Drought limits the long-distance transport of water in the xylem due to the reduced leaf-to-soil water potential difference and possible embolism-related losses of conductance, and of sugars in the phloem due to the higher viscosity of the dehydrated sugary solution. This condition can have cascading effects in water and carbon fluxes that may ultimately cause tree death. We hypothesize that the maintenance of xylem and phloem conductances is fundamental for survival also under reduced resource availability, when trees may produce effective and low C cost anatomical adjustments in the xylem and phloem close to the treetop where most of the hydraulic resistance is concentrated.

We analyzed the treetop xylem and phloem anatomical characteristics in coexisting Scots pine trees symptomatic and non-symptomatic of drought-induced dieback. We selected the topmost 55 cm of the main stem and selected several sampling positions at different distances from the stem apex to test for differences in the axial patterns between the two groups of trees. We measured the annual ring area (RA), the tracheid hydraulic diameter (Dh) and cell wall thickness (CWT), the conductive phloem area (PAcond) and the average lumen diameter of the 20 largest phloem sieve cells (Dph).

Declining trees grew less than the non-declining ones, and despite the similar axial scaling of anatomical traits, had larger Dh and lower CWT. Moreover, declining trees had wider Dph.

Our results demonstrate that even under drought stress, maintenance of xylem and phloem efficiencies is of primary importance for survival, even if producing fewer larger tracheids may lead to a xylem more vulnerable to embolism formation.

中文翻译：

---

苏格兰松树通过增加木质部和韧皮部电导率来应对干旱。

干旱限制了木质部中水的远距离运输，这是由于叶与土壤之间的水势差减小以及可能与栓塞相关的电导损失，以及由于脱水含糖溶液的较高粘度导致韧皮部中的糖分减少。这种情况可能对水和碳通量产生连锁反应，最终可能导致树木死亡。我们假设木质部和韧皮部电导的维持对于资源减少时的生存也是至关重要的，当树木可能在靠近大部分水力阻力集中的树顶附近的木质部和韧皮部中产生有效且低成本的C成本解剖调整时。

我们分析了干旱引起的枯死的有症状和无症状共存的苏格兰松树的树梢木质部和韧皮部解剖特征。我们选择了主茎的最上端55厘米，并选择了距茎尖不同距离的几个采样位置，以测试两组树木之间的轴向模式是否不同。我们测量了年轮面积（RA），气管水力直径（Dh）和细胞壁厚度（CWT），导电韧皮部面积（PAcond）以及20个最大韧皮部筛细胞（Dph）的平均管腔直径。

下降的树木的生长少于未下降的树木，尽管解剖特征的轴向缩放相似，但具有更大的Dh和更低的CWT。此外，下降的树木具有更大的Dph。

我们的结果表明，即使在干旱胁迫下，维持木质部和韧皮部的效率对于生存至关重要，即使产生较少的较大气管可能导致木质部更容易形成栓塞。