Seasonality of albedo and fraction of absorbed photosynthetically active radiation in the temperate secondary forest ecosystem: A comprehensive observation using Qingyuan Ker towers

Reforestation have been highlighted as a significant intervention for climate change by altering the land surface albedo and vegetation productivity. Previous studies have reported the effect of land-use/cover change on the land surface albedo, yet few studies have examined how changed understory and canopy structures derived from reforestation contribute to forest albedos and further influence fraction of absorbed photosynthetically active radiation (FAPAR, a proxy of gross primary productivity). Here we continuously measured canopy effective leaf area index (LAI_e) and understory greenness in the temperate secondary forest ecosystem. Albedos in three spectral regions (global solar radiation, SR; photosynthetically active radiation, PAR; near-infrared radiation, NIR) and FAPAR were monitored using the Qingyuan Ker Towers (three towers in a watershed; natural mixed broadleaved forest, T1-MBF; natural Mongolian oak forest, T2-MOF; larch plantation forest, T3-LPF), and their relationships to canopy LAI_e and understory greenness were examined. The result showed that SR and NIR albedos of MBF and MOF were greater than those of LPF, and their PAR albedos were comparable. SR and NIR albedos were comparably and positively correlated with canopy LAI_e and understory greenness, while PAR albedos were negatively correlated with canopy LAI_e and understory greenness. Understory greenness explained 75.6% of changes in NIR albedo of LPF in the peak growing season, indicating a strong control of understory vegetation on the albedos. In addition, SR albedos were significantly correlated with FAPAR, with stronger correlations in MBF (r = 0.69) and MOF (r = 0.93) than that in LPF (r = 0.3), showing a co-benefit of carbon uptake and albedo in broadleaved stands, but the co-benefit was weakened in coniferous plantation stand. These findings suggested that increasing vegetation green biomass and introduction of broadleaved trees into monocultural coniferous plantation may increase forest albedo and co-benefit of albedo and FAPAR to combat climate warming.