Unignorable carbon sequestration potential of afforested forests in the breadbasket of the Tibetan Plateau

The Middle Reach of the Yarlung Zangbo River (MRYZ) plays a crucial role as the primary grain-producing region of Tibet, and it is also known as the breadbasket of the Tibetan Plateau. However, this region is confronted with significant environmental challenges, primarily driven by its cold and arid climate, compounded by the exacerbating effects of human activities. One of the most pressing issues is aeolian desertification, which leads to the loss of soil and vegetation cover due to wind erosion, threatening both agricultural productivity and ecosystem stability. To address these issues, China has launched a series of large-scale afforestation programs in the region, including the Project of Grain to Green, the Project of Two Rivers and Four Streams, and the Ecological Safety Barrier Protection and Construction Plan for Xizang. These efforts have focused on mitigating desertification, improving soil conditions, enhancing vegetation greenness, and restoring ecological balance, resulting in notable improvements in land quality and agricultural productivity. Beyond these expected benefits, long-term afforestation may yield unforeseen ecological benefits, particularly with respect to carbon sequestration, which could significantly contribute to climate change mitigation and bolster regional carbon storage.

Carbon sequestration studies have traditionally concentrated on forest ecosystems with favorable climatic conditions.1 Due to their high biomass and large carbon storage capacity, these ecosystems are widely recognized as major contributors to the global terrestrial carbon sink. In contrast, arid and semi-arid regions such as the MRYZ have received far less attention, largely due to the assumption that harsh environmental conditions limit their potential for carbon sequestration, although recent studies suggest that carbon storage in global drylands may have been substantially underestimated.2 Afforestation in these regions can significantly alter local hydrothermal dynamics, reducing surface temperature, improving soil water retention, and enhancing vegetation growth, thereby promoting carbon sequestration. However, it may also intensify water stress by depleting deep soil moisture and groundwater and reducing downstream river flows, especially under high-density plantations of non-native species. Within this context, we explore whether long-term afforestation in this dry valley can mitigate desertification and promote carbon accumulation. Our findings highlight the broader significance of hydrological trade-offs associated with afforestation and underscore the need to reassess its carbon sequestration potential in water-limited regions.

Ecological transformation mechanism for carbon sequestration

Afforestation in the MRYZ has driven a profound ecological transformation that progressively mitigates desertification and modifies local hydrothermal conditions,3 thereby influencing regional carbon sequestration potential. To quantify these changes, we identified afforested and non-afforested areas across the seven counties (i.e., Sangzhuzi, Rinbung, Nyemo, Chushur, Gonggar, Danang, and Nanmulin) traversed by the MRYZ. Forest cover outside the afforested areas is very limited, and the surrounding non-afforested landscapes are mainly composed of shrublands and sparse grasslands. Using the MOD17A2 product, we estimated the regional gross primary productivity (GPP) for afforested and non-afforested areas by averaging the values of all pixels within each area. The annual GPP in afforested areas was found to increase from 200.01 gC m−2 year−1 in 2001 to 254.25 gC m−2 year−1 in 2025, representing an increase of 27.1%, whereas surrounding non-afforested landscapes increased by 15.0%. However, the contrast between afforested and non-afforested areas may also reflect differences in vegetation composition and stand age rather than afforestation effects alone. To establish a more robust benchmark, we further evaluated biomass carbon accumulation in the MRYZ against that of planted forests across China using the same age-biomass equation.4 Carbon density in afforested areas rose by 20.72 Mg C ha−1 between 2005 and 2020, exceeding the national average increase of 15.50 Mg C ha−1 observed in planted forests across China during the same period.4 This sustained biomass accumulation has substantially increased regional carbon storage, which was estimated at 2.79 Tg C in 2025 and projected to reach 4.82 Tg C by 2060, even without further afforestation.