Department of Natural Resources & the Environment, University of New Hampshire, 56 College Rd, Durham, NH 03824, USA.
Earth Systems Research Center, University of New Hampshire, 8 College Rd, Durham, NH 03824, USA.
Tree Physiol. 2023 Sep 6;43(9):1514-1532. doi: 10.1093/treephys/tpad070.
Tropical montane cloud forests (TMCFs) are expected to experience more frequent and prolonged droughts over the coming century, yet understanding of TCMF tree responses to moisture stress remains weak compared with the lowland tropics. We simulated a severe drought in a throughfall reduction experiment (TFR) for 2 years in a Peruvian TCMF and evaluated the physiological responses of several dominant species (Clusia flaviflora Engl., Weinmannia bangii (Rusby) Engl., Weinmannia crassifolia Ruiz & Pav. and Prunus integrifolia (C. Presl) Walp). Measurements were taken of (i) sap flow; (ii) diurnal cycles of stem shrinkage, stem moisture variation and water-use; and (iii) intrinsic water-use efficiency (iWUE) estimated from foliar δ13C. In W. bangii, we used dendrometers and volumetric water content (VWC) sensors to quantify daily cycles of stem water storage. In 2 years of sap flow (Js) data, we found a threshold response of water use to vapor pressure deficit vapor pressure deficit (VPD) > 1.07 kPa independent of treatment, though control trees used more soil water than the treatment trees. The daily decline in water use in the TFR trees was associated with a strong reduction in both morning and afternoon Js rates at a given VPD. Soil moisture also affected the hysteresis strength between Js and VPD. Reduced hysteresis under moisture stress implies that TMCFs are strongly dependent on shallow soil water. Additionally, we suggest that hysteresis can serve as a sensitive indicator of environmental constraints on plant function. Finally, 6 months into the experiment, the TFR treatment significantly increased iWUE in all study species. Our results highlight the conservative behavior of TMCF tree water use under severe soil drought and elucidate physiological thresholds related to VPD and its interaction with soil moisture. The observed strongly isohydric response likely incurs a cost to the carbon balance of the tree and reduces overall ecosystem carbon uptake.
热带山地云雾林（TMCF）预计在未来一个世纪将经历更频繁和更长时间的干旱，但与低地热带地区相比，对 TMCF 树木对水分胁迫的响应的理解仍然薄弱。我们在秘鲁的一个热带山地云雾林中进行了为期 2 年的穿透雨减少实验（TFR），以模拟严重干旱，并评估了几种优势物种（Clusia flaviflora Engl.，Weinmannia bangii（Rusby）Engl.，Weinmannia crassifolia Ruiz & Pav.和 Prunus integrifolia（C. Presl）Walp）的生理反应。测量了（i）液流；（ii）茎收缩、茎水分变化和水分利用的日周期；和（iii）从叶片 δ13C 估算的内在水分利用效率（iWUE）。在 W. bangii 中，我们使用树轮和体积含水量（VWC）传感器来量化茎水储存的日周期。在 2 年的液流（Js）数据中，我们发现无论处理方式如何，水分利用对水汽压亏缺（VPD）>1.07 kPa 的阈值响应，但对照树比处理树使用更多的土壤水。TFR 树木的日用水量下降与给定 VPD 下上午和下午 Js 速率的强烈降低有关。土壤水分也影响 Js 和 VPD 之间的滞后强度。在水分胁迫下，滞后性降低意味着 TMCF 强烈依赖于浅层土壤水。此外，我们认为滞后性可以作为植物功能对环境限制的敏感指标。最后，实验进行到 6 个月时，TFR 处理显著增加了所有研究物种的 iWUE。我们的结果突出了 TMCF 树木在严重土壤干旱下的保守用水行为，并阐明了与 VPD 及其与土壤水分相互作用相关的生理阈值。观察到的强烈等水响应可能会给树木的碳平衡带来成本，并降低整个生态系统的碳吸收。
