Phytoremediation capacity of hybrid aspen at sites affected by industry and agriculture.

Fast-growing Populus spp. are well-acknowledged to restore contaminated soils from heavy metals in industrial areas. Thus far, there is no knowledge about the phytoremediation capacity of Populus spp. plantations in hemiboreal Estonia conditions to restore industrially polluted areas. The objective of this study was to assess the soil contamination rate of heavy metals (As, Cd, Co, Cr, Cu, Fe, Mn, Mo, Ni, Pb and Zn) and their uptake by mature hybrid aspen (Populus tremula × Populus tremuloides Michx.) in plantations in different industrial pollution areas (e.g. cement factory, oil shale mining). For the reference, industrially polluted plantations were compared with the low pollution area hybrid aspen plantation on former agricultural soil, which was influenced by fertilization and liming before afforestation. Twenty-one years after afforestation, soil samples were collected from the 0-10 cm topsoil layer. Aboveground biomass sampling was performed for bark and stem wood by ingrowth cores to separate wood formed during early (1-10 years) and late (11-21 years) stand development. Two decades after the afforestation of industrially polluted areas, the heavy metal concentrations in the soil were higher than the reference plantation and the standard reference for unpolluted soils in most cases. The highest concentrations of heavy metals in woody biomass were in the oil shale quarry spoil; because of poor growth, the accumulated pools in aboveground biomass were low. Cd differed from other metals and accumulated less in wood and more in bark. The concentration of heavy metals (Cd, Cr, Cu, Fe, Mn, Ni and Zn) was higher in the first decade of stand formation (1-10 years) than in the last 10 years (11-21 years). High pools of heavy metals were accumulated in aboveground biomass in the reference plantation, indicating the considerable removal of heavy metal residues from the previous fertilization and liming source with harvest. Two decades of afforestation with hybrid aspen is too short for complete ecosystem restoration from heavy metals in industrially polluted areas.

Night-time water relations and gas exchange in cut shoots of five boreal dwarf shrub species: impact of soil water availability.

Recent findings suggest that drought may affect plants' daytime and night-time stomatal regulation differently. However, knowledge of night-time stomatal behaviour in dwarf shrubs growing in boreal ecosystems is lacking. We sampled cut shoots from dwarf shrub species to elucidate their capacity to transpire at night and the effect of drought on stomatal regulation. The shoots' water relations and gas exchange were measured under controlled conditions in a growth chamber. The studied species demonstrated considerable differences in their diurnal water use. The night-time water use percentage of daytime water use (NWU) reached up to 90% in Andromeda polifolia and Vaccinium uliginosum. In Rhododendron tomentosum, Vaccinium myrtillus and Chamaedaphne calyculata, the NWU was 62, 27 and 26%, respectively. The shoots of C. calyculata showed a significant increase (P < 0.001) in the transpiration rate (E) during the night. However, in R. tomentosum, a decrease (P < 0.05) in nightly E was observed. The shoot conductance (g) at the end of the night was lower than daytime g in all studied species, but the difference was not significant for V. uliginosum. Across the species, NWU was negatively related (P < 0.001) to the soil volumetric water content (SWC) in the plant habitat. However, daytime E and g were positively related (P < 0.05) to the habitat SWC. Only in V. myrtillus was night-time E higher (P < 0.05) in dry conditions than in wet conditions. Our results demonstrate high variability in diurnal water relations in dwarf shrubs, which can keep stomata open in the dark even when drought limits daytime g and E.

Effects of air humidity and soil moisture on secondary metabolites in the leaves and roots of Betula pendula of different competitive status.

Plant secondary metabolites (PSMs) defend plants against abiotic stresses, including those caused by climate change and against biotic stresses, such as herbivory and competition. There is a trade-off between allocating available carbon to growth and defence in stressful environments. However, our knowledge about trade-off is limited, especially when abiotic and biotic stresses co-occur. We aimed to understand the combined effect of increasing precipitation and humidity, the tree's competitive status, and canopy position on leaf secondary metabolites (LSMs) and fine root secondary metabolites (RSMs) in Betula pendula. We sampled 8-year-old B. pendula trees growing in the free air humidity manipulation (FAHM) experimental site, where treatments included elevated relative air humidity and elevated soil moisture. A high-performance liquid chromatography-quadrupole-time of flight mass spectrometer (HPLC-qTOF-MS) was used to analyse secondary metabolites. Our results showed accumulation of LSM depends on the canopy position and competitive status. Flavonoids (FLA), dihydroxybenzoic acids (HBA), jasmonates (JA) and terpene glucosides (TG) were higher in the upper canopy, and FLA, monoaryl compounds (MAR) and sesquiterpenoids (ST) were higher in dominant trees. The FAHM treatments had a more distinct effect on RSM than on LSM. The RSMs were lower in elevated air humidity and soil moisture conditions than in control conditions. The RSM content depended on the competitive status and was higher in suppressed trees. Our study suggests that young B. pendula will allocate similar amounts of carbon to constitutive chemical leaf defence, but a lower amount to root defence (per fine root biomass) under higher humidity.

Climate and Competitive Status Modulate the Variation in Secondary Metabolites More in Leaves Than in Fine Roots of Betula pendula.

Plant secondary metabolites have many important functions; they also determine the productivity and resilience of trees under climate change. The effects of environmental factors on secondary metabolites are much better understood in above-ground than in below-ground part of the tree. Competition is a crucial biotic stress factor, but little is known about the interaction effect of climate and competition on the secondary chemistry of trees. Moreover, competition effect is usually overlooked when analyzing the sources of variation in the secondary chemistry. Our aim was to clarify the effects of competitive status, within-crown light environment, and climate on the secondary chemistry of silver birch (Betula pendula Roth). We sampled leaves (from upper and lower crown) and fine roots from competitively dominant and suppressed B. pendula trees in plantations along a latitudinal gradient (56-67° N) in Fennoscandia, with mean annual temperature (MAT) range: -1 to 8°C. Secondary metabolites in leaves (SML) and fine roots (SMFR) were determined with an HPLC-qTOF mass spectrometer. We found that SML content increased significantly with MAT. The effect of competitive stress on SML strengthened in colder climates (MAT<4°C). Competition and shade initiated a few similar responses in SML. SMFR varied less with MAT. Suppressed trees allocated relatively more resources to SML in warmer climates and to SMFR in colder ones. Our study revealed that the content and profile of secondary metabolites (mostly phenolic defense compounds and growth regulators) in leaves of B. pendula varied with climate and reflected the trees' defense requirements against herbivory, exposure to irradiance, and competitive status (resource supply). The metabolic profile of fine roots reflected, besides defense requirements, also different below-ground competition strategies in warmer and colder climates. An increase in carbon assimilation to secondary compounds can be expected at northern latitudes due to climate change.

The initial overreaction of carbon cycle to elevated atmospheric humidity levels off over time - a FAHM study in a young birch forest.

Ecosystem responses to climate change are mainly predicted based on short-term studies. However, the first response can be a temporary overreaction, different from the later response of the more acclimated ecosystem. The current paper is a follow-up study of our previous article, where the effect of elevated atmospheric humidity on forest ecosystem carbon (C) balance was studied in a young silver birch (Betula pendula Roth) forest after two years of humidification. Here, we present the C balance of the same forest measured two years later when humidification treatment had been performed for four years. We revealed that the higher C sequestration capacity of the humidified birch forest ecosystem was an initial overreaction, which levelled off after four years of humidification, when the ecosystem became more acclimated to wetter conditions. Understorey production reacted rapidly and strongly by increasing belowground production more than twofold, but this reaction ceased after four years of humidification treatment. Trees responded to a lesser extent, and the initially decreased aboveground growth was recovered after four years of humidification, when the biomass allocation to tree fine-roots was increased. Our results showed that at early forest age, understorey plant production dominated in the whole ecosystem C sequestration capacity. But in the later stage, the most important C sink was biomass production of birches, and since the tree biomass production no longer differed between the treatments, C sequestration of the whole ecosystem did not differ either. The findings confirm that a preliminary reaction of an ecosystem can be different from the later response, which needs to be taken into account when prognosing the climate change consequences for carbon sequestration.

The competitive status of trees determines their responsiveness to increasing atmospheric humidity - a climate trend predicted for northern latitudes.

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.

Elevated air humidity affects hydraulic traits and tree size but not biomass allocation in young silver birches (Betula pendula).

As changes in air temperature, precipitation, and air humidity are expected in the coming decades, studies on the impact of these environmental shifts on plant growth and functioning are of major importance. Greatly understudied aspects of climate change include consequences of increasing air humidity on forest ecosystems, predicted for high latitudes. The main objective of this study was to find a link between hydraulic acclimation and shifts in trees' resource allocation in silver birch (Betula pendula Roth) in response to elevated air relative humidity (RH). A second question was whether the changes in hydraulic architecture depend on tree size. Two years of application of increased RH decreased the biomass accumulation in birch saplings, but the biomass partitioning among aboveground parts (leaves, branches, and stems) remained unaffected. Increased stem Huber values (xylem cross-sectional area to leaf area ratio) observed in trees under elevated RH did not entail changes in the ratio of non-photosynthetic to photosynthetic tissues. The reduction of stem-wood density is attributable to diminished mechanical load imposed on the stem, since humidified trees had relatively shorter crowns. Growing under higher RH caused hydraulic conductance of the root system (K R) to increase, while K R (expressed per unit leaf area) decreased and leaf hydraulic conductance increased with tree size. Saplings of silver birch acclimate to increasing air humidity by adjusting plant morphology (live crown length, slenderness, specific leaf area, and fine-root traits) and wood density rather than biomass distribution among aboveground organs. The treatment had a significant effect on several hydraulic properties of the trees, while the shifts were largely associated with changes in tree size but not in biomass allocation.

Impact of elevated atmospheric humidity on anatomical and hydraulic traits of xylem in hybrid aspen.

This study was performed on hybrid aspen saplings growing at the Free Air Humidity Manipulation site in Estonia. We investigated changes in wood anatomy and hydraulic conductivity in response to increased air humidity. Two hydraulic traits (specific conductivity and leaf-specific conductivity) and four anatomical traits of stem wood-relative vessel area (VA), vessel density (VD), pit area and pit aperture area-were influenced by the humidity manipulation. Stem hydraulic traits decreased in the apical direction, whereas branch hydraulic characteristics tended to be greatest in mid-canopy, associated with branch size. A reduction in VD due to increasing humidity was accompanied by a decrease in vessel lumen diameter, hydraulically weighted mean diameter (Dh), xylem vulnerability index and theoretical hydraulic conductivity. VA and Dh combined accounted for 87.4% of the total variation in kt of branches and 85.5% of that in stems across the treatments. Characters of branch vessels were more stable, and only the vessel-grouping index (the ratio of the total number of vessels to the total number of vessel groupings) was dependent on the interactive effect of the treatment and canopy position. Our results indicate that the increasing atmospheric humidity predicted for high latitudes will result in moderate changes in the structure and functioning of the hybrid aspen xylem.

Responses of sap flow, leaf gas exchange and growth of hybrid aspen to elevated atmospheric humidity under field conditions.

An increase in average air temperature and frequency of rain events is predicted for higher latitudes by the end of the 21st century, accompanied by a probable rise in air humidity. We currently lack knowledge on how forest trees acclimate to rising air humidity in temperate climates. We analysed the leaf gas exchange, sap flow and growth characteristics of hybrid aspen (Populus tremula × P. tremuloides) trees growing at ambient and artificially elevated air humidity in an experimental forest plantation situated in the hemiboreal vegetation zone. Humidification manipulation did not affect the photosynthetic capacity of plants, but did affect stomatal responses: trees growing at elevated air humidity had higher stomatal conductance at saturating photosynthetically active radiation (gs sat) and lower intrinsic water-use efficiency (IWUE). Reduced stomatal limitation of photosynthesis in trees grown at elevated air humidity allowed slightly higher net photosynthesis and relative current-year height increments than in trees at ambient air humidity. Tree responses suggest a mitigating effect of higher air humidity on trees under mild water stress. At the same time, trees at higher air humidity demonstrated a reduced sensitivity of IWUE to factors inducing stomatal closure and a steeper decline in canopy conductance in response to water deficit, implying higher dehydration risk. Despite the mitigating impact of increased air humidity under moderate drought, a future rise in atmospheric humidity at high latitudes may be disadvantageous for trees during weather extremes and represents a potential threat in hemiboreal forest ecosystems.

Climate change at northern latitudes: rising atmospheric humidity decreases transpiration, N-uptake and growth rate of hybrid aspen.

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.