The Effects of Biogeoclimatic Properties on Water and Nitrogen Availability and Douglas-Fir Growth and Fertilizer Response in the Pacific Northwest

Abstract

Coastal Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) is the predominant plantation species of the western Pacific Northwest, although soil and climate characteristics vary throughout the region. This study investigated the effects of water and nitrogen availability on Douglas-fir growth throughout the region. Sixty Douglas-fir plots were established from Vancouver Island, BC to southern Oregon. Biogeoclimatic properties and fertilizer response were investigated according to soil nutrient regimes (SNRs) and soil parent materials alone (SPMs) and split by state and province (RSPMs). The effects of climate, site, and soil properties were investigated on soil water, soil nitrogen (N), and foliar variables using boosted regression trees and generalized additive models. Soil water availability was affected by soil depth and texture and influenced total soil N content and needle area. Sedimentary SPMs and very rich SNRs were identified as the most productive due to fine textured soils with greater water and N availabilities, which resulted in greater site index values and low fertilizer response. Glacial SPMs and medium SNRs had the lowest soil productivity due to coarse-textured soils with low water and N contents. Height:DBH ratio was greatest on stands with glacial SPMs yielding greater than expected site index values. Low spring temperatures and low water availability led to high forest floor C:N ratios, which then led to lower foliar nitrogen concentrations. Forest floor C:N ratios and fertilizer response were higher on igneous SPMs and British Columbia glacial RSPMs. These studies highlighted the effects of soil water and N availability on Douglas-fir growth and fertilizer response throughout the region.