A Comparison of Metrics of Forest Structure and Above-Ground Carbon to Assess Restoration of Tropical Forest Ecosystems

Carbon accumulation and structural development are key indicators of the progress of forest-ecosystem restoration. However manual techniques of measuring them are time-consuming, labor-intensive and costly. Therefore, we tested four instrument-based metrics, as potential alternatives to conventional measurements, in a control site (non-restored), reference forest and 1½- and 11½-year-old forest undergoing restoration by the framework species method (FSM). Vegetation area index (VAI) from terrestrial LiDAR, leaf area index (LAI) from a plant canopy analyser, and canopy cover from hemispherical photography (CC_HP) and a densiometer (CC_D), clearly distinguished among the control site and 1½- and 11½-year-old-restoration (<0.05) (except for CC_HP in the control plot). They correlated well (R=0.58-0.80) with manual metrics (tree stocking density (TSD), basal area (BA) and above-ground carbon (AGC)), although the correlations weakened, with increasing structural development. However, the instrument-based metrics failed to reflect a doubling in AGC between 11½-year-old restoration and the reference forest, by under-estimating increases in structural development beyond canopy closure. CC_D is recommended for monitoring structural development, during early forest restoration, due to its cost-effectiveness, ease of use and minimal disturbance of the forest understory. After canopy closure, AGC remains the most useful metric to gauge how closely restoration achieves reference-forest conditions. After 11½ years of implementing the FSM, AGC had reached 49% (65.9 tC/ha, ±SD 30.44) of the reference forest level (137.4 tC/ha, ±SD 83.19).