Key message

Norway spruce operates with larger hydraulic safety margins (HSM) than beech and Douglas-fir despite the known drought sensitivity of spruce, questioning a pivotal role of HSM in drought tolerance.

Abstract

The exceptional 2018/2019 drought exposed Central Europe’s forests to severe stress, highlighting the need to better understand stomatal regulation strategies and their relationship to xylem safety under extreme drought. We studied diurnal, seasonal, and inter-annual variation in stomatal conductance (g_s) and leaf water potential (Ψ_Leaf) in co-occurring European beech (F. sylvatica), Norway spruce (P. abies), and Douglas-fir (P. menziesii) trees in the two summers and related them to hydraulic traits characterizing drought resistance. In 2018, F. sylvatica exhibited a continuous Ψ_Leaf decline from June to September, as is characteristic for an anisohydric strategy, while P. abies closed stomata early and reached the least negative Ψ_Leaf-values at the end of summer. P. menziesii showed low Ψ_Leaf-values close to P₁₂ (the xylem pressure at onset of embolism) already in July. Both conifers closed stomata when approaching P₁₂ and maintained low g_s-levels throughout summer, indicative for isohydric regulation. In 2019, all three species showed a linear decline in Ψ_Leaf, but F. sylvatica crossed P₁₂ in contrast to the conifers. The three species exhibited similar water potentials at turgor loss point (− 2.44 to − 2.51 MPa) and branch P₅₀ (xylem pressure at 50% loss of hydraulic conductance; − 3.3 to − 3.8 MPa). Yet, F. sylvatica and P. menziesii operated with smaller hydraulic safety margins (HSM means: 0.79 and 0.77 MPa) than P. abies (1.28 MPa). F. sylvatica reduced leaf size and specific leaf area in 2019 and increased Huber value. Our species comparison during extreme drought contradicts the general assumption that conifers operate with larger HSMs than angiosperm trees. Contrary to expectation, P. abies appeared as hydraulically less vulnerable than Douglas-fir.

中文翻译：

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严重干旱期间与欧洲山毛榉、挪威云杉和花旗松植物水势调节严格性相关的木质部安全

关键信息

尽管挪威云杉已知对干旱敏感，但挪威云杉的水力安全裕度 (HSM) 比山毛榉和花旗松更大，这对 HSM 在耐旱性方面的关键作用提出了质疑。

抽象的

2018/2019 年的异常干旱使中欧森林面临严重压力，凸显了需要更好地了解极端干旱下气孔调节策略及其与木质部安全的关系。我们研究了同时存在的欧洲山毛榉 ( F. sylvatica )、挪威云杉 ( P. abies ) 和花旗松的气孔导度 ( g _s ) 和叶水势 ( Ψ _Leaf )的日间、季节和年际变化。( P. menziesii ) 树在两个夏天并将其与抗旱水力性状相关。 2018 年，F. sylvatica从 6 月到 9 月表现出Ψ _{Leaf 的}持续下降，这是不等水策略的特征，而P. abies 则较早关闭气孔，并在夏末达到最小的Ψ _{Leaf负值。} P. menziesii在 7 月份就显示出接近P ₁₂（栓塞开始时的木质部压力）的低Ψ_叶值。两种针叶树在接近P ₁₂时都会关闭气孔，并在整个夏季保持较低的g _s水平，表明等水调节。 2019 年，所有三个物种的Ψ _Leaf均呈线性下降，但与针叶树相比，F. sylvatica跨越了 P _{12 。这三个物种在膨压损失点（− 2.44 至 − 2.51 MPa）和分支}P ₅₀（水力传导度损失 50% 时的木质部压力；− 3.3 至 − 3.8 MPa）表现出相似的水势。然而，F. sylvatica和P. menziesii 的水力安全裕度（HSM 指：0.79 和 0.77 MPa）比P. abies (1.28 MPa) 更小。F. sylvatica 2019 年的叶片尺寸和比叶面积减小，Huber 值增加。我们在极端干旱期间进行的物种比较与针叶树比被子植物树使用更大的 HSM 的普遍假设相矛盾。与预期相反，冷杉在水力方面似乎比花旗松更不易受到影响。