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We studied the physicochemical properties of stemwood in saplings of silver birch ( Roth) and hybrid aspen ( L. Ã 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

Decomposition of fine roots (

The aim of the present work was to investigate the nutrient (N,P,K) allocation and accumulation in grey alder (Alnus incana (L.) Moench) and hybrid alder (Alnus incana (L.) Moench x Alnus glutinosa (L.) Gaertn.) plantations growing on former agricultural land and to estimate the above-ground biomass production during 4 years after establishment. In August of the 4th year, when leaf mass was at its maximum, the amount of nitrogen accumulated in above-ground biomass of grey alder stand was 142.0 kg haâ1, the amount of phosphorus 16.3 kg haâ1 and the amount of potassium 49.5 kg haâ1. The amount of nitrogen accumulated in a hybrid alder stand totalled 76.8 kg haâ1, that of phosphorus 6.2 kg haâ1 and that of potassium 28.2 kg haâ1. The smaller amounts of N,P and K bound in the hybrid alder plantation are related to the smaller biomass of the stand. Still, the amounts of N,P and K consumed for the production of one ton of biomass were similar in the case of up to 4-year-old grey alder and hybrid alder stands. In the 4th year, the amount of nutrients consumed in one ton of biomass produced were: 16.0 kg N, 1.6 kg P and 5.4 kg K for grey alder and 14.6 kg N, 1.1 kg P and 5.2 kg K for hybrid alder. In the 4th year the total above-ground biomass (dry mass) of grey alder (15750 plants haâ1) amounted to 12.3 t haâ1, current annual increment being 6.7 t haâ1. In hybrid alder stands (6700 plants haâ1), the respective figures were 6.1 t haâ1 and 4.5 t haâ1. Comparison of the production capacity on the basis of mean stem mass in the 4th year revealed that the stem mass of grey alder exceeded that of hybrid alder (0.64 kg and 0.58 kg, respectively). Grey alder outpaced hybrid alder in height growth; in the 4th year after establishment, the mean height of the grey alder stand was 4.6 ± 0.9 m and that of the hybrid alder plantation 3.5 ± 0.9 m.

This study analyses long‐term changes in drought indices (Standardised Precipitation Index—SPI, Standardised Precipitation–Evapotranspiration Index—SPEI) at 1 and 3 months scales at 182 stations in 11 central and eastern European countries during 1949–2018. For comparative purposes, the necessary atmospheric evaporative demand (AED) to obtain SPEI was calculated using two methods, Hargreaves‐Samani (SPEIH) and Penman‐Monteith (SPEIP). The results show some relevant changes and tendencies in the drought indices. Statistically significant increase in SPI and SPEI during the cold season (November–March), reflecting precipitation increase, was found in the northern part of the study region, in Estonia, Latvia, Lithuania, northern Belarus and northern Poland. In the rest of study domain, a weak and mostly insignificant decrease prevailed in winter. Summer season (June–August) is characterized by changes in the opposite sign. An increase was observed in the north, while a clear decrease in SPEI, reflecting a drying trend, was typical for the southern regions: the Czech Republic, Slovakia, Hungary, Romania, Moldova and southern Poland. A general drying tendency revealed also in April, which was statistically significant over a wide area in the Czech Republic and Poland. Increasing trends in SPI and SPEI for September and October were detected in Romania, Moldova and Hungary. The use of SPEI instead of SPI generally enhances drying trends. [ABSTRACT FROM AUTHOR]

The decomposition of the leaf litter, fine roots (d < 2 mm) and coarser roots (2 ≤ d < 5 mm) of grey alder and silver birch, as well as of α-cellulose sheets using the litterbag method was studied in two experimental stands on Podzoluvisol soils in Southern Estonia. For both tree species, the coarser roots decomposed faster than the fine roots, (p < .05), tree species did not affect the decomposition rate of the roots (p > .5). The nitrogen (N) input to soil from aboveground litter was multiple times higher than the N flux from roots. The remaining relative ash-free mass of the leaves of grey alder and silver birch after three and a half years was similar. After 11 years the remaining relative ash-free mass of the fine and coarser roots of grey alder still accounted for around 10% of the initial value. For silver birch the remaining value was around 20% after 9 years. The litterbag method to underestimates in fertile soils the decomposition of organic matter and thus did not reflect the actual dynamics of decomposition. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]