Department of Biology, MSC03 2020, 1 University of New Mexico Albuquerque, New Mexico, 87131-0001.
Centre d'Ecologie Fonctionnelle et Evolutive CEFE, UMR5175, CNRS, Université de Montpellier, Université Paul-Valéry Montpellier, EPHE 1919 Route de Mende, Montpellier Cedex 5, 34293, France.
Ecol Evol. 2015 Apr;5(8):1618-38. doi: 10.1002/ece3.1422. Epub 2015 Mar 23.
Plant hydraulic conductance (k s) is a critical control on whole-plant water use and carbon uptake and, during drought, influences whether plants survive or die. To assess long-term physiological and hydraulic responses of mature trees to water availability, we manipulated ecosystem-scale water availability from 2007 to 2013 in a piñon pine (Pinus edulis) and juniper (Juniperus monosperma) woodland. We examined the relationship between k s and subsequent mortality using more than 5 years of physiological observations, and the subsequent impact of reduced hydraulic function and mortality on total woody canopy transpiration (E C) and conductance (G C). For both species, we observed significant reductions in plant transpiration (E) and k s under experimentally imposed drought. Conversely, supplemental water additions increased E and k s in both species. Interestingly, both species exhibited similar declines in k s under the imposed drought conditions, despite their differing stomatal responses and mortality patterns during drought. Reduced whole-plant k s also reduced carbon assimilation in both species, as leaf-level stomatal conductance (g s) and net photosynthesis (A n) declined strongly with decreasing k s. Finally, we observed that chronically low whole-plant k s was associated with greater canopy dieback and mortality for both piñon and juniper and that subsequent reductions in woody canopy biomass due to mortality had a significant impact on both daily and annual canopy E C and G C. Our data indicate that significant reductions in k s precede drought-related tree mortality events in this system, and the consequence is a significant reduction in canopy gas exchange and carbon fixation. Our results suggest that reductions in productivity and woody plant cover in piñon-juniper woodlands can be expected due to reduced plant hydraulic conductance and increased mortality of both piñon pine and juniper under anticipated future conditions of more frequent and persistent regional drought in the southwestern United States.
植物水力传导度（ks）是控制植物整体水分利用和碳吸收的关键因素，在干旱期间，它会影响植物的生存或死亡。为了评估成熟树木对水分可用性的长期生理和水力响应，我们在 2007 年至 2013 年期间操纵了生态系统尺度的水分可用性，在一个矮松（Pinus edulis）和刺柏（Juniperus monosperma）林地中进行了实验。我们使用超过 5 年的生理观测数据，研究了 ks 与随后的死亡率之间的关系，并研究了水力功能降低和死亡率对总木质冠层蒸腾（EC）和导度（GC）的后续影响。对于这两个物种，我们观察到在实验性干旱条件下，植物蒸腾（E）和 ks 显著降低。相反，补充水分的添加增加了这两个物种的 E 和 ks。有趣的是，尽管这两个物种在干旱期间表现出不同的气孔反应和死亡率模式，但它们在强制干旱条件下的 ks 下降幅度相似。整个植物 ks 的降低也降低了这两个物种的碳同化，因为叶片水平的气孔导度（gs）和净光合作用（An）随着 ks 的降低而强烈下降。最后，我们观察到，矮松和刺柏的慢性低整体植物 ks 与树冠萎蔫和死亡率增加有关，由于死亡率导致木质冠层生物量减少，对每日和年度冠层 EC 和 GC 有重大影响。我们的数据表明，在这个系统中，ks 的显著降低先于与干旱相关的树木死亡事件，其后果是树冠气体交换和碳固定的显著减少。我们的结果表明，由于水力传导度降低和矮松和刺柏的死亡率增加，预计在美国西南部地区未来更频繁和持续的区域干旱条件下，矮松-刺柏林地的生产力和木本植物覆盖率将会降低。
