Abstract 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.

中文翻译：

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在潮湿的北方地区，即使在短期干旱期间，蒸气压不足和太阳辐射也是香脂冷杉和黑云杉树种蒸腾作用的主要驱动因素

摘要 在植被覆盖的陆地表面，与土壤和冠层蒸发相比，树木蒸腾作用是将大量水分送回大气的主要过程。虽然已经在一系列生态系统中的一系列树种上研究了树木蒸腾作用的驱动力，但尚未对加拿大东部潮湿的北方森林中的香脂冷杉和黑云杉进行研究。因此我们研究了环境变量之间的关系位于加拿大魁北克省两个森林地点的这两种北方树种在多个生长季节（香脂冷杉为 2004 至 2013 年，黑云杉为 2006 至 2009 年）的树液流速（作为蒸腾作用的代表）。我们的结果表明，每日树液流量与两种物种的蒸汽压差 (VPD) 都具有很强的非线性关系。树液流也与两个物种的太阳辐射 (Rad) 密切相关，尽管与 VPD 的关系略弱。其他变量，如每日最高温度和降水，解释了树液流量变化的一小部分，而土壤含水量 (SWC) 和风速几乎没有影响。多年来每小时对树液流量和 VPD/Rad 之间的关系进行的分析表明，这两种物种都有很强的日照滞后现象。与先前提出的相反，这种滞后的大小似乎与异/异羟基的程度无关。最后，我们对香脂冷杉场地干旱期间树液流量与环境变量的关系的调查表明，尽管 SWC 显着下降，但树液流量仅略有减少。另一方面，VPD 和 Rad 仍然是树液流的主要驱动因素。这项研究强调，VPD 和 Rad 确实是生长季节以及北方潮湿地区干旱期间蒸腾作用的主要驱动因素。