Arvo Tullus

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 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.

Humidity-driven changes in growth rate, photosynthetic capacity, hydraulic properties and other functional traits in silver birch (Betula pendula)

A study was performed on saplings of silver birch (Betula pendula Roth) growing at the free air humidity manipulation site, which was established to investigate the effect of increased air humidity on tree performance and canopy functioning. The aim of the experiment was to simulate the impact of the increasing atmospheric humidity on forest ecosystems predicted for northern Europe. Artificially elevated relative humidity (RH), which causes transpirational flux to decrease, diminished nutrient supply to the foliage; leaf nitrogen content, phosphorus content and P:N ratio decreased. The changes in leaf nutritional status brought about a considerable decline in both photosynthetic capacity (Amax, Vcmax, Jmax) and tree growth rate. The manipulation induced diverse changes in tree hydraulic architecture and other functional traits. Different segments of the soil-to-leaf water transport pathway responded differently: leaf hydraulic conductance (KL) decreased, while hydraulic conductance of root systems (KR) and leaf-specific conductivity of stem-wood increased in response to elevated RH. Humidification caused the Huber values of stems to increase, thus reflecting changes in allocation patterns; relatively more resources were allocated to vascular tissue and less to foliage. The elevated RH induced substantial changes in specific leaf area (increased), branch- (decreased) and stem-wood density (decreased). The observed responses suggest that the expected climate-change-induced increase in the growth rate of trees at northern latitudes (boreal areas) due to the earlier start of the growing season in spring or higher carbon assimilation rate could be smaller or null if temperature rise is accompanied by a rise in atmospheric absolute humidity.

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.

Short-rotation Forestry for Supplying Biomass for Energy Production

In this chapter, we discuss the opportunities that short-rotation forestry may provide to meet the increasing demand for energy based on renewable resources. We present information on suitable species for northern Europe, their productivity, establishment and management. In this region, grey alder, hybrid aspen, poplars and willows are the most promising species for short-rotation forestry. The productivity of these species is around 5–12 Mg ha−1 a−1 in fertile sites. All of these species regenerate vegetatively, and they can be managed in several consecutive rotations without needing to repeat the establishment between rotations. No major negative environmental impacts have been found with the cultivation of these species. This is especially the case when plantations are established on abandoned agricultural land or otherwise degraded land.

Understorey vegetation in young naturally regenerated and planted birch (Betula spp.) stands on abandoned agricultural land

The abandonment of agricultural lands in Northern and Eastern Europe increases the area covered by first generation forests, which are either formed as an outcome of secondary succession or established as plantations. However, questions remain as to how these new stands develop and what kind of species they favour, which in turn has impacts on their ecological and economical value. Our aim was to compare understorey vascular plant and bryophyte vegetation characteristics between naturally regenerated and planted birch stands on abandoned agricultural sites in Estonia, focusing on the aspects of species richness and forest understorey recovery. Species richness and diversity of vascular plants were similar in both stand types but the number of forest vascular plant species was significantly higher in naturally regenerated stands. The bryophyte layer of naturally regenerated stands had a higher species richness, diversity, and number of forest bryophyte species. The higher number of forest vascular plant and bryophyte species in naturally regenerated stands can be explained by the longer undisturbed succession period. The recovery of the forest understorey was unaffected by former agricultural land use (crop field or grassland). The influence of soil properties on the recovery of the forest understorey was not detected, but the number of vascular plant species that grow in forests as well as in grasslands was negatively correlated with distance from forest. Overall, understorey vegetation of natural and planted birch stands did not reveal substantial differences. However, in the case of vigorous natural birch regeneration in the vicinity of forest land, unassisted reforestation should be favoured.

Growth of northern deciduous trees under increasing atmospheric humidity: possible mechanisms behind the growth retardation

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.

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 (KR) to increase, while KR (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.

Above-ground growth and temporal plant–soil relations in midterm hybrid aspen (Populus tremula L. × P. tremuloides Michx.) plantations on former arable lands in hemiboreal Estonia

ABSTRACT Short-rotation forestry with hybrid aspen is a novel silvicultural system in northern Europe on former arable lands. However, knowledge about hybrid aspen growth potential in different soil types and the impact of soil physico-chemical properties on tree productivity in the long term is still scanty. We used repeated monitoring of soil properties and tree growth in young (5-year) and midterm (15-year) hybrid aspen plantations in various soil types (corresponding to four forest site types) to determine the temporal changes in tree growth–soil relationships. Growth of midterm hybrid aspen plantations exceeded same-aged native European aspen stands about two-fold. Growth had improved on Aegopodium, remained moderate on moist Dryopteris and was increasingly suppressed on dry Hepatica soils. The pHKCl, available Ca, content of clay and layer thickness of the soil A-horizon had a significant effect on tree growth in young plantations, but these effects disappeared in the midterm age. The soil water-holding capacity and available P in the A-horizon had a significant growth-controlling effect on tree growth in both ages. We concluded that former arable soils provide a sufficient supply of major nutrients in midterm hybrid aspen plantations whereas minor changes have occurred in growth–soil relationships between young and midterm age.