Increasing Carbon Sequestration and Decreasing Carbon Emissions
By Mitch Lansky
In 2015, 196 countries agreed to act to limit global warming. To meet their climate goals, just reducing emissions may not be enough. We also need to increase carbon sequestration.
While sequestration opportunities exist with farm and pasture soils, Maine, which is 82.5 percent forested, has even more opportunities with its forest land. Forests sequester the equivalent of 13 percent of carbon emissions nationally and 30 percent globally.
In Maine there has been more emphasis on substituting biomass for fossil fuels, based on the assumption that biomass is “carbon neutral,” than on forest practices that increase carbon sequestration.
Forests sequester carbon through photosynthesis. Most of the sequestered carbon is either on or under the soil surface; only 25 percent is in live trees aboveground. Forest soil gets carbon from detritus that rots and becomes part of topsoil, and from microbial interactions with tree roots. Mycorrhizal fungi are a major factor in bringing carbon to lower soil levels, according to new research. The fungi help tree roots get more water and nutrients; in return trees provide carbon for the fungi.
Big trees and old forests offer good habitat for these fungi and effectively hold and sequester carbon. Having a full range of species of plants and animals in the forest and a full range of successional stages ensures not only that sequestration functions; such diversity also ensures resistance to and resilience from disturbances that could slow or interrupt sequestration.
Threats to sequestration include deforestation and intensive management – the most extreme example being whole-tree clearcuts followed by soil-disturbing site preparation. Heavy cutting on short rotations compounds the problem and does not allow enough time for ecosystem recovery. New research shows that forest soil subjected to heavy cutting can lose carbon for decades.
Forest Dynamics and Ownership
Maine’s forest is considered part of the Acadian Forest that makes up much of the Canadian Maritimes. The key species tolerate some shade and are adapted to growing in and under canopies. Pre-settlement catastrophic disturbance cycles (major fire or wind) were 800 or more years apart.
Management that emulates such natural disturbance cycles would cultivate multi-aged, continuous-canopy forests with relatively small gaps. Maine’s forests, however, are managed instead for ownership objectives. According to federal surveys, 58 percent of forest ownership in Maine is “corporate”; 31 percent, family; 6.3 percent, state; 3 percent, other private and 1 percent, federal. Corporate owners’ primary goal tends to be economic return. Other landowner types may have other objectives and longer time horizons.
Level and Type of Cut
Due to the recession and mill cutbacks and closures, cut has been declining and growth increasing in Maine forests in the last few years. Average stocking was around 17.7 cords per acre in 2014, far less than one would find in a mature forest. The average volume per acre in Maine’s northern three counties in 2008 was about 14 cords; in southern Maine, about 22 cords. Carbon sequestration and storage can be improved greatly, especially in northern Maine.
Cutting statewide has been relatively heavy, averaging 15 cords per acre. The Maine Forest Service classifies three types of cutting: partial, shelterwood and clearcuts. In 2014 clearcuts made up only 5.5 percent of cuts, but shelterwood (an even-aged method that can lead to complete removal of the overstory, leaving only advanced regeneration) and heavy partial cuts are leaving significant portions of the landscape in seedlings and saplings. Seedlings and saplings sequester less carbon than well stocked forests with big trees.
Due to decreased cut in recent years as a result of mill closures, growth has exceeded cut statewide. Most Maine timberland, especially in northern Maine, is certified as “sustainable.” Yet from 1999 to 2012, private certified timberland owners were cutting more than growth, especially high-valued, long-lived species such as red spruce and sugar maple.
Sustainability is not just keeping cut less than growth. Managers also need to account for changes in species ratios, quality and stand structure – all important factors in sequestration. Forest products are also important for sequestration. Longer-lasting products, such as lumber (or cross-laminated timbers) or furniture, keep carbon from the atmosphere and may substitute for products with more embodied energy.
Regardless of what the markets are for the cut wood, carbon sequestration needs to increase now – by increasing stand volume and the number of bigger and older trees, and by decreasing stand damage.
Most cutting in Maine is by mechanical harvesters – most commonly feller bunchers and grapple skidders, which remove whole trees with tops and branches attached and drag bunches of these “whole trees” to the yard.
With whole-tree harvesting systems, trails and big yard areas take up about 25 percent of forestland. The system is not well adapted to lighter cutting, which would be more appropriate for the Acadian Forest type. A better approach is lower-impact logging, which has a smaller footprint on the forest, leaves behind more trees and aims to minimize damage to soil and residual trees.
Slash (tops and branches) may make up 19 percent of the above-ground tree but holds over half of the essential plant nutrients, is an important source of organic matter and helps protect forest soils. Loss of woody debris is especially problematic in stands with poor soils and frequent entries. Dead, woody debris, both large and small diameter, is important to retain in forests, regardless of markets.
Markets influence what and how much gets cut, and that influences what remains to sequester carbon. Depending on how the wood is used, markets also impact carbon emissions.
In Maine 50 percent of wood cut goes to pulp, 27 percent to lumber, 20 percent to biomass and 3 percent to pellets and firewood. This means that three-fourths of all cut wood in Maine is chipped and/or burned. Even a portion of sawlogs (edgings, slabs, shavings, sawdust) is chipped and/or burned.
Markets are changing. Some Maine pulp mills have closed and are closing due to international competition – even though global paper consumption has increased. Remaining mills have shifted from mostly softwood to mostly hardwood. Biomass, which competes with fossil fuels, is threatened by low natural gas and oil prices.
Biomass markets create incentives for heavier cutting – not just by removing slash (which happens anyway with whole-tree harvesting) but also by removing smaller diameter wood that might not be suitable for pulp.
Cutting small trees that could become sawlogs later is a carbon issue, because lumber sequesters carbon in buildings, while burning biomass instantly releases CO2 to the atmosphere.
Biomass burning in stand-alone electric power plants puts out more CO2 per unit of electricity produced than do fossil fuels, even coal. Most biomass power plants have low efficiency; some convert less than 25 percent of the energy in wood to electricity. The rest is considered “waste heat.”
The concept of “carbon neutral” requires “offsetting” biomass burning in time (the cut forest grows back) or place (forests elsewhere are growing). Regarding time, wood is burned in minutes but takes decades to grow back. Regarding place, where? The same stand, owner, county, state, region? If there were no biomass harvest, there would be more sequestration. Forests sequester CO2 whether it comes from biomass plants or fossil fuel plants.
Resequestering the “debt” of higher emissions from biomass than from fossil fuels takes time, while we must start to reduce emissions now, regardless of the energy source used. Burning wood for heat or combined heat and power is much more efficient than stand-alone electric plants, although still less efficient than burning natural gas for the same purposes.
Increased biomass use would mean more cutting, which would lead to less sequestering and higher carbon emissions for decades. Calling this “carbon neutral” does not change results on the ground – failure to meet either of the double bottom lines.
Using wood for pulp and paper, rather than for biomass, could create even more emissions, because the pulp and paper industry is one of the most energy intensive in the country. Most paper is used once then discarded.
Paper production and consumption are global. Losing paper mills and stopping cutting here while adding mills and increasing cutting elsewhere does not necessarily reduce global carbon emissions or increase global carbon sequestration.
So, should we burn fossil fuels rather than biomass or vice-versa? Here’s another approach: negawatts. Before building any new power plants, first cut waste and inefficiency in production, transmission and use. This approach creates bigger, faster energy and economic paybacks than building new plants. The benefits go to consumers, not producers. Any fuel used (from any source) should be used efficiently.
Rather than trying to intensify management to meet ever-growing, wasteful demands (as if the forest has no limits), demand reduction can reduce emissions and increase opportunities for sequestration. Only 2 percent of Maine’s forest is now in reserved (uncut) land. A lot of opportunity exists for improvement.
Time matters. We need to progress toward these double goals now. Strategies that decrease carbon sequestration and increase carbon emissions now, with the promise of improvements decades from now, are not the solution.
We can assume that future generations would want
• more sequestration and lower carbon emissions
• more protection of biodiversity and less forest simplification
• more forests well stocked with big trees suitable for higher-value products and fewer acres of seedlings and saplings.
Practices that promote these goals will give future generations options. If we are wrong, future generations can always take the big trees and chip them. They can’t, however, turn little trees into big ones, unless they wait for decades.
We need to make real, measurable progress, not just rhetorical progress, now. We need to focus on the double bottom line, to manage forests and energy as if the future mattered.
About the author: Mitch Lansky wrote “Beyond the Beauty Strip: Saving What’s Left of our Forests.” Recently he has been investigating carbon sequestration in forests. This article is based on his longer piece, “Double Bottom Line: Managing Maine’s Forests to Increase Carbon Sequestration and Decrease Carbon Emissions” (March 2016), posted at https://www.meepi.org/lif/ .