and site productivity variables were tested using Pearson correlations. Fine, deep soils had the highest summer volumetric soil water content and contained high total soil N. Forest floor carbonitrogen (C/N) ratios were highly related to plant-available N, presumably due to climatic limitations on decomposition. Foliar N concentration was associated with plant-available N variables, whereas needle area was related to factors describing soil water-holding capacity. Douglas-fir site productivity and fertilizer response were related to the measured soil and foliar variables but were most strongly related to surface soil and forest floor C/N ratios.