Are greener forests stronger carbon sinks? Rising vapour pressure deficit instigates greening-productivity decoupling in a tropical biodiversity hotspot in India

Forests across the humid tropics are widely reported to be “greening”, leading to the assumption that they will remain reliable carbon sinks under changing climate. We explored this critically in the forests of North Eastern Region of India (NER), a climatically sensitive biodiversity hotspot containing ∼1/3rd of India's forest carbon. Using 24 years (2001–2024) of satellite-derived canopy structure (LAI), photosynthesis (GPP) and productivity (NPP) data, we designed a divergence framework to evaluate whether increases in vegetation activity translate into sustained carbon gains. Results revealed that despite stable canopy structure and modest increases in GPP (+1.76 g C m -2 yr -1 ), NPP declined across much of the region (-0.83 g C m -2 yr -1 ), suggesting that photosynthetic gains are increasingly decoupled from net carbon gain. Structural-functional divergence (LAI-NPP decoupling) affected ∼40% of forested areas, while photosynthetic-productive divergence (GPP-NPP decoupling) occurred across ∼30% of the area, revealing a progressive weakening of carbon-use efficiency. Breakpoint analysis identified a reorganization of forest carbon functioning beginning in the early 2010s. Coupling between canopy structure and photosynthesis weakened, while the conversion of photosynthetic gains into net carbon accumulation became increasingly constrained. Pixel-wise climatic causal analysis further identified vapour pressure deficit (VPD) as a primary driver of the productivity declines. Our results demonstrate that NER forests are not collapsing structurally but are undergoing a concealed weakening of carbon sink strength. Greenness-based assessments can substantially overestimate sink strength, underscoring the need to incorporate atmospheric stress, carbon-use efficiency, and functional vulnerability into carbon accounting and forest management under a warming climate. Our findings further highlight how biological responses to atmospheric dryness may decouple structural and functional forest carbon dynamics at landscape scale.