## 2 Materials  
### Study Area and Field Inventory of Plots  
The study area is located in a national tropical rainforest park in Hainan Province, China (108 deg36'-109 deg57' E, 18 deg23'-19 deg11' N), covering approximately 4900 km\({}^{2}\) with elevation ranging from 100 m to 1876 m (Figure 1). The region is characterized by a tropical maritime monsoon climate with annual average temperature of 22.5-26.0 \({}^{\circ}\)C and annual average precipitation of 1759 mm. Mixed broad-leaved forests and pure broad-leaved forests are the main land cover types in the study area where rubber tree (_Heva_ spp.), eucalyptus spp.), and Taiwan acacia (_Acacia_ spp.) are dominant among various tree species. To evaluate the relationship between remotely sensed estimated canopy structural variables (i.e., canopy cover and LAI) and forest inventory variables (i.e., tree height, diameter at breast height (DBH), crown diameter), we conducted a field campaign to measure the forest inventory variables. Three canopy coverage levels and five tree height levels were set in the selection of forest sample plots, resulting in 166 plots with a diameter of 30 m. These plots were positioned according to the trade-off between the accessibility and the random sampling rule.  
We used a real-time kinematic (RTK) GPS to accurately measure the plot center and each tree's position in every forest plot. The age group of each plot was recorded where the tree age was measured by counting the tree rings obtained by a growth cone. The dominant tree species in each mixed forest plot was also recorded where the dominant species was identified according to the ratio of the number of a specific tree species to the total. Tree height and DBH of each tree were separately measured by a laser altimeter (Haglof Vertex Laser developed by Haglof Sweden) and a tape measure. The crown diameter was estimated by averaging two measurements taken from two measuring tapes laid perpendicular to each other at the largest width of each crown. These plots were categorized into seven types according to the forest species which include 6 mixed coniferous and broad-leaved forest plots, 18 mixed broad-leaved forest plots, 49 _Eucalyptus robusta_ Smith plots, 30 _Acacia confusa_ [PERSON]. plots, 49 _Heeva brasiliensis_ plots, 2 Chinese fir plots, and 12 other coniferous trees plots. The AGBs of all tree species and forest types were calculated using allometry equations (Table 1) [31].  
\begin{table}
\begin{tabular}{c c c}
**Tree Species and Forest Types** & **Allometry Equation of Individual Tree** & **AGB of Plot (\#/ha)** \\ \hline Coniferous and broad-leaved mixed forest & \(AGB=0.2253\times D^{2.4213}\) & 124.6 \(\pm\) 106.4 \\ Broad-leaved mixed forest & \(AGB=0.1131\times\left(D^{2}H\right)^{0.8407}\) & 113.5 \(\pm\) 260.2 \\ Other coniferous trees & \(AGB=0.2309\times\left(D^{2}H\right)^{0.8683}\) & 185.0 \(\pm\) 174.6 \\ Chinese fir & \(AGB=0.0182\times\left(D^{2}H\right)^{0.9710}\) & 46.8 \(\pm\) 7.3 \\ _Eucalyptus robusta [PERSON]_ & \(AGB=0.0576\times\left(D^{2}H\right)^{0.8587}\) & 89.2 \(\pm\) 175.3 \\ _Acacia confusa [PERSON]_ & \(AGB=1.4240\times\left(D^{2}H\right)^{0.5680}\) & 127.6 \(\pm\) 170.0 \\ _Hexu brasiliensis_ & \(AGB=0.0524\times D^{2.461}\) & 46.5 \(\pm\) 87.7 \\ \end{tabular}
\end{table}
Table 1: Allometry equations of aboveground biomass (AGB) of individual trees for different tree species and forest types. \(D\) and \(H\) represent the DBH and tree height, respectively.  
Figure 1: Study area in the Hainan national tropical rainforest park with the background being a Sentinel-2 true color image. Yellow dots refer to the forest sample plots in the field inventory.