Vapour pressure deficit and solar radiation are the major drivers of transpiration of balsam fir and black spruce tree species in humid boreal regions, even during a short-term drought

On vegetation-covered land surfaces, tree transpiration, compared to soil and canopy evaporation, is a major process that sends large amounts of water back to the atmosphere. While the driving forces of tree transpiration have been studied over a range of tree species across an array of ecosystems, no work has been done on balsam fir and black spruce in the humid boreal forest of eastern Canada.We thus studied the relationships between environmental variables and sap flow velocity (as a proxy for transpiration) for these two boreal tree species located at two forest sites in Quebec, Canada over multiple growing seasons (2004 to 2013 for balsam fir and 2006 to 2009 for black spruce). Our results showed that daily sap flow had a strong non-linear relationship with vapour pressure deficit (VPD) for both species. Sap flow was also strongly correlated to solar radiation (Rad) for both species although with slightly weaker relationships than for VPD. Other variables such as daily maximum temperature and precipitation explain a smaller portion of the variance in sap flow while soil water content (SWC) and wind speed had almost no effect. An analysis of the relationships between sap flow and VPD/Rad on an hourly basis over multiple years showed strong diel hysteresis for both species. Contrary to what has been previously proposed, the magnitude of this hysteresis does not seem to relate to the degree of iso/anisohydricity. Finally, our investigation of sap flow relationships to environmental variables during a drought period at the balsam fir site showed that sap flow was only slightly reduced despite a significant decrease in SWC. On the other hand, VPD and Rad remained the main drivers of sap flow. This study emphasizes that VPD and Rad are indeed the major drivers of transpiration during the growing season as well as during drought in humid boreal region.