Wood serves for mechanical support, water transport and storage. These functions are provided for by different cells with a large variation in wood anatomy among species but also within individual trees. The latter often reflects ontogenetic adjustments, related to tree size or age, which can be studied by looking at patterns of radial variation in wood. We quantified radial variation in wood density (WD) and wood anatomy and ask how ontogenetic changes of wood functions are controlled in five canopy tree species in western Thailand. We ask if there are trade‐offs between these main functions of wood, how ontogenetic trends are linked to differences in growth trajectories and shade tolerance among tree species and if wood properties are mainly controlled by tree age or by size. In all species studied, vessel fraction, vessel size, theoretical hydraulic conductivity (Kh) and fibre wall thickness significantly increased with tree diameter. While the ray fraction also increased in all species except Neolitsea, axial parenchyma changed significantly only in Afzelia, the species with by far the largest axial parenchyma fraction. The average WD and Kh reflect the phenology, with deciduous and shade‐intolerant Toona and Melia having low WD and high Kh, and shade‐tolerant brevi‐deciduous Chukrasia and evergreen Neolitsea having higher WD and low Kh. Deciduous Afzelia, however, had the lowest Kh and second‐highest WD. The radial gradients in WD and Kh also reflect within‐species differences in growth rates during ontogeny. The relationship between WD and its underlying anatomical components varied substantially among species. Modulating fibre wall thickness and vessel size enables growing trees to increase water transport capacity and mechanical strength at the same time. Across species, tree diameter had a stronger effect than age on all parameters except for fibres. Given the very substantial within‐tree size‐related variation in wood traits, tree size is an essential parameter to include in comparative studies on the functional ecology of wood. Analysing ontogenetic changes in wood can advance our understanding of the different ecological strategies of trees. A http://onlinelibrary.wiley.com/doi/10.1111/1365-2435.12970/suppinfo is available for this article.

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Drought shapes the distribution and survival of trees even in tropical wet forests, and the wood and leaf trait spectra are used to understand drought adaptations. However, trait variation may result from ontogenetic adjustment or be related to tree size and not reflect evolutionary adaptations. Intraspecific variation in adaptations to drought can be an important factor in a species’ distribution and response to climate change, but excluding potentially confounding factors and proving adaptive evolution are challenging. Provenance trials can identify hereditary variability. We analysed wood and leaf traits in rubber (Hevea brasiliensis) tree clones from 15 locations in the Amazon basin that were planted in northern Thailand, controlled for tree size, tested for genetic relatedness and the phylogenetic signal in traits and compared trait variations with the climate at the location of origin. Correlations between traits and tree size were low. Intraspecific trait variation was similar to relationships in published among‐species comparisons, and correlations among wood traits and correlations among leaf traits were stronger than between wood and leaf traits. Genotype explained 30%–70% of the trait variation, and traits differed in how much of this variation was controlled by location or the relatedness among clones. There was no correlation with mean temperature or total annual rainfall. However, rainfall in the driest quarter (19–199 mm) was strongly related to leaf mass per area, carbon isotopic composition and area‐based nitrogen content (r2 = 0.54–0.70) and weaker to wood traits (vessel density and vessel lumen fraction). Trees from locations with a stronger dry season also had higher growth rates in Thailand. All traits correlating with climate showed a significant phylogenetic signal. We found no evidence of increased drought tolerance, but the trait spectrum and higher growth in trees from drier locations suggest that deciduous rubber trees have adapted via drought avoidance rather than tolerance. Our study also underlines the importance of looking at a suite of traits rather than individual ones to understand adaptive strategies. A http://onlinelibrary.wiley.com/doi/10.1111/1365-2435.13203/suppinfo is available for this article.

Vessels are responsible for an efficient and safe water transport in angiosperm xylem. Whereas large vessels efficiently conduct the bulk of water, small vessels might be important under drought stress or after winter when large vessels are embolized. Wood anatomy can adjust to the environment by plastic adaptation, but is also modified by genetic selection, which can be driven by climate or other factors. To distinguish between plastic and genetic components on wood anatomy, we used a Quercus robur trial where trees from ten Central European provenances were planted in three locations in Austria along a rainfall gradient. Because wood anatomy also adjusts to tree size and in ring-porous species, the vessel size depends on the amount of latewood and thereby ring width, we included tree size and ring width in the analysis. We found that the trees’ provenance had a significant effect on average vessel area (VA), theoretical specific hydraulic conductivity (Ks), and the vessel fraction (VF), but correlations with annual rainfall of provenances were at best weak. The trial site had a strong effect on growth (ring width, RW), which increased from the driest to the wettest site and wood density (WD), which increased from wet to dry sites. Significant site x provenance interactions were seen only for WD. Surprisingly, the drier site had higher VA, higher VF, and higher Ks. This, however, is mainly a result of greater RW and thus a greater proportion of latewood in the wetter forest. The average size of vessels > 70 μm diameter increased with rainfall. We argue that Ks, which is measured per cross-sectional area, is not an ideal parameter to compare the capacity of ring-porous trees to supply leaves with water. Small vessels (<70 μm) on average contributed only 1.4% to Ks, and we found no evidence that their number or size was adaptive to aridity. RW and tree size had strong effect on all vessel parameters, likely via the greater proportion of latewood in wide rings. This should be accounted for when searching for wood anatomical adaptations to the environment.