Data from: Variation of xylem vessel diameters across a climate gradient: insight from a reciprocal transplant experiment with a widespread boreal tree

Xylem vessel diameters represent an important plant hydraulic trait to ensure sufficient water supply from the roots to the leaves. The ability to adjust the hydraulic pathway to environmental cues is key in order to satisfy transpirational demands and maximize growth and survival. We evaluated the variability of vessel diameters in trembling aspen in a reciprocal transplant experiment. We tested six provenances from three ecological regions of North America planted at four test sites in western Canada. All test sites were established at the same time with the same provenances arranged in a randomized complete block design. Vessel diameter showed a strong interaction of population and test site suggesting a high degree of phenotypic plasticity in this trait. Gaussian kernel density estimates support plastic as well as genetic contributions in vessel diameter control trending from bimodal distributions at the most northern test site towards unimodal distributions at the warmest and mildest test site. Furthermore, we used test site-specific climate data in form of a 2-year, 5-year and 10-year average of 21 directly and derived climatic variables and found that average site-specific vessel diameters correlated strongly with summer moisture availability. A previously found negative relationship with vessel diameter and tree height in central Alberta was also found at two other boreal test sites but reversed at a wetter and milder sub-boreal test site. In summary, vessel diameters were highly plastic in response to different environments and varied with summer moisture availability. The variability of vessel diameter and tree height correlations suggests that vessel diameter alone cannot serve as a reliable proxy for long-term growth performance beyond boreal environments. Instead, selecting aspen populations with a high degree of plasticity in this trait appears to be the safest option for assisted migration and seed transfer programmes under climate change.

木质部导管直径（xylem vessel diameter）是保障水分从根系向叶片高效运输的关键植物水力性状。植物调节水力通路以响应环境信号的能力，是满足蒸腾需求、最大化生长与存活的核心前提。本研究依托互易移植实验（reciprocal transplant experiment），以颤杨（trembling aspen）为研究对象，探究其导管直径的变异特征：我们选取了来自北美3个生态区的6个种源，将其种植于加拿大西部的4个试验点。所有试验点均于同期建立，供试种源的田间布置采用随机完全区组设计（randomized complete block design）。分析结果显示，导管直径表现出种群与试验点的显著交互效应，表明该性状具备较高水平的表型可塑性（phenotypic plasticity）。高斯核密度估计（Gaussian kernel density estimates）结果证实，导管直径的调控同时受可塑性与遗传因素共同影响，其分布模式呈现梯度变化：从最北部试验点的双峰分布，逐渐向最温暖温和的试验点过渡为单峰分布。此外，本研究采集了各试验点的气候数据，涵盖2年、5年及10年平均的21个直接气候变量与衍生气候变量，分析发现，各试验点的平均导管直径与夏季水分可获得性显著相关。此前在阿尔伯塔省中部观测到的导管直径与树高的负相关关系，在另外2个北方针叶林试验点中同样被发现，但在一个更湿润温和的亚北方针叶林试验点中则发生了逆转。综上，导管直径对不同环境具有高度可塑性，且其变异与夏季水分可获得性紧密相关。导管直径与树高的相关性存在环境依赖性，这表明仅依靠导管直径无法作为北方针叶林以外环境中植物长期生长表现的可靠替代指标。在气候变化背景下，选取该性状可塑性较高的颤杨种源，应为辅助迁移（assisted migration）与种子调拨计划的最优选择。