Elevated atmospheric CO2 and humidity delay leaf fall in Betula pendula, but not in Alnus glutinosa or Populus tremula × tremuloides

Context: Anthropogenic activity has increased the level of atmospheric CO2, which is driving an increase of global temperatures and associated changes in precipitation patterns. At Northern latitudes, one of the likely consequences of global warming is increased precipitation and air humidity. Aims: In this work, the effects of both elevated atmospheric CO2 and increased air humidity on trees commonly growing in northern European forests were assessed. Methods: The work was carried out under field conditions by using Free Air Carbon dioxide Enrichment (FACE) and Free Air Humidity Manipulation (FAHM) systems. Leaf litter fall was measured over 4 years (FACE) or 5 years (FAHM) to determine the effects of FACE and FAHM on leaf phenology. Results: Increasing air humidity delayed leaf litter fall in Betula pendula, but not in Populus tremula × tremuloides. Similarly, under elevated atmospheric CO2, leaf litter fall was delayed in Betula pendula, but not in Alnus glutinosa. Increased CO2 appeared to interact with periods of low precipitation in summer and high ozone levels during these periods to effect leaf fall. Conclusions: This work shows that increased CO2 and humidity delay leaf fall, but this effect is species specific

Climate Change at Northern Latitudes: Rising Atmospheric Humidity Decreases Transpiration, N-Uptake and Growth Rate of Hybrid Aspen

<div><p>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 (<em>Populus tremula</em> L.×<em>P. tremuloides</em> 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.</p> </div

FAHM experimental area: a) locations (<i>X</i>: distance from the west edge, <i>Y</i>: distance from the south edge) of control (empty circles) and humidification plots (filled circles), b) general layout of a humidification plot comprising trees - hybrid aspens (filled triangles) and silver birches (empty triangles), <i>L</i> indicates the distance between opposite vent pipe pairs and c) photo of the humidification plot taken in July 2009.

<p>FAHM experimental area: a) locations (<i>X</i>: distance from the west edge, <i>Y</i>: distance from the south edge) of control (empty circles) and humidification plots (filled circles), b) general layout of a humidification plot comprising trees - hybrid aspens (filled triangles) and silver birches (empty triangles), <i>L</i> indicates the distance between opposite vent pipe pairs and c) photo of the humidification plot taken in July 2009.</p

Chemical properties of the soil humus horizon in the experimental plots.

<p>Chemical properties of the soil humus horizon in the experimental plots.</p

Comparison of foliar characteristics (mean ± standard error) in control (C) and humidified (H) plots during individual study years (asterisks indicate <i>p</i>-values based on <i>t</i>-test: 0.01<<i>p</i><0.05*; 0.001<<i>p</i><0.01**; <i>p</i><0.001***) and summary effect of the treatment through the years: “True” if <i>q</i><0.05.

<p>Comparison of foliar characteristics (mean ± standard error) in control (C) and humidified (H) plots during individual study years (asterisks indicate <i>p</i>-values based on <i>t</i>-test: 0.01<<i>p</i><0.05*; 0.001<<i>p</i><0.01**; <i>p</i><0.001***) and summary effect of the treatment through the years: “True” if <i>q</i><0.05.</p

Comparison of physiological traits: a) <i>F</i> – total sap flow, b) <i>F_D</i> – sap flux density, c) WUE – water-use efficiency of hybrid aspens at the ages of 3 and 4 years in control (C) and humidified (H) plots (<i>t</i>-test) and summary effect of the treatment over two years (model), whiskers denote ± standard error.

<p>Comparison of physiological traits: a) <i>F</i> – total sap flow, b) <i>F_D</i> – sap flux density, c) WUE – water-use efficiency of hybrid aspens at the ages of 3 and 4 years in control (C) and humidified (H) plots (<i>t</i>-test) and summary effect of the treatment over two years (model), whiskers denote ± standard error.</p

Weather conditions expressed as daily averages in the study area during the growing seasons (May–October) of the study period (2007–2010).

<p>Weather conditions expressed as daily averages in the study area during the growing seasons (May–October) of the study period (2007–2010).</p

The interaction between treatment and size of the trees at the end of the previous (<i>t</i>−1) growing season for predicting the current year (<i>t</i>) increment in control (C) and humidified (H) plots: a) stem diameter (<i>D</i>), b) stem volume (<i>V</i>).

<p>Points represent individual measurements across three years of the experiment.</p