## 1 Introduction  
### Tropical Forest Biomass and Importance of Mapping  
Approximately 31% of the Earth's surface is covered by forests [1], which provide essential ecosystem services and host enormous biodiversity. Forests hold an estimated 861 \(\pm\) 66 Pg of carbon stock, of which 42% resides in biomass [2], a fact which is particularly noticeable in tropical forests [3]. Therefore, tropical forests play a paramount role in global carbon sequestration and climate change; this has been a major reason for countries and organizations to report periodically on their forest status through schemes such as the Reducing Emissions from Deforestation and Forest Degradation (REDD).  
Despite the fact that the rate of forest loss has decreased in recent years, it continues to take place, with an estimated 3% reduction in global forested area from 1990 to 2015 [4], mainly due to clearings undertaken to prepare for agriculture expansion. Therefore, monitoring human-induced fluctuation in forest biomass is of paramount importance. Nevertheless, quantifying Above Ground Biomass (AGB) or below ground biomass in the tropics is especially challenging due to the large AGB stock, arduous accessibility to dense forest structures and the associated lack of inventory data. It is noteworthy that AGB is indirectly related to below-ground biomass, and species-specific models have been developed in this framework based on related field campaigns. For example, [PERSON] et al. [5] showcased the relationship between these two traits in a bamboo forest in northeast India; [PERSON] et al. [6] demonstrated an accurate estimation of the above-ground to below-ground biomass ratio in \(L\). _Ieuccephalus_; and [PERSON] et al. [7] derived a coefficient of determination, (R\({}^{2}\)) = 0.89, and a corresponding significance level, (\(p\)) \(<\) 0.001, between below- and above-ground biomass cumulatively for 12 species in a natural coniferous and broadleaf mixed forest in Northeastern China.  
The quantification of forest biomass is traditionally conducted with destructive sampling, which involves harvesting tree samples, requiring the felling, dissecting, oven-drying, and weighting of trees. While several studies have been based on this methodology (e.g., [8; 9; 10]), it is unavoidably a resource- and labor-intensive procedure [11], especially in tropical areas where carbon accumulation occurs in the biomass instead of the soil [2]. Tree growth, on the other hand, is traditionally assessed either by repeated measurements of marked trees in permanent sample plots or tree ring analysis (dendrochronology). The latter technique is applicable primarily in temperate environments and species, as it requires tree samples with well-defined annual growth rings [12]. Alternatively, allometric equations have been suggested to estimate AGB (e.g., [7; 13; 14; 15]) and related to height, diameter at breast height (DBH), species, age, crown density, and bioclimatic variables [16]. However, allometric equations are dependent on species composition and landscape characteristics; nonetheless, a few studies have proposed robust equations, such as that of [PERSON] et al. [17], which introduced a pan-tropical model incorporating wood-specific gravity, trunk diameter, and tree height. Consequently, interest in estimating AGB on large spatial scales has brought remote sensing to the forefront of current research, and numerous attempts have been made to approximate AGB based on Earth Observation (EO) data.