Science1 publisher2 min readPublished
Logging that stripped residue and forest floor left soils a third below preharvest carbon after 20 years
Researchers on British Columbia's long-term logging experiment found soil carbon fell 52% in five years on plots stripped of residue and forest floor. Twenty years later those plots were still 33% below preharvest levels, so the way a stand is cut stays in its soil carbon account for decades.
The Scientist · Science desk
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What happened
- The experiment split each of 12 British Columbia sites, spread over four biogeoclimatic zones, into nine plots that varied organic matter removal and soil compaction.
- Where the forest floor stayed intact, its carbon was down only 5% after 20 years, and carbon in the mineral soil beneath it had risen 14%.
- Taking only the trunks, known as stem-only harvesting, kept about 15% more forest floor carbon than whole-tree harvesting, where canopy residue leaves the site too.
- On sites where the whole forest floor was removed, estimates of the time for soil carbon to return to preharvest levels ranged from 22 to 86 years.
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Why it matters
- decision Operators choosing between stem-only and whole-tree harvesting now have a 20-year field measurement of what that choice costs in forest floor carbon on British Columbia sites.
- constraint Carbon inventories that apply one generic harvest-loss rate will misstate both kinds of plot, since stripped plots remain past the top of the 15-30% range at 20 years while intact-floor plots lost little.
- cost Where the floor is stripped, the soil carbon debt runs for decades, so a carbon account has to carry it across a recovery window whose length depends on the site.
Soil carbon after logging is hard to pin on any one cause. Wildfire, insects, disease and past management all change forest soil carbon stores, and tree type and climate change how much a harvest releases [3]. The BC-LTSP design deals with this by putting every treatment on the same ground. Nine plots on each of 12 sites makes 108 plots [14], and the plots on one site share its trees, weather and history. Each was sampled before harvest and again at 5, 10 and 20 years [6]. Every change is measured against preharvest levels [9].
The stripped plots give a rate as well as a loss. Between year 5 and year 20, total soil carbon on plots that lost both residue and forest floor regained 19 percentage points of its preharvest stock [15]. That is roughly 1.3 points a year [16]. Recovery estimates across those sites differ by nearly a factor of four [17]. The study set out to examine how latitude and climate affect soil carbon [4], and I would look there first for the spread.
On the plots that kept their forest floor, the two 20-year figures describe different pools: the organic layer on top and the mineral soil beneath it [7]. Netting them into one site total needs the stock sizes in the paper itself, published this year in the Journal of Geophysical Research: Biogeosciences by Wallace and colleagues [12]. The direction holds without the netting. With the floor left in place, mineral soil on logged ground gained carbon over two decades [7].
The thing this summary doesn't tell you is what compaction did. The experiment varied compaction alongside organic matter removal [5], and the study set out to examine both [4], but every figure in the Eos account comes from the removal treatments. On these sites, removing the forest floor clearly sets how much soil carbon is kept and how long it takes to come back [9] [10]. Whether compaction alone moves carbon by a comparable amount is a question for the full paper.
I think the residue result is the part a carbon ledger can use now. Forest soils hold an estimated 40% of the world's terrestrial carbon [1]. According to the Eos summary, the authors suggest that keeping organic residue on site and protecting the forest floor from heavy disturbance are key to preserving soil carbon during harvests [11]. These plots justify putting a harvest-method term into a stand's carbon account. That holds for 12 sites in one province, across four biogeoclimatic zones [5], with removal of the entire forest floor plus residue as the most severe case in the design [13] [9].
What to watch
- The compaction results in the full JGR Biogeosciences paper, which would show whether compacting the soil alone shifts carbon as much as removing the forest floor.
- Further sampling of the BC-LTSP plots past year 20, to test the 22-to-86-year recovery estimates against measured soil carbon.
- Whether forest carbon accounting protocols add a harvest-method or residue-retention term based on long-term plot data like these.