Abstract
At northern latitudes a rise in atmospheric humidity and precipitation is predicted as a consequence of global climate change. We studied several growth and functional traits of hybrid aspen (Populus tremula L.×P. tremuloides Michx.) in response to elevated atmospheric humidity (on average 7% over the ambient level) in a free air experimental facility during three growing seasons (2008–2010) in Estonia, which represents northern temperate climate (boreo-nemoral zone). Data were collected from three humidified (H) and three control (C) plots, and analysed using nested linear models. Elevated air humidity significantly reduced height, stem diameter and stem volume increments and transpiration of the trees whereas these effects remained highly significant also after considering the side effects from soil-related confounders within the 2.7 ha study area. Tree leaves were smaller, lighter and had lower leaf mass per area (LMA) in H plots. The magnitude and significance of the humidity treatment effect – inhibition of above-ground growth rate – was more pronounced in larger trees. The lower growth rate in the humidified plots can be partly explained by a decrease in transpiration-driven mass flow of NO3 − in soil, resulting in a significant reduction in the measured uptake of N to foliage in the H plots. The results suggest that the potential growth improvement of fast-growing trees like aspens, due to increasing temperature and atmospheric CO2 concentration, might be smaller than expected at high latitudes if a rise in atmospheric humidity simultaneously takes place.
Highlights
Global warming will be accompanied by a change in atmospheric water vapour content and precipitation rate, there will be pronounced regional differences in the magnitude and direction of these events [1]
H plots during the individual study years revealed that the growth rate in H plots was higher during the first year of the experiment but remained significantly lower compared to C plots after the second and third year (Fig. 3)
Stems of the trees of the same size grew slower in H plots compared to C plots, while such a distinction was greater in bigger trees (Fig. 5)
Summary
Global warming will be accompanied by a change in atmospheric water vapour content and precipitation rate, there will be pronounced regional differences in the magnitude and direction of these events [1] This has been validated by modern climate models [2,3,4], as well as by studies on previous global warming periods in the Earth’s history [5]. Water vapour acts as one of the most important greenhouse gases [3,11,12], which absorbs infrared radiation and increases air temperature, and atmospheric water-holding capacity according to the Clausius-Clapeyron relation [4] Such a process is known as a water vapour feedback and its potential role in future climate warming is considered to be substantial [11,13]
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