CHAPTER 5 SAR Methods for Mapping and Monitoring Forest Biomass Sassan Saatchi, Senior Research Scientist, Carbon Cycle and Ecosystems Section, Jet Propulsion Laboratory, California Institute of Technology ABSTRACT Forests play a major role in the global carbon cycle, sequestrating more than 25% of the carbon emitted to the atmosphere from fossil fuel consumption and land- use changes. The accumulation of carbon in forests has therefore become an efective strategy for mitigating climate change and an important mechanism for countries to meet their emission requirements under many international protocols and agreements. Remote sensing techniques are considered the most promis- ing approach for providing up-to-date information on the status of forest cover and carbon stocks at diferent scales. Among remote sensing techniques, Synthetic Aperture Radar (SAR) sensors at long wavelengths have the advantage of strong sensitivity to the forest Above Ground Biomass (AGB) and the ability to quantify and monitor carbon stocks at the scale in which human activities occur. This chapter provides a summary of the methodologies and techniques for estimating forest AGB and monitoring changes from existing and future SAR satellite systems. The material in this chapter is designed to help both practitioners and remote sensing students and experts use SAR imagery for mapping and monitoring forest biomass. The examples and the bibliography capture the state of the art in SAR remote sensing of vegetation structure and biomass, and provide resources for enthusiasts to follow future developments in the technology and the methodology. 5.1 Background put together the information from diferent types of carbon, water, and nutrients). Additionally, there is measurements on a global scale captures the overall an increased need to understand local to global stor- 5.1.1 GLOBAL DISTRIBUTION OF FOREST distribution of forest Above Ground Biomass (AGB) age and dynamics of carbon in ecosystems, as carbon BIOMASS and carbon stored in global ecosystems (Fig. 5.1). storage is a prerequisite to understanding the cou- The structure of forests (i.e., the three-dimension- pling of the biosphere to other components of Earth Vegetation in terrestrial ecosystems takes up a al arrangement of individual trees) is a direct indica- systems. For example, the amount of carbon in a sys- signifcant fraction (~30%, or 3 PgC year–1) of carbon tor of how much carbon is stored in the ecosystem. tem determines how much is eventually emitted to released to the atmosphere from fossil fuel and de- Carbon stored in an ecosystem has a profound efect the atmosphere (as CO , CO, and CH through burning forestation (LeQuere et al. 2018, Schimel et al. 2015) 2 4 on how the ecosystem functions (i.e., how it cycles and decay) when ecosystems are disturbed due to and creates the land residual sink with a destiny dependent on future climate conditions and human activities (Ciais et al. 2013, Bonan 2008). Almost all of this sink is in forests, covering about 3.8 billion ha (FAO 2015) of the land surface (~30%) and storing large reservoirs of carbon, approximately double the amount in the atmosphere (Canadell & Raupach 2008, Sabine et al. 2004). Together, the carbon stored and sequestered in these ecosystems are major con- tributors to mitigating climate change and the eco- Biomass nomic benefts of emission Reductions from Defor- Megagrams/hectare estation and Degradation (REDD) (IPCC 2007, Gibbs (Mg ha-1) et al. 2014). There are, however, large uncertainties surrounding the magnitude of the carbon stored in Figure 5.1 Distribution of forest AGB density in global ecosystems showing the high biomass in tropical rainforest regions and relatively lower biomass in extratropics extending to temperate and boreal forests, particularly at landscape scales (1–100 ha) regions with vast areas of forest cover. Map is produced at 1-km spatial resolution using a combination where mitigation benefts and ecosystem services of ground, lidar, and radar measurements by Saatchi’s team at the Jet Propulsion Laboratory, California are evaluated (Gibbs et al. 2007). A recent attempt to Institute of Technology. THE SAR HANDBOOK deforestation and degradation or from climate-driv- Ground inventory density en stress and fre. The amount of carbon stored in the Total forest/shrub area Inventory density (plots/1000 km ) system can be estimated from AGB, which is estimat- -5 Vegetation carbon ed from measurements of structure (e.g., the size and x 10 -2 density of trees) and the mass of trees. As such, AGB is considered a crucial variable for a range of applica- tions, including forest fre assessment, management of the timber industry, monitoring land-use change, and other ecosystem services such as biodiversity and production of food and fber, as well as green- -2 Vegetation carbon storage (PgC) ) house gas accounting. Total forest/shrub area (km Although many of these applications may be accounted for by using operational satellite obser- vations of forest cover change, the understanding of changes in terrestrial AGB remains rudimentary Latitude (Saatchi et al. 2011). For example, it is known that Figure 5.2 The distribution of woody (forest and shrubland) area and biomass derived from a changes in land use, largely from tropical deforesta- variety of sources from feld and remote sensing data. The red histogram shows forest inventory tion and fre, are estimated to have reduced biomass plot density in 1,000 km2 grid cells (Schimel et al. 2015b), suggesting an uneven distribution globally, while the global carbon balance suggests of inventory plots in the Northern Hemisphere and a lack of data in tropical regions. that terrestrial carbon storage has increased; albe- it the exact magnitude, location, and causes of this plot size, number of plots, and plot locations that from ground-based forest censuses that are based residual terrestrial sink are still not well quantifed have not been worked out for tropical forests. on labor-intensive feldwork (plot inventories) con- (Schimel et al. 2015a, Sellers et al. 2018). There is • Conventional NFI can provide accurate estimates ducted by trained operators. As such, these plot in- strong evidence that the residual sinks are spread in of forest carbon density at the national and po- ventories cannot be repeated frequently or at a low diferent forest ecosystems with locations that may tentially subnational levels depending on the cost everywhere. Thus, plot inventories are limited to change due to climate change and anomalies. Yet density of the plots. However, they cannot pro- managed forests in a number of developed countries the magnitude and fate of these terrestrial sinks are vide spatial maps unless combined with remote in the Northern Hemisphere where systematic sam- crucial to future climate projections, and any uncer- sensing data. pling of forest inventories are performed on a regu- tainties in the spatial locations or the temporal be- • In tropical and unmanaged forests, implementa- lar basis (5- to 10-year cycles). Information on most havior of them directly infuences the current status tion of NFI is extremely difcult, because of limit- carbon-rich global forests is missing, particularly in of global carbon cycle and climate (Houghton et al. ed access to the site and the cost of establishing developing and tropical countries, even though this is 2018, Schimel et al. 2015a). and monitoring plots over time. Using the proto- where most living biomass is located (63% of carbon cols of the U.S. or northern Scandinavian NFI to in intact tropical forests versus 15% in boreal forests 5.1.2 GROUND INVENTORY OF FOREST the tropics requires a large number of plots. and 13% in temperate forests, according to a recent BIOMASS • Conventional NFI data include 5–10 years of and comprehensive estimate (FAO 2015)). Further- Knowledge of the distribution and amount of repeated measurements, and the timing of the more, land-use activities, along with increasing dis- AGB is based almost entirely on ground inventory measurements is not coordinated among the turbances from climate and human stresses, are rap- measurements over an extremely small (and possibly countries, making it difcult to conduct a global idly changing plot inventory requirements to include biased) set of samples, with many regions left un- assessment for any period. For Greenhouse Gas more frequent observations of forest ecosystems. measured (Fig. 5.2). Conventional forest inventory (GHG) emissions, the use of a national inventory 5.1.3 REMOTE SENSING OF FOREST BIOMASS data known as the National Forest Inventory (NFI) along with remote sensing estimation of forest are based on systematic sampling of forests and are cover change can provide national-level emis- There is a strong synergism between ground and mainly designed for monodominant, evenly aged sions estimates, but those estimates may involve remote sensing measurements for quantifying AGB forests in managed temperate and boreal regions. uncertainty due to the lack of forest estimates in (Fig. 5.3). Ground data (generally consisting of Although the basic statistical techniques can be used areas where deforestation occurs. all tree diameters above a threshold, a sampling of for tropical forests, there are diferences in terms of At large scales, robust AGB estimates are acquired tree heights, and species identifcation that permits THE SAR HANDBOOK inference of wood densities) are more comprehen- Figure 5.3 sive locally than remote sensing data that generally Spaceborne Ground and lidar & radar measure aggregate canopy height (in the case of li- remote sensing measurement dar sensors) or some indicators of forest height and techniques to volume (in the case of radar sensors). In contrast, quantify forest airborne or satellite remote sensing-based data are Large footprint Small footprint lidar lidar structure far more extensive, with millions of measurements and AGB. over regional or continental scales compared to Airborne radar POLinSAR plots and providing a more spatially comprehensive measure of forest biomass variations.