Forest soil carbon storage in 10-year-old Douglas-fir plantations of western Oregon and Washington remains similar to pre-harvest

Abstract

Forests around the world, and in the case of this study, the coastal Pacific Northwest United States, store large amounts of carbon, both above ground in the trees and below ground in soils. Understanding the effects of forest disturbance, including timber harvesting, is important in order to evaluate the role that forestry plays in the global carbon cycle. Soil carbon can be difficult to assess with enough precision to detect the kinds of changes that are expected, yet a series of small changes over time in the same direction could have important cumulative effects. In this study, eight randomly selected Douglas-fir forest stands in Oregon and Washington were sampled at 300 points each using a fixed-depth sampling approach to attempt to detect a 5% or higher change in soil carbon storage to 1 m, longitudinally from pre-harvest to 10 years post-harvest. There was moderate variability in results over time at individual sites, with some sites decreasing slightly and others increasing slightly. Only two sites achieved lower than the 5% minimum detectible difference target. The remaining six sites were able to detect 5.7%–10.7% differences. In one case, an unexpectedly large increase in mineral soil carbon 10 years post-harvest occurred without clear explanation. On average, forest floor carbon stores were 20% larger 10 years post-harvest than pre-harvest. Even with the large increases excluded, both the fixed-depth approach and equivalent soil mass correction showed there was no significant change in mineral soil carbon stores to 1 m at 10 years post-harvest in the region.