The competitive status of trees determines their responsiveness to increasing atmospheric humidity – a climate trend predicted for northern latitudes ARVO TULLUS 1 , PRIIT KUPPER 1 , ANTS KAASIK 1 , HARDI TULLUS 2 , KRISTA L ~ OH M U S 1 , ANU S ~ OB E R 1 andARNE SELLIN 1 1 Institute of Ecology and Earth Sciences, University of Tartu, Lai 40, Tartu 51005, Estonia, 2 Institute of Forestry and Rural Engineering, Estonian University of Life Sciences, Kreutzwaldi 5, Tartu 51014, Estonia Abstract The interactive effects of climate variables and tree–tree competition are still insufficiently understood drivers of forest response to global climate change. Precipitation and air humidity are predicted to rise concurrently at high lati- tudes of the Northern Hemisphere. We investigated whether the growth response of deciduous trees to elevated air humidity varies with their competitive status. The study was conducted in seed-originated silver birch and mono- clonal hybrid aspen stands grown at the free air humidity manipulation (FAHM) experimental site in Estonia, in which manipulated stands (n = 3 for both species) are exposed to artificially elevated relative air humidity (6–7% over the ambient level). The study period included three growing seasons during which the stands had reached the competitive stage (trees were 7 years old in the final year). A significant ‘treatment9competitive status’ interactive effect on growth was detected in all years in birch (P < 0.01) and in one year in aspen stands (P = 0.015). Competi- tively advantaged trees were always more strongly affected by elevated humidity. Initially the growth of advantaged and neutral trees of both species remained significantly suppressed in humidified stands. In the following years, dominance and elevated humidity had a synergistic positive effect on the growth of birches. Aspens with different competitive status recovered more uniformly, attaining similar relative growth rates in manipulated and control stands, but preserved a significantly lower total growth yield due to severe initial growth stress. Disadvantaged trees of both species were never significantly affected by elevated humidity. Our results suggest that air humidity affects trees indirectly depending on their social status. Therefore, the response of northern temperate and boreal forests to a more humid climate in future will likely be modified by competitive relationships among trees, which may poten- tially affect species composition and cause a need to change forestry practices. Keywords: Betula pendula, climate change, climate manipulation, elevated air humidity, interactive effects, intraspecific competi- tion, Populus tremula 9 P. tremuloides, tree–tree competition Received 6 July 2016; revised version received 7 October 2016 and accepted 14 October 2016 Introduction Improving our understanding of the impact of climate change on plant communities requires consideration of the effects of environmental variables on plant– plant interaction processes, such as competition (Klan- derud, 2005; Brooker, 2006; Purves & Pacala, 2008; Adler et al., 2012; Liancourt et al., 2013). Competition for resources (primarily for irradiance, water and nutrients) is a key biotic factor that affects the growth and functioning of plants at the individual, popula- tion and community levels (Berger et al., 2008). In for- ests, competition affects tree mortality, size structure and species composition (Bugmann & Solomon, 2000; Reynolds & Ford, 2005; Pretzsch & Sch€ utze, 2014). However, the interactive effects of climate change variables and competition in forests are still largely unknown. The interactive effects of tree–tree competition and climate variables have been analysed typically by com- paring single- and mixed-species plots in climate manipulation experiments, that is by contrasting the average population-level responses (Liu et al., 2004; Kozovits et al., 2005; Kubiske et al., 2007; Smith et al., 2013). Such studies have demonstrated that trees responsiveness to climate change factors depends lar- gely on whether trees are subject to inter- or intraspeci- fic competition. When growing in mixtures, some species might have a competitive advantage over others, thereby changing community species composi- tion. For instance, the response of Populus tremuloides to elevated [CO 2 ] and [O 3 ] differs depending on whether it is grown in a mixture with Betula papyrifera or Acer Correspondence: Arvo Tullus, tel. +372 7376 176, fax +372 7376 380, e-mail: arvo.tullus@ut.ee 1961 © 2016 John Wiley & Sons Ltd Global Change Biology (2017) 23, 1961–1974, doi: 10.1111/gcb.13540