Mature stands of exotic tree species consume less water while sequester comparable carbon as those of native tree species: A 4-year monitoring study in South China

The wide application of fast-growing exotic and native tree species for reforesting degraded lands not only improves the ecological environment but more importantly, increases carbon sequestration to mitigate increasing atmospheric CO₂. Nevertheless, the controversial issue of changes in hydrology and carbon sequestration capacity that inevitably arise with the maturity of these fast-growing forests requires long-term monitoring research to clarify. This study investigated the water use and carbon assimilation patterns of exotic Acacia auriculiformis, Eucalyptus citriodora, and native Schima superba forests, based on 4-year sap flow monitoring combined with the multi-layer canopy conductance-constrained carbon assimilation model, and assessed their responses to environmental factors. The results reveal: (1) S. superba exhibited the highest average daily water use (Q_d) and forest transpiration (E_g). The average annual E_g for E. citriodora, A. auriculiformis and S. superba was 305.3, 359.9, 596.6 mm, respectively. (2) The average carbon assimilation rate (A_net) was 1.11, 1.13, 0.86 μmol·m⁻²·s⁻¹ and the mean annual forest carbon assimilation amount (C_stand) was 9.0, 10.7, 10.8 × 10³ t·ha⁻¹, for A. auriculiformis, E. citriodora and S. superba, respectively. (3) Environmental factors, along with the sap flux density and leaf photosynthetic parameters, explained 59 % to 75 % of forest A_net variance. The interaction of these factors explained 15 %, 9 %, and 18 % of A_net variance for A. auriculiformis, E. citriodora and S. superba, respectively. Despite consuming less water, mature exotic A. auriculiformis and E. citriodora forests sequestered comparable carbon to native S. schima forest, highlighting that exotics enhance carbon sequestration without adverse hydrological consequences as they mature.