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

1. 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. 2. 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. 3. 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. 4. 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. 5. 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.