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

Within-crown variation in leaf conductance of Norway spruce: effects of irradiance, vapour pressure deficit, leaf water status and plant hydraulic constraints

Responses of leaf conductance (gL) to variation in photosynthetic photon flux density, leaf-to-air vapour pressure difference, shoot water potential and soil-to-leaf hydraulic conductance (GT) were studied in Picea abies (L.) Karst. foliage with respect to shoot age and position within the canopy. The upper canopy shoots demonstrated on average 1.6 times higher daily maximum gL as compared to the lower canopy shoots growing in the shadow of upper branches. Functional acclimation of the shade foliage occurred in the form of both a steeper initial slope of the light-response curve and a lower light-saturation point of gL. The mean GT was 1.6–1.8 times bigger for the upper canopy compared to the lower canopy. We set up an hypothesis that stomatal conductance at the base of the live crown is constrained not only by low light availability but also by plant’s inner hydraulic limitations.Variation de la conductance foliaire dans les couronnes de l’Epicea : effets de l’éclairement, du déficit de vapeur d’eau dans l’air, de l’état hydrique des feuilles et des contraintes hydrauliques des arbres. Les réponses de la conductance foliaire (gL) aux variations de la densité de flux photosynthétique de photons, du déficit de saturation de l’air, du potentiel hydrique des rameaux et de la conductance hydraulique (GT) dans le transfert Sol-feuille ont été étudiées chez Picea abies (L) Karst. En relation avec l’âge des rameaux et leur position dans la canopée. Les rameaux de la partie supérieure de la canopée présentent des valeurs journalières maximum moyennes de gL1,6 fois plus élevées que les valeurs correspondantes de gL des rameaux des parties basses de la canopée se développant à l’ombre des branches les plus hautes. Une acclimatation fonctionnelle du feuillage à l’ombre se manifeste par une pente initiale plus élevée de la courbe de réponse à la lumière et un point de saturation de gL plus bas. La moyenne de GT était de 1,6 à 1,8 fois plus grande pour la partie basse de la canopée. Nous avançons l’hypothèse que la conductance stomatique à la base de la couronne vivante est conditionnée par les bas niveaux de lumière disponible mais aussi par les limitations hydrauliques internes de l’arbre

Coppicing improves the growth response of short-rotation hybrid aspen to elevated atmospheric humidity

Aspens are fast-growing clonal trees with a wide circumboreal distribution range, suitable for the production of pulp and bioenergy. The adaptability of aspen short-rotation coppice systems to climate change has rarely been investigated. For a large part of aspens' northern range, climate models predict an increase in precipitation and, consequently, in atmospheric humidity. Our aim was to clarify the long-term effect of elevated air humidity on vegetative reproduction capacity and dynamics of above-ground growth and size structure in aspen stands. We analysed tree growth data from two consecutive 6-year rotations (a planted and a coppice generation) in experimental short-rotation hybrid aspen (Populus tremula L. x P. tremuloides Michx.) stands in the Free Air Humidity Manipulation (FAHM) experiment in Estonia. In three plots, mean relative air humidity was elevated by 7% and three plots were controls. Across two rotation periods, the humidification effect on tree height and/or stem basal area increment was year-dependent (p < 0.001): negative in 4 years, positive also in 4 years and non-significant in 3 years. Mean basal area of humidified (11.6 +/- 0.8 cm(2)) and control trees (15.0 +/- 1.0 cm(2)) differed significantly (p = 0.035) at the end of the first but not the second rotation period (9.3 +/- 0.9 cm(2) and 9.3 +/- 1.2 cm(2), respectively). Average growth differences levelled out already in the beginning of the second rotation, suggesting that some root-level acclimation must have taken place. The annual size-growth relationships (SGR) indicated a more size-symmetric growth in humidified (SGR = 1.00 +/- 0.05) and a size-asymmetric growth (SGR = 1.12 +/- 0.04) in control stands, implying a greater role of root-competition in humidified stands. In humidified stands, the growth of re-sprouting trees was more strongly determined by parent tree size, indicating a stronger carry-over of size hierarchy. The tree height diversity fluctuated more in control stands, where mortality was higher, especially after dry years. To summarise, short- and long-term responses of hybrid aspen to elevated air humidity varied, emphasizing the importance of long-term climate manipulations with trees. Generally, hybrid aspen short-rotation coppice forests showed promising acclimation capacity with future more humid climate predicted for northern latitudes

Long-term effect of elevated air humidity on seasonal variability in diurnal leaf conductance and gas exchange in silver birch

Environmental conditions and photoperiod length drive the seasonal variability of gas exchange in plants. Still, little is known about trees’ adaptation to climate change, expressed as a delay in decreasing photosynthetic capacity at the end of the growing season. We investigated the effect of elevated air humidity (RH) and sampling period (from July to September) on the variability of net photosynthesis (An), dark respiration (R), daytime (gl_day) and night-time (gl_night) leaf conductance, an index of leaf chlorophyll content (SPAD) and An:SPAD ratio in cut shoots of silver birch. Measurements of cut shoots were conducted in a climate chamber to eliminate the direct effect of field conditions. The An, An:SPAD ratio and gl_day were higher in the humidification (H) than in the control (C) treatment (PThe accepted manuscript in pdf format is listed with the files at the bottom of this page. The presentation of the authors' names and (or) special characters in the title of the manuscript may differ slightly between what is listed on this page and what is listed in the pdf file of the accepted manuscript; that in the pdf file of the accepted manuscript is what was submitted by the author

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

Height (a), diameter (b) and stem volume (c) increments (Δ<i>H</i>, Δ<i>D</i> and Δ<i>V</i> respectively) of hybrid aspens in control (C) and humidified (H) plots during the study period.

<p>The significance of treatment effect (<i>t</i>-test) in individual years is indicated with asterisks and <i>q</i>-values (model) show the summary effect over the years when humidification was applied (age 2–4 years). Whiskers denote ± standard error.</p