The intensification of climate change-induced drought results in unprecedented tree and forest die-offs worldwide, increasingly driven by compound droughts. In this review, we examine the impacts of emerging compound droughts, which involve co-occurring stressors like soil drought and high temperature, along with elevated vapour pressure deficit over prolonged periods and at higher frequency. We explore the physiological and ecological mechanisms underlying tree water and carbon regulation during these extreme conditions, focusing on the balance between water demand and supply, the role of acclimation, and its consequences for ecosystem-level functions. By examining the mechanisms at play from the organ to the ecosystem-scale, we provide a comprehensive understanding of how trees and forests are likely to respond to an increasingly unpredictable climate with a higher likelihood of compound droughts.