Silviculture and Carbon in the Cloquet Woods
What are the connections between forests and atmospheric carbon? How do different silvicultural interventions affect forest-based carbon uptake and storage? On February 25, about 75 foresters, researchers, and others took to the Cloquet Forestry Center woods to find out.
The tour included visits to red pine, aspen, and mixed aspen-spruce stands with a variety of management histories, from benign neglect to a century of intensive management. At each stop, we discussed the stand history, rates of carbon sequestration and accumulation, and the impact of past and possible future silvicultural treatments.
Although the discussions took us much deeper into detail than is presented here, this post offers top-level take-home messages from the tour.
The tour was part of the February 2009
Forest Values and Carbon Markets conference.
Pure red pine: The student thinnings
This is one of the more impressive examples of the impact of active management in red pine that I know of. The stand originated naturally around 1910, so it’s about 100 years old. On one side of the road, the 40-acre stand has been thinned four times: in 1950, 1960, 1970, and 1985. On the other side of the road, the stand has never been thinned.
Focusing on carbon dynamics, both stands have sequestered approximately the same amount of carbon during the past 100 years. However, in the unthinned stand, almost 40% of that carbon has returned to the atmosphere, or is in process of doing so, through mortality and decomposition of dead wood. In the thinned stand, almost all of that natural mortality has been “captured” through thinning and turned into wood products.
In the early thinnings, nearly all of the harvested material went into pulp production. Pulp is a short-lived wood product, so much of this carbon would have returned to the atmosphere within 5-10 years. However, in the later thinnings, larger and larger proportions of the harvested wood went into long-lived wood products such as construction materials. Long-lived products store carbon on a nearly permanent basis.
Although not the focus of this tour, the four thinnings have also
dramatically increased the financial return. Including returns
from products sold (compounded at 5% annually) plus the value of
standing timber on the sites, the thinned stand has a total value of
over $6,000 per acre, many times that of the unthinned stand.
Comparing the two stands drove home the potential of active forest management to do three important things: 1) reduce atmospheric emissions of carbon through mortality and decomposition, 2) increase long-term carbon storage by increasing the proportion of harvestable wood products that are long-lived rather than short-lived, and 3) produce a dramatic financial return.
Aspen-spruce mixOur next stop was at a 22-year old mix of aspen and white spruce. The stand originated from a 1987 clearcut. Natural regeneration was almost pure aspen, and in the same year as the harvest, 800 white spruce seedlings per acre were planted on the site. Like most of the CFC soils, this is a low-productivity site for aspen, with a site index of only about 55 (meaning 50 year old aspen would be about 55 feet tall).
This stand has interesting silvicultural potential. Perhaps the most likely treatment would be to harvest the aspen when it becomes merchantable, likely around age 45-50, leaving the spruce intact. Depending on the pattern of harvest (e.g. uniform vs. patches), this would lead to some regeneration of aspen and more shade-tolerant conifers such as white spruce or balsam fir.
From a carbon storage perspective, this system would retain a relatively high level of carbon storage on site after the sale and harvest of the aspen. White spruce is relatively long-lived in Minnesota and could easily be managed on an 80-120 year rotation. The extended rotation, combined with the increase in growing space from the aspen removal, would also increase the growth rate of the spruce, producing larger trees and a higher proportion of long-lived wood products at the final harvest.
Based on research conducted in this area, on stands of similar age and composition, this stand is estimated to accumulate carbon at a rate of about 2.35 tonnes of CO2 equivalent per acre per year. (Accumulation is sequestration minus respiration.)
Mixed reserve stand
Just across University Road from the aspen-spruce stand is a reserved (unmanaged) mixture of aspen, birch, balsam fir, white spruce, and scattered other species. (This stand is in reserve status on the CFC management plan.)
This stand is very similar to the stand that was clearcut in 1987 to produce the mixed aspen-spruce stand described above.
Typical of older stands, this one is breaking up fairly rapidly. Dominant birch and aspen are nearing the end of their natural lives, particularly for northern Minnesota sandy sites. Decadent stands like this one have a number of important ecological benefits: they provide coarse woody debris for forage, den sites, and cover as well as a different kind of habitat from intensively managed stands.
From a carbon dynamics perspective however, stands like this one are less than optimal. Even before they fall, the dying trees begin to decay and emit carbon through the respiration activities of decomposition.
Based on research conducted in this area, on stands of similar age and composition, this stand is estimated to accumulate carbon at a rate of only about 0.4 tonnes of CO2 equivalent per acre per year. This is just over 1/4 the rate of the aspen-spruce mix across the road. The primary difference is the high rate of decomposition-related respiration in the reserve stand. (For more on this difference, click here.)
Young pure aspen
The final stand we visited was a young pure aspen stand. By this point of the tour, most of the big ideas were clear: This stand is accumulating carbon at a relatively fast rate, which is great. However, the likely silvicultural trajectory for pure aspen in this part of the world is a 40-55 year rotation followed by clearcut. This pattern, while creating important benefits for wildlife habitat and local production of renewable wood products, does not lead to a high level of long-term storage of atmospheric carbon.
This point is clarified by comparison with the mixed aspen-spruce stand described above. In that stand, after harvest of the merchantable aspen, a large standing stock of carbon remains in storage in the stand. The tradeoff, of course, is lower production of aspen, which is important to Minnesota’s wood products industry.
After visiting all of the stands, we visited the B4WARMED experiment. This experiment, led by Peter Reich with a number of other University of Minnesota collaborators, simulates the projected warming trend and will monitor impacts on native trees. The study uses a sophisticated system to carefully monitor and manipulate soil and above-ground temperature fluctuations. (Want to learn more about the project? Check out this KAXE interview with Rebecca Montgomery.)
Summing it up
The tour included several hours of discussion, in the woods, of practical issues associated with forest-based carbon accumulation, the role of silviculture, and related issues. We also discussed possible carbon credit payments and associated (and complex) issues like additionality, leakage, and carbon credit protocols. The take-home messages about carbon are as follows:
- Increasing stocking of a long-lived shade tolerant species can increase the stand’s potential for long-term carbon storage.
- Longer rotations, regardless of species, can increase long-term carbon storage as long as they don’t exceed the natural lifespan of the dominant species.
- Increasing the proportion of harvestable products that are long-lived (e.g. construction materials) as opposed to short-lived can increase long-term storage after harvest.
- Frequent thinning can capture mortality, turning trees that would otherwise die and decompose into products that can be harvested and sold, and at least some of which will contribute to long-term post-harvest storage.
Update: Minnesota Public Radio ran a story today called Northwoods hold an answer to slowing effects of climate change that covers similar ground.