Young secondary tropical forests occupy a larger area than mature forests nowadays but our understanding of their ecohydrological functioning, particularly with respect to tree water uptake, remains poor. Deep soil water uptake may make mature forests resilient to periods of water stress, but little is known in this regard for young forests with possibly less extensive root networks. We, therefore, studied sap flow dynamics for one year in two 50 m x 50 m forest plots: a young secondary forest (YSF, 5–7 years) and a semi-mature forest (SMF; 20 years) in montane eastern Madagascar. Temporal variations in the depth of water uptake were inferred from the stable isotope compositions of soil- and xylem water. Transpiration rates were low for both forest sites (265 and 462 mm y⁻¹ for the YSF and SMF, respectively). Vapour pressure deficit and global radiation explained most of the variation in transpiration rates at both sites. There was little evidence of transpiration limitation by soil water, despite an extended dry season. Trees in the YSF extracted water mostly from the intermediate soil depth (30–70 cm) during the dry season. In the SMF, the depth of uptake increased as the dry season progressed for some species (Abrahamia, Brachylaena and Cryptocaria), but not for others (Ocotea and Eugenia). Although the transpiration rates are low for both forests, they are comparable to results reported for other tropical montane sites after normalising for net energy input and leaf area. Estimated evapotranspiration totals (including interception loss, understorey and litter evaporation) were 679 mm and 1063 mm y⁻¹ for the YSF and SMF, respectively (42% and 61% of precipitation, respectively). These results suggest that the stage of forest regrowth affects water uptake, and thus the water balance during forest succession.

现在，年轻的次生热带森林比成熟森林占据更大的面积，但我们对其生态水文功能的了解，特别是在树木吸水方面的了解仍然很差。深层土壤吸水可能使成熟森林能够抵御水分胁迫，但在这方面，对于根系网络可能不太广泛的年轻森林知之甚少。因此，我们在两个 50 m x 50 m 的森林地块中研究了一年的液流动力学：马达加斯加东部山区的一个年轻次生林（YSF，5-7 年）和一个半成熟森林（SMF；20 年）。从土壤和木质部水的稳定同位素组成推断出吸水深度的时间变化。两个森林地点的蒸腾速率都很低（265 和 462 mm y ^-1分别用于 YSF 和 SMF）。蒸汽压不足和全球辐射解释了两个地点蒸腾速率的大部分变化。尽管旱季延长，但几乎没有证据表明土壤水分会限制蒸腾作用。YSF 中的树木在旱季主要从中等土壤深度（30-70 厘米）中提取水分。在 SMF 中，某些物种（ Abrahamia、Brachylaena和Cryptocaria）的吸收深度随着旱季的进展而增加，但其他物种（ Ocotea和Eugenia ）则没有）。尽管这两种森林的蒸腾速率都很低，但在净能量输入和叶面积标准化后，它们与其他热带山地地区报告的结果相当。YSF 和 SMF 的估计蒸散总量（包括截流损失、林下和枯枝落叶蒸发）分别为 679 mm 和 1063 mm y ^-1（分别为降水的 42% 和 61%）。这些结果表明，森林再生阶段会影响水分吸收，从而影响森林演替过程中的水分平衡。