Changes in soil physical and chemical properties following organic matter removal and compaction: 20-year response of the aspen Lake-States Long Term Soil Productivity installations

Soil functions that control plant resource availability can be altered by management activities such as increased organic matter (OM) removal and soil compaction during forest harvesting. The Long Term Soil Productivity study was established to evaluate how these practices influence soil and site productivity using experimental treatments that span a range of forest types and soil conditions at sites across North America. Here we report on the effects of these treatments on soil properties after 20 years at three of the oldest sites in the study. The sites all are located in aspen (Populous tremuloides) forests of the Lake States region USA, and span a soil texture (silt loam, sand, and clay) and productivity gradient. Treatments were applied in a 3 × 3 factorial design that included three levels of OM removal (stem only harvest, SOH; whole tree harvest, WTH; and WTH plus forest floor removal, FFR) and three levels of soil compaction (no additional, intermediate, and high). After 20 years, effects of OM removal were primarily associated with the extreme FFR treatment, and generally limited to the lower productivity sand and clay texture sites. At the sand texture site with low initial pools of C and nutrients, FFR resulted in soil C and Ca reductions over the 20-year period, which may have caused large reductions in aspen growth that were previously observed at that site. Although treatment effects of SOH and WTH were limited, soil P tended to decrease at all sites during the study period, which may affect future productivity at these sites. Effects of soil compaction treatments were generally linear and only apparent at the silt loam and sand texture sites. At all sites, bulk density in the upper 10 cm had fully recovered from harvest- and treatment-induced increases after 20 years, but remained elevated and increased with increasing compaction at depths below 10 cm. Previous work indicates that soil compaction had neutral to positive effects on growth at the sand texture site, but strongly negative effects on growth at the silt loam texture site. These 20-year results demonstrate that the effect of OM removal and soil compaction on soil properties is site-specific, which generally aligns with concepts of soil quality and its influence on vegetation growth. Although the LTSP study has proved invaluable in clarifying these linkages across North America, there are some limitations with measurement protocols that limit the overall utility of the soil assessment. These limitations inhibit the development of soil-based indices to identify high risk sites and practices at odds with sustainable forest management.