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No evidence of coordination between leaf hydraulic function and stomatal characteristics was observed in young hybrid aspen stands attributable to the near-anisohydric behaviour and the fast-growing nature of this species. Within forest canopies, there are environmental gradients that enhance water losses in the direction of treetops. Therefore, the foliar morpho-anatomical plasticity, which allows plant acclimation to environmental variations, is of great importance, but empirical information about the relationship between leaf structural and functional traits is inconsistent, varying with ecological requirements and the life strategy of the species. In this study, leaf hydraulic conductance, anatomical structure, and stomatal morphology were investigated in young hybrid aspen (Populus tremula × P. tremuloides) stands to elucidate the coordination between structural and hydraulic traits within the canopy. Stomatal size and density demonstrated opposite trends with respect to leaf location, leading to a relatively uniform spatial distribution of potential gas exchange capacity in the canopy. The hydraulic system of branches was adjusted to maximise the potential water supply in the midcanopy, containing nearly half of the trees’ total photosynthetic surface. Most vascular traits in both the petioles and laminae demonstrated stronger development towards the treetop that should enhance the water supply of the upper foliage, which is exposed to higher irradiance and atmospheric evaporative demand. Small increases in the total vein density expressed per unit lamina area in the upper canopy could not compensate for substantial leaf thickening, resulting in decreased vein density expressed per unit lamina volume and increasing the resistance of the leaf extravascular pathway. No solid evidence of coordination was found between leaf hydraulic function and stomatal characteristics attributable to the near-anisohydric behaviour and the fast-growing nature of this species.

Predawn water potential disequilibrium in hybrid aspen coppice growing in humid environment was mostly determined by hydraulic conductance and the ratio of nocturnal to daily water loss was relatively small. Water relations are of crucial importance for biomass accumulation in trees and forest productivity, yet the significance of nocturnal water use is still debated. We investigated which environmental factors influence nocturnal transpiration, development of the equilibrium between soil and plant water potentials and plant hydraulic conductance in hybrid aspen (Populus tremula L. × P. tremuloides Michx.) coppice. Predawn leaf water potential and sap flow were measured simultaneously with relevant environmental factors on 2-year-old coppice shoots during the growing season of 2015. Nocturnal water loss constituted on average 4–5% of daily total transpiration and almost 2/3 of nocturnal sap flow. The nocturnal water loss was mainly driven by atmospheric evaporative demand, while maximum sap flux densities could extend 35% of the daytime values on dry nights. Predawn leaf water potential was constantly more negative than soil water potential, resulting in predawn disequlibrium between leaf and soil water potentials (PDD). Total soil-to-leaf hydraulic conductance was the primary factor explaining the variation in PDD. Our results suggest that in humid conditions plant hydraulic capacity is more substantial determining plant predawn water status than stomatal behaviour with respect to current environmental drivers.

Climate change scenarios predict an increase in air temperature and precipitation in northern temperate regions of Europe by the end of the century. Increasing atmospheric humidity inevitably resulting from more frequent rainfall events reduces water flux through vegetation, influencing plants' structure and functioning. We investigated the extent to which artificially elevated air humidity affects the anatomical structure of the vascular system and hydraulic conductance of leaves in Betula pendula. A field experiment was carried out at the Free Air Humidity Manipulation (FAHM) site with a mean increase in relative air humidity (RH) by 7% over the ambient level across the growing period. Leaf hydraulic properties were determined with a high-pressure flow meter; changes in leaf anatomical structure were studied by means of conventional light microscopy and digital image processing techniques. Leaf development under elevated RH reduced leaf-blade hydraulic conductance and petiole conductivity and had a weak effect on leaf vascular traits (vessel diameters decreased), but had no significant influence on stomatal traits or tissue proportions in laminae. Both hydraulic traits and relevant anatomical characteristics demonstrated pronounced trends with respect to leaf location in the canopy-they increased from crown base to top. Stomatal traits were positively correlated with several petiole and leaf midrib vascular traits. The reduction in leaf hydraulic conductance in response to increasing air humidity is primarily attributable to reduced vessel size, while higher hydraulic efficiency of upper-crown foliage is associated with vertical trends in the size of vascular bundles, vessel number and vein density. Although we observed co-ordinated adjustment of vascular and hydraulic traits, the reduced leaf hydraulic efficiency could lead to an imbalance between hydraulic supply and transpiration demand under the extreme environmental conditions likely to become more frequent in light of global climate change.

The study was performed on coppice shoots and 9-year-old saplings of hybrid aspen (Populus tremula L. × Populus tremuloides Michx.) growing at the Free Air Humidity Manipulation site in 2013 and 2014. The main aims of our study were to determine the differences in water potentials between soil and leaves before dawn and the major environmental factors preventing the overnight equilibration of water potential. Secondly, we investigated the impact of increasing relative air humidity—a climate trend predicted for northern Europe—on the equilibration. A relatively small predawn disequilibrium (PDD) was recorded both for coppice shoots and saplings: on average 0.042 and 0.066 MPa, respectively. We found a significant correlation between PDD and the vapour pressure deficit of ambient air (R 2 = 0.80 and 0.66, respectively). The experiment with bagged leaves revealed that 76% (coppice shoots) and 47% (saplings) of the PDD are attributable to nocturnal transpiration. In coppice shoots, the magnitude of PDD is best explained by soil water availability. After cool and moist nights, when leaves were usually dew-wetted, the PDD was close to zero, supporting the idea of foliar uptake of water through the cuticle. After drier nights, predawn leaf water potential does not represent a reliable estimate of soil water potential even in moderately humid climates. Increasing atmospheric humidity does not affect the magnitude of PDD in hybrid aspen. However, the stomata of hybrid aspen remain open at night, which enables nocturnal transpiration and water loss from the foliage.

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

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.

Variation in leaf and shoot hydraulic conductance was examined on detached shoots of silver birch (Betula pendula Roth), cut from the lower third (shade leaves) and upper third of the crown (sun leaves) of large trees growing in a natural temperate forest stand. Hydraulic conductances of whole shoots (K S), leaf blades (K lb), petioles (K P) and branches (i.e. leafless stem; K B) were determined by water perfusion using a high-pressure flow meter in quasi-steady state mode. The shoots were exposed to irradiance of photosynthetic photon flux density of 200–250 μmol m−2 s−1, using different light sources. K lb depended significantly (P

Spatial patterns in vessel diameter, vessel density and xylem conducting efficiency within a crown were examined in closed-canopy trees of silver birch (Betula pendula). The variation in anatomical and hydraulic characteristics of branches was considered from three perspectives: vertically within a crown (lower, middle and upper crown), radially along main branches (proximal, middle and distal part), and with respect to branch orders (first-, second- and third-order branches). Hydraulically weighted mean diameter of vessels (D h) and theoretical specific conductivity of the xylem (k t) exhibited no vertical trend within the tree crown, whereas leaf-specific conductivity of the xylem (LSCt) decreased acropetally. Variation in LSCt was governed by sapwood area to leaf area ratio (Huber value) rather than by changes in xylem anatomy. The acropetal increase in soil-to-leaf conductance (G T) within the birch canopy is attributable to longer path length within the lower-crown branches and higher hydraulic resistance of the shade leaves. D h, k t and LSCt decreased, while vessel density (VD) and relative area of vessel lumina (VA) increased distally along main branches. A strong negative relationship between vessel diameter and VD implies a trade-off between hydraulic efficiency and mechanical stability of xylem. D h and VD combined explained 85.4% of the total variation of k t in the regression model applied to the whole data set. Xylem in fast-growing branches (primary branches) had greater area of vessel lumina per unit cross-sectional area of sapwood, resulting in a positive relationship between branch radial growth rate and k t. D h, k t and LSCt decreased, whereas VD increased with increasing branch order. This pattern promotes the hydraulic dominance of primary branches over the secondary branches and their dominance over tertiary branches. In this way crown architecture contributes to preferential water flow along the main axes, potentially providing better water supply for the branch apical bud and foliage located in the outer, better-insolated part of the crown.

Responses of leaf conductance (gL) to variation in environmental and plant hydraulic factors were examined on intact and detached shoots of little-leaf linden (Tilia cordata Mill.) with respect to branch position in the crown. Using detached shoots, we manipulated leaf water supply and light availability in order to separate the effects of insufficient hydraulic supply and low irradiance. The intact upper-crown leaves demonstrated 2.0–2.2 times higher (P < 0.001) daily maxima of gL compared to the lower-crown leaves growing in the shadow of upper branches. Mean soil-to-leaf conductance (GT) was 1.9 times higher (P < 0.001) for the upper-crown foliage compared to that of the lower crown. The total hydraulic resistance was distributed: soil to distal shoots—41–51%, 25-cm distal shoots—10–15% and leaves—39–44%. In lower branches, gL was constrained by both low light availability and limited water supply; in upper branches, only by irradiance. Artificial reduction of hydraulic constraints raised bulk leaf water potential (ΨL) and made gL less sensitive to changes in both atmospheric and plant factors. Stomatal responses to leaf-to-air vapour pressure difference (VPD) were significantly modified by leaf water status: high ΨL seemingly inverted the gL versus VPD relationship. Enhanced water supply increased gL and transpiration rate (E) in the lower-crown foliage, but not in the upper-crown foliage. The results support the idea that leaves in the lower canopy are hydraulically more constrained than those in the upper canopy.

Spatial variation in sapwood area to leaf area ratio (Huber value, HV) and specific leaf area (SLA) was examined in branches of closed-canopy trees of silver birch (Betula pendula Roth). HV increased basipetally within a crown and decreased with increasing branch order, but exhibited no significant radial trend along a primary branch. HV was primarily determined by branch position in a crown and branch diameter at the sampling point, being independent of the size of the tree and branch. Greater HV in the lower-crown branches is considered a means to mitigate differences in hydraulic transport capacity between the branches located in different canopy layers. Beside branch position and sampling location on a branch, SLA depended significantly on several other variables characterising tree and branch size. SLA increased basipetally within a crown and along a primary branch, but exhibited no significant trend with branch orders. Because height caused leaf area (AL) to diminish more rapidly than leaf dry weight, AL primarily determined the vertical variation in SLA.

Water relations and gas exchange were studied in the crowns of small European larch (Larix decidua Mill.) trees with respect to branch position. The upper-crown branches showed significantly higher branch sap flux rate (Fla) and branch conductance (gb) compared to the lower crown (P

Nutrient conservation in plants and soil fertility may be intricately linked. We studied nitrogen conservation in small Scots pine (Pinus sylvestrisL.) trees growing in stands on organogenic Dystric Histosols and on mineral Podzols. Nitrogen-resorption efficiency (NRE) and proficiency (NRP) of senescent needles, and mean residence time of nitrogen (MRT) were studied in relation to needle surface area, needle longevity, and leaf mass per area (LMA). Trees on Podzols had higher nitrogen concentration in green needles than the trees on Dystric Histosols, but the nitrogen concentration of yellowing needles was similar for trees on both soil types. NRE averaged 65±3.5% (mean±SD) and 56±7.2% for the trees on Podzols and Dystric Histosols, respectively. Neither NRP (0.44±0.05% and 0.35±0.07%, respectively) nor MRT (8.4±2.3 and 6.1±1.2 years) differed significantly between the stands on the two soil types. Mean needle surface area was significantly smaller in trees on Dystric Histosols (76±29 mm2) than on Podzols (131±38 mm2), whereas needle longevity varied between 2 and 4 years independently of the soil type. Trees invested, on average, the same amount of dry matter per unit of needle area on both soil types. Growth of trees, measured as increment of shoot length, was more restricted on Dystric Histosols (55±18 mm yr−1) than on Podzols (184±44 mm yr−1). The results of the correlation analysis applied to pooled data were inconsistent with the relations between traits of stress resistance syndrome observed in inter-specific comparisons. The study indicated that Scots pine trees relocated nitrogen from senescent foliage more efficiently on mineral Podzols than on organogenic Dystric Histosols, but the minimum nitrogen concentration of needles appeared to be similar on both soil types.

Different calculation methods, based on needle geometry, for estimating both projected area (PLA) and total surface area (TLA) of foliage in Norway spruce [Picea abies (L.) Karst.] were compared. Seventy-eight shoots of four age classes were sampled from both the basal and top thirds of crowns. Three dimensions (the length, minor and major diameters) of needles were taken, and the needle shape was approximated to a parallelepiped or ellipsoid. There was a perfect coincidence of the measured and estimated values of PLA calculated as the width of the needle projection multiplied by needle length, and corrected for needle taper (method III), or when the needle projection was treated as a rectangle joined with half-ellipses at both ends (method IV). The most reliable estimations of TLA resulted from treating the needle sides as faces of the parallelepiped tapering at their ends in the form of half-ellipses. The ratio of TLA to PLA ranged from 2.2 to 4.0 depending on the needle morphology. Needle minor diameter (anatomical width; D 1) was found to be a better morphological index of the spruce foliage than needle flatness, i.e. the ratio of major to minor diameter. Expressing the factor for converting PLA to TLA as a function of D 1 considerably improved the precision of the estimates. Close relationships were established between specific leaf area, expressed on both a projected area (SLAP) and total surface area basis (SLAT), needle dry weight (R 2 was 0.799 and 0.852, respectively) and minor diameter of needles (R 2 was 0.701 and 0.554, respectively).

Variation in base water potential (Ψb, a daily maximum level of plant water potential, which is presumed to correspond to the condition of equilibrium between the soil and plant water potentials) was examined in shoots of Norway spruce trees growing in well-drained and waterlogged soils. The influence of soil water content, air temperature, and vapour pressure deficit of the atmosphere on Ψb was studied using the pressure chamber technique. Maximum daily water potentials were not always observable before dawn; some were registered up to two hours later. This tendency being characteristic of trees growing under stress (shade, waterlogging) conditions, increased with declining soil water availability. In trees growing in well-drained soil, Ψb depended asymptotically on the available soil water storage (R2=0.73), while the values were slightly influenced by vapour pressure deficit of the atmosphere as well. In trees growing in waterlogged soil, Ψb was independent of the soil water storage, but sensitive to the vapour pressure deficit.

The relative hydraulic conductivity (k) of xylem and resistance (R) to water flow through trunk, primary roots and branches in Picea abies trees growing under contrasting light conditions were investigated. The xylem permeability to water was measured by forcing 10 mM water solution of KC1 through excised wood specimens. From the values of k, the sapwood transverse area and the length of conducting segments, R of the whole trunk, branches and roots was calculated. The relative conductivity of xylem in open-grown trees exceeded that of shade-grown trees by 1.4–3.1 times, while k was closely correlated with the hydraulically effective radius (Re) of the largest tracheids (R2 was 0.85–0.94 for open- and 0.51–0.79 for shade-grown trees). Because of both a low k and a smaller sapwood area in shade-grown trees the resistance to water movement through their trunk, roots and branches was many times higher. The distribution of R between single segments of the water-conducting pathway differed considerably in trees from different sites. At high water status the largest share of the total resistance in open- as well as shade-grown trees resides in the apical part of the trunk. The contribution of the branches to total xylem resistance is supposed to increase with developing water deficit.