Biogeochemical Cycling in Conifer Forests of the Pacific Northwest, USA: Long-term Interactions of Soil Nitrogen, Mineralogy, and Logging Disturbance

Nutrient limitation constrains tree growth in many managed forests. Nitrogen (N) is the most common limiting nutrient, but high N supply can shift limitation to other nutrients, particularly phosphorus (P) and base cations (Ca, Mg, K). As different soil minerals have different capacities to supply P and base cations, soil mineralogy is a major factor that determines likelihood of nutrient limitation. Disturbance via logging can also intensify nutrient limitation by removing the nutrients contained in biomass from the ecosystem and by accelerating rates of nutrient leaching. The influence of logging on nutrient depletion can vary by harvest type and frequency, though detection of harvest effects can be difficult due to their emergence over long time frames. We used the process-based biogeochemical model NutsFor to evaluate how soil N (low N vs. high N) and soil mineralogy (sedimentary vs. basaltic) interact to drive soil nutrient limitation in conifer forests of the Pacific Northwest, USA. Ongoing changes in forest management on private vs. public lands led us to further examine how interactions between harvest type (bole-only vs. whole-tree-harvest) and rotation length (40 yr vs. 80 yr) influence the sustainability of soil fertility. Our preliminary findings indicate forests growing on low N soil-maintained N-limited conditions under all logging regimes, although whole-tree harvest also led to K limitation on basaltic mineralogy. In naturally high N sites where N did not limit tree growth, model results suggested Ca limitation on sedimentary sites and K limitation in basalt sites. Logging led to greater reductions of tree-growth on high N sites than on low N sites, where nutrient losses in harvested biomass exceeded leaching losses. Surprisingly, longer 80 yr rotations depleted more nutrients than 40 yr rotations in many simulations, due to high biomass accrual in these forests. Overall, the findings suggest that historically productive high N sites may be particularly sensitive to nutrient depletion and reduced tree growth in as few as one or two rotations. These simulated interactions of site N status and mineralogy on nutrient limitation provide a set of testable predictions to guide monitoring and management aimed at sustaining long-term forest productivity across a range of soil biogeochemical conditions.