Forest Floor Carbon Holds On After Logging, Long Study Finds

Forests are among the planet's biggest carbon vaults, and most of that wealth sits out of sight. Forest soils hold an estimated 40% of the world's terrestrial carbon, yet harvesting a stand can release roughly fifteen to thirty percent of the carbon stored in its soil. A new 20-year experiment in British Columbia was built to find out which logging practices keep forest floor carbon buried. The result, reported in the Journal of Geophysical Research: Biogeosciences, points to a thin layer most logging plans barely mention: the forest floor, the mat of needles, leaves and decaying wood lying on top of the mineral soil.

That floor is easy to overlook because the timber industry measures success in trunks. But soil scientists have long suspected that what happens to the ground layer decides whether a harvested forest keeps acting as a carbon sink or starts leaking the forest floor carbon it spent decades storing. The British Columbia Long-Term Soil Productivity study gave researchers a setup that rarely exists in forestry science to test the question directly. Twelve sites spread across four biogeoclimatic zones were each split into nine plots testing different combinations of soil compaction and organic matter removal. On some plots, crews took only the tree trunks, a practice called stem-only harvesting that leaves branches and treetops where they fall. On others, standard whole-tree harvesting carried the trunks and the canopy residue off the site. On the most extreme plots, the entire forest floor was stripped down to bare mineral soil. Researchers collected forest floor and mineral soil samples before the harvest and returned for three more rounds of sampling across the two decades that followed, according to Eos, which covered the study.

What the plots showed after two decades

Where the forest floor stayed intact, the carbon books stayed nearly balanced. Forest floor carbon slipped by only 5% over the full study period, while carbon in the mineral soil underneath actually rose by 14%. The way the trees came out mattered too: stem-only harvesting preserved roughly 15% more forest floor carbon than whole-tree harvesting, because the branches and needles left behind kept feeding the floor they fell on. The design also varied soil compaction, which let the team separate the damage done by heavy equipment from the damage done by residue removal. In the paper, the authors suggest that keeping organic residue on site and shielding the floor from heavy disturbance is the simplest way to hold carbon through a timber harvest.

The losses piled up fast where both the harvest residue and the forest floor were removed. Total soil carbon dropped 52% within five years of the harvest and still sat 33% below preharvest levels at the end of the study. Sites that lost their forest floor faced recovery times estimated at 22–86 years before soil carbon returned to preharvest levels. The pattern across the plots was consistent: treatments that kept the floor kept the carbon, and treatments that removed it paid for the difference over decades.

Why the floor matters beyond one province

Pinning down logging's carbon cost has never been simple. Wildfire, insects, disease and a forest's own management history all shift soil carbon stores, and tree species, climate and harvest method change how much is lost at each cut. That noise is what makes a controlled comparison across a dozen sites over two decades unusual: instead of estimating losses from models, the team measured the same ground before and after, across zones with different latitudes and climates. The study was designed to track exactly those variables, including how soil carbon and the forest floor recover after heavy harvesting.

The practical message lands on the people who plan harvests. Whole-tree harvesting is standard practice in many operations because it leaves a clean site, but this study shows that clean comes with a carbon bill measured in decades. Forest managers weighing climate targets against timber output now have measured numbers to work with: leave the residue and the floor alone, and the soil keeps most of its forest floor carbon; strip them, and the ground needs decades to recover what a single harvest took away.

The study, "Forest Floor Protection: The Critical Defense Against Post-Harvest Soil Carbon Loss" by Brian M. Wallace and colleagues, appears in the Journal of Geophysical Research: Biogeosciences (2026, DOI: 10.1029/2026JG009694). Eos, the American Geophysical Union's news site, covered the findings on October 5.