Knowledge of plant hydraulics facilitates our understanding of the capabilities of forests to withstand droughts. This common-garden study quantified the hydraulic response to variation in sandy soil conductivity and atmospheric vapor pressure deficit (VPD) of five morphologically contrasting, wide-ranging pine species (Pinus virginiana, P. echinata, P. taeda, P. elliottii, P. palustris) of the Southeastern US, a region experiencing relatively high occurrence of hydrological droughts, which are projected to increase in frequency and severity. We employed a Bayesian hierarchical model to estimate xylem hydraulic parameters associated with drought vulnerability curves (VC) for terminal branches and shallow roots. We found that branches in all of the pine species were more resistant to cavitation-induced embolism and had greater hydraulic safety margin than roots. Among all species, P50 (i.e., water potential at which 50 % conductivity is lost) and S50 (i.e., the slope of VC centered on P50) of roots showed an increasing trend from shorter- to longer-needle species. By contrast, hydraulic conductivity at saturation (ksat) of either branches or roots did not exhibit any trend with needle length. We devised a simplified index for daily average canopy conductance (GcI), computed from high-frequency sap flux measurement. Regression of GcI showed that mean daytime VPD accounted for most of the variation in GcI (> 60 %), followed by unsaturated soil hydraulic conductivity (ksoil; 24 %), whereas in situ root conductivity accounted for the least (< 5 %). All species exhibited a linear-log relationship between the variation in GcI unexplained by VPD and ksoil, a pattern consistent with preventing soil water conditions from dropping to low levels where ksoil declines dramatically. We concluded that the shorter-needle species (P. virginiana and P. echinata) are likely to tolerate drought better than the other species, due to more resistant roots and a moderate-to-high sensitivity of GcI to VPD and ksoil.

中文翻译：

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在形态相反但同时出现的 5 种松树种中，冠层导度对干旱导致水汽压亏缺增加和土壤电导率降低的响应相似


植物水力学知识有助于我们了解森林抵御干旱的能力。这项公共花园研究量化了美国东南部五种形态对比鲜明、分布广泛的松树种（弗吉尼亚松、松果树、泰达松、埃利奥蒂松、帕鲁斯特里松）对沙质土壤电导率和大气蒸气压亏缺 （VPD） 变化的水力响应，该地区水文干旱的发生率相对较高，预计干旱的频率和严重程度会增加。我们采用贝叶斯分层模型来估计与末端分支和浅根的干旱脆弱性曲线 （VC） 相关的木质部水力参数。我们发现所有松树树种的树枝都比根更能抵抗空化诱导的栓塞，并且具有更大的水力安全边际。在所有物种中，根的 P50 （即失去 50 % 电导率的水势） 和 S50 （即以 P50 为中心的 VC 斜率） 显示出从短针物种到长针物种的增加趋势。相比之下，枝条或根的饱和水力传导率 （ksat） 没有表现出任何随针长变化的趋势。我们设计了一个简化的日平均冠层导度 （GcI） 指数，该指数由高频树液通量测量计算得出。GcI 回归显示，平均白天 VPD 占 GcI 变化的大部分（> 60 %），其次是非饱和土壤水力传导率（ksoil;24 %），而原位根传导率占最少（< 5 %）。 所有物种在 VPD 和 kssoil 无法解释的 GcI 变化之间都表现出线性对数关系，这种模式与防止土壤水分条件下降到低水平一致，而 ksoil 急剧下降。我们得出结论，较短的针叶物种 （P. virginiana 和 P. echinata） 可能比其他物种更耐干旱，因为根系更具抵抗力，并且 GcI 对 VPD 和 kssoil 具有中度至高度敏感性。