Résumé
Premise: Xylem must perform several functions to ensure tree survival and the design of the xylem space leads to trade-offs among water transport, mechanical support and carbohydrate storage. However, it is not known how these trade-offs are affected by inherent xylem design (such as porosity and parenchyma arrangement) and whether the coordination among xylem traits is consistent across species and climates. Testing hypotheses about the design of the xylem space is facilitated by studying both stem and root xylem: regardless of porosity (ring, semi- and diffuse porous) in the stem, lateral roots tend to be diffuse porous. Methods: We sampled 60 adult angiosperm tree species from temperate, Mediterranean and tropical climates. A total of 13 traits linked to multiple xylem functions were measured in stems and roots, including wood density and fiber fraction (mechanical traits), non-structural carbohydrates (NSC), ray (RP) and axial parenchyma (AP) arrangement (storage traits), vessel size and density (anatomical traits associated with water transport). We also evaluated the strength of phylogenetic signals for each xylem trait calculated.Results: We found NSC and specific xylem hydraulic conductivity (Kth) were positively correlated in stems, but negatively in roots, highlighting trade-offs in the xylem space between shoots and roots, due to their different functions. Wood density was lowest in temperate species and across all species, it was positively correlated with total parenchyma fraction in both stems and roots, but was negatively correlated with vessel fraction and Kth. Species with paratracheal AP and ring-porous xylem tended to have greater Kth, and Kth was positively related to AP but negatively to RP fraction in roots across all species. Mediterranean species had the lowest Kth, while tropical species had the lowest vessel density but the largest vessels. Phylogenetic trends affected xylem traits, but was not the main factor impacting the coordination of trade-offs among traits.Conclusions: The design of the xylem space is optimised for multifunctionality and that space is reorganised depending on the main functions that xylem must perform. Trade-offs occur among mechanical, storage and hydraulic traits in stems and roots, with roots possessing traits more linked to storage capacity and hydraulics. Climate is a major factor affecting both the design of the xylem space and the trade-offs that occur, with evolutionary history playing a minimal role in the coordination among traits.