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To evaluate the impact of stand age on the ecosystem's C budget, as well as the post-harvest recovery of the C storages and fluxes, a chronosequence of Scots pine stands from the clear-cut stage up to the age of 110 years was studied. An age-related trend of net primary production (NPP) demonstrated effective C accumulation in the young and middle-aged stands and their levelling out thereafter. The understorey vegetation contributed 8-46% to total NPP, being lower in the pole and middle-aged stands, but without a clear age related trend. Annual cumulative soil heterotrophic respiration (Rh) demonstrated stable values along the chronosequence, varying between 3.8 and 5.4 t C ha

Thinning changes the functioning of the whole forest ecosystem, including carbon and nitrogen (N) cycling. The input of organic matter and N into soil, as well as soil temperature and moist...

Hemiboreal forests form a transitional belt between boreal and temperate forests in Eurasia, and due to long-term climate warming they could potentially expand in a northerly direction. However, carbon (C) exchange studies in this transitional forest type are scarce. In 2014–2015 we analyzed CO2 exchange in a hemiboreal mixed forest at SMEAR Estonia (Station for Measuring Ecosystem-Atmosphere Relations) using eddy covariance (EC), soil chamber technique and biometric methods. Employing two plots that differ in forest stand composition (mixed (MP) and coniferous (CP)) located within the footprint of the EC tower, we evaluated heterotrophic and total soil respiration (Rh and Rs, respectively). Measurements showed that C, nitrogen (N), and soil organic matter (SOM) contents in topsoil were significantly higher in the coniferous plot, and the total C stock until 20 cm of mineral soil was higher by a factor of two (i.e., 170.6 and 94.8 t C ha−1 for the CP and MP, respectively). Only in 2015 there was a significant difference in Rs between the plots, while Rh exhibited no detectable difference. The cumulative Rs during the growing season was similar for both plots (CP and MP) and years: 532 and 548 g m−2, respectively, in 2014; 636 and 592 g m−2, respectively, in 2015. The same applied to Rh: 418 and 430 g m−2, respectively, in 2014; 406 and 387 g m−2, respectively, in 2015. The forest as a whole was a C sink. In 2015, net ecosystem exchange (NEE) was -585.62 ± 45.16 g C m−2 yr−1 (the average and SD of two gap-filling methods: MDS and nonlinear regressions). Gross primary production (GPP), and ecosystem respiration (RE) were -1280.62 ± 53.36, and 696.00 ± 98.51 g C m−2 yr−1, respectively (the average and SD of two flux partitioning methods: nighttime data-based and daytime data-based). NEE and GPP suggested a greater similarity to temperate forests, while RE was closer to more boreal forest stands. The mean value of net ecosystem production (NEP) of both plots, estimated using forest stand biometric, litter and chamber data was similar to the annual NEE value: 6.3 and −5.9 t C ha−1 yr−1, respectively.

Grey alder (Alnus incana) is a highly productive indigenous tree species, potential for short-rotation forestry in the Baltic and Nordic countries. The aim of the study was to investigate t...

Estimation of soil-related carbon (C) fluxes is needed to understand the dynamics of the soil organic carbon pool, to determine changes in the carbon balance and functioning of forest ecosystems, and to support climate change policies. The objective of the study was to analyse the variation in the most dynamic soil C input (tree and understory above- and belowground litter production) and output (soil respiration) fluxes, in addition to the forest floor, understory and fine root biomass stocks, in eight different Scots pine (Pinus sylvestris L.) and Norway spruce (Picea abies (L.) Karst) sites growing on mineral soils in Estonia. Further, the impact of soil C input and output fluxes on the soil organic carbon (SOC) pool was examined, and the net ecosystem production (NEP) of the stands was estimated. Fine root production (FRP) of the trees constituted 53% and 28% and needle litter constituted 25% and 28% of the total annual C input to the soil in the Norway spruce and Scots pine stands, respectively. The total FRP of the trees and the understory roots and rhizomes ranged from 211 to 1040 g m−2 yr−1, of which the understory comprised up to 28%. The mean annual soil respiration (Rs) rate was 5.7 ± 0.3 and 6.5 ± 0.3 Mg C ha−1 yr−1 in the pine and spruce stands, respectively, and did not differ significantly between the two groups of stands. The SOC pool of the studied stands depended significantly on both the above- and belowground C input fluxes. Tree-derived litter had the strongest effect on the SOC pool, while the Rh as the main soil C output flux showed no significant impact. The NEP ranged from 4.2 to −1.8 Mg C ha−1 yr−1 and demonstrated a strong negative correlation with stand age. The results affirm the importance of belowground as well as aboveground litter production on carbon accumulation in forest soils.

Clear-cutting is a conventional method of forest management which significantly changes carbon (C) cycling at the ecosystem level for a long time. Estimation of the interim period during which the ecosystem turns from a C source to a C sink is crucial for clarifying the environmental effects of management on forest C cycling. The current study provided new knowledge of C cycling in young pine stand and demonstrated the recovery of C sequestration of the forest ecosystem during the post harvesting period. We estimated the C balance in a 6-year-old Scots pine stand by using two different methods: carbon budgeting, for estimating annual net ecosystem production (NEP), and eddy covariance (EC), for estimating net ecosystem exchange (NEE). For C budgeting, the above- and belowground biomass production of the ecosystem, as well as the soil heterotrophic respiration efflux at the studied site was estimated. Annual NEE at the studied young forest ecosystem was 1.19 ± 0.36 t C ha−1, gross primary ecosystem production was 9.87 and total ecosystem respiration was 11.06 t C ha−1. Estimated NEE was in good accordance with the results of NEP (1.37 t C ha−1), which confirms the relevance of the C budgeting method. Increased annual woody biomass production is the main factor which induced the young Scots pine ecosystem to act as a C sink: annual C accumulation in tree biomass in a 6-year-old stand was 1.0 t C ha−1 but reached already 2.4 t C ha−1 in the following year. Assuming that the annual Rh flux is of the same magnitude in the subsequent years, the ecosystem will become a C sink already during a short period after clear-cut. Annual soil respiration (Rs) and heterotrophic soil respiration (Rh) were 6.0 and 4.2 t C ha−1, respectively and the Rh/Rs ratio was 0.70. However, at this stage also the understorey vegetation contributed essentially to NEP, making up 56% of the annual C uptake accumulated in the plants. The methane flux and the leached C flux were negligible, 0.004 and 0.015 t C ha−1 yr−1, respectively. Our results demonstrate that well regenerated young Scots pine stand on a former clear-cut area will be able to turn into a C sequestering ecosystem already before ten years after cutting.

Methane (CH4) has high global warming potential, and its atmospheric concentration is increasing rapidly at the present rate of 0.3% yr-1. Forest ecosystems cover a large part of the biosphere and play a significant role in climate change. Upland forest soils are considered as important terrestrial sinks for atmospheric CH4; however, the complex interactions between microbial processes of CH4 production and oxidation, and environmental drivers are not well understood. Balance of CH4 in the forest ecosystems depends on two main natural processes, i.e., anaerobic methanogenesis and aerobic methanotrophy, driven by multiple environmental factors. A forest ecosystem's ability to exchange CH4 depends on the soil type, environmental conditions, species composition, living trees and deadwood, age and health conditions of the tree stand, and their CH4 balance can vary between seasons and years.In this study, we present long-term CH4 fluxes (from 2015 to 2019) in a 60-200-year-old coniferous forest site of Scots pine (Pinus sylvestris) grown on loose sandy soil in Soontaga research station (58°01'N 26°04'E) in Estonia. The fluxes of CH4 were measured every two weeks, using a manual static soil chamber (n = 6) and gas chromatography method. Air temperature, precipitation and humidity, and soil moisture and temperature (10 cm depth) were measured continuously. The average annual temperature and precipitation recorded were 7.3 + 1.0 °C and 54.3 + 3.9 mm, respectively.The results showed that mature pine forest soil was an annual net sink of CH4: −21.14 + 0.59 g ha−‍1 yr-1 (mean + SE). No significant difference (p 4 uptake and tree age. Methane uptake correlated negatively (r2 = 0.61, p 4 uptake (−36.93 g ha−‍1) was observed in July 2018, the warmest and driest month during the overall period. Even though soil moisture was only weakly correlated (r2 = 0.15, p 4 uptake, the CH4 flux was affected by precipitation. As a result of this, it is noticed that CH4 uptake in the cold and wet conditions decreased with increasing precipitation in winter and increased with warming during warm and dry conditions in summer.Concluded, our coniferous pine forest was sequestering CH4 during the investigated five years. The soil CH4 uptake could be explained by CH4 oxidation at optimal temperature in the water-unsaturated surface soil regulating the soil's microbial activity.

Peatlands are unique wetland ecosystems that cover approximately 3% of the world’s land area and are mostly located in boreal and temperate regions. Around 15 Mha of these peatlands have been drained for forestry during the last century. This study investigated soil archaeal and bacterial community structure and abundance, as well as the abundance of marker genes of nitrogen transformation processes (nitrogen fixation, nitrification, denitrification, and dissimilatory nitrate reduction to ammonia) across distance gradients from drainage ditches in nine full-drained, middle-aged peatland forests dominated by Scots pine, Norway spruce, or Downy birch. The dominating tree species had a strong effect on the chemical properties (pH, N and C/N status) of initially similar Histosols and affected the bacterial and archaeal community structure and abundance of microbial groups involved in the soil nitrogen cycle. The pine forests were distinguished by having the lowest fine root biomass of trees, pH, and N content and the highest potential for N fixation. The distance from drainage ditches affected the spatial distribution of bacterial and archaeal communities (especially N-fixers, nitrifiers, and denitrifiers possessing nosZ clade II), but this effect was often dependent on the conditions created by the dominance of certain tree species. The composition of the nitrifying microbial community was dependent on the soil pH, and comammox bacteria contributed significantly to nitrate formation in the birch and spruce soils where the pH was higher than 4.6. The highest N2O emission was recorded from soils with higher bacterial and archaeal phylogenetic diversity such as birch forest soils. This study demonstrates that the long-term growth of forests dominated by birch, pine, and spruce on initially similar organic soil has resulted in tree-species-specific changes in the soil properties and the development of forest-type-specific soil prokaryotic communities with characteristic functional properties and relationships within microbial communities.

The greenhouse-gas (GHG) balance of CH4 and N2O emissions and CO2 respiration in peatlands plays a key role in climate change. A GHG balance is mostly calculated from point measurements. In this study we propose and test a geophysical method of low frequency domain electromagnetic method (FDEM) as a proxy for forest-floor CO 2 respiration and CH4 emissions in two 50 × 100 m drained peatlands in Estonia: a Downy birch and a Scots pine stand from April to September 2015. The FDEM measurement campaign in September 2015 yielded a detailed map of topsoil electrical conductivity. We explain the predictive power of FDEM on GHG emissions through relationships with dry bulk density of soil . Although we over- or underestimated some fluxes owing to soil heterogeneity, we composed reasonably credible GHG emission maps. The initial results seem promising and we recommend further application of FDEM methods.

Peatland drainage based on ditch systems is a widely used forestry management practice in the boreal and hemiboreal forests to improve tree growth. This study investigated the morphological variation in absorptive roots (first- and second-order roots) across the distance gradient from the ditch with four sampling plots (5, 15, 40, and 80 m) in six drained peatland forests dominated by Downy birch and Norway spruce. The dominating tree species had a significant effect on the variation in absorptive root morphological traits. The absorptive roots of birch were thinner with a higher specific root area and length (SRA and SRL), higher branching intensity (BI), and lower root tissue density (RTD) than spruce. The distance from the ditch affected the absorptive root morphological traits (especially SRA and RTD), but this effect was not dependent on tree species and was directionally consistent between birch and spruce. With increased distance from the ditch (from plot 5 to plot 80), the mean SRA increased by about 10% in birch and 5% in spruce; by contrast, the mean RTD decreased by about 10% in both tree species, indicating a potential shift in nutrient foraging. However, soil physical and chemical properties were not dependent on the distance from the ditch. We found a species-specific response in absorptive root morphological traits to soil properties such as peat depth, pH, and temperature. Our results should be considered when evaluating the importance of morphological changes in absorptive roots when trees acclimate to a changing climate.

The decomposition of the leaf litter, fine roots (d

Thinning is the main silvicultural method for improving stand growth and wood quality, however, despite the relevance and extensive use of thinning in forest management, its effect on stand carbon (C) balance is still poorly studied at the ecosystem level. The present case study estimated the two-year post-thinning effect on the C balance of a pole stand and a middle-aged Scots pine stand growing on mesotrophic sandy soils. Moderate thinning from below reduced the stand C storage by 21–24%, however, the amount of C accumulated in woody biomass, which was removed by logging, is expected to recover in both stands in the following four years. The reduced biomass of the trees contributed to the decreased annual net primary production (NPP) of the stand by 9–11%. The absolute value of net ecosystem production decreased by 0.9 and 0.7 t C ha−1 yr−1 in the pole and the middle-aged stand, respectively; still, both thinned plots maintained their C sink status. The production of the herbaceous understorey as well as the production of needles increased in the younger stand after thinning, but this could not compensate for C loss at the stand level. The effect of thinning on the production of mosses and dwarf shrubs was not expressed in either stand, probably due to the too short post-thinning period. Thinning did not significantly affect either total soil respiration or the heterotrophic respiration (Rh). However, it increased the contribution of Rh to total soil respiration, which can be attributed to decreased fine root biomass and root respiration, while the aboveground litterfall was not significantly changed after thinning. Fine root production, which accounted for the main belowground litter input, was significantly lower in both thinned plots. Moderate thinning in the pole and the middle-aged Scots pine stand did not change the ecosystem into a C source and the induced C loss will be compensated during a short post-thinning period.

Gap fraction measurements with digital a hemispherical camera were carried out in 2007–2016 in a Scots pine (Pinus sylvestris L.) stand growing on transitional Sphagnum bog in Jarvselja, Estonia. The measurements were done at the time when (1) the pines had maximum foliage, or (2) after autumn needle fall or before bud burst. Data for gap fraction calculation were extracted from camera raw files. For data recording, the sensor output signal maximum was kept according to the image brightness histogram within the interquartile range of the sensor dynamic range. A linear conversion method (LinearRatio) was used for image processing. The relative needle loss in autumn (20–30%) increased gap fraction by 0.02–0.05 at view zenith angles from 10° to 60° and decreased the mean plant area index from 1.86 to 1.68 after autumn litter fall. The measurement results were in good agreement with a theoretical gap fraction model simulation and with gap fraction estimates made from cover photos. The measurement and data processing protocol is appropriate for long-term monitoring in permanent sample plots.

Estimation of the carbon (C) storages and fluxes in different forest ecosystems is essential for understanding their C sequestration ability. The net ecosystem production (NEP) and the net primary production (NPP) in five downy birch ( Betula pubescens ) stands, aged between 12 and 78 years, growing on fertile well-drained Histosols, were studied. Drainage of swamp forests is a large-scale manipulation, which causes significant shifts at the ecosystems level, altering C and nutrient cycling a great deal. Young and middle-aged downy birch stands (12–30-year-old) acted as C sink ecosystems, accumulating 1.4–3.0 t C ha −1 yr −1 . In the 38-year-old stand NEP was roughly zero; annual C budget was almost in balance. The over-matured downy birch stand (78-year-old) acted as a C source emitting 0.95 t C ha −1 yr −1 . Annual woody biomass increment of the stand was the main factor which affected the forest to act as a C accumulating system. Although the highest heterotrophic respiration (Rh) values were measured in the middle-aged stands, mean soil C emission did not differ significantly between the studied stands. Annual total soil respiration (Rs) and Rh ranged from 7.4 to 8.8 t C ha −1 and 4.7 to 6.2 t C ha −1 , respectively. Soil temperature appeared to be the dominant driver of the soil CO 2 effluxes. Temperature sensitivity (Q 10 value) of respiration rates (3.0–5.5), as well as the Rh/Rs (0.6–0.7) varied irrespective of stand age. Both the annual aboveground litter (1.5–1.9 t C ha −1 yr −1 ) and fine root litter (0.9–1.5 t C ha −1 yr −1 ) input fluxes were quite similar for the studied stands. However, the annual organic C input into the soil via above- and belowground litter was smaller than the annual Rh efflux, indicating that continuous mineralization of the peat layer reduces the soil organic C pool. The main share of the C stock in the drained swamp downy birch stands was soil C; the storage of C accumulated in the woody biomass of the trees accounted for only 5–20% of the total C storage of the ecosystem.

Estimation of the carbon (C) storages and fluxes in different forest ecosystems is essential for understanding their C sequestration ability. Grey alder (Alnus incana (L.) Moench) is a fast growing tree species with a great potential for short-rotation forestry in the Nordic and Baltic countries and its stands are considered C accumulating ecosystems. We hypothesized that grey alder stands growing at fertile sites act as C sinks in the young and middle-age stages, while mature stands become C sources as a consequence of declined net primary production (NPP). Net ecosystem production (NEP) was studied in five grey alder stands aged between 9 and 40 years. It was found that the NEP of the studied grey alder stands of different ages varied from −1.98 to +4.14 t C ha−1 yr−1. The oldest grey alder stand proved to be a weak C source (−0.77 t C ha−1 yr−1). However, also young alder stands regenerated in a clear-cut area may emit C in the earlier stage, owing to previous cutting and decomposition of organic residues matter. In this aspect, the land use history is of great significance. Leaf litter accounted for the largest (44–61%) annual organic C input to soil, the fine roots contributed with 12–32% and the herbaceous understorey (aboveground + belowground), with 14–26% to it. Annual cumulative soil heterotrophic respiration (Rh) in the studied stands ranged between 2.6 and 5.8 t C ha−1 yr−1; C leaching was negligible, being 10–30 kg C ha−1 yr−1. The C sequestration to soil depended on previous land use; in the first forest generation, i.e. a stand growing on previous agricultural land, the annual Corg input into soil exceeded Rh, which indicated C accumulation in soil. Although grey alder stands are generally C accumulating ecosystems, the annual biomass production of the studied stands was highly variable; thus annual NEP depended on the effect of the year rather than on stand age. The current study demonstrates that use of repeated time-series measurements applied for stands of different development stages can provide more detailed estimation of C accumulation in forest ecosystems.

Although thinning is a widely used silvicultural method, its effect on stand carbon (C) cycling is still poorly studied at the ecosystem level. The present case study estimated the two-year post-thinning effect on the C balance of a pole and a middle-aged silver birch stand. The results demonstrate the multifaceted impact of thinning on the different C fluxes of deciduous forest ecosystems. The effect of thinning on the C budget of the studied stands was modest: net ecosystem production (NEP) decreased by 1.2 and 1.6 t C ha−1 yr−1 in the pole and the middle- aged stand, respectively; still, both stands remained C sinks. Lower annual production in the thinned stands as a result of the decreased standing biomass of the trees was the main factor for reduced C sequestration capacity. Thinning increased the C accumulation of the herbaceous plants in both stands, however, it did not compensate for the lower C accumulation by the trees. In general, thinning did not affect significantly the soil respiration fluxes; the small post-thinning increase of the annual soil heterotrophic flux, 0.33–0.68 t C ha −1, was most probably related to elevated soil temperature during the active growing season. The annual aboveground litter flux, i.e. the labile C source of Rh, was not significantly changed by thinning. Fine root production and the belowground C input to the soil remained at the same level in the pole stand and decreased slightly in the middle-aged stand. We conclude that the high production ability and fast C accumulation recovery of silver birch stands growing on fertile soils leads to a balanced C budget already during the short post-thinning period.

Understanding the linkage of soil respiration (Rs) with forest development is essential for long-term C cycle models. We estimated the variation and temperature sensitivity (Q10 value) of Rs and its hetero-, (Rh) and autotrophic (Ra) components in relation to abiotic and biotic factors in Norway spruce stands of different ages, and the effect of trenching on microbial and soil characteristics. Trenching method was used to partition Rs into Rh and Ra. Ingrowth core method was used to estimate fine root production. Soil microbial biomass was measured using manometric respirometers. Rs varied in differently aged stands demonstrating non-linear response to development stage. The variation of Rs was explained by changes in biotic factors rather than by changes in soil microclimate. Rh was more sensitive to Ts than Rs or Ra. After 4 years of trenching soil pH, N, SOM and dehydrogenase activity were significantly changed in trenched plots compared to control plots. Different Q10 values of Rh and Ra in stands of different ages indicate the importance of Rs partitioning. Trenching should be used during a limited number of years because of the possible changes in chemical characteristics of soil and in the activity of soil microbial community.

Stumps of conifer trees are a prospective source of bioenergy and stump harvesting is a novel practice in forestry management in the Baltic and Nordic countries. However, as stump harvesting may cause possible environmental risks there has emerged a clear need for research focusing on sustainable forest management. Three Norway spruce (Picea abies) clear-cut areas on different soils in Estonia were selected for the present study. We analysed the effect of stump harvesting on net nitrogen mineralization (NNM) and on nutrient leaching. On dry and sandy Endogleyic Arenosol (Oxalis site type), stump harvesting reduced the annul NNM flux significantly; 134 and 202 kg N ha yr−1 at the harvested and at the control site, respectively. In clear-cut area where Endogleyic Cambisol was dominating (Hepatica site type), stump harvesting had no effect on NNM (92 vs 88 kg N ha yr−1). However, in a clear-cut area where the soil type was Endogleyic Albic Podzol (Myrtillus site type), stump harvesting increased the total annual NNM flux: 102 vs 70 kg N ha yr−1 at the harvested and at the control site, respectively. Stump harvesting affected also the proportion of nitrification and ammonification processes in NNM. At the Myrtillys site type stump harvesting increased the annual nitrogen (N) leaching flux. One year after stump harvesting, leaching at the harvested site was 11.7 vs 4.5 kg N ha−1 yr−1 at the control site. In the second year N leaching decreased and the difference levelled off. Increased N leaching was induced by a larger amount of water; average N concentration of the harvested and control sites did not differ. Although at the Oxalis site N leaching was larger at the harvested than at the control site, the total annual leached N flux was small (∼2 kg N ha−1). At the fertile Hepatica site type treatment had no impact on N leaching, which was only ca 1 kg N ha−1 yr−1. Phosphorus (P) leaching was very small in all study areas, being below 0.1 kg P ha−1 yr−1. The effect of stump harvesting on annual NNM as well as on N leaching was soil specific and highly variable. Stump harvesting affected also the proportion of the nitrification and ammonification processes in total NNM. Considering the first short-term results obtained from different site types, we can conclude that harvesting of spruce stumps does not induce serious environmental hazards in relation of N cycling.

A generic regression model for above-ground biomass of forest stands was constructed based on published data (R2= 0.88,RSE= 32.8 t/ha). The model was used 1) to verify two allometric regression models of trees from Scandinavia applied to repeated measurements of 275 sample plots from database of Estonian Network of Forest Research (FGN) in Estonia, 2) to analyse impact of between-tree competition on biomass, and 3) compare biomass estimates made with different European biomass models applied on standardized forest structures. The model was verified with biomass measurements from hemiboreal and tropical forests. The analysis of two Scandinavian models showed that older allometric regression models may give biased estimates due to changed growth conditions. More biomass can be stored in forest stands where competition between trees is stronger. The tree biomass calculation methods used in different countries have also substantial influence on the estimates at stand-level. A common database of forest biomass measurements from Europe in similar to pan-tropical tree measurement data may be helpful to harmonise carbon accounting methods.

Stumpwood as a resource of renewable bioenergy has considerable potential in the Nordic and Baltic countries. So far, however, it has largely been left unutilised in Estonia. This study reports the results of the height growth, needle and twig traits, and Heterobasidion sp. and Armillaria sp. infection in emerging Norway spruce (Picea abies) stands after stump harvesting. After six and eight growing seasons, a significant positive effect of stump harvesting on the height growth of the trees was revealed, compared to control plots with no ground preparation. In one study site stump harvesting was revealed to have a positive effect on the morphologic parameters of twigs and needles, while no differences were noted in the other two sites. The nutrient content of the needles was similar between the treatments across all study sites. Stump harvesting had no effect on the spread of Heterobasidion sp. and Armillaria sp. in the new forest generation. Root infections in young trees with Heterobasidion sp. were detected as early as within two growing seasons from planting. The infection was most severe in the Hepatica site type stand, where 90% of the trees in the previous forest generation had been infected by Heterobasidion sp. In the two other stands studied, where the share of infected trees in the previous generation had been 26% and 16%, Heterobasidion sp. was significantly less widespread. The main causal agent of root rot on all the sampling sites was H. parviporum, but the presence of Armillaria sp. was also remarkable.

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.

The main aim of the current study was to estimate the annual net nitrogen mineralization (NNM) flux in stands of different tree species growing on drained peatlands, as well as to clarify the effect of tree species, soil properties and litter on annual NNM dynamics. Three study sites were set up in May 2014: a downy birch ( Ehrh.) stand and a Norway spruce ( (L.) Karst.) stand in full-drained swamp (ODS) and a Scots pine ( L.) stand in full-drained swamp (MDS). The NNM flux was estimated using the method with incubated polyethylene bags. The highest value of NNM was found in stands that were growing on fertile ODS: 127.5 kg N ha yr and 87.7 kg N ha yr, in the downy birch stand and in the Norway spruce stand, respectively. A significantly lower annual NNM flux (11.8 kg N ha yr) occurred in the Scots pine stand growing in MDS. Nitrification was highest at fertile ODS sites and ammonification was the highest at the low fertility MDS site. For all study sites, positive correlation was found between soil temperature and NNM intensity. The difference in annual NNM between the downy birch stand and the Norway spruce stand growing on similar drained fertile peatlands was due to litter quality. The annual N input into the soil through leaf litter was the highest at the downy birch site where also the C/N ratio of litter was the lowest. The second highest N input into the soil was found in the spruce stand and the lowest in the pine stand.Betula pubescensPicea abiesOxalisPinus sylvestrisMyrtillusin situ–1–1–1–1–1–1

Grey alder is a wide spread tree species in the Baltic region and a promising species for short rotation forestry. The symbiotic dinitrogen (N2) fixation ability makes this tree important for the regulation of nitrogen (N) cycle in forested areas. In a homogeneous 32-year-old natural grey alder stand (GAS) and an adjacent clear-cut (CC) in South-East Estonia (58°17′ N; 27°17′ E; set up in May 2011) we analyzed net nitrogen mineralization (NNM; with incubated bags), N leaching (with plate lysimeters), and nitrous oxide (N2O) fluxes (with static chambers). The total annual NNM did not intensify in the CC area: in the upper 0–20 cm soil layer the NNM was 169.9 and 157.0 kg ha−1 in the (GAS) and in the CC, respectively. In both cases, the share of nitrification was 100% and NNM intensity was the highest in July. During the snow-melt in April, both in the GAS and in the CC site the leaching of total N was up to 25 kg N month−1, whereas in the rest of study period it was negligible in both sites. Harvesting slightly decreased N2O emission, however, it was low at both study sites (−0.55 to 19.75 and −0.77 to 7.43 kg N2O ha−1 yr−1 in the GAS and the CC, respectively). Management of grey alder stands by traditional silvicultural methods (clear-cuts) did not to increased hazardous N losses through leaching.

Carbon (C) budgets were compiled for silver birch chronosequence stands by synthesizing the above- and belowground C pools and fluxes with the aim to assess the impact of age on stand C sequestration and cycling. The fine root (d Soil respiration (Rs) was measured during the two growing seasons of 2010 and 2011; the trenching method was used to estimate the contribution of heterotrophic respiration (Rh) to total soil respiration. Soil temperature was the driving environmental factor of the temporal variation of Rs and Rh. Stand age affected significantly respiration rates. Annual Rs was 6.2, 9.7 and 8.2 t C ha−1 and annual Rh was 2.97, 4.21 and 3.61 t C ha−1 in the pole, middle-aged and premature silver birch stands, respectively. The annual contribution of Rh to Rs was similar in each stand ranging from 0.43 to 0.48. Total annual net primary production (NPP) of the ecosystems of 7.4, 7.9 and 8.5 t C ha−1 year−1 was estimated in the pole, middle-aged and premature stands. After balancing the C input and output fluxes in the silver birch stands of different development stages, it appeared that they all acted as effective C sinks. Net ecosystem production (NEP) was 4.4, 3.7 and 4.9 t C ha−1 year−1 in the pole, middle-aged and premature stands, respectively. In the premature stand the second layer of the spruce contributed significantly to the increased C input. The annual organic C input into the soil through above- and belowground litter occurred to be of the same magnitude as C loss via annual heterotrophic respiration. Thus, annual increment of woody biomass in fertile silver birch stands reflects annual organic C sequestration in the ecosystem. Among the studied stands, estimated NEP values were the lowest in the middle-aged stand and the highest in the oldest stand, indicating non-linear relationship between stand NEP and stand age.

Stump removal is a novel potential source of woody biomass for renewable energy in the Nordic and Baltic countries. In Estonia, a prospective resource of Norway spruce ( Picea abies ) stumps is available for bioenergy. The main aims of the present study were to estimate the biomass and volume of harvested spruce stumps; also to evaluate nutrient and carbon loss from a site due to stump removal; and to estimate the effect of stump harvesting on soil respiration. In this pilot study an allometric model was elaborated for estimation of the biomass and volume of harvested stumps. The volume of harvested spruce stumps reached up to 130 m 3 ha −1 and potential energy content amounted to 290 MW h ha −1 . The average calorific value of different stump and root fractions varied between 19.0 and 19.3 kJ g −1 and there was no significant difference between the stump fractions. The weighted average density of the harvested spruce stumps was 430 kg m −3 . Fertile automorphic forest types were selected for this pilot study and the nitrogen (N) storage removed from clear-cuts with stumps was not appreciable, being 1.2–4.4%, in comparison with soil N storages. Removed phosphorus (P) accounted for 8–28% of total soil lactate soluble P and removed potassium (K) accounted for 6–23% of the soil K storage. The loss of carbon (C) made up 11–33% of the soil C storage. However, the concentration of lactate soluble P evidently reflected the balance between solubilized and assimilated P in the soil rather than actual P limitation. Hence, at fertile sites stump removal does not caused considerable N and C depletion, however, removal of P and K may be appreciable compared with corresponding soil storage values. Stump harvesting did not increase soil respiration intensity. Total annual soil respiration at the Oxalis site type was 4.9 and 5.1 t C ha −1 yr −1 in the uprooted and control plots, respectively. At the Myrtillus site type the respective values were lower, 3.7 and 3.9 t C ha −1 yr −1 . Also soil heterotrophic respiration in the Oxalis site type was practically the same at the uprooted and control sites, being 3.3–3.2 t C ha −1 yr −1 . However, in the Myrtillus site type the values of the heterotrophic respiration efflux was higher from the control plots than from the uprooted plot, at 2.5 and 1.8 C ha −1 yr −1 , respectively. Norway spruce stumps are a potential bioenergy resource in Estonian forests and stump harvesting at fertile autotrophic sites does not cause problems in terms of depletion of nutrients and carbon from the ecosystem.

The tree root–mycorhizosphere plays a key role in resource uptake, but also in the adaptation of forests to changing environments. The adaptive foraging mechanisms of ectomycorrhizal (EcM) and fine roots of Picea abies, Pinus sylvestris and Betula pendula were evaluated along a gradient from temperate to subarctic boreal forest (38 sites between latitudes 48°N and 69°N) in Europe. Variables describing tree resource uptake structures and processes (absorptive fine root biomass and morphology, nitrogen (N) concentration in absorptive roots, extramatrical mycelium (EMM) biomass, community structure of root-associated EcM fungi, soil and rhizosphere bacteria) were used to analyse relationships between root system functional traits and climate, soil and stand characteristics. Absorptive fine root biomass per stand basal area increased significantly from temperate to boreal forests, coinciding with longer and thinner root tips with higher tissue density, smaller EMM biomass per root length and a shift in soil microbial community structure. The soil carbon (C) : N ratio was found to explain most of the variability in absorptive fine root and EMM biomass, root tissue density, N concentration and rhizosphere bacterial community structure. We suggest a concept of absorptive fine root foraging strategies involving both qualitative and quantitative changes in the root–mycorrhiza–bacteria continuum along climate and soil C : N gradients.

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.×P. 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.

In the present study the chronosequence approach was applied to the analysis of the dynamics of the growth of 2–45-year-old grey alder stands ( Alnus incana ) growing at fertile sites as well as the dynamics of carbon (C) and nitrogen (N) accumulation in woody biomass and in the soil. A new yield table was elaborated for natural grey alder stands growing at fertile sites. The average growing stock and increment of grey alder stands were lower than those presented in earlier yield tables compiled in Estonia or in the other Baltic or Nordic states. It can be explained by an uneven spatial distribution of trees in natural grey alder stands; because of this, the production of different alder stands of the same age was highly variable. However, some stands were characterised by remarkably high production, which indicates the high growth potential of this tree species. The bulk maturity of grey alder stands is achieved at the age of 12–13 years. However, the recommendable optimal harvesting age of grey alder stands could be 20–25 years, as annual woody biomass increment was high even after the age of 20 years. The average stemwood density found in this study was 396 ± 3.6 kg m −3 and the average density of the branches was 460 ± 2.9 kg m −3 . Regarding of CN accumulation in the stands, different land use histories demonstrated different patterns. First-generation grey alder stands growing on previously non-forested lands accumulated effectively C and N in the soil. The average increase of the C pool in the 10 cm topsoil layer was in the range 1.1–1.2 t ha −1 yr −1 in young stands and 0.97–0.84 t ha −1 yr −1 in older stands. In the case of second-generation grey alder stands the soil C pool in young and old stands was of same magnitude and these stands accumulated C mainly in woody biomass. The share of the soil C pool in middle-aged and old grey alder stands was roughly two times as large as the C storage accumulated in the above-ground woody biomass of the trees. The average N accumulation in first-generation grey alder stands of the age of 14–35 years was 74 kg N ha −1 yr −1 . Also a significant acidifying effect of alders on the soil pH status was revealed. The study confirmed that grey alder has the potential to be a high yielding tree in short-rotation forestry for woody biomass production. Grey alder stands serve as efficient forest ecosystems accumulating C both in biomass and in the soil. However, previous land use history plays an essential role in this respect.

An essential carbon (C) pool of forest ecosystems is stored in belowground tree biomass; however, data about belowground C and nutrient accumulation is scarce. It is crucial to involve belowground part for developing regional and global C or nitrogen (N) budgets for different forest ecosystems. Fine root ( D D ⩾ 2.0 mm) biomass were studied in a chronosequence in four silver birch ( Betula pendula Roth) stands aged 6–60 years. The soil coring method was used for estimation of birch fine root biomass and necromass in the upper 40 cm topsoil layer. Stump biomass and coarse root biomass were estimated by excavation of the root systems of model trees in a 6-year-old stand and in a 32-year-old silver birch stand. For a 14-year-old stand and for a 60-year-old stand, the developed root-shoot ratio approach was used. Carbon and nitrogen accumulation in belowground biomass was estimated as well. Fine root biomass varied between 103 g m −2 and 409 g m −2 and fine root necromass, between 22 g m −2 and 93 g m −2 ; both increased with stand age. Fine root vertical distribution showed a gradual decrease with soil depth in all profiles, however, fine root biomass shifted more to the deeper soil layers with increasing stand age. The fine root live/dead ratio decreased hyperbolically with stand age while the change was most rapid before pole age. The most active part of the fine root system – ectomycorrhizal biomass (EcMB) – was estimated for three stands (6-, 14- and 32-year-old); EcMB per basal area varied between 8 and 14 kg m −2 and was not related to stand age. Coarse root biomass increased with stand age from 7.9 Mg ha −1 in the 6-year-old stand to 40.2 Mg ha −1 in the 60-year-old stand. The root-shoot ratio decreased in silver birch stands drastically between 6 and 14 years but stabilized thereafter. Hence the use of a root-shoot ratio of 21% for calculation of belowground biomass from aboveground biomass for silver birch stands is justified. Carbon accumulation in the belowground tree biomass of silver birch stands varied between 4.0 Mg ha −1 and 19.8 Mg ha −1 , being the highest in the 60-year-old stand. The C accumulated in belowground biomass accounted for 3–4% of the total C storage of the total C storage of the forest ecosystem in the young stands and for 9.5–11% in the middle-aged and mature stands. Fine root mortality and the corresponding C flow as fine-root litter input increased with stand age. As the biomass of the trees was the main C sink, a significant share (3–28%) of C was accumulated in the belowground part of the stand. C accumulation in the fine roots was low in chronosequence birch stands; however the C flux into the soil through fine root litter may be considerable.

In the Northern and Baltic countries, grey alder is a prospective tree species for short-rotation forestry. Hence, knowledge about the functioning of such forest ecosystems is critical in order to manage them in a sustainable and environmentally sound way. The 17-year-long continuous time series study is conducted in a grey alder plantation growing on abandoned agricultural land. The results of above- and below-ground biomass and production of the 17-year-old stand are compared to the earlier published respective data from the same stand at the ages of 5 and 10 years. The objectives of the current study were to assess (1) above-ground biomass (AGB) and production; (2) below-ground biomass: coarse root biomass (CRB), fine root biomass (FRB) and fine root production (FRP); (3) carbon (C) and nitrogen (N) accumulation dynamics in grey alder stand growing on former arable land. The main results of the 17-year-old stand were as follows: AGB 120.8 t ha−1; current annual increment of the stem mass 5.7 t ha year−1; calculated CRB 22.3 t ha−1; FRB 81 ± 10 g m−2; nodule biomass 31 ± 19 g m−2; fine root necromass 11 ± 2 g m−2; FRP 53 g DM m−2 year−1; fine root turnover rate 0.54 year−1; and fine root longevity 1.9 years. FRB was strongly correlated with the stand basal area and stem mass. Fine root efficiency was the highest at the age of 10 years; at the age of 17 years, it had slightly reduced. Grey alder stand significantly increased N and Corg content in topsoil. The role of fine roots for the sequestration of C is quite modest compared to leaf litter C flux.

Silver birch ( Roth.) is one of the main pioneer tree species occupying large areas of abandoned agricultural lands under natural succession in Estonia. We estimated aboveground biomass (AGB) dynamics during 17 growing seasons, and analysed soil nitrogen (N) and carbon (C) dynamics for 10 year period in a silver birch stand growing on former arable land. Main N fluxes were estimated and nitrogen budget for 10-year-old stand was compiled. The leafless AGB and stem mass of the stand at the age of 17-years were 94 and 76 Mg ha respectively. The current annual increment (CAI) of stemwood fluctuated, peaking at 10 Mg ha yr at the age of 15 years; the mean annual increment (MAI) fluctuated at around 4–5 Mg ha. The annual leaf mass of the stand stabilised at around 3 Mg ha yr. The stand density decreased from 11600 to 2700 trees ha in the 8- and 17-year-old stand, respectively. The largest fluxes in N budget were net nitrogen mineralization and gaseous N-N emission. The estimated fluxes of NO and N were 0.12 and 83 kg ha yr, respectively; N leaching was negligible. Nitrogen retranslocation from senescing leaves was approximately 45 kg ha, N was mainly retranslocated into stembark. The N content in the upper 0–10 cm soil layer increased significantly (145 kg ha) from 2004 to 2014; soil C content remained stable. Both the woody biomass dynamics and the N cycling of the stand witness the potential for bioenergetics of such ecosystems.Betula pendula–1–1–1–1–1–1–1222–1–1–1–1

The two alder species, black alder (Alnus glutinosa (L.) Gaertn.) and grey alder (Alnus incana (L.) Moench) are known to be pioneer species in the succession of new land areas, and important tree species in renewable biomass production, the restoration of post-mining sites and riparian forest ecosystems. We analyzed the influence of soil physico-chemical characteristics (soil water content, pHKCl, total N, soluble P, organic matter content, C and N ratio, and elemental content) on bacterial community structure based on pyrosequencing analysis of the 16S rRNA gene V2 and partly V3 region in two black and two grey alder stand soils with different management histories. The analyses revealed clear differences in all the measured chemical characteristics of studied soils. All the studied stands also had distinct soil bacterial communities, and the number of shared species was low. In all stands species from phylum Proteobacteria were dominant, and the next phyla by percentage were Bacteroidetes and Actinobacteria. At the family level, Chitinophagaceae and Bradyrhizobiaceae species dominated. The obtained bacterial community Inverted Simpson's diversity indices showed no difference between the studied sites. The alder species did not affect soil bacterial community structure. Distance-based regression analysis indicated that soil pH value, water content, soluble phosphorus concentration and also total boron, cadmium, and aluminium content were related to the variation of soil bacterial community structure in alder stand soils. The results of this study emphasize the importance of soil geomorphological properties in addition to soil physical and chemical characteristics in the formation of soil bacterial community structure during restoration of exhausted open mining areas, management of abandoned agricultural lands, and short rotation forests with different alder species.

The influence of forest ageing on fine-root morphology and relations between fine-root and leaf characteristics is poorly studied. The aim of this study was to analyse age-driven changes in ectomycorrhizal roots (EcM roots) and leaf morphology in a chronosequence of silver birch (Betula pendula Roth.), which would provide a better understanding of adaptation responses and acclimation capacity of tree roots and leaves. The chronosequence included six age classes (3, 6, 14, 32, 45, and 60 years.). All stands had regenerated naturally and grew in a highly productive Oxalis forest site type in Estonia. Most changes in the morphology of EcM roots and leaves of silver birch occur faster at a young age. The functional parameters—mean specific area of EcM root (SRA) and leaf specific area (SLA) as well as leaf N—decreased with age. EcM root SRA and specific root length (SRL) decreased with stand age as a result of increased mean diameter and tissue density. In age classes of 6, 14, and 32 years, the total number of dominating EcM taxa was 34, and the distribution of four different dominating EcM exploration types (contact-, short-, medium-, long-distance) was similar. We conclude that high values of SRA, SLA, and leaf N measured in young silver birch stands indicate high activity of physiological processes necessary for fast-growing young trees. A decrease of SLA and SRA and N in the chronosequence of fertile stands of silver birch is most probably caused by down-regulation of growth, affecting simultaneously leaves and fine roots.

Owing to its ability to produce large amounts of biomass in a short period of time, grey alder can be considered to be a prospective tree species for short-rotation forestry (SRF) in Eastern Europe and the Nordic countries. Relatively scanty data is available about grey alder yield and growth dynamics. Seven yield-tables from six countries and several published studies have been included in this review. The main aim of the review was to sum up and analyze published data; to evaluate the potential for biomass production and to summarize the existing relevant knowledge for giving recommendations about the optimal principles on managing alder stands. According to different yield-tables, the mean annual increment (MAI) of 20-year-old stands varied from 2.56 m3 ha−1 to 4.75 m3 ha−1 (dry matter). In favourable conditions, the growth of alder stands can be rapid and biomass production high. The highest woody biomass of annual production reported in literature amounts to 17 t ha−1 y−1. A rotation length of 15…20 years is recommended by the majority of authors. The rotation period is longer in northern countries (Norway, Finland) than in southern countries. According to yield-tables, it coincides with the start of the decrease in MAI in most cases. Approximately 60 t ha−1–90 t ha−1 of stemwood can be produced during one rotation. The density of the natural grey alder stand is typically very high. The optimal initial density of grey alder may not exceed 10,000 ha−1 in the case of plantations and the optimal number of trees per hectare before harvesting should range between 3000 ha−1 and 6000 ha−1.

During recent decades, studies of the carbon (C) balance of forest ecosystems have became more actual, mainly in connection with the global increase of CO2 in the atmosphere. In the present study the stand chronosequence approach was applied to analyse C sequestration dynamics. Study was made of C accumulation both in biomass and in the soil in 6–60-year-old silver birch (Betula pendula) stands growing at fertile (Oxalis) sites. As the growth of the studied stands was vigorous, their yield was higher than that presented in several yield tables for earlier periods. The C concentration (C%) in different compartments of the trees varied between 47% and 55%. However, the weighted average of C concentration in the silver birch trees was approximately 50% regardless of stand age. The average C concentration of the herbaceous understorey plants was 43.3 ± 0.5%. The soil Corg pool was independent of stand age, and so far there occurred no C accumulation during stand succession, expressed as Corg values or stage of forest floor formation. This might indicate fast Corg turnover in the soils of the Oxalis site. The total C pool in a mature silver birch stand was 185 t ha−1 of which 50% was accumulated in the aboveground part of the trees. In young birch stands the C pool in aboveground biomass and in the soil accounted for 21–39% and 53–71%, respectively, of the total C pool of a stand. In pre-mature and mature stands the corresponding share accounted for 50–59% of the aboveground C of the trees and 29–38% of the soil C pool. Due to closed canopies, the role of herbaceous understorey plants as a C sink was modest, constituting 1% or even less of the total C pool of the older stands. The annual C flux 1.6 t ha−1 yr−1 into the soil via litter fall was the largest in the middle-age stand. Our results show that the main C sink in fertile silver birch stands is located in the wooden parts of trees. The C pool in tree biomass increased with stand age, whereas the soil Corg pool remained stable. For a more profound understanding of C cycling in silver birch forest, soil respiration fluxes should be measured.

A facility for free air humidity manipulation (FAHM) was established to investigate the effect of increased air humidity on trees’ performance and their canopy functioning with respect to rising air humidity predicted for Northern Europe. The FAHM system enables air relative humidity (RH) to be increased up to 18 units (%) over the ambient level during mist fumigation, depending on the wind speed inside the experimental stand. Water was dispersed inside 14 × 14 m experimental plots in the form of mist with an average particle size of 50 μm from June to August in 2008, and from May to September in 2009. The average increase in RH was 7 units (%) over the whole period of humidification in 2008 ( P F ) in silver birch ( Betula pendula Roth.) trees was 24.8% ( P P P > 0.05) in silver birch on the days without misting. In hybrid aspen ( Populus tremula L. × P. tremuloides Michx.) the average difference in F between C and H plots was 61.1% ( P P F as a result of long-term acclimation to fumigation. The leaves of silver birch in a humidified plot demonstrated up to 2.4 °C lower ( P T L ) compared to the control plot in 2009. The decline in T L decreased the humidity gradient between leaf and air by about 1/3, whereas 2/3 of the effect was caused directly by changes in air humidity in the leaf boundary layer. Our preliminary data suggest that the FAHM experimental facility enables water fluxes through a deciduous tree canopy to be reduced and this effect is attributable both to the increased air humidity and decreased leaf temperature. Changes in these two basic factors may create considerable differences in the physiology, anatomy and nutrition of a whole tree, also affecting forest functioning in the light of global climate change.

Fine root acclimation to different environmental conditions is crucial for growth and sustainability of forest trees. Relatively small changes in fine root standing biomass (FRB), morphology or mycorrhizal symbiosis may result in a large change in forest carbon, nutrient and water cycles. We elucidated the changes in fine root traits and associated ectomycorrhizal (EcM) fungi in 12 Norway spruce stands across a climatic and N deposition gradient from subarctic-boreal to temperate regions in Europe (68°N–48°N). We analysed the standing FRB and the ectomycorrhizal root tip biomass (EcMB, g m−2) simultaneously with measurements of the EcM root morphological traits (e.g. mean root length, root tissue density (RTD), N% in EcM roots) and frequency of dominating EcM fungi in different stands in relation to climate, soil and site characteristics. Latitude and N deposition explained the greatest proportion of variation in fine root traits. EcMB per stand basal area (BA) increased exponentially with latitude: by about 12.7 kg m−2 with an increase of 10° latitude from southern Germany to Estonia and southern Finland and by about 44.7 kg m−2 with next latitudinal 10° from southern to northern Finland. Boreal Norway spruce forests had 4.5 to 11 times more EcM root tips per stand BA, and the tips were 2.1 times longer, with 1.5 times higher RTD and about 1/3 lower N concentration. There was 19% higher proportion of root tips colonized by long-distance exploration type forming EcM fungi in the southern forests indicating importance of EcM symbiont foraging strategy in fine root nutrient acquisition. In the boreal zone, we predict ca. 50% decrease in EcMB per stand BA with an increase of 2 °C annual mean temperature. Different fine root foraging strategies in boreal and temperate forests highlight the importance of complex studies on respective regulatory mechanisms in changing climate.

Mining activities create wastelands that require reclamation. The relief of abandoned opencast oil shale mining area is rugged, and the mining spoil is extremely stony and alkaline (pH 8), with low N and organic content. Planting of fast-growing deciduous tree species such as silver birch (Betula pendula) on post-mining area is the best means to accelerate the development of a new forest ecosystem in such harsh conditions. A chronosequence of silver birch stands (1, 2, 3, 5, 29, 40 years old) was investigated to reveal changes in bulk soil (S) and rhizosphere (R) properties, in rhizosphere effect on bacterial activity and diversity, and in fine-root morphological adaptations in relation to stand development. The rhizosphere effect on bacterial activity was measured as a rhizosphere/soil (R/S) ratio and on species diversity as a similarity (%) between rhizosphere and bulk soil bacterial communities. Bacterial species diversity was determined by denaturing gradient gel electrophoresis (DGGE) technique and was expressed as Shannon diversity index. Biolog EcoPlates were used to determine the summed activity of cultivable bacteria in rhizosphere and bulk soil. Short-root morphological parameters were measured using WinRHIZO™ Pro. Soil pH and available P concentration decreased logarithmically, and N% and organic matter concentration increased linearly with increasing stand age. During the first 30 years of stand development SIR increased an order, from 0.18 to 1.90 mg C g−1. Bulk soil bacterial diversity increased logarithmically with stand age. The bacterial diversity was higher in rhizosphere than in bulk soil. Rhizosphere effect on bacterial activity was low a year after planting, increased more than two times in the next 2 years, and decreased thereafter rapidly with stand age. Rhizosphere effect, indicating plant support to rhizosphere microbial communities, was highest when soil conditions were still poor, but trees had already overcome the transplant shock. All short-root morphological parameters showed certain trends with age. Specific short-root length varied between 56 and 313 m g−1 and decreased logarithmically with stand age and soil improvement. The fastest changes in short-root morphology, rhizosphere effect, and soil pH occurred during the early development of silver birch stands – in the first 5 years; P nutrition and N use efficiency improved simultaneously. Rhizosphere effect and short-root morphological adaptation have an important role in soil and stand development on oil shale post-mining area, and silver birch is a promising tree species for reclamation of alkaline mining spoil.

Short-rotation energy forestry is one of the potential ways for management of abandoned agricultural areas. It helps sequestrate carbon and mitigate human-induced climate changes. Owing to symbiotic dinitrogen (N 2 ) fixation by actinomycetes and the soil fertilizing capacity and fast biomass growth of grey alders, the latter can be suitable species for short-rotation forestry. In our study of a young grey alder stand ( Alnus incana (L.) Moench) on abandoned arable land in Estonia we tested the following hypotheses: (1) afforestation of abandoned agricultural land by grey alder significantly affects the soil nitrogen (N) status already during the first rotation period; (2) input of symbiotic fixation covers an essential part of the plant annual N demand of the stand; (3) despite a considerable N input into the ecosystem of a young alder stand, there will occur no significant environmental hazards (N leaching or N 2 O emissions). The first two hypotheses can be accepted: there was a significant increase in N and C content in the topsoil (from 0.11 to 0.14%, and from 1.4 to 1.7%, respectively), and N fixation (151.5 kg N ha −1 yr −1 ) covered about 74% of the annual N demand of the stand. The third hypothesis met support as well: N 2 O emissions (0.5 kg N ha −1 yr −1 ) were low, while most of the annual gaseous N losses were in the form of N 2 (73.8 kg N ha −1 yr −1 ). Annual average NO 3 –N leaching was 15 kg N ha −1 yr −1 but the N that leached from topsoil accumulated in deeper soil layers. The soil acidifying effect of alders was clearly evident; during the 14-year period soil acidity increased 1.3 units in the upper 0–10 cm topsoil layer.

Soil respiration resulting from microbial and root respiration is a major component of the forest carbon cycle. The response of soil respiration to varying environmental factors (soil temperature and soil moisture) was studied in a Norway spruce chronosequence composed of four age classes (4, 27, 36, and 84 year old) on Gleyic Podzol. Soil respiration was measured monthly with closed dynamic chamber system, soil temperature and soil moisture were measured simultaneously. Mean soil respiration rate averaged over three years was 3.3 μmol CO2 m-2s-1, ranging from 0.6 to 5.4 μmol CO2 m-2s-1, with the maximum occurring in August and the minimum in December. Stand age had a significant effect on soil respiration: the highest respiration rate was found in 27-year-old stand. Over three years an exponential relationship between soil respiration and soil temperature accounted for 68-81% of the seasonal variation, Q10 (the factor by which the respiration rate differs for a temperature interval of 10 °C) for the individual stands ranged between 4.4 and 5.4. The influence of soil moisture content on soil respiration was weak and revealed in dry conditions only. The results of this study can be used to help understand and predict the effect of harvest on soil respiration and how the respiration might respond to changing climate conditions.

Limited nutrient acquisition from soil is a key process limiting productivity in boreal forest. We investigated short-root morphological adaptations and rhizosphere effect in relation to site conditions in interaction with tree mineral nutrition. We studied seven young (8- to 14-year-old) silver birch (Betula pendula Roth.) stands on abandoned agricultural land in Estonia. Soil pH varied from 3.8 to 7.0, and soil N % from 0.07 to 0.26%. Tree nutrient (NPK) status was expressed by leaf nutrient concentrations. Leaf N correlated negatively with short-root specific length and area. Summed activity (SA) and metabolic diversity of bacteria (by BIOLOG Ecoplate™), bacterial community diversity (by DGGE) and pHKCl were determined for rhizosphere (R) and bulk soil (S) to reveal the extent of the rhizosphere effect. Bacterial activity in rhizosphere was 1.4–4.7 times higher than in bulk soil. Ratio SAR/SAS indicating root support to the rhizosphere bacterial communities decreased with increasing bulk soil pH; however, when bulk soil pH was ≥5, the decrease in SAR/SAS was insignificant, i.e. the rhizosphere effect stayed at a stable level. Diversity of bacterial community was 6% higher in bulk soil than in rhizosphere. Rhizosphere acidification occurred in studied stands when bulk soil pHKCl ≥ 5. Short-root N % correlated positively with SAR/SAS. We concluded that tree N-nutritional status was related to short-root morphological parameters but not to studied microbiological characteristics in the soil of young silver birch stands.

Eight-year-old plantations (11) of silver birch were studied on abandoned agricultural land in Estonia. Trees were planted on uncovered soil and on polyethylene. After eight growing seasons, the height and diameter at breast height of the trees grown on polyethylene were significantly (P 0.05) between the values of the live crown ratio. The height growth of silver birch grown without mulching as well as with mulching was found to be more intensive on Glossic Podzoluvisol, Calcaric Cambisol, Calcaric Luvisol and Dystric Gleysol; however, the height growth was more intensive on mulched soil. The height growth of the birches was modelled on the single-tree and stand levels for five soil types.

Mõningate kiirekasvuliste lehtpuupuistute kasv ja produktsioonivõime endisel põllumaal Several studies about stands growing on abandoned agricultural lands are induced by extensive afforestation of agricultural lands and more intensive use of biomass. Overview of above-ground biomass production of grey alder, hybrid alder and silver birch young stands growing on former agricultural areas are presented in current paper. The results of 16- and 6-year period of alders and silver birch stands, respectively, are reported. Above-ground biomass and biomass production of stands were estimated. The growing stock and current annual increment (CAI) of 16-years-old grey alder stand were 250 m3 ha-1 and 35.6 m3 ha-1, respectively. Due to fast growth and high biomass production capacity grey alder is promising tree species for short-rotation forestry in Estonia. Hybrid alder is productive tree although not exceeding the productivity of grey alders. Stem volume and CAI of the 16-years-old hybrid alder stand were 155 m3 ha-1 and 21 m3 ha-1, respectively. Our results supported earlier data reported in literature: rotation period of hybrid alder is longer than for grey alder and CAI of hybrid alder stands culminate later. The productivity of young silver birch stands on abandoned agricultural land is varying in a broad range. One young silver birch stand growing on abandoned agricultural land was involved into study. Growing stock and CAI in 13-years old stand were 118 m3 ha-1 and 15 m3 ha-1, respectively. These values are exceeding respective values of several yield tables of silver birch and biomass production capability of silver birch stands in favourable conditions is high.

We estimated the global warming potential of Estonian peatlands (transitional fens and ombrotrophic bogs) based on greenhouse gases (GHG) CO2, CH4, and N2O and carbon (C) accrual in biomass, and the effects of drainage on these processes. Data were derived from a review of the literature of boreal peatlands. Areal estimates of peatland types were multiplied with the values of the interquartile range of literature-derived GHG fluxes. The effect of drainage and radiative forcing of Estonian peatlands were also evaluated. Annual emission of CO2, CH4, and N2O is estimated to be 278 to 1,056×103 of CO2 equivalent (eq), of which CO2 makes up 22 to 44%, CH4 53 to 73%, and N2O 3 to 5%. The annual efflux is 419 to 676×103 CO2 eq year−1 from drained peatlands, and 2141 to 380×103 CO2 eq year−1 from the undrained peatlands. The annual loss of C from peatlands is estimated to be 38 to 86 tons C×103 year−1. Thus due to drainage, Estonia’s transitional fens and ombrotrophic bogs have gone from being sinks to sources of C.

Summary The dynamics of the above-ground biomass production of a grey alder plantation on abandoned farmland was investigated during 11 years after establishment. In the 12-year-old stand, the total biomass of the above-ground part of the stand was 68.8 t dry matter (DM) ha 1 and the current annual production (CAP ) was 14.0 t DM ha 1 year 1 . The predicted mean annual increment (MAI) reached is maximum at the age of 16 years, which indicates bulk maturity (the stand age when CAI = MAI) and appropriate rotation time for obtaining maximum biomass production. In the case of short-rotation forestry, initial stand density should not be higher than 6500 – 6000 trees per hectare. Below-ground biomass accounted for 18 and 16 per cent of total stand biomass at a stand age of 5 and 10 years, respectively. The biomass of the nodules was estimated at 155 ± 63 kg DM ha 1 and the biomass of the fi ne roots was estimated at 870 ± 130 kg DM ha 1 in the 10-year-old grey alder stand. Of the fi ne roots, 80 per cent and almost all nodules were located in the upper 0 – 20 cm soil layer in both the 5-year-old and the 10-year-old stand. The value of leaf area index increased with stand age, ranging between 1.38 and 5.43 m 2 m 2 during the development of the stand. Specifi c leaf area varied in different years from 11.1 to 13.5 m 2 kg 1 .

Biomassi produktsioon jänesekapsa kasvukohatüübi arukaasikutes Present study was carried out in three silver birch (Betula pendula Roth.) stands in the Oxalis site type in South-Estonia. The above-ground biomass, above-ground biomass production and foliar characteristics of 14-, 32- and 45-year-old silver birch natural stands were studied. The above-ground biomass and production of the stand was estimated in August 2008 using dimension analysis (model tree method). The above-ground biomasses in the 14-, 32- and 45-year-old silver birch stands growing on forest land were 39.9; 140.3 and 224.3 t ha-1 respectively. All results are given in dry mass (DM). Current annual production (CAP) of stem mass ranged 4.7-7.5 t ha-1. Leaf area index (LAI) ranged 2.5-4.4 m2 m-2 in the studied stands. Based on these results, stands of silver birch growing in Oxalis site type are very productive and may exceed the respective values presented in yield tables used in Estonian forestry.