Five-year growth, biomass, and nitrogen pools of Douglas-fir following intensive forest management treatments

Abstract

In young Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco var. menziesii) stands, intensive forest management has resulted in variable responses in survival and growth and therefore stand biomass due to effects on microclimate and competition for limited resources. As seedling genetics and management practices develop over time, there is a need to re-evaluate how Douglas-fir survival and growth varies with intensive practices. Five-year Douglas-fir diameter at 15 cm (D15) and height growth, biomass, and nitrogen (N) pools after organic matter removals and soil compaction at a recently established site in central Oregon (NARA) were compared to five-year results from three Douglas-fir sites with organic matter removal and vegetation control treatments that were established 10–14 years earlier in central Washington (Fall River and Matlock) and northern Oregon (Molalla). The NARA site contained up to two times greater individual-tree foliar and branch biomass for the same stem D15 than the previously established sites, possibly due to a lower planting density, a warmer climate, and/or improved genetics/different seed sources. Prediction of stem biomass was similar at all sites using D152*height. Periodic D15 and height growth from 1 to 3 years and five-year Douglas-fir biomass was lower in the bole-only (BO) without compaction treatment at NARA compared to other treatment combinations. Greater periodic D15 and height growth after five years of annual vegetation control (Fall River, Matlock, and Molalla) resulted in larger Douglas-fir biomass compared to the initial vegetation control treatments. These results indicate the beneficial effects of vegetation control on early growth across a wide range of conditions. With one exception, whole-tree removal treatments did not affect five-year-old Douglas-fir biomass or N pools, indicating site resilience to increased OM removal across the region. A principal component analysis (PCA) clearly separated all four sites according to climate, soil temperature and moisture, N availability, D15 and height growth, and Douglas-fir and competing vegetation biomass. Within each site, the PCA showed that vegetation control was associated with the greatest differences in Douglas-fir and competing vegetation biomass. This supports the notion that site factors and vegetation control treatments are critically important in mitigating response to intensive management practices.