Forests

Forest management activities have the potential to sustain and enhance the role of the North American forest sector in mitigating rising GHG concentrations over the next century. Key opportunities include 1) avoided deforestation emissions, 2) carbon uptake with afforestation and management to enhance stock growth, and 3) harvest removals directed toward clean energy options, including using logging residues and waste wood as a substitute for fossil fuels and long-lived wood products to replace building materials such as cement and steel that are more carbon emissions intensive (Birdsey et al., 2006; Lemprière et al., 2013).

Slowing deforestation and targeting clearings toward lands with lower carbon density could reduce carbon emissions substantially (Lemprière et al., 2013). Reducing harvest intensity, lengthening harvest rotations, and increasing stand densities are additional leading options because they generally increase carbon stocks in the absence of severe disturbance (Creutzburg et al., 2017; D’Amato et al., 2011; Harmon and Marks 2002; Perez-Garcia et al., 2007; Taylor et al., 2008). McKinley et al. (2011) reported that a combination of longer harvest intervals, management to increase vegetation growth rates, and establishment of preserves may increase carbon uptake by 30 to 105 Tg C per year in the United States alone. Important to note, however, is that slowing deforestation and harvesting in one region may simply displace such activities (i.e., leakage) if unmatched by a change in the demand for associated land uses and forest products. Moreover, increased carbon stocks in areas prone to severe disturbance may not act as a lasting sink for atmospheric carbon.

Forestry activities also may be adapted to promote soil carbon maintenance and transfer by minimizing disturbances to soil and stand structure and increasing forest productivity and the inputs to the soil (Canadell and Raupach 2008; Jandl et al., 2007). Other forestry efforts can minimize impacts to belowground carbon stocks associated with some management and harvesting activities (Nave et al., 2010; Noormets et al., 2015). Fuel reduction treatments that aim to lower severe fire risk may constitute a limited future sink for atmospheric carbon if expected future fire emissions could be reduced more than the carbon emissions from prescribed burning and mechanical removal (Hurteau and North 2009). Treatments that utilize wood removals for bioenergy may have additional mitigation benefits depending on the type of woody material used (harvest residues versus whole trees) and the fate of that material in the absence of fuel-reduction treatments (Dale et al., 2017). However, treatment areas tend to be much larger than the area they ultimately protect, so the net benefits over large landscapes may not be realized (Boer et al., 2015; Campbell et al., 2012; Hudiburg et al., 2013; Loehman et al., 2014).

Regarding afforestation, the potential for increasing carbon uptake in the United States alone is high, given that 1) the country’s current forestland amounts to about 72% of that in 1630 (Smith et al., 2009) and 2) 60% of the CO₂ emitted from forest harvesting in the United States a century ago has yet to be resequestered (McKinley et al., 2011). U.S. afforestation alone could yield 1 to 225 Tg of additional forest carbon uptake per year in coming decades (McKinley et al., 2011). However, there are major practical limits to widespread implementation since the higher levels of afforestation would require taking land from other uses such as food production (Ray et al., 2009). In Canada, afforestation could add up to 59 Tg C per year (Lemprière et al., 2013). In Mexico, minimal data are available on the carbon uptake potential of afforestation, or even forest management in general.

Another potential opportunity for reducing carbon emissions is shifting harvested wood from short-lived products toward uses with slower or no carbon release to the atmosphere (Bellassen and Luyssaert 2014; Lemprière et al., 2013; Oliver et al., 2014). An additional possibility is the use of forest biomass as a substitute for fossil fuels for energy production (Miner et al., 2014). Worth noting, however, is that long time frames, accurate counterfactuals, and full life cycle assessments often are needed to estimate the mitigation benefits of these and other carbon management activities, including bioenergy (Hudiburg et al., 2013; McKechnie et al., 2011; Perez-Garcia et al., 2007).

Estimates of the potential for forest management to mitigate rising GHGs vary widely because of uncertainties, mainly in natural disturbances, leakage effects, and carbon markets (Anderegg et al., 2015b; ECCC 2016; Gough et al., 2016; Harmon et al., 2011). Climate change effects are also uncertain and differ by forest type and location, making climate-adaptive forest management increasingly important (Duveneck and Scheller 2015). Assessment of carbon management opportunities may need to include consideration of vulnerability to disturbances. For example, locating carbon uptake activities in low-disturbance environments may be appropriate, along with perhaps focusing carbon emission actions (e.g., harvesting and land clearings) in higher-disturbance environments.

In the future, forest carbon management likely will be a co-benefit of many other forest uses and values. Owners and managers may decide to maintain lower carbon stocks as a side effect of pursuing other values, such as promoting habitat for select wildlife and reducing risk of severe wildfires.