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Recent studies have suggested that predawn stomatal opening may enhance early-morning photosynthesis (A) and improve the relative growth rate of trees. However, the causality between night-time stomatal conductance, A, and tree growth is disputable because stomatal opening in darkness can be mediated by previous day photosynthate loads and might be a consequence of growth-related processes like dark respiration (R). To identify linkages between night-time leaf conductance (gl_night), A, R, and tree growth, we conducted an experiment in hybrid aspen saplings grown under different air relative humidity (RH) conditions and previous day irradiance level (IR_pday). Predawn leaf conductance (gl_predawn) depended on RH, IR_pday and R (P < 0.05), whereas early-morning gross A (Agross_PAR500) depended on IR_pday and gl_predawn (P < 0.001). Daytime net A was positively related to Agross_PAR500 and leaf [N] (P < 0.05). Tree diameter and height increment correlated positively with gl at the beginning and middle of the night (P < 0.05) but not before dawn. Although our results demonstrate that gl_night was related to tree growth, the relationship was not determined by R. The linkage between gl_predawn and Agross_PAR500 was modified by IR_pday, indicating that daily CO2 assimilation probably provides feedback for stomatal opening before dawn.

Cuticular wax layer is the first barrier against the outside environment and the first defense encountered by herbivores and pathogens. The effects of environmental factors on cuticular chemistry, and on the formation of glandular trichomes that account for the storage and secretion of lipophilic compounds to the leaf surface are poorly understood. Low vapor pressure deficit (VPD) has shown to reduce the nitrogen (N) status of plants. Thus, we studied the effects of elevated air humidity, indicated as VPD, and the effect of N fertilization on cuticular waxes and glandular trichome density in silver birch (Betula pendula Roth). Experiments were carried out in growth chambers with juvenile plants and in a long-term field experiment with older trees. Low VPD reduced the glandular trichome density in both experiments, in chamber and in field. The contents of the major triterpenoid and flavonoid aglycones correlated positively with glandular trichome density, which supports the role of trichomes in the exudation of secondary compounds to the leaf surface. A closer examination of the cuticular wax chemistry in the chamber experiment revealed that low VPD and N supply affected the composition of cuticular waxes, but not the total wax content. The deposition of different wax compounds followed a co-ordinated pattern in birch leaves, but different compound groups varied in their responses to N fertilization and low VPD. Low VPD reduced the hydrophobicity of cuticular waxes, as demonstrated by lower alkane content and less hydrophobic flavonoid profile in low VPD than in high VPD. Reduced hydrophobicity of the wax layer is presumed to increase leaf wettability. Together with reduced trichome density in low VPD it may enhance the susceptibility of trees to fungal pathogens and herbivores. High N supply under low VPD reduced the effect of low VPD on the cuticular wax composition. Total fatty acid content and the expression of β-amyrin synthase were lower under high N supply than under moderate N supply irrespective of VPD treatment. Nitrogen availability and decreasing VPD will modify leaf surface properties in silver birch and thereby affect tree defence against abiotic and biotic stress factors that emerge under climate change.

At northern latitudes, a rise in atmospheric humidity and precipitation is predicted as a consequence of global climate change. We investigated the impact of high (H) and moderate (M) daytime air relative humidity (RH) on water relation parameters and foliar nutrient status in saplings of silver birch, to elucidate the interactions between water and nutrient uptake. A ~40% lower daytime vapour pressure difference between the leaf interior and atmosphere caused significantly (P

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 latitudes of the Northern Hemisphere. We investigated if 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 monoclonal 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 ‘treatment×competitive 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). Competitively 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 the future will likely be modified by competitive relationships among trees, which may potentially affect species composition and cause a need to change forestry practices. This article is protected by copyright. All rights reserved.

Increasing atmospheric humidity—a climate trend predicted for northern Europe—will reduce water flux through vegetation. Diminished transpirational water flux impacts various physiological processes, causing growth decline in deciduous trees. We propose, based on the results obtained from the long-term free air humidity manipulation experiment, concurrent mechanisms to explain the growth deceleration due to increases in relative air humidity. Reduced atmospheric evaporative demand diminishes nutrient uptake and leads to lower leaf nutritional status and to an unbalanced foliar phosphorus/nitrogen ratio (P:N), resulting in a decline in leaf photosynthetic capacity. Elevated relative humidity induces readjustment of foliar metabolism: disturbed N metabolism, accumulation of starch and changes in secondary metabolite contents probably impair both photosynthetic performance and growth. Increased carbohydrate content in the leaves suggests that sink strength of trees is reduced under elevated humidity. As a consequence of the stress, foliar development is hindered, observed at individual leaf or whole-tree foliage levels, lowering production potential of trees proportionally to their foliar area. Larger investments in stem xylem in relation to foliage cause an increase in the ratio of non-photosynthetic to photosynthetic tissues, leading to larger maintenance respiration costs determined by the volume of parenchymatous tissue. An increase in the proportion of living parenchyma cells in relation to dead xylem elements in sapwood additionally enhances respiration costs. Disproportionate changes in hydraulic versus stomatal conductance become a critical factor in the case of weather extremes, which limit canopy conductance and may induce dysfunction of the hydraulic system. Increasing environmental humidity creates favourable conditions for development of pathogens, increasing frequency of fungal damage.

Although night-time water relations have been studied in different plant species in the last decade, there is limited information about the impact of climate variables on endogenous regulation of night-time water relations in trees. The aim of the current study was to elucidate how long-term exposure to increased air humidity impacts night-time gaseous and liquid phase conductance and gas exchange in cut shoots of hybrid aspen (Populus tremula L. × P. tremuloides Michx.) sampled from the free air humidity manipulation experimental site and measured at constant air relative humidity (RH) level in a growth chamber. Neither the early-night leaf conductance (g n1) nor canopy conductance (g c) differed (P > 0.05) between the humidification (H) and control (C) treatments. However, there was a significant (P

Processes determining the carbon (C) balance of a forest ecosystem are influenced by a number of climatic and environmental factors. In Northern Europe, a rise in atmospheric humidity and precipitation is predicted. The study aims to ascertain the effect of elevated atmospheric humidity on the components of the C budget and on the C-sequestration capacity of a young birch forest. Biomass production, soil respiration, and other C fluxes were measured in young silver birch (Betula pendula Roth) stands growing on the Free Air Humidity Manipulation (FAHM) experimental site, located in South-East Estonia. The C input fluxes: C sequestration in trees and understory, litter input into soil, and methane oxidation, as well as C output fluxes: soil heterotrophic respiration and C leaching were estimated. Humidified birch stands stored C from the atmosphere, but control stands can be considered as C neutral. Two years of elevated air humidity increased C sequestration in the understory but decreased it in trees. Humidification treatment increased remarkably the C input to the soil. The main reason for such an increase was the higher root litter input into the soil, brought about by the more than two-fold increase of belowground biomass production of the understory in the humidification treatment. Elevated atmospheric humidity increased C sequestration in young silver birch stands, mitigating increasing CO2 concentration in the atmosphere. However, the effect of elevated atmospheric humidity is expected to decrease over time, as plants and soil organisms acclimate, and new communities emerge.

Several climate change scenarios predict increasing precipitation for northern latitudes and several other regions in the globe, leading to increase in atmospheric water vapour content [expressed as increased relative humidity (RH) or as decreased water vapour pressure deficit (VPD)] and environmental wetness. Plants are known to be sensitive to high humidity (low VPD) as indicated by changes in stomatal function, transpiration, mineral nutrient uptake, growth, development, sugar metabolism and leaf epicuticular wax composition in several species and studies. To understand the impact of increasing humidity (lower VPD) on forest ecosystems, a long-term and large-scale field experiment (FAHM, Free Air Humidity Manipulation) is conducted in Estonia with silver birch and hybrid aspen, representing widespread deciduous species in northern Europe. The experiment revealed that high humidity is an important climatic factor, causing reduction in growth rate of the aboveground parts, leaf biomass and area, bud size, sap flux and earlier bud break and delayed leaf fall. In the belowground parts, root biomass (fine-root biomass in particular) was increased although the root compartment was exposed to anaerobic conditions, increased soil pH and soil water potential, with lowered soil respiration and altered microbial and fungal communities. In the leaves, high humidity shifted the metabolism towards nonstructural carbohydrates, antioxidants and phenolic compounds, while nutrient (N and P) content, photosynthesis, dark respiration, hydraulic conductance and the density of glandular trichomes were reduced. The change in the chemical composition of the surface wax layer resulted in lower hydrophobicity, exposing the leaves for fungal pathogen attacks. In the stem, N and P content in wood and number of living parenchyma cells were increased, while the stem wood density was reduced and wood chemistry altered. Reduction in the growth rates under high humidity was more pronounced in hybrid aspen than in silver birch. Birch showed more efficient adjustment of leaf and root morphology, hydraulic architecture, primary carbon and nitrogen metabolism and leaf surface properties mitigating the negative impacts of high humidity. At the ecosystem level, high humidity altered soil processes (moisture content, microbiota, nitrification), with potential to affect competition and species composition. A complementary laboratory study with birch demonstrated that additional N supply can counteract the effects of low VPD on cellular metabolism and leaf surface wax quality.

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. 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.

Climate change is predicted to bring about a rise in precipitation and atmospheric humidity at northern latitudes, which could have a considerable impact on forest ecology and management. This study investigates the effect of elevated relative air humidity (RH) on tree growth and biomass accumulation and allocation in six-year-old seedling-originated silver birch (Betula pendula Roth) and monoclonal hybrid aspen (Populus tremula L.×P. tremuloides Michx.) stands. The study was conducted in the unique Free Air Humidity Manipulation experimental facility in Estonia. Two understory vegetation types (forest and pasture) were used in the experimental plots. As a short-term response during the first manipulation years, a small rise (6–7%) in RH reduced the growth of both tree species. In the fifth humidification year, suppression of growth continued in aspen, but birch acclimated effectively to elevated RH – differences in biomass and stem diameter increments levelled out in humidified (H) and control (C) plots. The H birches ensured biomass accumulation by changing the biomass and morphology of the physiologically most active tree parts: increasing fine-root biomass, as well as specific areas of fine roots (SRA) and leaves (SLA). The effect of elevated RH on most leaf and fine root characteristics of aspens was in a similar direction as that of birches, but weaker, and the plasticity of the functional traits was also lower compared to birches, probably due to the lack of genetic variation in monoclonal aspens. The biomass of young aspens was 22% lower in H plots. The relative aboveground biomass allocation was not affected by elevated RH. The aboveground biomass of humidified birches was higher in plots with forest understory, where root competition was lower compared to the dense root systems of grasses in pasture understory plots. Aspen biomass was not affected by understory vegetation type. We conclude that young silver birches have sufficient phenotypic plasticity to acclimate to elevated RH and the forests retain biomass accumulation. However, the growth acclimation of planted forests of clonal plant material is more complicated and humidity-tolerance should be considered in the selection of new clones for areas where an increase in RH is anticipated.

Although the impact of nitrogen availability on the night-time water relations of plants has received a lot of attention during the last decade, knowledge of how these two traits are interrelated is contradictory and still limited. The aim of the current study was to investigate the impact of leaf nitrogen concentration on night-time (Enap) and daytime (Ed) transpiration rate, nightly water-use percentage of daytime water use (NWU), and increase in night-time transpiration rate (INT) in artificial predawn hours in the cut shoots of 16 woody species measured in the controlled conditions of a growth chamber. Two distinct patterns of night-time water use associated with leaf nitrogen concentration ([N]) were observed: shoots with high NWU were characterised by significantly (P 0.05). Our results suggest that high NWU could potentially compensate limited nitrogen uptake in species able to grow in nutrient-poor habitats. Furthermore, night-time stomatal regulation mechanisms may differ between species according to their [N] and this may explain the contradictory results between previous studies.

Acclimation of light sensitivity of hydraulic conductance of shoots of silver birch (Betula pendula) and hybrid aspen (Populus × wettsteinii) to growth environments with three different air humidities was studied. Hydraulic conductance of shoots kept for 1–2 h in darkness (D) or in light (L) was measured by the pressure chamber method, and light sensitivity was defined as a significant difference between D and L shoots. Light sensitivity of shoots grown in three different air humidities was found to vary. Amongst shoots grown in current natural air, only the hydraulic conductance of the whole shoot and that of the leaf blades of birch upper foliage were significantly light sensitive. Amongst shoots grown in decreased air humidity, hydraulic conductance of the whole shoot, the leaf blades, and the stem and petioles of birch upper foliage, the conductance of the whole shoot and the leaf blades of birch lower foliage, and the conductance of the whole shoot of aspen upper foliage were light sensitive. None of the shoots grown in increased air humidity were significantly light sensitive. We predict that light sensitivity will become more widespread among species in regions where air humidity decreases as a result of global climate change, and vice versa. Low white light always caused the same increase in hydraulic conductance as high white light, and blue and white light always caused an increase in conductance about two times greater than red light, indicating that growth environment did not markedly modify the mechanism of light sensitivity.

Climate change is predicted to bring about a rise in precipitation and air humidity at northern latitudes, which could have considerable impact on forest management. This paper investigates the effect of increased air humidity and understory composition on short root morphology and on the relative abundance of colonizing ectomycorrhizal (EcM) fungal associates in silver birch (Betula pendula Roth.) stands. Short root morphological traits of silver birch were analyzed at increased humidity and ambient conditions for two different understories (early-successional grasses and diverse “forest” understory) in three consecutive years (2009–2011). The fungal community was determined in 2010 (after three seasons of misting) using molecular methods. The study was conducted on the Free Air Humidity Manipulation (FAHM) experimental facility established in Estonia. Silver birches responded to the rise in air humidity by forming longer and thinner short roots, which can be interpreted as a morphological adaptation leading to an increase in the absorptive area. The response was stronger when humidification concurred with the species-poor understory of pioneer grasses. The inter- and intra-treatment variation in short root morphological parameters decreased by the third year. Using molecular methods, overall 64 EcM taxonomic units were distinguished. Hydrophilic fungal morphotypes dominated significantly in humidified plots, hydrophobic morphotypes in control plots. Our results suggest that rising air humidity causes a morphological stress response in EcM short roots. Young trees show the ability to adapt to climate change with great plasticity by modifying short root length, diameter and specific root length (SRL). Humidification leads to a shift in the fungal colonizers towards the dominance of hydrophilic taxa, which may alter ecosystem functioning.

Highlight Metabolite profiling and mineral nutrient analysis revealed that long-term exposure to decreased vapour pressure deficit (VPD) promotes production of starch, phenolic compounds, and antioxidants, and affects nutrient status in tree leaves., Relative air humidity (RH) is expected to increase in northern Europe due to climate change. Increasing RH reduces the difference of water vapour pressure deficit (VPD) between the leaf and the atmosphere, and affects the gas exchange of plants. Little is known about the effects of decreased VPD on plant metabolism, especially under field conditions. This study was conducted to determine the effects of artificially decreased VPD on silver birch (Betula pendula Roth.) and hybrid aspen (Populus tremula L.×P. tremuloides Michx.) foliar metabolite and nutrient profiles in a unique free air humidity manipulation (FAHM) field experiment during the fourth season of humidity manipulation, in 2011. Long-term exposure to decreased VPD modified nutrient homeostasis in tree leaves, as demonstrated by a lower N concentration and N:P ratio in aspen leaves, and higher Na concentration and lower K:Na ratio in the leaves of both species in decreased VPD than in ambient VPD. Decreased VPD caused a shift in foliar metabolite profiles of both species, affecting primary and secondary metabolites. Metabolic adjustment to decreased VPD included elevated levels of starch and heptulose sugars, sorbitol, hemiterpenoid and phenolic glycosides, and α-tocopherol. High levels of carbon reserves, phenolic compounds, and antioxidants under decreased VPD may modify plant resistance to environmental stresses emerging under changing climate.

Previous investigations have been demonstrated that night-time water flux may increase or remain unchanged at low mineral nutrient availability. At the same time, it is a well-known fact that night-time water flux is relatively high in fast-growing species, which typically grow in fertile soil. To test the impact of soil nutrient deficiency on night-time water flux and fine-root acclimation in saplings of hybrid aspen ( Populus tremula L. × Populus tremuloides Michx.), a growth chamber experiment was carried out. We set up a hypothesis that night-time sap flux density ( F ) and night-time water use percentage from daytime water use (NWU) are more intense in fertile conditions, in order to enhance or sustain the high intrinsic growth potential of hybrid aspen. The main limiting element in the low nutrient availability treatment (low-n) was nitrogen. The nitrogen concentrations of leaves and fine-roots exhibited the strongest ( R 2 = 0.95; P F and NWU were several times higher in the case of fertilized soil (high-n treatment) compared to low-n treatment ( P F disappeared at the end of the period of sap flow measurements, when the foliage area of trees was almost full-grown. Endogenous increase in water flux during predawn hours was observable only in the high-n treatment. Significantly greater NWU ( P P P

A facility for free air humidity manipulation (FAHM) was established to investigate the effect of increased air humidity on trees’ performance and their canopy functioning with respect to rising air humidity predicted for Northern Europe. The FAHM system enables air relative humidity (RH) to be increased up to 18 units (%) over the ambient level during mist fumigation, depending on the wind speed inside the experimental stand. Water was dispersed inside 14 × 14 m experimental plots in the form of mist with an average particle size of 50 μm from June to August in 2008, and from May to September in 2009. The average increase in RH was 7 units (%) over the whole period of humidification in 2008 ( P F ) in silver birch ( Betula pendula Roth.) trees was 24.8% ( P P P > 0.05) in silver birch on the days without misting. In hybrid aspen ( Populus tremula L. × P. tremuloides Michx.) the average difference in F between C and H plots was 61.1% ( P P F as a result of long-term acclimation to fumigation. The leaves of silver birch in a humidified plot demonstrated up to 2.4 °C lower ( P T L ) compared to the control plot in 2009. The decline in T L decreased the humidity gradient between leaf and air by about 1/3, whereas 2/3 of the effect was caused directly by changes in air humidity in the leaf boundary layer. Our preliminary data suggest that the FAHM experimental facility enables water fluxes through a deciduous tree canopy to be reduced and this effect is attributable both to the increased air humidity and decreased leaf temperature. Changes in these two basic factors may create considerable differences in the physiology, anatomy and nutrition of a whole tree, also affecting forest functioning in the light of global climate change.

The diurnal changes in light-saturated photosynthesis (Pn) under elevated CO(2) and/or O(3) in relation to stomatal conductance (g(s)), water potential, intercellular [CO(2)], leaf temperature and vapour-pressure difference between leaf and air (VPD(L)) were studied at the Aspen FACE site. Two aspen (Populus tremuloides Michx.) clones differing in their sensitivity to ozone were measured. The depression in Pn was found after 10:00 h. The midday decline in Pn corresponded with both decreased g(s) and decreased Rubisco carboxylation efficiency, Vc(max). As a result of increasing VPD(L), g(s) decreased. Elevated [CO(2)] resulted in more pronounced midday decline in Pn compared to ambient concentrations. Moreover, this decline was more pronounced under combined treatment compared to elevated CO(2) treatment. The positive impact of CO(2) on Pn was relatively more pronounced in days with environmental stress but relatively less pronounced during midday depression. The negative impact of ozone tended to decrease in both cases.

The effect of elevated CO(2) and O(3) on apparent quantum yield (varphi), maximum photosynthesis (P(max)), carboxylation efficiency (V(cmax)) and electron transport capacity (J(max)) at different canopy locations was studied in two aspen (Populus tremuloides) clones of contrasting O(3) tolerance. Local light climate at every leaf was characterized as fraction of above-canopy photosynthetic photon flux density (%PPFD). Elevated CO(2) alone did not affect varphi or P(max), and increased J(max) in the O(3)-sensitive, but not in the O(3)-tolerant clone. Elevated O(3) decreased leaf chlorophyll content and all photosynthetic parameters, particularly in the lower canopy, and the negative impact of O(3) increased through time. Significant interaction effect, whereby the negative impact of elevated O(3) was exaggerated by elevated CO(2) was seen in Chl, N and J(max), and occurred in both O(3)-tolerant and O(3)-sensitive clones. The clonal differences in the level of CO(2)xO(3) interaction suggest a relationship between photosynthetic acclimation and background O(3) concentration.

Water-use strategies of Populus tremula and Tilia cordata, and the role of abscisic acid in these strategies, were analysed. P. tremula dominated in the overstorey and T. cordata in the lower layer of the tree canopy of the temperate deciduous forest canopy. Shoot water potential (Ψ), bulk-leaf abscisic acid concentration ([ABA]leaf), abscisic acid concentration in xylem sap ([ABA]xyl), and rate of stomatal closure following the supply of exogenous ABA (v) decreased acropetally through the whole tree canopy, and foliar water content per area (w), concentration of the leaf osmoticum (c), maximum leaf-specific hydraulic conductance of shoot (L), stomatal conductance (g s), and the threshold dose per leaf area of the exogenous ABA (d a) required to reduce stomatal conductance increased acropetally through the tree canopy (from the base of the foliage of T. cordata to the top of the foliage of P. tremula) in non-stressed trees. The threshold dose per leaf dry mass of the exogenous ABA (d w) required to reduce stomatal conductance, was similar through the tree canopy. After a drought period (3 weeks), the Ψ, w, L, g s, d a and d w had decreased, and c and v had increased in both species. Yet, the effect of the drought period was more pronounced on L, g s, d a, d w and v in T. cordata, and on Ψ, w and c in P. tremula. It was concluded that the water use of the species of the lower canopy layer—T. cordata, is more conservative than that of the species of the overstorey, P. tremula. [ABA]leaf had not been significantly changed in these trees, and [ABA]xyl had increased during the drought period only in P. tremula. The relations between [ABA]leaf, [ABA]xyl and the stomatal conductance, the osmotic adjustment and the shoot hydraulic conductance are also discussed.

Summary 1. The impacts of elevated atmospheric CO 2 and/or O 3 have been examined over 4 years using an open-air exposure system in an aggrading northern temperate forest containing two different functional groups (the indeterminate, pioneer, O 3 -sensitive species Trembling Aspen, Populus tremuloides and Paper Birch, Betula papyrifera , and the determinate, late successional, O 3 -tolerant species Sugar Maple, Acer saccharum ). 2. The responses to these interacting greenhouse gases have been remarkably consistent in pure Aspen stands and in mixed Aspen/Birch and Aspen/Maple stands, from leaf to ecosystem level, for O 3 -tolerant as well as O 3 -sensitive genotypes and across various trophic levels. These two gases act in opposing ways, and even at low concentrations (1·5 × ambient, with ambient averaging 34‐36 nL L − 1 during the summer daylight hours), O 3 offsets or moderates the responses induced by elevated CO 2 . 3. After 3 years of exposure to 560 µ mol mol − 1 CO 2 , the above-ground volume of Aspen stands was 40% above those grown at ambient CO 2 , and there was no indication of a diminishing growth trend. In contrast, O 3 at 1·5 × ambient completely offset the growth enhancement by CO 2 , both for O 3 -sensitive and O 3 -tolerant clones. Implications of this finding for carbon sequestration, plantations to reduce excess CO 2 , and global models of forest productivity and climate change are presented.

Correlations between leaf abscisic acid concentration ([ABA]), stomatal conductance (g s ), rate of stomatal opening in response to an increase in leaf water potential (s i ), shoot hydraulic conductance (L) and photosynthetic characteristics were examined in saplings of six temperate deciduous tree species: Acer platanoides L., Padus avium Mill., Populus tremula L., Quercus robur L., Salix caprea L. and Tilia cordata Mill. Species-specific values of foliar [ABA] were negatively related to the mean values of g s , s i , L and light- and CO 2 - saturated net photosynthesis (P max ), thus providing strong correlative evidence of a scaling of foliar gas exchange and hydraulic characteristics with leaf endogenous [ABA]. In addition, we suggest that mean g s , s i , L and P max for mature leaves may partly be determined by the species-specific [ABA] during leaf growth. The most drought-intolerant species had the lowest [ABA] and the highest g s , suggesting that interspecific differences in [ABA] may be linked to differences in species-specific water-use efficiency. Application of high concentrations of exogenous ABA led to large decreases in g s , s i and P max , further underscoring the direct role of ABA in regulating stomatal opening and photosynthetic rate. Exogenous ABA also decreased L, but the decreases were considerably smaller than the decreases in g s , s i and P max . Thus, exogenous ABA predominantly affected the stomata directly, but modification of L by ABA may also be an important mechanism of ABA action. We conclude that interspecific variability in endogenous [ABA] during foliage growth and in mature leaves provides an important factor explaining observed differences in L, g s , s i and P max among temperate deciduous tree species.

We studied the physicochemical properties of stemwood in saplings of silver birch ( Roth) and hybrid aspen ( L. × Michx.), grown for four years under artificially elevated relative air humidity (on average by 7%) in field conditions, using the Free Air Humidity Manipulation (FAHM) research facility in Estonia. Altogether 91 sample trees from three experimental plots with manipulated air humidity and from three control plots were cut in the dormant season and sampled for the analysis of cellulose, hemicellulose, acid detergent lignin, macronutrients (N, P, K), ash content, density, and calorific value of wood. The analysed trees grew significantly more slowly under elevated humidity conditions, with a more pronounced effect on aspens. Significantly higher concentrations of N and P were observed in the stemwood of both aspens and birches grown under elevated humidity. This could be the result of a change in the content of living parenchyma cells and/or enhanced retranslocation of nutrients into wood parenchyma. Additionally, humidification resulted in a significantly higher concentration of cellulose and a lower concentration of hemicellulose in aspen stemwood, and in significantly lower concentrations of cellulose and K in birch stemwood. Elevated humidity did not affect lignin concentration, ash content, basic density and calorific value of stemwood. Results from the FAHM experiment suggest that the increasing air humidity accompanying global warming at northern latitudes will affect the growth and functioning of deciduous trees and forests, with obvious consequences also for forest management and industry.Betula pendulaPopulus tremulaP. tremuloides

Two aspen (Populus tremuloides Michx.) clones, differing in O3 tolerance, were grown in a free-air CO2 enrichment (FACE) facility near Rhinelander, Wisconsin, and exposed to ambient air, elevated CO2, elevated O3 and elevated CO2+O3. Leaf instantaneous light-saturated photosynthesis (PS) and leaf areas (A) were measured for all leaves of the current terminal, upper (current year) and the current-year increment of lower (1-year-old) lateral branches. An average, representative branch was chosen from each branch class. In addition, the average photosynthetic rate was estimated for the short-shoot leaves. A summing approach was used to estimate potential whole-plant C gain. The results of this method indicated that treatment differences were more pronounced at the plant- than at the leaf- or branch-level, because minor effects within modules accrued in scaling to plant level. The whole-plant response in C gain was determined by the counteracting changes in PS and A. For example, in the O3-sensitive clone (259), inhibition of PS in elevated O3 (at both ambient and elevated CO2) was partially ameliorated by an increase in total A. For the O3-tolerant clone (216), on the other hand, stimulation of photosynthetic rates in elevated CO2 was nullified by decreased total A.

Leaf gas exchange parameters and the content of ribulose1,5-bisphosphate carboxylase/oxygenase (Rubisco) in the leaves of two 2-year-old aspen (Populus tremuloides Michx.) clones (no. 216, ozone tolerant and no. 259, ozone sensitive) were determined to estimate the relative stomatal and mesophyll limitations to photosynthesis and to determine how these limitations were altered by exposure to elevated CO2 and/or O3. The plants were exposed either to ambient air (control), elevated CO2 (560 p.p.m.) elevated O3 (55 p.p.b.) or a mixture of elevated CO2 and O3 in a free air CO2 enrichment (FACE) facility located near Rhinelander, Wisconsin, USA. Light-saturated photosynthesis and stomatal conductance were measured in all leaves of the current terminal and of two lateral branches (one from the upper and one from the lower canopy) to detect possible age-related variation in relative stomatal limitation (leaf age is described as a function of leaf plastochron index). Photosynthesis was increased by elevated CO2 and decreased by O3 at both control and elevated CO2. The relative stomatal limitation to photosynthesis (ls) was in both clones about 10% under control and elevated O3. Exposure to elevated CO2 + O3 in both clones and to elevated CO2 in clone 259, decreased ls even further ‐ to about 5%. The corresponding changes in Rubisco content and the stability of Ci/Ca ratio suggest that the changes in photosynthesis in response to elevated CO2 and O3 were primarily triggered by altered mesophyll processes in the two aspen clones of contrasting O3 tolerance. The changes in stomatal conductance seem to be a secondary response, maintaining stable Ci under the given treatment, that indicates close coupling between stomatal and mesophyll processes.

Controls on leaf stomatal conductances imposed by soil water availability and foliage acclimation to long‐term integrated irradiance were studied in a natural mixed deciduous stand composed of shade‐intolerant Populus tremula L. and shade‐tolerant Tilia cordata Mill. Positive relationships between maximum stomatal conductance and seasonal integrated average daily quantum flux density ( \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape $Q_{\mathrm{int}\,}$ \end{document} , mol m \documentclass{aastex} \usepackage{amsbsy} \usepackage...

To clarify interactions between stomatal responses to two simultaneous environmental changes, the rates of change in stomatal conductance were measured after simultaneously changing two environmental factors from the set of air humidity, leaf water potential (hydraulic environmental factors), air CO(2) concentration and light intensity (photosynthetic environmental factors). The stomatal responses to changes in leaf water potential were not significantly modified by any other simultaneous environmental change. A decrease in air humidity was followed by a decrease in stomatal conductance, and an increase in air humidity was followed by an increase in the conductance, irrespective of the character of the simultaneous change in the photosynthetic environmental factor. If the simultaneous change had an opposite effect on stomatal conductance, the rate of change in stomatal conductance was higher than the theoretical summed rate-the sum of the rate following one environmental change and the rate following another environmental change, measured separately. That is, the stomatal response to air humidity dominated over the responses to photosynthetic environmental factors. Yet, if the simultaneous change in photosynthetic factors had a codirectional effect on stomatal conductance, the rate of stomatal conductance change was lower than the theoretical summed rate. After a simultaneous change of two photosynthetic environmental factors, the rate of stomatal conductance change was very similar to the theoretical rate, if both the environmental changes had a codirectional effect on stomatal conductance. If the changes in the photosynthetic factors had opposite effects on stomatal conductance, the conductance increased, irrespective of the character of the increasing environmental factor. In drought-stressed trees, the rates of change in stomatal conductance tended to differ from the theoretical summed rates more than in well-watered trees. Stomatal closure following an increase in CO(2) concentration was the stomatal response that was most strongly suppressed by the response to another simultaneous environmental change. Six species of temperate deciduous trees were shown to be similar in their relations between the stomatal responses to two simultaneous environmental changes. The mechanism and ecological significance of the interactions between the two signal response pathways of stomata are discussed.

Photosynthetic acclimation under elevated carbon dioxide (CO2) and/or ozone (O3) has been the topic of discussion in many papers recently. We examined whether or not aspen plants grown under elevated CO2 and/or O3 will acclimate after 11 years of exposure at the Aspen Face site in Rhinelander, WI, USA. We studied diurnal patterns of instantaneous photosynthetic measurements as well as A/Ci measurements monthly during the 2004–2008 growing seasons. Our results suggest that the responses of two aspen clones differing in O3 sensitivity showed no evidence of photosynthetic and stomatal acclimation under either elevated CO2 ,O 3 or CO2 þ O3. Both clones 42E and 271 did not show photosynthetic nor stomatal acclimation under elevated CO2 and O3 after a decade of exposure. We found that the degree of increase or decrease in the photosynthesis and stomatal conductance varied significantly from day to day and from one season to another. Published by Elsevier Ltd.

The balance of mechanistic detail with mathematical simplicity contributes to the broad use of the Farquhar, von Caemmerer and Berry (FvCB) photosynthetic rate model. Here the FvCB model was coupled with a stomatal conductance model to form an [A,g(s)] model, and parameterized for mature Populus tremuloides leaves under varying CO(2) and temperature levels. Data were selected to be within typical forest light, CO(2) and temperature ranges, reducing artifacts associated with data collected at extreme values. The error between model-predicted photosynthetic rate (A) and A data was measured in three ways and found to be up to three times greater for each of two independent data sets than for a base-line evaluation using parameterization data. The evaluation methods used here apply to comparisons of model validation results among data sets varying in number and distribution of data, as well as to performance comparisons of [A,g(s)] models differing in internal-process components.

Paper birch (Betula papyrifera Marsh.) and three trembling aspen clones (Populus tremuloides Michx.) were studied to determine if alterations in carbon gain in response to an elevated concentration of CO 2 ([CO 2 ]) or O 3 ([O 3 ]) or a combination of both affected bud size and carbohydrate composition in autumn, and early leaf development in the following spring. The trees were measured for gas exchange, leaf size, date of leaf abscission, size and biochemical characteristics of the overwintering buds and early leaf development during the 8th-9th year of free-air CO 2 and O 3 exposure at the Aspen FACE site located near Rhinelander, WI. Net photosynthesis was enhanced 49-73% by elevated [CO 2 ], and decreased 13-30% by elevated [O 3 ]. Elevated [CO 2 ] delayed, and elevated [O 3 ] tended to accelerate, leaf abscission in autumn. Elevated [CO 2 ] increased the ratio of monosaccharides to di- and oligosaccharides in aspen buds, which may indicate a lag in cold acclimation. The total carbon concentration in overwintering buds was unaffected by the treatments, although elevated [O 3 ] decreased the amount of starch by 16% in birch buds, and reduced the size of aspen buds, which may be related to the delayed leaf development in aspen during the spring. Elevated [CO 2 ] generally ameliorated the effects of elevated [O 3 ]. Our results show that both elevated [CO 2 ] and elevated [O 3 ] have the potential to alter carbon metabolism of overwintering buds. These changes may cause carry-over effects during the next growing season.

Leaf hydraulic conductance (K(leaf)) and several characteristics of hydraulic architecture and physiology were measured during the first 10 weeks of leaf ontogeny in Populus tremula L. saplings growing under control, mild water deficit or elevated temperature conditions. During the initial 3 weeks of leaf ontogeny, most measured characteristics rapidly increased. Thereafter, a gradual decrease in K(leaf) was correlated with a decrease in leaf osmotic potential under all conditions, and with increases in leaf dry mass per area and bulk modulus of elasticity under mild water deficit and control conditions. From about Week 3 onward, K(leaf) was 33% lower in trees subjected to mild water deficit and 33% higher in trees held at an elevated temperature relative to control trees. Mild water deficit and elevated temperature treatment had significant and opposite effects on most of the other characteristics measured. The ontogenetic maximum in K(leaf) was correlated positively with the width of xylem conduits in the midrib, but negatively with the overall width of the midrib xylem, number of lateral ribs, leaf dry mass per area and bulk modulus of elasticity. The ontogenetic maximum in K(leaf) was also correlated positively with the proportion of intercellular spaces and leaf osmotic potential, but negatively with leaf thickness, volume of mesophyll cells and epidermis and number of cells per total mesophyll cell volume, the closest relationships being between leaf osmotic potential and number of cells per total mesophyll cell volume. It was concluded that differences in protoplast traits are more important than differences in xylem or parenchymal cell wall traits in determining the variability in K(leaf) among leaves growing under different environmental conditions.

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 NO(3) (-) 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 CO(2) concentration, might be smaller than expected at high latitudes if a rise in atmospheric humidity simultaneously takes place.

Stomatal density and size were measured along the light gradient of a Betula pendula Roth. canopy in relation to microclimatic conditions. The theoretical stomatal conductance was calculated using stomatal density and dimensions to predict to what degree stomatal conductance is related to anatomical properties and relative stomatal opening. Stomatal density was higher and leaf area smaller in the upper canopy, whereas epidermal cell density did not change significantly along the canopy light gradient, indicating that stomatal initiation is responsible for differences in stomatal density. Stomatal dimensions – the length of guard cell on the dorsal side and the guard cell width – decreased with declining light availability. Maximum measured stomatal conductance and modelled stomatal conductance were higher at the top of the crown. The stomata operate closer to their maximum openness and stomatal morphology is a more important determinant of stomatal conductance in the top leaves than in leaves of lower canopy. As stomata usually limit photosynthesis more in upper than in lower canopy, it was concluded that stomatal morphology can principally be important for photosynthesis limitation in upper canopy.

The seasonal course of maximum hydraulic conductance of leaf laminae (K lamina) of shoots correlated strongly with the seasonal course of the maximum hydraulic conductance of leaf laminae of HgCl2-treated shoots (K lamina(HgCl2)), and with the seasonal course of the difference (dK lamina) between K lamina and K lamina(HgCl2). However, it did not correlate strongly with the seasonal course of the hydraulic conductance of stem and petioles of the shoot (K stpt) in six temperate deciduous tree species. The species ranked according to K lamina as follows: Populus tremula L. > Salix caprea L. > Padus avium Mill. > Quercus robur L. > Tilia cordata Mill. > Acer platanoides L. The species-specific maxima of K lamina correlated positively with the simultaneous values of K lamina(HgCl2), dK lamina and K stpt; the correlation was strongest with K lamina(HgCl2). It was concluded that the seasonal dynamics of maximum hydraulic conductance of leaf laminae was determined almost equally by the seasonal dynamics of the hydraulic conductance of foliar protoplasts and apoplast, but the inter-specific differences in K lamina were mainly caused by the different apoplastic hydraulic conductance in leaves of these species. The relative contribution of dK lamina (in K lamina) was highest in slow-growing species (~55% in A. platanoides) and the lowest in fast-growers (~30% in S. caprea).

Some anatomical characteristics in leaves relating to hydraulic conductance and stomatal conductance were examined in six temperate deciduous tree species. The fourth power of the radius of the conducting elements in xylem (r4) and the area of mesophyll and epidermal cells per unit length of leaf cross-section (u) were high in leaves with high hydraulic conductance (L). Stomatal conductance (gs) and stomatal sensitivity to an increase in leaf water potential (si) correlated positively with the length of stomatal pore (l), but negatively with the guard cell width (z) and the length of the dorsal side of the guard cells (ld). Stomatal sensitivity to a decrease in leaf water potential (sd) correlated negatively with l and positively with z and ld. The anatomical characteristics associated with hydraulic conductance (r4 and u) and those associated with stomatal conductance and sensitivity to changes of leaf water potential (l, z and ld) were correlated. We conclude that hydraulic conductance may depend on anatomical characteristics of xylem, mesophyll and epidermis, and stomatal conductance and its sensitivity to changing water potential may depend on anatomical characteristics of stomata. The correlation of shoot hydraulic conductance with stomatal conductance and its sensitivity may be based largely on the correlation between the anatomical characteristics of the water conducting system and stomata in these trees.