<b>Domke</b>, G., C. A. <b>Williams</b>, R. Birdsey, J. Coulston, A. Finzi, C. Gough, B. Haight, J. Hicke, M. Janowiak, B. de Jong, W. A. Kurz, M. Lucash, S. Ogle, M. Olguín-Álvarez, Y. Pan, M. Skutsch, C. Smyth, C. Swanston, P. Templer, D. Wear, and C. W. Woodall, 2018: Chapter 9: Forests. In Second State of the Carbon Cycle Report (SOCCR2): A Sustained Assessment Report [Cavallaro, N., G. Shrestha, R. Birdsey, M. A. Mayes, R. G. Najjar, S. C. Reed, P. Romero-Lankao, and Z. Zhu (eds.)]. U.S. Global Change Research Program, Washington, DC, USA, pp. 365-398, https://doi.org/10.7930/SOCCR2.2018.Ch9.
Many of the factors identified in SOCCR1 (CCSP 2007) continue to be important drivers of change in carbon stocks of forest ecosystems and wood products (CCSP 2007). North American forests are highly diverse, and many are changing rapidly. Management (e.g., timber harvesting and cyclical forest uses) is a major driver of carbon dynamics. Land conversions may cause net carbon emissions in North America, even in the United States where gross gains in forestland exceed gross losses. The changing climate and atmospheric chemistry (e.g., nitrogen deposition, tropospheric ozone, and rising atmospheric CO2 concentrations) are modifying forest growth rates, growth potential, and mortality. Natural disturbances (e.g., wind, fire, and insects and disease) are generally accelerating mortality and modifying forest composition. All these drivers, and their ongoing trends, have important implications for forest carbon policy and management.
9.4.2 Land Use and Land-Use Change
Land use and land-use change can have major implications for land carbon stocks and fluxes and thus are key requirements for UNFCCC reporting. Land-use change, including conversion of nonforestland to forestland, in European nations (Nabuurs et al., 2013) and the United States (Woodall et al., 2015), has taken up a sizeable amount of atmospheric CO2 since 1990, but this effect is expected to slow in the near future (Coulston et al., 2015; Nabuurs et al., 2013).
The current rate of land-use change in Canada is small, with about 0.02% of Canada’s forest area lost each year through deforestation (Dyk et al., 2015; ECCC 2016) or about 30,000 ha of forest lost per year from 2006 to 2015 (ECCC 2017). The gain in forest area through afforestation, vegetation thickening, and expansion of tree lines northward and to higher elevations is not known, so the net balance of forest area change cannot be determined.
In Mexico, land converted to forest contributes a sink of atmospheric carbon of 3.4 Tg C per year. This sink is more than offset by carbon losses from forest conversion, leading to net carbon emissions of about 8.8 Tg C per year from the balance of forest gains and losses in Mexico (see Table 9.3; INECC/SEMARNAT 2015).
Deforestation in the United States occurs at a rate of about 0.12% per year, or 355,000 ha per year (Masek et al., 2011), but is more than offset by forest gain from afforestation. The net effect is a gain in U.S. forest land area of about 0.15% per year, or 430,000 ha per year (Smith et al., 2009; U.S. EPA 2018) between 2006 and 2015, largely converted from grasslands and croplands (U.S. EPA 2018). This nationwide assessment of net changes in forest area masks important region-specific patterns, with the North and Rocky Mountains seeing net gains in forest land area over the past couple decades and the Pacific Coast and South seeing net losses (Smith et al., 2009). The estimated net carbon flux in the United States associated with forestland conversion is approximately zero, with gains in forestland constituting a sink of atmospheric carbon of 23 Tg C per year and losses resulting in emissions of 23 Tg C per year (see Table 9.3.S. EPA 2018).
9.4.3 Forest Management
Nearly two-thirds of Canada’s forests and nearly all forests in the conterminous United States are considered managed lands. Human activities directly influence these lands, and management is mainly for wood products, water, and recreation services, with carbon uptake a secondary outcome. In many of these regions, forest carbon stocks are recovering from historical clearing and thinning dating back to as early as the 1600s. This recovery stimulates forest carbon uptake from both afforestation and carbon accumulation in still-maturing stands. Forest management also has 1) altered forest species composition (e.g., with the establishment of plantations); 2) generally accelerated carbon accumulation rates (Erb et al., 2013); and 3) modified forest soil fertility, both through nutrient gains from fertilizer application and nutrient losses from erosion caused by some harvesting practices. The net effect of such activities on forest carbon stocks and fluxes is unclear. Fire suppression activities have tended to increase forest carbon stocks, and, along with grazing practices, may contribute to woody encroachment. Fuel reduction treatments (e.g., prescribed fire and thinning) often are intended to lower the risk of severe wildfire by reducing crown density, thinning the understory, and reducing fuel loads, all of which may contribute to short-term carbon losses. However, these treatments often lead to carbon storage in wood products, protection of residual trees, and increased growth through reduction of resource competition. Collectively, therefore, fuel reduction treatments may contribute to greater long-term carbon storage than untreated stands (Hurteau et al., 2008; Loudermilk et al., 2016).
9.4.4 Climate and Atmospheric Chemistry
Climate change and extreme weather events, as well as changes in atmospheric chemistry (e.g., nitrogen deposition, tropospheric ozone, and rising atmospheric CO2 concentrations), affect carbon cycling in forests (Ollinger et al., 2002; Sun et al., 2015; Templer et al., 2012). In general, rising temperatures (Melillo et al., 2011) and atmospheric CO2 concentrations (Norby et al., 2005) stimulate forest productivity, but the magnitude of these effects depends on soil fertility, particularly nitrogen and phosphorous availability, and the composition of the soil microbial community (Drake et al., 2011; Finzi and Schlesinger 2002; Terrer et al., 2016). Atmospheric nitrogen deposition can increase soil fertility (Thomas et al., 2010), counteract soil resource limitations (e.g., Johnson et al., 1998; Oren et al., 2001), and directly enhance tree growth (Thomas et al., 2010). Climate-induced changes in precipitation may alter soil carbon dynamics and vegetation carbon uptake during periods of inundation, lead to flooding-related tree mortality, and cause soil erosion with losses of particulate and dissolved organic carbon from forests (Frank et al., 2015).
Although some climatic and atmospheric changes can stimulate productivity, they also can negatively affect forest carbon sinks. High temperatures can induce heat-related stress in plants (Peng et al., 2011), worsen drought conditions (Diffenbaugh et al., 2015), and lead to higher mortality and lower productivity in ecosystems (Anderegg et al., 2015a; Birdsey and Pan 2011). Climate warming also increases night-time ecosystem respiration and reduces net ecosystem production (NEP; Anderegg et al., 2015b). Similarly, the positive effect of rising atmospheric CO2 and nitrogen availability on net primary production (NPP) can be moderated by elevated tropospheric ozone, which damages plants, reducing their health and productivity (Karnosky et al., 2003; Loya et al., 2003; Pan et al., 2009). Rates of sulfur deposition have declined in recent years, but acid deposition from excess nitrogen remains elevated and contributes to lower soil pH; depletion of labile cations, such as calcium, needed for plant growth (Likens et al., 1996, 2001); and mobilization of aluminum, which is toxic to plants (Aber et al., 1998). The effects of acid deposition on forest carbon storage are mediated through stand age, soil type (e.g., cation-poor sandstones versus calcium-rich limestone), and ultimately the fate of deposited nitrogen. Excess nitrogen deposition can result in nitrogen saturation of biotic and abiotic sinks, altering ecosystem carbon allocation, and lead to a cascade of negative effects on water and air quality that decrease forest productivity. The United States is a global hotspot of nitrogen emissions and deposition, with a steady rate of wet deposition of dissolved inorganic nitrogen from 1985 to 2012. However, the contribution from ammonium has increased relative to nitrate, and deposition is higher in the Midwest and Northeast than in the South and West (Du et al., 2014).
Stimulatory effects of rising CO2 on aboveground forest productivity have not been matched by a concomitant increase in soil carbon, the largest carbon pool in forests and one that does not turn over very quickly (Lichter et al., 2008; van Groenigen et al., 2014). Thus, larger litter inputs to soils without an increase in soil carbon stocks implies an accelerated rate of carbon cycling in global forest ecosystems (Pan et al., 2013). Moreover, GHGs are returned to the atmosphere through emissions of CO2 from harvested products; emissions of CO2, CH4, and nitrous oxide (N2O) from biomass burning; and evasion of CO2 from streams and rivers (Kim and Tanaka 2003; Turner et al., 2013). These emissions are expected to offset a portion of the gains in productivity from afforestation following disturbance and climatic and atmospheric changes (Turner et al., 2013). Furthermore, severe warming of forest soils has been shown to accelerate soil organic matter decay and result in net loss of soil carbon emitted as CO2 (Melillo et al., 2017). Given the wide range of forest responses, better understanding of the effects of climatic and atmospheric changes continues to be a high research priority in the United States.
9.4.5 Natural Disturbances
Natural disturbances are widespread across North America (see Figure 9.3) and play an important role in the forest carbon cycle (Hicke et al., 2012; Odum 1969; Williams et al., 2016), affecting NPP and heterotrophic respiration, transferring carbon from live to dead pools, and involving direct emissions (e.g., from fires [French et al., 2011; Ghimire et al., 2012]). These disturbances include wildfires, insects and pathogens, droughts, floods, and severe wind events (Frank et al., 2015; Tian et al., 2015). Severe disturbances typically cause an immediate reduction in stand-level productivity, transfer carbon from live to dead stores, and increase decomposition. These effects generally are followed by a gradual increase in productivity and decrease in decomposition as the stand recovers. Initial net carbon release immediately after severe disturbances gives way to net carbon uptake as a forest regrows, but the full effect on atmospheric CO2 depends also on the timing of disturbance-induced CO2 releases. Carbon impacts of disturbance vary with several key features including disturbance type and severity, temporal sequence of events, and biotic and climatic conditions of regeneration (Hicke et al., 2012; Williams et al., 2016).
Figure 9.3: Satellite-Derived Distribution of Major Forest Disturbances by Type for Canada (a) and the United States (b)
The extent, severity, and frequency of natural disturbances have increased in recent decades (Allen et al., 2010; Hicke et al., 2013; see Figure 9.4), likely influenced by recent climate change and human activities. Western regions of Canada and the United States have experienced substantial die-offs recently from wildfire, insect outbreak, and drought disturbances. These events have led to widespread tree mortality, with fire and insects alone affecting up to 9% of the live tree carbon stocks in western U.S. forests (Ghimire et al., 2012, 2015; Hicke et al., 2013) and with insects also having a substantial and prolonged effect in British Columbia (Kurz et al., 2008a, 2008b). Disturbance impacts on region-wide carbon dynamics can be large and result in sizeable interannual variability in the forest carbon balance (see Figure 9.5), and landscapes often contain offsetting effects of large carbon releases in small areas that recently experienced severe disturbance and modest carbon uptake in larger areas at various stages of recovery from prior disturbance. In eastern North America, native and invasive forest insects play important roles locally (Clark et al., 2010) and regionally (Kurz and Apps 1999). Insect damage in the United States is estimated to result in the loss of about 20 Tg of live carbon stocks per year, though release to the atmosphere through decomposition can be delayed for decades. Similar, if not larger, losses have been reported for Canada (Kurz et al., 2008a, 2008b). U.S. wildfires lead to emissions of about 40 Tg C per year, with large year to year variability. Windstorms cause an average annual loss of about 35 Tg of live carbon stocks in the United States alone (Williams et al., 2016), largely from hurricanes in the Southeast that have major individual impacts (Chambers et al., 2007; Fisk et al., 2013). Windstorm losses of live biomass are released to the atmosphere only gradually and typically are offset by forest regrowth, leading to a steady long-term effect on atmospheric carbon (Fisk et al., 2013; Zeng et al., 2009). Droughts in the United States and Canada have resulted in punctuated and widespread reductions in forest productivity (Schwalm et al., 2010) as well as tree mortality (Anderegg et al., 2013a, 2013b; Hogg et al., 2008; Michaelian et al., 2011; Peng et al., 2011; Potter 2016; van Mantgem et al., 2009) that together can cause sizeable declines in NEP and the strength of the forest carbon sink (Brzostek et al., 2014; Ma et al., 2012; Schwalm et al., 2012).
Figure 9.4: Teragrams (Tg) of Carbon in Western U.S. Trees Killed by Disturbances
Figure 9.5: Effects of Natural Disturbances on Carbon Dynamics in Canada’s Managed Forests
Accounting for land-use change, management, disturbance, and forest aging, some models project that U.S. forests will continue taking up carbon but at declining rates, largely because of land-use dynamics and aging forests (USDA-OCE 2016; Wear and Coulston 2015). After 20 years of net gains, forest area is projected to level and then decline gradually after 2030 due to ongoing population growth and declining afforestation on agricultural lands (U.S. Forest Service 2012; Wear and Coulston 2015), though projections differ depending on assumptions about how macroeconomic and market trends will drive land use. In the western United States, aging forests coupled with disturbance dynamics are projected to diminish carbon uptake to negligible levels by midcentury. In the East, younger productive forests are expected to have high carbon uptake rates, though harvest-related emissions substantially reduce the net effect on atmospheric carbon.
Climate change defines complex and uncertain adjustments to net carbon accumulation in forests. Several studies suggest that atmospheric enrichment from CO2 and nitrogen could increase biomass growth by 0% to 2% annually (Fang et al., 2014; Schimel 2007; Shevliakova et al., 2013). Meanwhile, climate change generally is expected to increase the frequency and severity of natural disturbances in North America in the coming decades, potentially reducing forest carbon stocks considerably (Peterson et al., 2014; U.S. Forest Service 2012). Other climate change impacts—including shifts in growing season length, water availability, and temperature—will interact with atmospheric changes to determine forest growth responses (Gedalof and Berg 2010; McCarthy et al., 2006). Projection experiments that include a trend of increased productivity (+0.4%), coupled with forest age, disturbance, and management dynamics, indicate some potential for additional carbon uptake over baseline levels described previously (+5.1% from 2015 to 2050; Wear and Coulston 2015). However, increases are small relative to the projected changes for all other driving variables. Forest sink strength is likely to diminish gradually over the next 20 years as forest area gains tail off and forests continue to age. Uncertainty regarding the future carbon balance of North American forests increases with time. There is some potential for enhanced productivity resulting in a larger carbon sink, but disturbance rates and other elements of global change could increase carbon emissions from forests (Kurz et al., 2013; Lemprière et al., 2008). Uncertainties about the impacts of global change remain high. Increased sinks are unlikely to be of sufﬁcient magnitude to offset higher emissions from increased disturbances and enhanced release of carbon from decomposition (Kurz et al., 2013). However, the forest sink in the eastern temperate zone of North America is expected to be relatively stable despite these pressures (Wear and Coulston 2015).
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