Vermont uses wood for a large portion of its heating and electricity needs, and a host of new policies and proposals under development could increase wood-burning significantly. While Vermont already has the highest percentage of homes heating with wood in the nation, and the negative health impacts to show for it, policymakers want more. Vermont’s 2022 Comprehensive Energy Plan sets a goal of meeting 35% of the state’s thermal energy needs from wood by 2030, up from current levels of about 24%. Vermont’s new Clean Heat Standard could fuel this growth by promoting “advanced wood heating” and allowing wood-burning district heating facilities to qualify as “eligible clean heat measures,” a provision apparently intended to support the proposed expansion of the 50 MW McNeil biomass power plant in Burlington.
With forests covering nearly three quarters of the Green Mountain State, the Energy Plan states that “From a wood energy perspective, Vermont has sufficient material available to keep adding to its existing portfolio of wood energy systems without risking forest health or sustainability.” We asked the question, what might more biomass logging mean for Vermont’s forests and their ability to fight climate change?
Vermont’s forests still hold a lot of carbon. However, according to Vermont’s Greenhouse Gas Emissions Inventory, Vermont’s forest carbon “sink” – the amount of CO2 taken out of the atmosphere each year by forests – has declined 32% since 1990 (Figure 1).
The sink is estimated as the difference between forest carbon stocks year-to-year (stocks are estimated by repeat measurements at permanent forest plots). Net forest uptake of CO2 from the atmosphere is reported as a negative number. In Figure 1, the upward trend shows that Vermont’s forests are taking up less CO2 than they used to. This is bad news for the climate, because if forests are taking up less atmospheric CO2, more CO2 remains in the atmosphere, contributing to global warming.
Vermont already burns a lot of wood for both heat and electricity and has the emissions to show for it. In conformance with UNFCCC annual GHG reporting guidelines, CO2 emissions from wood-burning are accounted for as changes in the forest carbon sink, not as direct emissions. However, the direct emissions are still reported separately. According to EPA’s national greenhouse gas inventory, wood-burning in Vermont emitted 2.21 million tonnes of CO2 in 2020. If these were accounted for as direct emissions, they would represent a 21% increase over the 10.2 million tonnes of CO2e that Vermont reported as emitting that year.
New incentives for biomass energy could drive more logging, further increasing CO2 emissions and decreasing forest carbon stocks. What might this mean for Vermont’s climate mitigation efforts?
Exploring what’s driving the current decline in the forest sink can help predict how forests would respond to more logging. The sink’s decline is likely linked to multiple factors, potentially including land use change; (according to the Vermont Agency of Natural Resources (ANR), Vermont loses 2,000 to 4,000 acres of its approximately 4.5 million acres of forest per year); commercial logging; decreases in the carbon stored in harvested wood products; and changes in storm frequency, pests, temperature, or water availability that alter tree, deadwood, and soil carbon stocks. The overview from the Vermont ANR suggests that the decline is likely due to “similarly aged and aging trees” and that “While older forests store much more carbon than younger trees, they sequester carbon at a slower rate.”
Vermont’s GHG inventory data on “Forest Carbon Flux” (“forests remaining forests”, excluding land converted to forests) breaks out the sink by carbon pool. The data show a somewhat surprising trend: the biggest decline in Vermont’s forest carbon sink occurs not in aboveground biomass, as might be expected if land use change, logging, or forest aging were the main drivers, but instead in the categories of mineral soils, wood products (including wood carbon stored in landfills), and deadwood. Together, these categories account for 87% of the decline in Vermont’s carbon sink from 1990 to 2020.
Categories and percent changes are:
- Mineral soil: -84% since 1990, 37% of total decline
- Wood products/landfill: -62% since 1990, 33% of total
- Deadwood: -36% since 1990, 17% of total
- Aboveground biomass: -6% since 1990, 9% of total
- Litter (leaves, needles, twigs): -41% since 1990, 2% of total
- Belowground biomass (roots of live biomass > 2 mm diameter): -6% since 1990, 2% of total
Importantly, while the decline in the aboveground biomass sink is not the largest driver, representing 9% of the total decline, forest management may still be a factor in driving the decline in Vermont’s forest carbon sink, as explained below.
Mineral Soil (37% of the decline)
Vermont’s ANR points out that “Soils store more than half of the carbon in the forest: 946 MMt (million metric tonnes) CO2e compared to 796 MMt CO2e for the four other pools combined.” This means that relatively small percent changes in soil carbon accumulation can result in relatively large changes in the absolute size of the carbon sink. Declines in forest mineral soil carbon accumulation may be due to global warming. For example, an artificial soil warming experiment found that warming soil increased CO2 emissions by a third. Forest soil carbon may also be lost via leaching, where dissolved carbon filters out of the forest in water runoff, and into nearby rivers. As Vermont has gotten both warmer and wetter in the last fifty years, both of these factors may have contributed to the decline in mineral soil carbon accumulation.
It’s important to consider how increased forest logging, particularly for woody biomass, can make soils warmer (via increased light penetration) and wetter (via reduced tree transpiration). Biomass logging tends to strip more wood and carbon out of forests because wood quality is not as much of a consideration for biomass fuel as for other products. The extensive removal of so-called low-quality trees and logging residues means soils can be left far more exposed after biomass logging than under traditional logging that leaves more trees and residues onsite. Indeed, an analysis of multiple studies found that biomass logging depletes soil carbon more than conventional logging. The impact extends to nutrient removal, as well, which can potentially affect forest regeneration.
Wood products and landfills (33% of the decline)
Declines in the wood products and landfill category could be due to changes in economic demand, trends in recycling and waste disposal, or other causes. While Vermont’s Greenhouse Gas inventory reported on carbon accumulated in wood products and landfills, it wasn’t possible to explore the wood products and landfill data in more depth because the methods reported were unclear. Third-party data sources, e.g., the specialized timber products output (TPO) data from the US Forest Service, are not available for Vermont.
Deadwood in forests (17% of the decline)
The US Forest service estimates the deadwood pool using the volume of deadwood found in random plot samples. Declines in the deadwood carbon pool could be due to changes in accumulation rates, decay rates, or increased removal, since large-diameter deadwood can be collected as firewood or chipped and burned for biomass fuel. Deadwood accumulation changes with the species of trees present, their sizes and ages, and management. One study in European deciduous forests found that forest management and annual mean temperature were important drivers of deadwood decline in managed forests. As is the case for soil carbon, logging and thinning activities may increase surface temperatures, increasing decomposition rates. Field research studies may be required to identify the exact causes of deadwood decline in Vermont forests.
Aboveground biomass accumulation (9% of the decline)
Some of the decline in aboveground biomass can be attributed to forest conversion, which the Vermont ANR estimates occurs on 2,000 – 4,000 acres per year. However, many other factors could be contributing to this decline, including logging, pests, disease, and climate stress.
Litter in forests (2% of the decline)
The relatively small decline in the carbon sink provided by forest litter is likely linked to whatever is driving the larger declines in other carbon pools.
Belowground biomass accumulation (2% of the decline)
Belowground biomass accumulation is likewise linked to aboveground trends.
Vermont has extensive forests and prides itself on its sustainable forest management. However, Vermont’s forest carbon sink declined by 32% between 1990 and 2020. During that time period, as Figure 2 shows, CO2 emissions from wood burning residential heating and electricity have increased by more than 400%. Proponents of plans to expand the use of biomass heat and energy in Vermont claim that these proposals will help the state meet its climate goals, but keeping carbon in the forests, rather than the atmosphere, is vital for achieving climate goals. Vermont policymakers would do well to observe the present trends of biomass burning and the forest carbon sink in Vermont.
