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Increasing air humidity -a climate trend predicted for northern latitudes -alters the chemical composition of stemwood in silver birch and hybrid aspen. Silva Fennica vol. 48 no. 4 article id 1107. 16 p. Highlights• Hybrid aspen and silver birch trees grew more slowly under increased air humidity conditions and had higher concentrations of N and P and a lower K to N ratio in stemwood. • Minor species-specific changes were detected in stemwood concentrations of cellulose and hemicellulose. • Density, calorific value and concentrations of lignin and ash in stemwood were not affected by elevated humidity.

We investigated crown development in young trees, assessing whether the plasticity in parent shoot and bud growth is mostly related to the position within a crown (upper versus lower branches) or driven by local light conditions. Both larger shoots and buds were associated with increased light availability, whereas leaf-stem allometry was influenced by branch height. The allometry of total bud mass and stem cross-sectional area varied little across the crown, but it changed throughout the summer, driven by bud rather than stem growth. This suggests that bud-stem relationships are regulated locally, at the shoot level, instead of crown-level processes.

Elevated air humidity modulates bud size and the frequency of bud break in fast-growing deciduous trees: silver birch (Betula pendula Roth.) and hybrid aspen (Populus tremula L. 3 P. tremuloides Michx.) Abstract Key message Elevated atmospheric humidity reduced bud size by restricting primordium growth and increased the frequency of bud break in fast-growing deciduous trees, but the responses are species-specific. Abstract The initiation, development, and growth potential of buds determine the structure of a tree crown. Climate change scenarios project increasing amounts of precipitation at high latitudes in the future, but the effects of a more humid climate on bud size and growth potential remain mostly unexplored. This study investigates the effects of atmospheric humidity on fast-growing deciduous trees, silver birch (Betula pendula Roth.) and hybrid aspen (Populus tremula L. 9 P. tremuloides Michx.), planted in a unique free-air experimental facility, designed for increasing air humidity during the growing season. Over four consecutive years (2009)(2010)(2011)(2012), the size of overwintering buds was assessed in the upper crown of young trees, and additionally, annual height increments were assessed during the study period. Furthermore, bud contents, probabilities of bud death and break, and subsequent shoot growth were examined. In silver birch, misting effect on height growth depended on year, possibly due to acclimation or variable weather conditions, but bud size was consistently reduced. Nevertheless, misting restricted the growth rather than the initiation of leaf primordia in the bud, and decreased bud size did not translate into changes in the leaf area of future shoots. In contrast, the size of hybrid aspen buds was markedly reduced by misting only in 2009; however, increased humidity promoted bud break, reducing the proportion of dormant buds. The underlying mechanisms causing reduced bud size may involve interactions with shoot growth, but require further study. Although the effect of stimulated bud break is subtle in a given year, cumulative effects may modulate crown structure in the long term, facilitating the acclimation of tree growth to rising humidity in the future.

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