Site-level estimates of Douglas-fir foliage retention from climate, soil, and topographic variables

Abstract

Cohort-based live foliage retention of Douglas-fir (Pseudotsuga menziesii), repeatedly shown to be directly related to tree growth, was estimated for areas in western Oregon and Washington by correlating nearly 1000 plot-level estimates of foliage retention with variables of climate, soils, and topography. Sampling of Douglas-fir foliage retention, motivated by the Swiss needle cast-induced loss of foliage (causal agent Nothophaeocryptopus gaeumanii) in western Oregon and Washington, covered the period from 1998 to 2021, with estimates collected from the Oregon Coast Range near the California border to the Olympic Peninsula and the foothills of the northern Oregon Cascades and the foothills near the border of Washington and Canada. Douglas-fir foliage retention was found to be correlated with average June/July relative humidity, December maximum temperature, and May precipitation averaged for the periods of budbreak for the 2–4 yr old needle cohorts, a transformation of slope and aspect, the number of meters above 400 m elevation, topographic position index, and soil cation exchange capacity. Using the constructed equation, foliage retention was estimated across a systematic grid of points in western Oregon and Washington and combined with a regional growth model to estimate potential volume growth loss across the region. The greatest implied average growth losses (∼23%) were in the zones closest to the Pacific Ocean between 46° and 48°N. Premature loss of Douglas-fir foliage in the Pacific Northwest has been primarily associated with Swiss needle cast, and this work may help explain conditions that favor Swiss needle cast or describe additional foliage retention drivers.