Air temperature and vapor pressure deficit (VPD) have been continuously rising under global warming. These two factors influence vegetation productivity via different physiological and ecological pathways. However, their individual effects have remained unclear at larger spatial scales.

In a study published in Journal of Hydrology, researchers from the Xishuangbanna Tropical Botanical Garden (XTBG) of the Chinese Academy of Sciences, and their collaborators, assessed the individual effects of air temperature and VPD on vegetation productivity across China.

Researchers used solar-induced chlorophyll fluorescence and gross primary productivity as proxies for vegetation health. They disentangled the effects of air temperature and VPD on gross primary productivity through statistical methods including convergent cross mapping and partial least squares structural equation modeling.

VPD was found to consistently have a significant negative effect on vegetation productivity which was especially strong in arid and semi-arid regions. Air temperature was found to have a generally positive effect on productivity when its interactions with other factors were statistically isolated. But when high temperatures co-occurred with high VPD, this positive effect was significantly weakened, highlighting a critical synergistic effect.

Furthermore, researchers found that the negative effect of VPD is expected to intensify in the future, while the positive effect of air temperature is projected to diminish. These shifts are forecasted to be most significant in mid-latitude regions.

"Our work shows that VPD is not only a side effect but a critical regulator of ecosystem function. Its growing negative effect can severely undermine the ability of vegetation to act as a carbon sink in the future, particularly in mid-latitude regions," said YANG Jie of XTBG.

To enhance ecosystem and agricultural drought resilience, researchers recommend planting drought-tolerant species in humid regions, establish integrated early warning systems for agricultural drought, modernizing irrigation infrastructure, and optimizing water resource allocation.