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 (g(l_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 (g(l_predawn)) depended on RH, IR_pday and R (P < 0.05), whereas early-morning gross A (A(gross_PAR500)) depended on IR_pday and g(l_predawn) (P < 0.001). Daytime net A was positively related to A(gross_PAR500) and leaf [N] (P < 0.05). Tree diameter and height increment correlated positively with g(l) at the beginning and middle of the night (P < 0.05) but not before dawn. Although our results demonstrate that g(l_night) was related to tree growth, the relationship was not determined by R. The linkage between g(l_predawn) and A(gross_PAR500) was modified by IR_pday, indicating that daily CO2 assimilation probably provides feedback for stomatal opening before dawn.

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.

Our aim was to investigate the responses of night and daytime water fluxes to environmental stimuli and endogenous drivers in 5 tree species with different water-use strategies. Data analysis revealed that air vapour pressure deficit (VPD) and wind speed were the main drivers of night-time sap flux density (F-night) in all studied species. For Populus x wettsteinii, Populus tremula, Betula pendula, and Alnus glutinosa, VPD was also the major driver of daytime sap flux density (F-day). In Alnus incana, VPD explained less from the total variation in F-day than the photosynthetic photon flux density (Q(P)). The F-day versus VPD regression slope decreased significantly (p<.001) in the following sequence P. x wettsteinii = P. tremula > B. pendula > A. glutinosa > A. incana. However, the F-night versus VPD regression slope declined (p<.05) in sequence P. x wettsteinii > B. pendula > P. tremula > A. incana = A. glutinosa. P. x wettsteinii and B. pendula demonstrated highest net photosynthesis rates (P-n) among the all investigated species. Multiple regression analysis (independent factors: leaf dark respiration rate and sucrose, glucose, and starch contents) revealed that sucrose content was the only factor, which explained variation (R-2=0.35; p<.01) in predawn leaf conductance (g(pd)). Our findings suggest that trees ability to open stomata and lose water at night does not depend directly on their daily water-use strategy but is determined probably by species photosynthetic capacity, growth potential, and nitrogen-use strategy.

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 < 0.05) higher canopy conductance to water vapour in daytime, but also at night. The linearized regression slopes of the relationships between dependent variable (daytime cumulative sap flow (Q c), night-time Q c, total leaf area, total leaf biomass) and week of study were significantly higher in the H treatment than in the M treatment (P < 0.05), indicating greater increase in these variables under high RH during the study period. Although the foliar phosphorous (P) content was significantly (P < 0.05) higher in the H than in the M treatment, the total foliar nitrogen (N) content did not differ (P > 0.05) between the treatments. Our results suggest that the 32.3% higher total foliar P in the H treatment could be partly induced by 28.2% greater transpiration-driven mass flow of water-soluble P and/or by 25.4% higher number of absorptive root tips. Our results confirm that elevated daytime atmospheric humidity increases the potential for night-time water flux and might also facilitate the uptake of mineral nutrients in silver birch, a dominating deciduous tree species in boreal forests.

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 < 0.01) difference in predawn leaf conductance (g n5) and shoot hydraulic conductance (K 5) between the treatments, with the shoots from the H treatment opening their stomata more efficiently before dawn. Both the early-night dark respiration (R 1) and height increment of stump sprouts were significantly higher (P < 0.01) in the control than in the H treatment. Although the relationship between g n5 and predawn dark respiration (R 5) was statistically significant (P < 0.01) in the H treatment, the gn5 did not depend on R 5 in the C treatment. Our findings suggest that endogenous increase in predawn water flux associates with decreased growth rate of hybrid aspen grown at elevated RH. Thus, regional changes in air humidity may potentially impact night-time water relations in fast-growing tree species like hybrid aspen.

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 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 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 future will likely be modified by competitive relationships among trees, which may potentially affect species composition and cause a need to change forestry practices.

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.

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.

Plant architecture is shaped by endogenous growth processes interacting with the local environment. The current study investigated crown development in young black alder trees, assessing the effects of local light conditions and branch height on individual bud mass and contents. In addition, we examined the characteristics of parent shoots [the cross-sectional area (CSA) of stem and total leaf area, shoot length, the number of nodes, the number and total mass of buds per shoot] and leaf-stem as well as bud-stem allometry, as several recent studies link bud development to hydraulic architecture. We sampled shoots from top branches and two lower-crown locations: one subjected to deep shade and the other resembling the upper branches in light availability. Sampling was carried out three times between mid-July and late October, spanning from the early stages of bud growth to dormancy. Individual bud mass and shoot characteristics varied in response to light conditions, whereas leaf-stem allometry depended on branch height, most likely compensating for the increasing length of hydraulic pathways. Despite the differences in individual bud mass, the number of preformed leaves varied little across the crown, indicating that the plasticity in shoot characteristics was mainly achieved by neoformation. The relationship between total bud mass and stem CSA scaled similarly across crown locations. However, scaling slopes gradually decreased throughout the sampling period, driven by bud rather than by stem growth. This suggests that the allometry of total bud mass and CSA of stem is regulated locally, instead of resulting from crown-level processes.

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.

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 (E-nap) and daytime (E-d) 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 ([NI) were observed: shoots with high NWU were characterised by significantly (P<0.05) lower [N], whereas shoots with highest INT had greater [N]. The forward stepwise regression analysis revealed that variability in E-d, NWU and INT depended on [N], whereas the impact of other predictor variables (leaf phosphorus concentration, habitat soil water content and light availability) was insignificant (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. (C) 2013 Elsevier B.V. All rights reserved.

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

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 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.xP. 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. (C) 2014 Elsevier B.V. All rights reserved.

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. (C) 2013 Elsevier B.V. All rights reserved.

The light sensitivity of the shoot hydraulic conductance in five temperate deciduous tree species was measured using two methods to clarify the role of light sensitivity and the suitability of the methods used to study it. The light sensitivity measured using a method that included an interruption of ≤10min in shoot light acclimation did not differ from that measured using a method with continuous illumination. The 'noncontinuous light' methods are suitable for measuring hydraulic conductance and its light response. Light sensitivity correlated with other leaf water traits as follows: positively with the ion-mediated increase in xylem hydraulic conductance; a relative decrease in the hydraulic conductance of the laminae in response to HgCl2; a relative change in stomatal conductance in response to changes in PAR intensity or atmospheric CO2 concentration, or to a decrease in air humidity or leaf water potential; and with instantaneous water use efficiency. The traits correlated negatively with shoot hydraulic conductance, stomatal conductance and relative increases in stomatal conductance in response to increases in leaf water potential. We suggest that high light sensitivity should be considered as one of the characteristics of conservative water use in trees. Low blue light increased shoot hydraulic conductance to a similar extent to moderate white light and twice as much as moderate red light. Blue light perception is important in the light sensitivity mechanism.

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.