Compound hot extremes exacerbate forest growth decline in dry areas but not in humid areas in the Northern Hemisphere.

• Compound hot extremes did not exacerbate tree growth across 70% of the forests. • Compound hot extremes have started to exacerbate tree growth decline in forests in dry regions. • Growth reduction by compound hot extremes showed a significant increase over the past decades. Under climate warming, the adverse effects of daytime or nighttime hot extremes on forest productivity have been widely reported. However, the impact of compound hot extremes (i.e., combination of daytime and nighttime hot extremes) on tree growth remains unclear. Combining long-term and large-scale tree-ring width and remote-sensing datasets between 1949 and 2020, we examined and compared the effects of individual and compound hot extremes on tree growth and forest productivity in the Northern Hemisphere. Here we showed that compound hot extremes did not exacerbate tree growth and productivity decline compared with individual daytime hot extremes across 70% of the forested area. However, we observed a stronger adverse effect of compound hot extremes on tree growth and productivity compared with individual daytime hot extremes in dry regions. Due to the increased severity and frequency of compound hot extremes, we predicted that compound hot extremes may impose a greater threat to forest productivity and global carbon cycling as compared to individual hot extremes under future climate change. Our results call for effective policies and actions to mitigate the rapidly growing adverse impact of compound hot extremes on forest ecosystems and carbon cycling. [Display omitted] Compound hot extremes generally exacerbate tree growth decline compared with daytime hot extremes in forests in dry regions but not in humid regions across the Northern Hemisphere, possibly due to the counterbalancing effects of daytime and nighttime hot extremes on water availability. [ABSTRACT FROM AUTHOR]