Forests

9.8.1 Synthesis

Net carbon uptake by North American forests is well documented. Its strength varies regionally, with about 80% of the North American forest sink for atmospheric carbon occurring within the United States. Attributing North America’s forest carbon sink to drivers remains difficult. Forest regrowth following historical clearing plays a role, but studies also suggest sizeable contributions from growth enhancements such as CO₂ fertilization, nitrogen deposition, or climate trends supporting accelerated growth. Resolving each factor’s contribution is a major challenge and critical for developing reliable predictions. Several factors driving this sink are expected to decline over coming decades, and an increasing rate of natural disturbance could further diminish current net carbon uptake in the near term, possibly giving way to increased net carbon uptake in the more distant future if forests fully recover from today’s disturbance trends.

Intensive forestry in select regions causes large annual reductions in forest carbon stocks that are eventually compensated for by forest regrowth, often over decades, if biomass recovers to preharvest conditions. However, carbon releases from the associated decay of harvested wood products offset a substantial portion (about half) of the net carbon sink in North American forests. Recent trends in natural disturbance rates have diminished the strength of net forest carbon uptake across much of North America. Net loss of forest carbon stocks from land conversions also reduces sink strength across the continent, with carbon losses from forest conversion exceeding carbon gains from afforestation and reforestation.

9.8.2 Gaps

Forests across North America are quite diverse. Although much is known about this diversity, datasets are still needed to characterize forest conditions at the scale of disturbance and management units (e.g., stand scale, ~30 m × 30 m). Such data would provide managers with the information necessary to design and implement effective carbon policy and management aiming to increase carbon uptake or reduce emissions. Maps of site productivity, stand age, and biomass at a stand scale (e.g., 30 m) would be particularly valuable, offering practical improvements to current assessment capabilities.

Remeasurement data on tree- and stand-scale carbon stocks—including standing dead and downed wood and soil carbon pools and their turnover rates—are needed to record contemporary rates of carbon accumulation, improve under­standing of net carbon uptake drivers, and aid assessment frameworks and models required for prediction. Also needed are analyses of expected shifts in forest composition in response to trends in climate; atmospheric composition; disturbances; the establishment and spread of invasive and/or exotic insects, pathogens, and plants; and management to improve projections of future carbon dynamics beyond an assumption of steady forest compositions and static ecotones. Conclusive evaluation of the rate and magnitude of woody encroachment is still lacking. Delivery of forest carbon to wetlands and waterways via erosion and drainage also is poorly quantified, despite its importance for continental-scale carbon budgeting and management.

Basic understanding of carbon flux and stock dynamics following disturbance is still limited, with some studies suggesting a substantial impact to fluxes (Edburg et al., 2011) and other studies reporting a more muted response (Moore et al., 2013; Reed et al., 2014). Predictions of future disturbance trends are hampered by limited understanding of disturbance interactions involving legacies of flammability and host species presence and absence, as well as active management responses such as fuel reduction treatments or preemptive and salvage logging. Also needed is knowledge of how belowground carbon stocks change as lands transition across uses over time (Domke et al., 2016). These gaps challenge assessments of legacy emissions and post-disturbance recovery and hamper attempts to quantify the potential of management activities to promote long-lived forest carbon sinks and reduce carbon emissions.

The use of remote sensing (e.g., Landsat) has led to major advances over the past decade in monitoring aspects of disturbance and land-use change (Bachelet et al., 2015; Hansen et al., 2013), but major research gaps remain. Disturbance histories at the stand scale and attribution to disturbance type and severity remain poorly characterized, as are rates of forest conversion. Improved estimates of the location, severity, and timing of natural disturbances are needed, particularly in Mexico. Degradation of forest stocks (e.g., from selective logging, low-severity disturbances, and stress) also remain poorly characterized at the scales needed for assessing carbon dynamics and managing forest carbon. Landscape-scale records of management practices such as replanting, selective harvesting, cyclical use, and agroforestry also are needed. Integration of a range of remote-sensing technologies, including light detection and ranging (LIDAR), with field plot data and carbon cycle modeling, promises to substantially improve the ability to measure and monitor forest carbon dynamics at large scales. Addressing these and other gaps ultimately will lead to spatially explicit estimates of carbon stocks and fluxes that comprehensively assess impacts of disturbance, management, and environmental changes on carbon fluxes.

Coupled experiments and models as well as multifactor manipulations are needed to better understand carbon cycling in forest ecosystems and the drivers contributing to carbon dynamics. Full life cycle analyses are required to improve understanding of today’s carbon sinks in a longer temporal context, account for the full effects of management and global change drivers, and evaluate the costs and benefits of substituting wood products for other building materials or energy sources. Also needed is better information on the origin and fate of harvested wood products, which should enable more accurate and comprehensive estimation of harvesting impacts.

Collectively, the large uncertainties and substantial variation in model predictions and GHG inventory estimates can be attributed to the gaps identified in this section. Future assessments should attempt to better integrate data sources and products and move beyond a focus on forest carbon exchange with the atmosphere toward full climate impact assessment such as in Anderson-Teixeira et al. (2012). Considerations are needed of 1) albedo changes from forest change, 2) CH₄ and N₂O fluxes, and 3) dynamics of other radiatively active atmospheric constituents such as aerosols and black carbon.

Also needed are management and planning tools (e.g., see Figure 9.6) designed to help develop and evaluate alternative landscape-scale strategies for managing forests to address a range of ecosystem services including carbon. Platforms, such as the Forest Vegetation Simulator (FVS; www.fs.fed.us/fmsc/fvs/) and i-Tree (www.itreetools.org), enable assessment of impacts from disturbance trends and management scenarios in the context of uncertain global environmental changes to inform policymakers, land managers, industry, and the public. Such platforms can be designed to consider a wide range of ecosystem values beyond carbon to assess full climate forcing (i.e., albedo impacts), as well as biodiversity, habitat, water quality and quantity, timber production, disturbance avoidance, and other goods and services. Moreover, these platforms can be designed to flexibly handle uncertainty in forest responses to changes in climate and interactive trends in management and natural disturbance regimes.

9.8.3 Outlook

Climate change is influencing forest carbon in diverse ways, supporting enhanced carbon uptake in some regions by lengthening growing seasons and elevating CO₂ supply to photosynthesis. However, climate change also is leading to plant stress that reduces growth, increases the likelihood of mortality, and supports more extensive and severe disturbance-induced releases of carbon. All these drivers are altering the ecology and natural resources of North America’s forests. How these processes and their net effect will unfold over coming decades remains unclear.

Harvesting is the dominant forest management activity affecting carbon dynamics in North American forests; it has a net effect of reducing land carbon stocks and emitting carbon to the atmosphere. Slowing harvesting rates or modifying cutting practices could affect future forest carbon stocks significantly.

Several management activities could increase forest uptake of atmospheric carbon and decrease emissions in the forest sector (Birdsey et al., 2006; McKinley et al., 2011; Post et al., 2012). These activities include delaying or avoiding emissions from wood products by producing renewable building materials and developing energy sources with lower life cycle emissions than their GHG-intensive alternatives. Management through afforestation also may promote rapid regrowth of carbon stocks within forests (Erb et al., 2013) and even expand forestlands (Birdsey et al., 2006). However, practical limits are likely to severely constrain implementation, along with competition with other management and use objectives (Ray et al., 2009). Although climate mitigation activities, and associated carbon markets, remain highly uncertain, they clearly have the potential to substantially influence the priority placed on forest management to promote forest sector carbon storage.