Hybrid Aspen Plantations Research Articles

Soil mycobiomes in native European aspen forests and hybrid aspen plantations have a similar fungal richness but different compositions, mainly driven by edaphic and floristic factors.

The cultivation of short-rotation tree species on non-forest land is increasing due to the growing demand for woody biomass for the future bioeconomy and to mitigate climate change impacts. However, forest plantations are often seen as a trade-off between climate benefits and low biodiversity. The diversity and composition of soil fungal biota in plantations of hybrid aspen, one of the most planted tree species for short-rotation forestry in Northern Europe, are poorly studied. The goal of this study was to obtain baseline knowledge about the soil fungal biota and the edaphic, floristic and management factors that drive fungal richness and communities in 18-year-old hybrid aspen plantations on former agricultural soils and compare the fungal biota with those of European aspen stands on native forest land in a 130-year chronosequence. Sites were categorized as hybrid aspen (17-18-year-old plantations) and native aspen stands of three age classes (8-29, 30-55, and 65-131-year-old stands). High-throughput sequencing was applied to soil samples to investigate fungal diversity and assemblages. Native aspen forests showed a higher ectomycorrhizal (EcM) fungal OTU richness than plantations, regardless of forest age. Short-distance type EcM genera dominated in both plantations and forests. The richness of saprotrophic fungi was similar between native forest and plantation sites and was highest in the middle-aged class (30-55-year-old stands) in the native aspen stands. The fungal communities of native forests and plantations were significantly different. Community composition varied more, and the natural forest sites were more diverse than the relatively homogeneous plantations. Soil pH was the best explanatory variable to describe soil fungal communities in hybrid aspen stands. Soil fungal community composition did not show any clear patterns between the age classes of native aspen stands. We conclude that edaphic factors are more important in describing fungal communities in both native aspen forest sites and hybrid aspen plantation sites than forest thinning, age, or former land use for plantations. Although first-generation hybrid aspen plantations and native forests are similar in overall fungal diversity, their taxonomic and functional composition is strikingly different. Therefore, hybrid aspen plantations can be used to reduce felling pressure on native forests; however, our knowledge is still insufficient to conclude that plantations could replace native aspen forests from the soil biodiversity perspective.

Multivariate mixed-effects models for stand characteristics of hybrid aspen plantations in southern Finland and southern Sweden

Hybrid aspen, a hybrid between the European aspen and North American trembling aspen (Populus tremula L. × P. tremuloides Michx.), is a promising species because of its fast-growth and its suitability for multi-purpose use. However, models for predicting the age-dependent development of stand characteristics are still missing. The main objectives of this study were therefore to develop the models for predicting stand characteristics of hybrid aspen plantations and to validate the model applicability. The target response variables were stand basal area (BA), basal area-weighted mean diameter (DG) and basal area-weighted mean height (HG). Data were obtained from clonal hybrid aspen trials in southern Finland and southern Sweden. Multivariate mixed-effects modelling was used to estimate the parameters of seemingly unrelated regression for BA, DG, and HG. Model fit provided the following predictor variables: stand age (AGE), the number of trees per hectare (TPH), site index (SI), growing degree-days (GDD5), soil and site type, and thinning treatment. The chosen predictors differed slightly by response variable, but all parameters were highly significant (P < 0.0001), and model goodness-of-fit statistics presented high accuracy: RMSE of 2.59 m2 ha−1 for BA, 1.21 cm for DG, 1.05 m for HG in arithmetic scale. The applied simulations illustrated clear differences in the predicted development of stand characteristics when input variables SI, TPH or GDD5 changed. The developed models were assessed to be easily applicable and useful for predicting the stand and tree characteristics of clonal hybrid aspen plantations, especially for the stands with AGE ≤ 30 years and TPH ≤ 2000 trees ha−1.

Isoprene and monoterpene emissions from alder, aspen and spruce short-rotation forest plantations in the United Kingdom

Abstract. An expansion of bioenergy has been proposed to help reduce fossil-fuel greenhouse gas emissions, and short-rotation forestry (SRF) can contribute to this expansion. However, SRF plantations could also be sources of biogenic volatile organic compound (BVOC) emissions, which can impact atmospheric air quality. In this study, emissions of isoprene and 11 monoterpenes from the branches and forest floor of hybrid aspen, Italian alder and Sitka spruce stands in an SRF field trial in central Scotland were measured during two years (2018–2019) and used to derive emission potentials for different seasons. Sitka spruce was included as a comparison as it is the most extensive plantation species in the UK. Winter and spring emissions of isoprene and monoterpenes were small compared to those in summer. Sitka spruce had a standardised mean emission rate of 15 µgCg-1h-1 for isoprene in the dry and warm summer of 2018 – more than double the emissions in 2019. However, standardised mean isoprene emissions from hybrid aspen were similar across both years, approximately 23 µgCg-1h-1, and standardised mean isoprene emissions from Italian alder were very low. Mean standardised total monoterpene emissions for these species followed a similar pattern of higher standardised emissions in the warmer year: Sitka spruce emitting 4.5 and 2.3 µgCg-1h-1 for 2018 and 2019, aspen emitting 0.3 and 0.09 µgCg-1h-1, and Italian alder emitting 1.5 and 0.2 µgCg-1h-1, respectively. In contrast to these foliage emissions, the forest floor was only a small source of monoterpenes, typically 1 or 2 orders of magnitude lower than foliage emissions on a unit of ground area basis. Estimates of total annual emissions from each plantation type per hectare were derived using the MEGAN 2.1 model. The modelled total BVOC (isoprene and monoterpenes) emissions of SRF hybrid aspen plantations were approximately half those of Sitka spruce for plantations of the same age. Italian alder SRF emissions were 20 times smaller than from Sitka spruce. The expansion of bioenergy plantations to 0.7 Mha has been suggested for the UK to help achieve net-zero greenhouse gas emissions by 2050. The model estimates show that, with such an expansion, total UK BVOC emissions would increase between &lt;1 % and 35 %, depending on the tree species planted. Whereas increases might be small on a national scale, regional increases might have a larger impact on local air quality.

Models for diameter distribution and tree height in hybrid aspen plantations in southern Finland

Hybrid aspen ( L. × Michx.) is known with outstanding growth rate and some favourable wood characteristics, but models for stand management have not yet been prepared in northern Europe. This study introduces methods and models to predict tree dimensions, diameter at breast height (dbh) and tree height for a hybrid aspen plantation using data from repeatedly measured permanent sample plots established in clonal plantations in southern Finland. Dbh distributions using parameter recovery method for the Weibull function was used with Näslund’s height curve to model tree heights. According to the goodness-of-fit statistics of Kolmogorov-Smirnov and the Error Index, the arithmetic mean diameter () and basal area-weighted mean diameter () provided more stable parameter recovery for the Weibull distribution than the median diameter () and basal area-weighted median diameter (), while showed the best overall fit. Thus, Näslund’s height curve was modelled using with Lorey’s height (), age, basal area (), and tree dbh (Model 1). Also, Model 2 was tested using all predictors of Model 1 with the number of trees per ha (). All predictors were shown to be significant in both Models, showing slightly different behaviour. Model 1 was sensitive to the mean characteristics, and , while Model 2 was sensitive to stand density, including both and as predictors. Model 1 was considered more reasonable to apply based on our results. Consequently, the parameter recovery method using and Näslund’s models were applicable for predicting tree diameter and height.Populus tremulaP. tremuloidesDDGDMDGMDGDGHGBATPHDGHGBATPHDG

Where and when are plantations established? Land-use replacement patterns of fast-growing plantations on agricultural land

The location of fast-growing plantations for energy reveals valuable information concerning farmers' preferences and land use change patterns, necessary to have adequate environmental and economic assessments. The present study analyses the last decades’ data concerning the establishment of willow, poplar and hybrid aspen plantations in Sweden. The analysis includes extension planted, a geo-statistical analysis of core location patterns, and the type of land being replaced or replacing plantations, for the period 1986–2017. The results show a steady decrease of willow plantations in recent years, which can be explained by changes in the policy framework (after 1996) and the increase in cereal prices (after 2007). The decline is partially offset by the establishment of new poplar and hybrid aspen plantations. New plantations tend to spread in southern areas; willow tends to be planted on higher productivity agricultural areas, and poplar on less productive land. There is a trend towards preferring smaller plantations (<1 ha) versus large ones (>10 ha). Although many willow plantations have been established on previous cereal land (particularly on spring barley and winter wheat), this pattern changed after 2007, preferring grasses and fallow land. The latter is the most common land use replaced by poplar plantations. These shares may have important applications in economic and environmental assessments; the general spatial patterns of the main fast-growing species for biomass can provide a valid reference for the future implementation of bioenergy production systems.

Site index models with density effect for hybrid aspen (Populus tremula L. × P. tremuloides Michx.) plantations in southern Finland

This study was conducted to examine the characteristics of dominant height growth and develop site index models for clonal hybrid aspen plantations in southern Finland. Data were obtained from repeatedly measured clonal hybrid aspen trials with varying initial spacing: 2.5 m × 2.5 m (1600 trees ha−1), 3.0 m × 3.0 m (1200 trees ha−1), 3.5 m × 3.5 m (800 trees ha−1), and 5.0 m × 5.0 m (400 trees ha−1). The total number of data points in the analysis was 389 for the age of 3–20. Within the range of observed data, the dominant height grew linearly over age and was significantly different due to the initial planting density; growth was higher when the planting was denser. Using the initial density effect, dominant height growth models were developed based on the Chapman-Richards function through nonlinear mixed-effects modelling. The density variable was found to be statistically the best variable when modifying only the shape parameter of the Chapman-Richards function. All fixed-effects were significant for both models, with and without the density effect. The residual plots of the model did not show any bias over the predicted value, stand age or planting density. The predicted dominant height was higher with increasing initial density. The predicted dominant height increment was faster with higher planting densities until the age of 14 years. The anamorphic site index curves were presented with base age of 20 years including the planting density effect. The overall pattern of site index curves was consistent with those observed in previous studies. The models developed in this study can be used to estimate the dominant height and site index of hybrid aspen plantations in southern Finland.

CUT-AWAY PEATLAND RE-CULTIVATION WITH FAST GROWING WOODY PLANTATIONS: COST-BENEFIT ANALYSIS

In Latvia peat production is historically important economic sector, giving significant contribution to rural economic development and providing work places in rural areas, where other job opportunities are limited. Every peat extraction field has certain lifespan, based on economic, social or ecological assumptions. Every peat field could be extracted for set period of time, after which peat extraction is not feasible any more. Current legislation in Latvia requires re-cultivating of places, where mineral resources, including peat, were extracted. Aim of the study is to evaluate afforestation as one of the cut-away peatland re-cultivation possibility. Economic analysis were done for extracted peat field afforestation with birch, black alder, grey alder, pine, spruce, hybrid aspen, poplar and willow. The economic calculations were done comparing different timber products from plantations, different planting materials and plantation lifespans. Lowest investment costs for plantation establishment on cut-away peatlands are for pine and spruce plantations and highest for willow plantations. Highest revenues in plantations with 20 years rotation cycle are in poplar plantations with 0.2 m cuttings and lowest in grey alder plantations. In plantations with rotation cycle 40 years highest revenues are in hybrid aspen plantations and lowest in pine plantations.&nbsp;

Carbon and nitrogen stabile isotope ratio and heavy metals in Leccinum aurantiacum in a hybrid aspen plantation in agricultural land

Edible mycorrhizal fungi can be harvested in the fourth year after establishment of a hybrid aspen plantation in previous agricultural land at hemiboreal conditions. It is important to understand the role of fungi in element cycling at the ecosystem level as well as the amounts of elements, including heavy metals, that are accumulated in fruitbodies of edible fungi in the context of food safety. Therefore we evaluated the carbon and nitrogen stable isotope ratio and content of heavy metals (Cr, Pb, Mn, Ni, Cd, Cu and Zn) in Leccinum aurantiacum (Bull.) Gray in a juvenile hybrid aspen (Populus tremuloides Michx. × Populus tremula L.) plantation in agricultural land initially fertilised with biogas production residues (digestate), sewage sludge and wood ash, which are potential sources of pollution of heavy metals. The research site was established in the spring of 2011, but the natural occurrence of fruitbodies of L. aurantiacum was observed in 2014. The mean isotopic data for fruitbodies of L. aurantiacum, collected in the 2014–2018 monitoring period, ranged between -27.4 and -24.5‰ for δ13C and between 7.8 and 10.1‰ for δ15N. The mean content of investigated heavy metals in fruitbodies of L. aurantiacum were up to 129 mg kg-1 for Zn, up to 99 mg kg-1 for Cu, up to 30 mg kg-1 for Mn, up to 1.5 mg kg-1 for Ni, up to 1.7 mg kg-1 for Cd, up to 1.1 mg kg-1 for Cr and up to 0.6 mg kg-1 for Pb. We concluded that the application of digestate, sewage sludge and wood ash as fertilisers to improve soil quality in hybrid aspen plantations in agricultural land did not result in increased heavy metal accumulation in fruitbodies of L. aurantiacum, but N isotopic compositions were altered (15N-enriched) for a period of up to five years after application of the organic fertilizers (digestate and sewage sludge). &#x0D; &#x0D; Keywords: agricultural land, hybrid aspen, Leccinum aurantiacum (Bull.) Gray, heavy metals, stable isotope ratio

First record of Entoleuca mammata in hybrid aspen plantations in hemiboreal Estonia and stand–environmental factors affecting its prevalence

Entoleuca mammata [causing Hypoxylon canker (HC)] is one of the most serious pathogens that are colonising Populus species. The area of fast-growing hybrid aspen plantations has remarkably increased in northern Europe because of the high demand for woody biomass, but the prevalence of HC and the stand-related and environmental factors that favour its presence have rarely been monitored in such monocultural plantations. The presence of E. mammata in Estonia was proven by ITS sequence on hybrid aspen. Repeated monitoring after 4 years in 24 hybrid aspen plantations found that the share of visually damaged trees by HC increased from 0.6% (15-year-old trees) to 1.6% (19-year-old trees). The mortality rate of the infected trees during the 4 years was 100%. The probability of HC incidence was favoured by tree vigour and higher soil acidity. The higher susceptibility of more vigorous trees might be related to their trade-off between productivity and defence compounds. The share of trees with clear visual symptoms of HC can be considered marginal, but the aggressive nature of the pathogen suggests the need to extend the monitoring period, especially in recently thinned sites, and to expand the observations to native Populus tremula stands.

Macronutrient leaching in a fertilized juvenile hybrid aspen (Populus tremula L. × P. tremuloides Michx.) plantation cultivated in an agroforestry system in Latvia

Abstract Leaching of macronutrients such as nitrogen, phosphorus and potassium from soils is of particular interest in the Baltic Sea region because of its adverse effects on water quality. The objective of this study was to evaluate macronutrient leaching in a juvenile hybrid aspen (Populus tremula L. × P. tremuloides Michx.) plantation cultivated in an agroforestry system and fertilized with biogas production residues, wastewater sludge and wood ash in hemi-boreal climate conditions. Analysis of macronutrient concentrations in the soil solution showed that annual macronutrient leaching decreased over time after the establishment of the plantation and application of fertilizers. Moreover, macronutrient leaching was affected not only by meteorological conditions, but also by the type of fertilizer used. During the five years after establishment of the plantation, nitrate-nitrogen leaching decreased up to 99.6%, phosphate-phosphorus leaching decreased up to 97.1%, but potassium ions leaching decreased up to 76.5%. Nevertheless, juvenile hybrid aspen plantations cultivated in an agroforestry system remain an important part in mitigation of leaching of macronutrients from agricultural lands in the Baltic Sea region.

Diversity of lichens and bryophytes in hybrid aspen plantations in Estonia depends on landscape structure

The importance of single-species forest plantations in sustaining biodiversity could be bigger than expected. We described the diversity of lichens and bryophytes in 15 midterm (16- to 17-year-old) hybrid aspen (Populus tremula × Populus tremuloides) plantations in Estonia. Species richness and composition data were linked with environmental and landscape-scale variables. Altogether, 44 lichen and 37 bryophyte species were recorded from plantations; richnesses of bryophytes and lichens were positively correlated. Lichen species composition was significantly affected by landscape-scale parameters (distance to the nearest present forest, distance to the nearest forest continuously occurring in the same place since the 1930s, and forest area in the study plot vicinity in the 1930s). Bryophyte species composition was affected mainly by light conditions and forest area in the 1930s. Among lichens, the sexual crustose species functional group dominated in midterm plantations; appearance of asexual lichens of di...

Impact of Populus Plantations on Water and Soil Quality

Trees of genus Populus (in our context primarily poplars) are predominantly grown in Sweden in small plantations on arable land in southern and central parts of the country to produce biomass for energy and other purposes. This study evaluated the effects (i) of poplar plantations on groundwater quality, by determining differences in leaching of nitrogen and phosphorus to groundwater, and (ii) of poplar and hybrid aspen plantations on soil quality in terms of carbon in the top- and subsoil. The study was conducted comparing Populus plantations in Sweden with adjacent fields with cereals and grasslands. The experiment concerning the groundwater leaching was conducted in eight poplar plantations along three growing seasons (2012–2015). For the soil carbon experiments, 19 poplar and two hybrid aspen plantations and the respective reference fields were sampled. NO3-N leaching from poplar plantations was significantly lower than that from reference fields with cereals, but not when compared with grasslands. Spring NO3-N leaching was significantly lower in poplars than in the reference fields, whereas leaching of NO3-N in autumn did not differ. Concentrations of PO4-P in the groundwater of poplar plantations were lower compared to the respective ones of the reference fields. There were no clear trends observed when comparing carbon concentrations in the topsoil of the poplar and hybrid aspen plantations compared to the respective adjacent reference fields. For the subsoil, the average carbon concentrations in the poplar and hybrid aspen plantations were equal to the respective ones of cereals, but were higher when compared to grassland.

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

ABSTRACTShort-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.

Development of stem cracks in young hybrid aspen plantations

Abstract Cracks expose wood to fungal infections that significantly affects wood quality, while rapid wound occlusion decreases probability of infections. Assessment of scars was done at four grade scale in three adjacent hybrid aspen trials at the age of 8-10 years in central part of Latvia three years after bark crack occurrence. Occluded wounds were found for 95% of damaged trees, regardless of tree age. Among trees that had cracks wider than 1 cm, 42% had uniformly healed bark, but 7% still had open wounds. Wound development was significantly affected by crack width and length (both p &lt; 0.001), but had no clear relation with tree DBH (diameter at breast height) and relative DBH increment (both p &gt; 0.05). At clonal mean level, scar grade was significantly affected by grade of crack three years earlier and clone (both p &lt; 0.001), but mean DBH of clone had no relation (p &gt; 0.05) to proportion of trees evaluated by any of the scar grades. The results suggest that three years after the bark crack formation most of them had successfully occluded and selection of clones with better diameter growth has no influence on development of cracks.

The impact of former land-use type to above- and below-ground C and N pools in short-rotation hybrid aspen (Populus tremula L. × P. tremuloides Michx.) plantations in hemiboreal conditions

Short-rotation forestry with fast-growing hybrid aspen (Populus tremula L.×P. tremuloides Michx.) on former arable lands is a novel land-use system in northern Europe that has substantially increased in the region during the last few decades. The objective of this study was to assess the potential of hybrid aspen plantations to sequestrate C and N to above- and below-ground pools from the age of 5 (young) to 15 years (midterm) depending on former land-use type. Data were collected from permanent experimental plots on former croplands (n=28) and grasslands (n=23). Based on repeated soil monitoring and destructive model tree sampling, the following C and N pools were estimated: above-ground biomass of trees, soil uppermost layer (A-horizon), coarse roots and subsoil (below A-horizon up to a depth of 75cm). On average, A-horizon SOC and Ntot pools had decreased significantly on former grasslands, while no change was observed on croplands. Unexpectedly, considerable changes had occurred in subsoil, where SOC and Ntot pools increased significantly on both former land-use types. Therefore, grasslands’ A-horizon C loss was compensated for by increased coarse-root fraction and subsoil gains. About one-third of below-ground C pool was stored in subsoil. In above-ground leafless part of 15-year-old hybrid aspen model trees the weighted average C concentration was 45.8% and the N concentration was 0.309%. From the age of 5–15, hybrid aspen plantations acted as C sinks because total ecosystem C pool increased significantly by 3.17MgCha−1yr−1 on former croplands and by 2.56MgCha−1yr−1 on former grasslands. The main C sequestration had taken place in the above-ground pool (for croplands 73.8% and for grasslands 94.9%). To conclude, hybrid aspen plantations already showed during the first 15years a great potential to sequestrate C and N at the ecosystem level, whereas former SOC-exhausted croplands have a higher ability to sequestrate new C to the below-ground pool than already SOC-rich grasslands. Deeper subsoil should definitely be taken into account in SOC estimations.

The impact of short-rotation hybrid aspen (Populus tremula L. × P. tremuloides Michx.) plantations on nutritional status of former arable soils

The area of short-rotation forest (SRF) plantations with hybrid aspen (Populus tremula L.×P. tremuloides Michx.) for pulp and energy wood production in approximately 25-year rotation period has increased considerably during the last few decades in Northern Europe on previous agricultural lands. However, little is known about how this new land use will alter the chemical properties of soils in the long-term and how long nutrients from the land’s previous use will sustain high productivity of trees. Little is also known about the dynamics of soil micronutrients and the plantations’ influence on subsoil layers. Repeated soil monitoring was carried out in hybrid aspen plantations at the ages of 5 (young) and 15years (midterm) to detect changes in soil chemical characteristics, including soil reaction (pHKCl), and concentrations of nutrients and organic carbon (Corg) in A- and B-horizon on 51 permanent experimental plots. The main soil- and stand-related factors responsible for changes in soil chemical properties, were analyzed. Significant changes were observed both in soil A-horizon and B-horizon. In A-horizon, pHKCl (−0.2 units), Ca (−19.4%) and B (−25.8%) had decreased while total N (+5.8%), Cu (+20.3%) and Mn (+21.3%) had increased. In B-horizon, Ca (−23.4%), Mg (−12.8%) and B (−45.0%) had decreased, while Corg (+17.2%) and Cu (+53.8%) had increased. The concentrations of available P, K and Corg in A-horizon had remained unchanged. Reduction of soil chemical characteristics mainly occurred in plantations where their initial level had been high (this relationship was observed in A-horizon for total N and B and in B-horizon for Corg, Mg and B). Several soil chemical characteristics decreased more (or increased less) in plantations where tree growth was more vigorous (in A-horizon: pHKCl, total N and B; in B-horizon: Mg and B). Soil cation exchange capacity, base saturation and moisture conditions also explained some of the observed variation in changes. Relatively high loss of B in faster growing plantations indicates possible future limitations of this essential micronutrient. To summarize, first generation hybrid aspen plantations planted on former agricultural fields had significantly altered soil reaction (pHKCl) but did not show significant depletion of primary macronutrients (N, P, K) and soil organic carbon.

Vascular plant and bryophyte flora in midterm hybrid aspen plantations on abandoned agricultural land

Short-rotation forest plantations with hybrid aspen on abandoned agricultural land are a novel use of land in eastern and northern Europe; however, to date, their impact on floristic diversity has not been thoroughly studied. Our aim is to determine the kind of vascular plant and bryophyte (including epiphytes) species that benefit from such plantations. Data on understorey and soil humus layer properties were collected as part of the repeated monitoring of 204 permanent vegetation plots in twenty-four 13- to 14-year old commercial hybrid aspen plantations in boreonemoral Estonia. Data analysis indicated that the understorey of midterm hybrid aspen plantations is formed of species with different ecological requirements, ranging from typical fallow species to shade-tolerant forest species. Midterm plantations were dominated by common grassland species; however, occasionally less frequent and protected grassland species were also found. No rare species of epigeic or epiphytic bryophytes were recorded. Overall, a slow succession towards a shade-tolerant understorey was observed when comparing midterm and young plantations, with the occurrence of forest species in negative correlation with the amount of canopy-transmitted total solar radiation. The number of species characteristic of forests growing on similar soil types was low. Further studies will clarify how close the habitat in such intensively managed plantations on abandoned agricultural lands can approach that of natural forests in hemiboreal conditions.

Short-rotation forestry with hybrid aspen (Populus tremula L.×P. tremuloides Michx.) in Northern Europe

This review summarises the knowledge about the ecology, breeding and management of hybrid aspen (Populus×wettsteinii Hämet-Ahti=P. tremula L.×P. tremuloides Michx.). The review is restricted mainly to Northern Europe, where hybrid aspen has been most intensively studied and cultivated and where it has proved to be one of the fastest-growing hardwoods, suitable for the production of pulp- and energy-wood using the principles of short-rotation forestry. During recent decades over 4500 ha have been cultivated with hybrid aspen in the region. A number of research articles and domestic project reports involving hybrid aspen have been published, providing the basis for this review. Breeding has resulted in clones with high productivity and improved resistance to pests and diseases. Thus, hybrid aspen has fulfilled the preconditions for becoming an economically valuable hardwood in Northern Europe. Hybrid aspen plantations can be established on abandoned agricultural land, on forest land, and for the reclamation of exhausted surface mines. However, fast growth rate occurs only in fertile sites with good nutritional and hydrophysical properties. An increased area of Populus plantations on forest or agricultural land can have both positive and negative impacts on biodiversity, depending on landscape context, management activities and considered organisms. Further studies are needed concerning silviculture, site-growth relations, stability of clones, environmental and biodiversity impacts in large-scale plantations at various sites and adaptation of hybrid aspen to climate change.

Possibilities of establishing hybrid aspen plantations in Latvia

Possibilities of establishing hybrid aspen plantations in Latvia

The impact of available water content in previous agricultural soils on tree growth and nutritional status in young hybrid aspen plantations in Estonia

The impact of soil available water content (AWC) on the growth and foliar nutrient concen- trations of trees in 7-yr-old hybrid aspen (Populus tremula L. x P. tremuloides Michx.) plantations established on abandoned agricultural lands was studied. AWC in the topmost 75 cm soil layer was significantly related to height growth and foliar N concentration of hybrid aspens. The correlations were stronger on well-drained automorphic soils and in the case of a dry growing season with insufficient precipitation. AWC over 150-160 mm can be considered sufficient, and below 120 mm insufficient for fast growth of hybrid aspen. The differences in AWC were less pronounced in the top 25 cm soil layer but were more noticeable in the 25-50 and 50- 75 cm soil layers. AWC estimated as a function of soil specific surface area and bulk density was shown here to be a significant indicator for site selection for establishing hybrid aspen plantations. Foliar N con- centration over 2.7% can be considered optimal and below 2.4%, insufficient for hybrid aspen on the studied soils. Foliar concentrations of P (on average 0.24%) and K (on average 0.79%) varied little and, thus, did not correlate with tree growth. The most suitable previous agricultural soils for afforestation with hybrid aspen would be moderately drained Albeluvisols, Luvisols and Planosols.