Hardi Tullus

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

Abstract 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.

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

At northern latitudes a rise in atmospheric humidity and precipitation is predicted as a consequence of global climate change. We studied several growth and functional traits of hybrid aspen (Populus tremula L.×P. tremuloides Michx.) in response to elevated atmospheric humidity (on average 7% over the ambient level) in a free air experimental facility during three growing seasons (2008–2010) in Estonia, which represents northern temperate climate (boreo-nemoral zone). Data were collected from three humidified (H) and three control (C) plots, and analysed using nested linear models. Elevated air humidity significantly reduced height, stem diameter and stem volume increments and transpiration of the trees whereas these effects remained highly significant also after considering the side effects from soil-related confounders within the 2.7 ha study area. Tree leaves were smaller, lighter and had lower leaf mass per area (LMA) in H plots. The magnitude and significance of the humidity treatment effect – inhibition of above-ground growth rate – was more pronounced in larger trees. The lower growth rate in the humidified plots can be partly explained by a decrease in transpiration-driven mass flow of NO3 − in soil, resulting in a significant reduction in the measured uptake of N to foliage in the H plots. The results suggest that the potential growth improvement of fast-growing trees like aspens, due to increasing temperature and atmospheric CO2 concentration, might be smaller than expected at high latitudes if a rise in atmospheric humidity simultaneously takes place.

Partitioning Of Carbohydrates And Biomass Of Needles In Scots Pine Canopy

The study was aimed at the quantitative evaluation of the temporal and spatial partitioning of non-structural carbohydrates and needle biomass in a canopy of Scots pine (Pinus sylvestris L.) growing in a Myrtillus site type forest stand (predominant in Estonia). The tree canopy was divided into ten equal layers and the material for the spatial partitioning of the investigated characteristics was sampled from all layers. Our findings revealed a significant variation in morphology and in the partitioning of carbohydrates in needles in different layers of the canopy. The study of the temporal dynamics of carbohydrates showed that starch content in needles started to increase in early spring before budbreak, which was accompanied by a decline in soluble carbohydrates. In October, the starch content of needles was low, but the concentration of soluble sugars started to increase attaining a maximum in winter. Regression analysis indicated that before budbreak, the partitioning of soluble sugars in different canopy layers was relatively weakly correlated with the height of the layer; however, a strong correlation was observed for starch. In autumn, when the growth of trees stopped and daily temperatures decreased, the allocation of soluble sugars was correlated with the height of the canopy layer

Short-rotation Forestry for Supplying Biomass for Energy Production

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

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

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

Change from Agriculture to Forestry: Floristic Diversity in Young Fast-Growing Deciduous Plantations on Former Agricultural Land in Estonia

The understorey vascular plant cover and its relations with the overstorey tree species and site properties in young silver birch and hybrid aspen plantations were studied. Understorey vegetation was similar in both plantation types in terms of species richness, diversity, sensitivity to human impact, life-span and habitat preference. Nevertheless, in denser silver birch plantations some signs indicated a faster vegetation development overall, e.g., a higher share of shade tolerant plant species. The concentration of total N was higher in the humus layer of silver birch plantations consequently affecting the nutritional status of the understorey vegetation. The significant impact of the plantation type on the understorey vegetation was confirmed by the NMDS analysis. The hypothesis that semi-exotic hybrid aspen plantations may support the spread of alien species or may show a tendency towards smaller indigenous species richness was not confirmed. Irrespective of the overstorey tree species, a strong previous land use impact, i.e. disturbance history, on the ground vegetation was eminent.

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

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