Forest soils are a major carbon reservoir, holding roughly 40% of the world's terrestrial carbon. But logging can disturb that store: earlier estimates suggest 15%–30% of soil carbon is lost when a forest is harvested. A 20-year experiment in British Columbia now shows how much depends on what happens to the forest floor.
Researchers used 12 long-term sites across four biogeoclimatic zones, each divided into plots with different levels of organic matter removal and soil compaction. In some plots only tree trunks were removed (stem-only harvesting); in others whole trees and canopy residue were taken; in the most severe treatment the entire organic forest floor was scraped away. Soil samples taken before harvest and again at 5, 10, and 20 years revealed clear differences.
Where the forest floor stayed intact, soil carbon remained stable: forest floor carbon declined by only 5% over 20 years, while mineral soil carbon increased by 14%. Stem-only harvesting preserved about 15% more forest floor carbon than whole-tree harvesting. In contrast, removing the forest floor caused total soil carbon to drop 52% within five years, and it was still 33% below preharvest levels after two decades.
Recovery times for the most disturbed plots were estimated at 22–86 years. The authors conclude that keeping organic residue on site and protecting the forest floor from heavy disturbance are key to limiting carbon losses during timber harvests.