Quantification of forest degradation and belowground carbon dynamics: ongoing challenges for monitoring, reporting and verification activities for REDD+

Keywords::

• carbon fluxes
• heterogeneity
• net ecosystem carbon balance
• remote sensing
• soil carbon

Proper implementation of reducing emissions from deforestation and forest degradation, plus improving forest management, carbon stock enhancement and conservation (REDD+) requires addressing challenges related to social, technological, ecological and economical issues [1,2]. These challenges are scale dependent (both in time and space) but must be addressed to achieve the viability of short- and long-term REDD+ initiatives. Furthermore, the details for accounting carbon dynamics (to assess capture and emissions) under REDD+ are an ongoing international discussion. Multiple studies have addressed the importance of applying multi-scale and multi-method approaches for improving frameworks for monitoring, reporting and verification (MRV) for implementation of REDD+ [3,4]. However, there are still several challenges ahead for proper and successful carbon accounting in REDD+. Here, we briefly discuss three key issues related to: forest degradation, full accounting of carbon fluxes in forests and belowground carbon dynamics. We believe these represent ongoing important challenges that must be addressed to reduce uncertainties in estimating forest carbon emissions and removals, particularly in the light of future implementation of REDD+.

Forest degradation in heterogeneous landscapes

Recent advances in remote sensing techniques have improved the capacity to estimate forest cover change from different platforms (e.g., LiDAR, Landsat and Moderate Resolution Imaging Spectroradiometer). These approaches have different spatial and temporal resolutions that provide advantages and challenges while detecting these changes [5,6]. Many forested landscapes in developing countries are characterized by a mosaic of forests of different ages and patches of different land uses creating complex heterogeneous landscapes. Deforestation and changes in land use (e.g., changes from forest to agriculture) are relatively easy to detect by analyzing structural and spectral reflectance changes [7]; however, identifying and quantifying forest degradation (i.e., long-term lowering of biomass density within a forest) still remains challenging [2,6]. Quantifying forest degradation is probably more challenging when a forested landscape is represented by a heterogeneous mix of patches of secondary and degraded forests [8]. These complex (but not necessarily uncommon) landscapes may require higher land-based efforts through intensive forest inventories and may be subject to larger uncertainties while upscaling the effects of forest degradation. Novel approaches can detect higher spatial resolution changes (e.g., using LiDAR), but the extent of using these techniques is limited in time and space, which in turn is a challenge for upscaling at regional and national scales [9]. We believe that improving information about land-use change dynamics and carbon recovery rates after disturbances using multi-method approaches across landscapes will improve estimations about forest degradation, and will reduce uncertainties when upscaling and forecasting estimates of forest carbon emissions.

An additional challenge is to identify the past and current drivers of deforestation and forest degradation. In forested landscapes of developing countries, some of the main direct causes of deforestation are the transformation of vegetation cover toward agricultural and livestock purposes (e.g., grassland areas for cattle raising), and forest fires. In addition, forest degradation is potentially related to a nonregulated extraction of forest products (e.g., timber and firewood/charcoal), shifting cultivation, cattle grazing within the forest and low-intensity fires. Information about the historic drivers of forest change is valuable for estimating, interpreting and forecasting carbon dynamics across forested landscapes, and for determining policy decision and management actions.

Full accounting of carbon fluxes in forested landscapes

We argue that a full accounting of changes in carbon stocks and fluxes is needed to accurately estimate forest carbon emissions as part of MRV activities. The net ecosystem carbon balance (NECB) includes carbon fluxes such as fixation (i.e., gross primary production), ecosystem respiration and lateral transport of carbon by erosion or other sources, such as anthropogenic transport or harvest [10]. The quantification of NECB as a goal for MRV activities could help in establishing accurate baseline scenarios that can be upscaled in space and time for proper allocation of resources and management actions towards implementation of REDD+.

Failure to account for NECB as a quantitative measure for evaluation of REDD+ activities will lead to large uncertainty in estimating carbon emissions by forested landscapes. For example, if MRV activities are just focused on standing aboveground biomass, then, through forest inventories and remote sensing approaches, it is possible to determine aboveground net primary production (with its associated uncertainty due to techniques and upscaling of information). However, with this approach there is a lack of information from losses due to ecosystem respiration that include autotrophic and heterotrophic processes [11], or lateral transport of carbon [12], which are critical in determining ecosystem carbon dynamics especially after disturbances [13]. Therefore, using an NECB approach for determining carbon emissions will provide more information about forest sources and sinks after degradation and deforestation, and will provide information to define baseline scenarios and evaluation of implementation of REDD+ activities. Intensive monitoring sites could help in developing MRV approaches towards accounting for NECB, and information collected at these sites could be applied for upscaling and forecasting approaches.

Belowground carbon dynamics

Soils represent the largest carbon pool in terrestrial ecosystems [14], and belowground carbon could account for >50% of total carbon pools in tropical forests [15]. However, one of the most challenging issues for MRV activities is, arguably, the quantification of belowground carbon dynamics. Belowground carbon is influenced by plant roots, mycorrhizae, heterotrophic and autotrophic bacteria, soil organic carbon quantity distribution (e.g., labile pools) and quality (e.g., carbon:nitrogen ratios), lateral fluxes of soil carbon (e.g., erosion and dissolved organic carbon), and vertical CO₂ fluxes due to autotrophic (respiration from plant roots and mycorrhizae) and heterotrophic (respiration from microorganisms) sources. Furthermore, studies have shown that incorporating information about belowground carbon dynamics is important for estimating not only total ecosystem carbon fluxes [10], but also estimating regional climate [16]. Studies have identified changes in soil carbon after land-use change [17], but proper incorporation of changes in different components of belowground carbon dynamics is needed to accurately calculate NECB and reduce uncertainty in estimating forest carbon emissions.

Incorporating belowground carbon dynamics in MRV activities is challenging because of several reasons. First, traditionally most forest inventories mainly focus on the estimation of timber stocks, harvest and forest cover. Second, it is not possible to accurately measure belowground carbon dynamics from a remote sensing approach, which increases the labor costs and reduces the spatial coverage of manually collected measurements. Third, soils represent a three-dimensional space within a medium that it is not well mixed (in comparison with a water body or the atmosphere); therefore, multiple measurements are needed to cover the true domain of the soil, usually beyond 1 m depth [18]. In order to accurately estimate NECB and fully account for successes in reducing emissions, we strongly suggest that the scientific and policy community consider the important role of belowground carbon dynamics for NECB when discussing MRV activities.

Concluding remarks

The main goal of the UNFCCC is to contribute to “stabilize greenhouse gas concentrations in the atmosphere at a level that would prevent dangerous anthropogenic interference with the climate system.” To evaluate the success of this international treaty it is important to accurately estimate GHG dynamics as a result of the implemented policies and management actions. We believe that there is a need to synthesize information between above- and below-ground effects of deforestation and forest degradation, and to bring attention to the importance of multiple components of the NECB. Here, we highlighted the need to better quantify forest degradation and its drivers, and the importance of belowground carbon dynamics for quantifying NECB. Understanding of the NECB may motivate more countries to add belowground information during forest inventories towards reducing uncertainty in estimates of carbon emissions. Finally, we highlighted that there is a need to produce consistent historical field data on forest degradation to help in the interpretation of the current state of carbon dynamics across forested landscapes. More accurate data can be obtained during forest inventories regarding historical dates of major disturbances that may be identified and corroborated by using historical satellite records to develop disturbance regional databases.

We recognize that measuring forest degradation and its effects on forest carbon dynamics is complicated and expensive. Furthermore, a full accounting of changes in stocks and fluxes to constrain the NECB is likely impossible to perform across forests in all countries where REDD+ could be applied. We believe that, ultimately, efforts to quantify emissions could be restrained by a cost–benefit analysis in order to fully allocate economic resources across international treaties and financial transactions [19]. Therefore, it is important to close the knowledge gap in the aforementioned topics as this will provide critical information for evaluation of REDD+ activities and quantification of uncertainties, especially when upscaling this information across regions and nations.

Financial & competing interests disclosure

Support was provided by NASA under Carbon Monitoring System project no. NNX13AQ06G. The authors have no other relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript apart from those disclosed.

No writing assistance was utilized in the production of this manuscript.