**2. Methodology**

This work is a review of current themes under the UNFCCC process that are relevant to forestry. This review, in collating studies across different dimensions that can potentially contribution to CDR assessment and accounting in tropical forestry, investigates current trend of primary and secondary forest transitions in the context of tropical forest management and HWP, the approaches of quantifying anthropogenic activities and their contribution to CO2 emissions and removals, the integration of sustainable forest management models with livelihood opportunities and incentives for CDRs.

Making reference to different reviewed studies in literature, which only provide different single assessments of the components of forest GHG assessment, we conduct an multi-disciplinary and quantitative review of the potential of tropical forest as CO2 sinks in relation to nature- and made-induced changes in land use, and as well other socioeconomic needs fueled by local and global market transitions. With a target towards tropical forests, we review current themes under the UNFCCC process that are relevant to forestry. Thus, some aspects of GHG estimation in forests were not within the scope of this review. For instance, we do not include components such as atmospheric CO2 reduction by direct removal such as CO2 Capture and Removal and CO2 flux measurements.

Central to this review, among others, is the increasing CO2 fertilization of secondary vegetation growth which out-competing old forest trees, and the need for assessing CDR contribution from anthropogenic actions, which have high uncertainty and variability between local contexts and across geographic scales. Build upon reported evidence of atmospheric CO2 enrichment of vegetation growth and transition in tropical forests, we compare a Business As Usual (BAU) scenario of native forests without management interventions versus an alternative scenario of human interference with Sustainable Forest Management practices based on contemporary silviculture and HWP production and consumption. In the following sections, we present the different themes of our integrated review. Based on these multi-dimensional components, together with qualitative and quantitative insights from the reviewed studies, we highlight potential options for both sustainable forest managements and monitoring, and as well enhancement of GHG estimations for NDC and CDR incentives.

### **3. Results and discussion**

#### **3.1 Tropical forest biomass and GHG monitoring**

Forests have been recognized and acknowledge in both IPCC reports and the Paris Agreement to contribute substantially in achieving climate change mitigation goals [7–9]. However, it is currently challenging to spatially quantify and temporally monitor the extent to which forests impact atmospheric greenhouse gas (GHG) concentrations. In terms of atmospheric CO2, loss (CO2 emission) and gain (CO2 removal) can co-occur on pixels or areas undergoing forest management or other forms of disturbance and regrowth.6 These actions and dynamic land-use patterns occur at spatial and temporal scales not often captured by global models and estimates of GHG flux [10]. Nonetheless, estimates of GHG emissions and sinks by most developing countries, translated into NDCs, are mainly based on default emission factors that do not necessarily reflect country specifics in terms of forest structure and status of forest transitions.7

Global models and maps of GHG fluxes are based of inventory database that do not reliably represent the contexts in tropical forests with consideration of the high local or regional variability in forest structure and anthropogenic changes [10, 11].

<sup>6</sup> If we are unable to quantify them, we would not reach the goal of reliable monitoring and sustainable management. Regarding GHG fluxes, opposing fluxes simultaneously occur at local and regional scales at magnitudes that depend on the location and time of disturbance or management actions.

<sup>7</sup> For instance, the nature of forest degradation, the composition of intact forests, and state type of secondary forests.

#### *CDR and Tropical Forestry: Fighting Climate Change One Cubic Meter a Time DOI: http://dx.doi.org/10.5772/intechopen.109670*

Based on two decades temporal series of observational spatial data, Harris et al. [10] introduced a global spatial framework for GHG fluxes in forests of any geography. However, existing forest GHG flux assessment frameworks and models are unable to discriminated the contributions from anthropogenic versus non-anthropogenic effects and likewise, between managed and unmanaged land. To achieve such distinctions, adaptable combinations of field inventory and spatial data are needed to unravel and aggregate local to regional estimates. In the context of tropical forests, the use of spatial data from radar or synthetic aperture radar (SAR) systems have remarkable potentials in providing weather- and daylight-independent information of land features.

In spatial data modeling, remote sensing procedures offer unprecedented advantages (resources, time, and cost) in large-scale biomass and GHG estimation in forests and other land uses.8 However, by design, current satellite-based earth observation platforms have orbiting patterns and image capturing intervals over tropical forests that provide low temporal data, which do not guaranty the parallel monitoring of anthropogenic activities within tropical forests in a timely manner. Thus, there is large time-lapse between the on-going deforestation actions and potential remote sensing data9 to support the monitoring of both GHG emission and CO2 removal10 [12].

In the Brazilian Amazon for instance, most data used today are still from old studies carried out by RADAMBRASIL surveys, from the late 1950s to the early 1970s using side-looking airborne radar imagery combined with 1-ha ground plots at approximately 3000 points, often reached by helicopter. Even with these limitations, the use of the RADAMBRASIL surveys11 is still not easily compensated for by applying more sophisticated remote sensing interpretation to a small set of ground-based plots12 [13].

Unlike spatial data captured from optical sensors, radar sensors have the characteristic advantage of penetrating cloud cover, which is predominant over most tropical

<sup>8</sup> In the context of tropical forests, the application of remote sensing procedure for biomass mapping and monitoring is receiving wide attention and progress. Several compounding factors may be accountable for low rate of remote sensing application and technology transfer to tropical forest monitoring. Among these factors, technical capacity is increasingly a lesser hurdle compared the situation a decade prior. There are growing freely accessible archives of satellite-based remote sensing images (data) such as data provided from NASA Landsat missions and the operational mission of the European Space Agency (ESA) Copernicus program, which have jointly reduced the hurdle of access to remote sensing data.

<sup>9</sup> The closest being a minimum of 12 days across the tropics for ESA's Space-borne Sentinel operational satellites.

<sup>10</sup> Thus, in current times, most of the challenges in remote sensing monitoring of tropical forest GHG flux may center around the nature of available data for applications in tropical forest contexts—different data are needed for different contexts and as well in addressing the wide uncertainties for tropical forests in global projections and maps of GHG flux.

<sup>11</sup> It has been daunting to many research groups: the reports are a vast labyrinth of over 50,000 pages, written in Portuguese and historically with limited availability at any single location. However, ignoring this enormous body of work represents a loss that

<sup>12</sup> Unlike spatial data captured from optical sensors, radar sensors have the characteristic advantage of penetrating cloud cover, which is predominant over most tropical forests during seasonal monsoons and vegetation proliferation. Though radar images can potentially capture vegetation information across seasons, radar or SAR image processing workflows have been largely unreported for the myriad potential applications in tropical forests. Using satellite-based radar data.

forests during seasonal monsoons and vegetation proliferation. Though radar images can potentially capture vegetation information across seasons, radar or SAR image processing workflows have been largely unreported for the myriad potential applications in tropical forests. Using satellite-based radar data, there are increasing efforts in delineating and quantifying sectoral anthropogenic actions and land use [14, 15] and aboveground biomass [16, 17] in tropical forests.13 Notwithstanding these growing efforts, most estimates of forest biomass and GHG emission for project-based and national assessments still rely largely on extrapolations from often scant field inventories using either allometric models or application of remote sensing data at spatial scales that mask variability across geographies and local details—the scale of most anthropogenic activities. In tropical forest landscapes, the majority of anthropogenic actions and land use changes occur widely at smallholder scales that range in spatial extent between 0 and 2 hectares; this is undermining the increasing tendencies of large-scale plantation establishment beyond the aforementioned range. Context-dependent information on anthropogenic contributions to atmospheric CO2 emissions and removals are, therefore, needed to reliably account and aggregate impacts at a global level. Thus, multi- and cross-sectoral efforts towards climate change adaptation and achieving the objectives of the Paris Agreement should consider a per-hectare assessment, m3 production and monitoring frameworks to match the realities and needs in tropical forests, and offer a more inclusive incentive for communities to engage in and benefit from CDR activities.
