Carbon stock and sequestration of planted and natural forests along climate gradient in water-limited area: A synthesis in the China's Loess plateau

Large-scale afforestation has been widely implemented for restoring the degraded lands and mitigating climate change while naturally regenerated forests should be protected to preserve carbon (C) in land ecosystems. However, a critically needed comparison on the effectiveness of C sequestration between planted forest (PF) and natural forest (NF) in water-limited areas has rarely been conducted at a regional scale. This study synthesized 275 publications to examine the changes of C stock in above-ground biomass (AGB), below-ground biomass (BGB) and deep soils (0-200 cm) between PF and NF in the China's Loess Plateau, a typical water-limited region with extensive afforestation. We found that, NF stored more than twice as much C as did PF in both biomass and soil. But PF allocated relatively higher proportion of C stock to BGB and deep soil layer (> 100 cm) than NF. Moreover, the C sequestration rates in biomass and soil for PF were 1.42 and 1.72 Mg C ha⁻¹ yr⁻¹, respectively, much higher than that of NF (0.13 and -1.38 Mg C ha⁻¹ yr⁻¹). In addition, afforestation in areas with the mean annual precipitation (MAP) greater than 505 mm may promote both biomass and soil C accumulation without causing severe soil water deficit. Moreover, the increase in mean annual temperature (MAT) could significantly promote the C accumulation in both biomass and soil of PF but accelerated the soil C loss of NF. Overall, our results indicate that afforestation in water-limited areas may exhibit higher potential to sequester C than naturally regeneration, and afforestation is worth promoting with careful consideration of planting density and rainfall zones to avoid causing excessive water depletion. This study has important implications for the forest management in water-limited areas particularly under global climate change.