In the context of increasing heat periods and recurrence of droughts, and thus higher soil water depletion, we explored and quantified the role of understorey vegetation in ecosystem evapotranspiration in boreal and temperate forests. We reviewed and analysed about 200 papers that explicitly gave figures of understorey vegetation evapotranspiration relative to different stand features and traits. Understorey vegetation accounted on average for one-third of total ecosystem evapotranspiration during the growing season. Overstorey leaf area index (LAI) is the main variable that drives understorey evapotranspiration through radiation interception. Most data show that below an overstorey LAI of 2–3, the contribution of the understorey vegetation to ecosystem evapotranspiration increases exponentially, following the exponential increase of the climatic demand, i.e. potential evapotranspiration. Different factors have the potential to modulate this effect such as species composition and phenology, root distribution, and interaction with droughts. Consequently, managers must be aware that depending on understorey species present on site and stand structure, understorey vegetation can contribute significantly to a negative stand water balance.

在高温期增加和干旱再次发生以及土壤水分消耗增加的背景下，我们探索和量化了下层植被在北方和温带森林生态系统蒸散中的作用。我们回顾和分析了大约 200 篇论文，这些论文明确给出了与不同林分特征和性状相关的林下植被蒸散量数据。在生长季节，林下植被平均占生态系统总蒸散量的三分之一。上层叶面积指数（LAI）是通过辐射拦截驱动下层蒸散的主要变量。大多数数据表明，在 2-3 的上层 LAI 之下，下层植被对生态系统蒸散的贡献呈指数增长，随着气候需求的指数增长，即潜在蒸散量。不同的因素有可能调节这种影响，例如物种组成和物候、根系分布以及与干旱的相互作用。因此，管理人员必须意识到，根据现场存在的林下物种和林分结构，林下植被会显着导致林分水量负平衡。