Tree size mostly drives the variation of xylem traits at the treeline ecotone.

Key message: The axial structure of the hydraulic system in trees is relatively invariant and insensitive to temperature, while trees plastically adjust the number of cells within the tree ring. At higher elevations and latitudes in the treeline ecotone, reduction in the heat accrued during the growing season is reflected in gradually decreasing tree size. Due to low temperatures, treeline trees might produce smaller xylem cells and, as a consequence, tree growth could be limited. However, some xylem traits (i.e., cell lumen area) are considered relatively insensitive to climatic factors but highly dependent on tree size because of the natural widening of xylem conduits towards the stem base. We tested the hypothesis that earlywood cell lumen area is essentially invariant and depends largely on tree size. Tracheid traits in four conifer species from the lower ("timberline") and upper ("treeline") parts of the treeline ecotone (Picea engelmannii, Picea abies, Pinus cembra and Larix decidua) were measured in the Colorado Front Range (U.S.A.), Krkonoše Mts. (Czech Republic) and Dolomites (Italy). On transversal sections sampled at 1 m of stem height, we measured cell lumen areas, transversal cell size, cell wall thickness, tree-ring width and number of cells per radial file. Cell lumen areas were always greater at the timberline than treeline. When tree height is accounted for, the earlywood cell area did not differ between the two sites, thus showing that difference in temperature did not affect earlywood cell area in any of the four measured species. The number of cells within tree rings exhibited high inter-annual variability according to environmental factors. The fundamental hydraulic structure in trees is relatively rigid and insensitive to temperature, while trees plastically adjust the number of cells within the tree ring as a result of inter-annual climate variability and leaf production. [ABSTRACT FROM AUTHOR]