Assessing the effects of fertilization on Douglas-fir growth responses and edaphic variables in the Pacific Northwest
Zhang, X . 2025.
Abstract
Douglas-fir (Pseudotsuga menziesii var. menziesii.) is an important commercial tree species
in the coastal Pacific Northwest (PNW), and its biomass is also regarded as a significant
forest carbon (C) sink. Productivity of Douglas-fir varies regionally due to nutrient
availability (especially nitrogen). Nitrogen (N) fertilization can improve Douglas-fir
productivity, but knowledge gaps remain in the mechanisms affecting growth response to
fertilization, as well as in the effects of this management practice to the soil nutrient condition
itself in the long term due to the complex interactions between environmental variables. This
study leveraged a paired-tree fertilization study to examine the growth response of Douglas-
fir to soil nutrient availability and fertilization, plus the effects of the fertilization on the
edaphic variables ten to thirteen years after the N input. The objectives of this study were to
1) Predict Douglas-fir growth responses after N fertilization in the PNW; 2) Examine changes
in soil nutrient availability ten to thirteen years after fertilization. In this study, N fertilization
was applied across 15 sampled Douglas-fir installations throughout coastal Oregon and
Washington State, and basal area and ring area responses to the fertilization were analyzed
along with the initial and current soil macronutrient levels measured with Probes. Results of
this study show that 1) multiple factors affect the long-term response of Douglas-fir growth to
one-time N fertilization; 2) the levels of soil cation availability and surface C:N ratio affect
response extent; 3) the level of initial N content determines the responding patterns; 4) N
fertilization has limited long-term effect on soil nutrient availability; 5) stand-level
understanding of edaphic and geoclimatic conditions should contribute to better outcomes of
N fertilization in the PNW. Our findings provide critical references for accurately assessing
the long-term effects of N fertilization on Douglas-fir productivity and the ecosystem services
of plantation forests such as carbon sequestration.
