Mapping structural attributes of tropical dry forests by combining Synthetic Aperture Radar and high-resolution satellite imagery data

Aim Optical satellite imagery has been used for mapping the spatial distribution of vegetation structure attributes; however, obtaining accurate estimates with optical imagery can be difficult in tropical forests due to their dense canopy and multi-layered vegetation. Synthetic aperture radar imagery can be more suitable in this case, as the radar signal can penetrate the forest canopy and interact with stems, providing a better estimation of the vegetation structure. This study compared the accuracy of forest species richness, tree diameter, height, and basal area estimates obtained using Sentinel-2 and Advanced Land Observing Satellite -1 (ALOS) Phased Array type L-band Synthetic Aperture Radar (PALSAR) data, either combined or separately. Location The Yucatan Peninsula, Mexico. Methods Field data were collected in three 3600-km(2)-window areas with three different types of tropical dry forest. Three random forest regression models were fitted: one using explanatory variables derived from Sentinel-2 data, a second using predictor variables derived from ALOS PALSAR, and the third using a combination of explanatory variables from both sensors. A variance partitioning analysis was carried out to examine the percent variability of each vegetation attribute that was explained by the models combining the explanatory variables of the two sensors (ALOS PALSAR and Sentinel-2). Results Vegetation attribute estimation errors ranged from 13% to 38.5% when using ALOS PALSAR variables and from 11% to 33% when using Sentinel-2 variables. Combining variables from both sensors provided more accurate estimates of vegetation attributes. A 5% reduction of the estimated error, and an increase from 0.50 to 0.63 of the percentage of variation explained by the models (R-2) were achieved. Conclusions Our results suggest that both ALOS PALSAR and Sentinel-2 data provide accurate estimates of vegetation structure and species richness in tropical dry forests. However, combining explanatory variables from the two sensors improved the estimation accuracy of vegetation attributes.

Automated summary

Optical satellite imagery can have limitations in accurately estimating vegetation structure in tropical forests due to their dense canopy, while synthetic aperture radar imagery can provide better estimates by penetrating the canopy. This study compared the accuracy of forest species richness and attributes using data from Sentinel-2 and ALOS PALSAR, showing that combining variables from both sensors improved estimation accuracy.