Study to assess sources of Douglas-fir site resilience to intensive harvesting

Field studies have demonstrated that many forest sites are able to withstand whole-tree harvesting with little detectable effect on productivity. The Forest Service North American long-term soil productivity (LTSP) network, for example, found little overall effects of harvesting that included non-stem tree components on tree growth over the course of 10 years. However, some sites, particularly in boreal regions, have been shown to be more sensitive to intensive harvesting due to nutrient removals.

One hypothesis for site resilience to intensive harvesting is the accelerated input and decay of fine roots, leading to increased soil organic matter and microbial nutrient mineralization which may compensate for organic matter and nutrient removals during harvesting. In addition to direct root inputs, higher soil moisture and temperature in harvested stands may increase their decay and nutrient release.

NCASI is providing support for a study led by Dr. Jeff Hatten from the Department of Forest Engineering, Resources and Management at Oregon State University to assess the potential sensitivity of forested sites to whole-tree harvesting. The study is part of a larger effort supported by Weyerhaeuser Company and a large grant from the Northwest Advanced Renewables Alliance (NARA) to analyze effects of three intensities of harvest removal (bole-only, whole-tree, and whole-tree + forest floor) and two levels of compaction on the quantity and characteristics of soil organic matter using chemical biomarkers and stable isotopic techniques. The study will include measurements of density fractions and lignin, cutin/suberin, fatty acid biomarkers in pre- and post-harvest soil samples to determine relative contributions of needles and harvest residue, roots, and soil microorganisms to soil organic matter fractions. Data will be linked to characteristics of sites across the Pacific Northwest.


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