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. × P. tremuloides Michx.)

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. × P. tremuloides Michx.), planted in a unique free-air experimental facility, designed for increasing air humidity during the growing season. Over four consecutive years (2009–2012), 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.