What are the Potential CO2 Emission Offsets for Thorn Woodlands? A Promising Remote Sensing Approach for Mapping Carbon Sequestration

Quantifying carbon sequestration poses a challenge for monitoring and assessment of recently established woody plants with relatively small canopies. Multispectral remote sensing can increase efficacy of carbon assessment in woodlands for large spatial extents by using fine grained data. In this study, we demonstrate a novel crown identification algorithm using Digital Ortho Quarter Quadrangle (DOQQ) data, with 1 m spatial resolution and three bands in the green, red, and near-infrared wavelengths, to determine canopy attributes for woodlands in the Lower Rio Grande Valley, USA, which as part of the U.S. Fish and Wildlife South Texas Refuge Complex. We applied this algorithm for areas within the refuge covering 7496 ha of thorn woodlands which had been restored through natural regeneration and replanting since the early 1990’s. On average, the algorithm delineated 83% of individual crowns, though calibration of algorithm thresholds was necessary for different areas. From derived canopy attributes, carbon stored by individual plants was calculated from allometric equations developed for three shrub species for this biotic province. The remotely sensed data estimated plant densities of 25 individuals/ha in naturally regenerated and 20 individuals/ha for replanted areas. From these data, we calculated average aboveground carbon of 1.6 kg/plant for naturally regeneration area and 0.5 kg/plant for replanted areas. We estimated an average value of 1.41+ 0.01 Mg/ha of carbon stored by woody plants in natural regeneration areas compared to 1.1+0.01 Mg/ha in replanted areas. Belowground biomass estimated from aboveground carbon density literature values with 0.41+0.01 Mg/ha and 0.49+0.01 Mg/ha for natural regeneration versus replanted areas, respectively. Based on these derived values, we estimate that woody plants for the entire refuge complex have sequestered 20516 Mg carbon, with approximately 6% of this amount attributed to restoration.