Guérin Marceau, Martin-Benito Dario, von Arx Georg, Andreu-Hayles Laia, Griffin Kevin L, Hamdan Rayann, McDowell Nate G, Muscarella Robert, Pockman William, Gentine Pierre
Department of Earth and Environmental Engineering Columbia University New York NY USA.
Forest Ecology Department of Environmental Sciences Swiss Federal Institute of Technology ETH Zurich Zürich Switzerland.
Ecol Evol. 2018 Jan 5;8(3):1655-1672. doi: 10.1002/ece3.3743. eCollection 2018 Feb.
In the southwestern USA, recent large-scale die-offs of conifers raise the question of their resilience and mortality under droughts. To date, little is known about the interannual structural response to droughts. We hypothesized that piñon pines () respond to drought by reducing the drop of leaf water potential in branches from year to year through needle morphological adjustments. We tested our hypothesis using a 7-year experiment in central New Mexico with three watering treatments (irrigated, normal, and rain exclusion). We analyzed how variation in "evaporative structure" (needle length, stomatal diameter, stomatal density, stomatal conductance) responded to watering treatment and interannual climate variability. We further analyzed annual functional adjustments by comparing yearly addition of needle area (LA) with yearly addition of sapwood area (SA) and distance to tip (), defining the yearly ratios SA:LA and SA:LA/. Needle length () increased with increasing winter and monsoon water supply, and showed more interannual variability when the soil was drier. Stomatal density increased with dryness, while stomatal diameter was reduced. As a result, anatomical maximal stomatal conductance was relatively invariant across treatments. SA:LA and SA:LA/ showed significant differences across treatments and contrary to our expectation were lower with reduced water input. Within average precipitation ranges, the response of these ratios to soil moisture was similar across treatments. However, when extreme soil drought was combined with high VPD, needle length, SA:LA and SA:LA/ became highly nonlinear, emphasizing the existence of a response threshold of combined high VPD and dry soil conditions. In new branch tissues, the response of annual functional ratios to water stress was immediate (same year) and does not attempt to reduce the drop of water potential. We suggest that unfavorable evaporative structural response to drought is compensated by dynamic stomatal control to maximize photosynthesis rates.
在美国西南部，近期针叶树的大规模死亡引发了关于其在干旱条件下恢复力和死亡率的问题。迄今为止，对于干旱的年际结构响应了解甚少。我们假设矮松（）通过每年通过针叶形态调整来减少树枝中叶水势的下降来应对干旱。我们在新墨西哥州中部进行了一项为期7年的实验，采用三种浇水处理方式（灌溉、正常浇水和防雨）来验证我们的假设。我们分析了“蒸发结构”（针叶长度、气孔直径、气孔密度、气孔导度）的变化如何响应浇水处理和年际气候变率。我们还通过比较每年针叶面积（LA）的增加量与边材面积（SA）的增加量以及到树梢的距离（），定义每年的SA:LA和SA:LA/比值，进一步分析了年度功能调整。针叶长度（）随着冬季和季风供水量的增加而增加，并且在土壤较干燥时表现出更大的年际变率。气孔密度随着干燥程度的增加而增加，而气孔直径减小。结果，解剖学上的最大气孔导度在各处理间相对不变。SA:LA和SA:LA/在各处理间显示出显著差异，与我们的预期相反，随着水分输入减少，这些比值更低。在平均降水范围内，这些比值对土壤湿度的响应在各处理间相似。然而，当极端土壤干旱与高水汽压差（VPD）相结合时，针叶长度、SA:LA和SA:LA/变得高度非线性，这强调了高VPD和干燥土壤条件相结合时响应阈值的存在。在新的树枝组织中，年度功能比值对水分胁迫的响应是即时的（同年），并且不会试图减少水势的下降。我们认为，对干旱不利的蒸发结构响应通过动态气孔控制得到补偿，以最大化光合速率。
