• Cloud cover and altitude jointly shape sap flow responses. • Low clouds suppress sap flow; high cloud bases can enhance it. • Roles of sap flow drivers differ between low- and high-altitude cloud days. • Declining low cloud cover could increase boreal forest transpiration over time. Transpiration drives up to 40 % of terrestrial precipitation, with forests playing a critical role. This study combines long-term sap flow measurements and surface-based cloud observations to examine how different clouds affect tree transpiration across boreal and temperate European forests. Under specific cloudy conditions, sap flow can exceed clear-sky levels, reflecting distinct radiation effects of various cloud types. However, overall cloudiness reduces maximum sap flow by up to 40 %. A key finding is the contrasting influence of low- and high-altitude clouds: low-altitude clouds suppress transpiration by limiting incoming radiation, while high-altitude clouds have negligible impact even when overcast. Structural equation modelling further indicates a pathway linking transpiration to cloudiness when other meteorological and site factors are accounted for. Satellite data show a decline in low‑altitude cloud fraction over boreal forests, and a highly simplified model-based order‑of‑magnitude estimate suggests a potential associated increase in transpiration equivalent to ∼0.6–1.2 mm of precipitation annually. These results emphasize how climate change driven changes in cloud cover and type may alter the moisture flux to the atmosphere, impacting regional and global water cycles.
Talvinen et al. (Thu,) studied this question.