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Biomass Carbon Accounting and Harvests

A recent article by Katherine Tweed in Scientific American, “Cleaner than Coal? Wood Power Makes a Comeback,” rehashes yet again the controversy over exporting wood pellets to electricity-generating utilities in the UK and EU. Instead of bringing the light of science into the discussion, however, Tweed treats facts as if they are opinion and glosses over the most important issue: how do we practice carbon accounting in a comprehensive way, taking both the positive and negative (and not just the negative) effects into account?

The following paragraph falls into the former category—fact masquerading as opinion: Power plants that are switching to pellets counter [emphasis added] that higher-value lumber is being harvested anyway. Wood residues, which go into pellets, have a far lower value than lumber so the plants argue [emphasis added] that because they are only buying the low-value product, their market power is limited and it is the higher-value products that drive decisions on how forests are logged.”

It is surprising to me to see facts regarded as opinion in Scientific America of all places. The fact that harvest decisions are determined not by demand for logging residues and unmerchantable timber but by demand for sawtimber is not just a counter-argument; it is a verifiable, economic fact. Sawtimber-sized trees are harvested to make lumber, a durable good used to construct homes and businesses that last for decades. Because of the long-term value of the end product, sawtimber-sized trees are among the highest value forest products.

As a result, harvest timing decisions are made in accordance with the growth cycle of sawtimber. While it varies by species, in the South pine is on a 25-year rotation and hardwood is on a 50-year rotation. Pulpwood and biomass are by-products of these every 25-year or every 50-year events.

Falling into the second category—the failure to add anything substantive or scientific to the very serious work of developing a fair and comprehensive carbon accounting system—is the lack of credibility of one of the experts that Tweed chooses to include in the article. Nathanael Greene (director of renewable energy policy at the Natural Resources Defense Council (NRDC), for instance, freely admits on the NRDC blog that he has “just enough science, engineering, and economics to be dangerous.”

The following paragraph demonstrates just how dangerous: “If some pellets for Drax are not coming from residues , but from cutting whole trees as . . . NRDC asserts is already happening, the carbon payback can look worse compared with burning coal because more mature trees absorb more carbon than smaller trees do. If hardwood is harvested only for pellets, the carbon payback will take at least decades and possibly more than 100 years, according to some studies.”

I have two issues with this statement. First, it assumes that sawtimber is being used to manufacture pellets. It is not. The economics just don’t make sense. Why would a landowner sell to a pellet mill if he or she could get two to three times the price from a sawmill? My second issue is with the lack of understanding of growth rates in the South (the region from which most of Drax’s supply will come). The truth is that if whole trees are being used they are pulpwood-sized trees. Since pulpwood is a by-product of a once every 25-year pine harvest or a once every 50-year hardwood harvest, it should be noted the age of that pulpwood is roughly 15 years for pine and 25 years for hardwood, nowhere near the “more than 100 years” referenced in the article.

A second expert quoted by Tweed is Thomas Buchholz, a senior scientist at Spatial Informatics Group (one of the contributors to the Manomet study, which has been widely called into question). The particular source referenced by Tweed is a study that Buchholz and Spatial Informatics were paid by the Southern Environmental Law Center (SELC) to conduct. While Buchholz’s scientific credentials are strong, his perspective—at least inasmuch as it is outlined in the article—is neither fair nor comprehensive.

Here’s the section of the article: “Studies have found that burning wood over fossil fuels produces about 65 to 95 percent less greenhouse gas emissions. Those studies, however, do not take into account the change in forest carbon stocks,” according to Buchholz. “When researchers start adding in the change to the forest carbon stock, the picture changes, which worries environmentalists. Although trees can be replanted, whereas coal is a finite resource in the short term, there is debate as to how much carbon smaller trees take up compared with mature trees. Based on a study Buchholz did for the SELC, if Europe imports southeastern U.S. wood instead of burning coal, atmospheric greenhouse gases could add up to 300 percent in the first 50 years, although they would drop below fossil fuel levels eventually.”

Here are my issues with this section of the article. First, I need a lot more information in order to determine the accuracy of these statements. Is Buchholz counting only the carbon sequestered in the actual trees that are going into the pellets (not the sawtimber-sized trees that are going to sawmills and the pulpwood, bark and sawmill residues going to pulp or paper mills)? If he is doing a carbon accounting for wood pellets used to generate electricity, he should be. Does it really seem logical to include the carbon in a tree that was used to make lumber in the carbon accounting of electricity generated in Europe from a U.S.-made wood pellet?

I suspect this is exactly what he is doing, however.  While this doesn’t seem fair or logical, if he is doing so, is he at least taking into consideration the fact that many of the hundreds of products that are made from trees (lumber, OSB, plywood, paper, packaging and furniture, to name a few) continue to sequester carbon for years and even decades?

My issue with the carbon accounting schemes developed by environmentalists is that they generally only want to take into account the negatives. When a large tree is harvested, let’s not pretend that 100 percent of the carbon it sequesters is immediately released into the atmosphere. A large chunk continues to be sequestered for the life of the building products made from the tree. When a small tree is harvested and used to produce paper and packaging, some portion of the carbon in that tree continues to be sequestered for the life of those recyclable products. Just because, as Thomas Buchholz points out, the “forest carbon stock” has changed, it does not mean that the carbon has been released into the atmosphere. It is a scientific fact that some large portion of it is just stored somewhere else.

And to the observation that “there is debate as to how much carbon smaller trees take up compared with mature trees,” I would say it is pretty obvious that big trees sequester more carbon than small trees. However, the science I have seen concerning the rate at which carbon is sequestered by trees suggests smaller trees appear to be more efficient at doing so. As trees mature into older age classes, they grow more slowly, and their sequestration rate slows. A small tree in a well-managed forest will grow faster and sequester carbon at a faster rate.

It is issues such as these that I wish Scientific American would focus on; the publication is certainly capable of looking at the real science behind these very important issues and helping us resolve the controversy instead of throwing more fuel on the fire. Environmental concerns are certainly relevant to the discussion, but let’s make sure we acknowledge that facts are facts, not opinion, and that any carbon accounting discussions focus on the right raw materials and include the positives as well as the negatives. To do otherwise will lead us to make decisions and to craft policy based on an imbalanced ledger.