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Filling the Forest Carbon Sink

Updated: Aug 31, 2018

by John Gunn

Continuation of business-as-usual forestry will likely lead to Maine’s forest sector being a net carbon sink for the foreseeable future – meaning that when you add up all the carbon stored in the forest and also consider all the greenhouse gas emissions associated with harvesting trees and making products such as paper, lumber, and energy – there continues to be more carbon going into the system than going out. But, we can do better! In findings recently published in the Oxford University Press Journal Forestry, shifting to forestry practices that less frequently harvest smaller amounts from each acre leads to 14-33% more carbon being stored over the next 100 years. This happens because trees can grow older and larger and store more carbon than typically happens under current practices. There is an added benefit that when trees are eventually harvested, more of them will go to make higher-value products like lumber that also stores more carbon for a longer time when they are used to make buildings. On the other hand, harvesting more from each acre more frequently leads to a 30-70% reduction in carbon storage compared to current practices.   

In this paper, we used a spreadsheet-based life-cycle assessment (LCA) tool developed by Hennigar et al. (2013) that tracks carbon through time in the forest and through all the “life” stages after harvest. This includes use in products like lumber, paper, energy, and disposal in landfills. In the tool, acres of forest are allocated to different forest types and different management regimes that they will be subject to for 300 years.

For this study, we looked at forests across the entire state of Maine and allocated acreage according to the current distribution of forest types and ages. We then looked at trends in harvest practices as reported by the Maine Forest Service and distributed the acres accordingly. About 4% of the forest was set aside from harvest activities. This becomes the “baseline”. For comparison, we modified the acreage allocation of harvest activities to see how forest management practices of different intensity (i.e., how much is volume is removed and how frequently) influences the amount of carbon staying within the system. The six scenarios we evaluated are listed below in the table. The scenarios include two where the amount of set asides was doubled and quadrupled.

Our most significant finding is that reducing the amount and frequency of harvest could ultimately keep more carbon out of the atmosphere than current practices. But what is also interesting to me is the large role that assumptions about “substitution” plays in the outcomes. We know that when lumber is ultimately produced from harvest activities that this material (and its carbon) can stay stored in buildings for long periods of time – and that by substituting wood for steel and concrete we see an added benefit of avoiding the very high greenhouse gas emissions associated with the production of those materials. The question is, how much of the lumber that is produced can truly be considered a substitute right now, or in the future? It’s certainly not 100%. And it isn’t 0%. So, it is somewhere in between, but knowing that number matters for the bottom line. The greater the percentage, the better managed forests look (from a carbon perspective) compared to unmanaged forest.

As with all modelling exercises, the assumptions matter. We do the best we can in justifying the assumptions that are made to try to answer the questions we are using the model to address, but it is never perfect. Models such as this are helpful tools to understand the big “levers” that could make the outcomes move one direction or another. As we gain new knowledge, we can improve these assumptions and see how the outcomes might change. For example, newly-published research by Ethan Belair and Mark Ducey at UNH shows that recent harvest practices in Maine are likely different than the baseline we developed using more coarse reporting by the Maine Forest Service. This is something I will likely be exploring in a future research project.

The forests of New England will continue to play a significant role in efforts to mitigate climate change. How we manage the forest, and how we use the wood that is harvested, will significantly influence the climate benefits we can see from this important ecosystem.

Please email me for a .pdf copy of this paper.

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