Water use efficiency threshold for terrestrial ecosystem carbon sequestration in China under afforestation

• Afforestation gives rise to risk factors related to environmental degradation.
• WUE limitation on C sequestration under afforestation.
• Greatest C sequestration potential is the 400∼500 mm precipitation isoline.

Forests play a vital role in global carbon (C) cycling. Accordingly, afforestation engineering programs that promote increased terrestrial C stocks are an important means to help gradually decrease atmospheric CO2 emissions. China, however, had increased its afforested area bordering hydroclimatic zones to 275.71 million hm2 between 1949 and 2010. Ecosystem water use efficiency (EWUE) and plant water use efficiency (PWUE) provide data on ecosystem sensitivity to water availability across rainfall regimes. The water consumption cost of C sequestration (WCCC) is also an important parameter that gauges the cost of C sequestration under afforestation. However, abrupt changes in EWUE and PWUE (threshold values of 1.5 and 3.6 gC kg−1 H2O, respectively) have been measured within the 400–500 mm precipitation climatic isoline boundary situated between semi-humid and arid zones. The threshold value of the corresponding WCCC was 1.0 kg H2O gC−1. Forest ecosystems in China typically generate high EWUE and PWUE values (2.80 ± 0.77 and 4.25 ± 1.02 gC kg−1 H2O, respectively) but low WCCC values (0.52 ± 0.42 kg H2O g−1 C), providing proof that afforestation is the best choice in increasing terrestrial C stocks. However, China's major afforestation engineering programs have concentrated efforts toward low EWUE and PWUE and high WCCC in the western region of the 400–500 mm precipitation isoline boundary, belonging to the arid and semiarid zones, which introduced potential environmental risks. Therefore, policies related to large-scale C sequestration initiatives under afforestation must first fully consider the statuses of WCCC and WUE.