Xylem traits
At the final harvest, three to four seedlings per species were chosen randomly for xylem traits measurements. For each individual, one fully expanded leaf as well as the stem was pickled, and the middle part of each leaf (including the middle of the midvein) as well as the middle of each stem were cut transversely. The materials were embedded in 5% agar and progressively dehydrated in 50, 70 and 95% ethanol (2h per solution), after which the small blocks of agar were infiltrated for 15 days with resin JB 4 Polysciences (Polysciences Inc., Warington, Pa., USA). After polymerisation of the resin, 2 µm thick cross-sections were obtained with a glass ultra-microtome, then sections were stained with 5% toluidine blue and permanently mounted onto slides with DPX (dibutyl phthalate in xylene). The cross-sections of leaves and stems were studied with a light microscope (Zeiss Axioskop; Carl Zeiss, Jena, Germany) on a computer screen with image analysis software (Aequitas IA v. 1.25) (Castro-Díez, Puyravaud & Cornelissen, 2000, Castro-Díez, Puyravaud, Cornelissen & Villar-Salvador, 1998).
For stems, stem xylem area and stem xylem conductance area (stem xylem area minus cell wall area) were circled and measured. The proportion of cell wall area relative to xylem area in transverse section was measured in three to four microscopic fields per slide using Aequita tools (Castro-Díez et al. , 1998). For leaves, leaf midvein xylem area and minor vessel area were circled in light microscope images and measured, and the minor vessel area was calculated as the average area of the ten smallest vessels of the cross-section of leaves, which were defined as the distal conduits. The plant minor vessel number (N vessel) was theoretically approximated as:N vessel = A/( π*R s*R m), where A is stem xylem conductance area, R s is the radius of the biggest vessel in stem medium, and R m is the radius of the minor vessel in leaves. This calculation was based on the pipe model, which states that the sum of all vessel inner diameters at each vein order is equal (Shinozaki et al. , 1964). We used the stem (rather than leaf) xylem conductance area to calculateN vessel because it is difficult to gain the leaf xylem conductance area in a representative way from entire leaf cross-sections, especially for species that have big leaves.