Specific root length (SRL, m g71 ) is probably the most frequently measured morphological parameter of fine roots. It is believed to characterize economic aspects of the root system and to be indicative of environmental changes. The main objectives of this paper were to review and summarize the published SRL data for different tree species throughout Europe and to assess SRL under varying environmental conditions. Meta-analysis was used to summarize the response of SRL to the following manipulated environmental conditions: fertilization, irrigation, elevated temperature, elevated CO 2 , Al-stress, reduced light, heavy metal stress and physical disturbance of soil. SRL was found to be strongly dependent on the fine root classes, i.e. on the ectomycorrhizal short roots (ECM), and on the roots 50.5 mm, 51 mm, 52 mm and 1 -2 mm in diameter SRL was largest for ECM and decreased with increasing diameter. Changes in soil factors influenced most strongly the SRL of ECM and roots 50.5 mm. The variation in the SRL components, root diameter and root tissue density, and their impact on the SRL value were computed. Meta-analyses showed that SRL decreased significantly under fertilization and Al-stress; it responded negatively to reduced light, elevated temperature and CO 2. We suggest that SRL can be used successfully as an indicator of nutrient availability to trees in experimental conditions.

Nutrient conservation plays an important role in plants adapted to infertile environments. Nutrients can be conserved mainly by extending the life span of plant parts and\or by minimizing the nutrient content of those parts that are abscissed. Together these two parameters (life span and resorption) define the mean residence time (MRT ) of a nutrient. In this review we summarize available information on nitrogen resorption and life span, and evaluate their relationship to the MRT of nitrogen, both between and within species. Abundant information with respect to nitrogen resorption efficiency and life span is available at the leaf level. By definition, woody evergreen plants have a much longer leaf life span than species of other life-forms. Conversely, differences in resorption efficiency among life-forms or among plants in habitats differing in soil fertility appear to be small. Inter-specific variation in leaf life span is much larger than intra-specific variation (factor of 200 compared with 2, respectively), while resorption efficiency varies by about the same magnitude at both levels (factor of 3.8 compared with 2.7, respectively). The importance of resorption efficiency in determining leaf-level MRT increases exponentially towards and above the maximum resorption efficiency observed in nature. This effect is independent of leaf life span, which may explain the lack of life-form related differences in resorption efficiency. When scaling up from the leaf to the whole-plant level, fundamental differences in turnover rate among different plant organs must be considered. Woody species invest c. 50% of their net productivity into their low-turnover stems, while in herbaceous species the life span of stems is only slightly longer than that of leaves. As a result, nutrient turnover of woody (evergreen and deciduous) plants is generally lower than that of herbaceous species (herbs and graminoids) on a whole-plant basis. At the intra-specific level empirical data show that both biomass life span (i.e. the inverse of biomass loss rate) and resorption efficiency are important sources of variation in MRT. However, we argue that the relative importance of resorption efficiency in explaining variation in MRT is lower at the interspecific level, whereas the reverse is true for life span. This is because variation in MRT and life span is much larger at the inter-specific level compared with variation in resorption efficiency. Plant traits related to nutrient conservation are discussed with respect to their implications for leaf structure, plant growth, competition, succession and ecosystem nutrient cycling.

The area of forest plantations is increasing worldwide helping to meet timber demand and protect natural forests. However, with global change, monospecific plantations are increasingly vulnerable to abiotic and biotic disturbances. As an adaption measure we need to move to plantations that are more diverse in genotypes, species, and structure, with a design underpinned by science. TreeDivNet, a global network of tree diversity experiments, responds to this need by assessing the advantages and disadvantages of mixed species plantations. The network currently consists of 18 experiments, distributed over 36 sites and five ecoregions. With plantations 1–15 years old, TreeDivNet can already provide relevant data for forest policy and management. In this paper, we highlight some early results on the carbon sequestration and pest resistance potential of more diverse plantations. Finally, suggestions are made for new, innovative experiments in understudied regions to complement the existing network.

SummaryWhile both annual and perennial plants store nitrogen resources during the growing season, seasonal N cycling is a hallmark of the perennial habit. In Populus the vegetative storage proteins BSP, WIN4 and PNI288 all play a role in N storage during active growth, whereas BSP is the major form of reduced N storage during winter dormancy. In this review we explore cellular and molecular events implicated in seasonal N cycling in Populus , as well as environmental cues that modulate both the phenology of seasonal N cycling, and the efficiency and proficiency of autumn N resorption. We highlight recent advances that have been made using Populus genomics resources to address processes germane to seasonal N cycling. Genetic and genecological studies are enabling us to connect our understanding of seasonal N cycling at molecular and cellular levels with that at ecophysiological levels. With the genomics resources and foundational knowledge that are now in place, Populus researchers are poised to build an integrative understanding of seasonal N cycling that spans from genomes to ecosystems.New Phytologist (2005) 167 : 19-30

Through litter decomposition enormous amounts of carbon is emitted to the atmosphere. Numerous large-scale decomposition experiments have been conducted focusing on this fundamental soil process in order to understand the controls on the terrestrial carbon transfer to the atmosphere. However, previous studies were mostly based on site-specific litter and methodologies, adding major uncertainty to syntheses, comparisons and meta-analyses across different experiments and sites. In the TeaComposition initiative, the potential litter decomposition is investigated by using standardized substrates (Rooibos and Green tea) for comparison of litter mass loss at 336 sites (ranging from -9 to +26 °C MAT and from 60 to 3113 mm MAP) across different ecosystems. In this study we tested the effect of climate (temperature and moisture), litter type and land-use on early stage decomposition (3 months) across nine biomes. We show that litter quality was the predominant controlling factor in early stage litter decomposition, which explained about 65% of the variability in litter decomposition at a global scale. The effect of climate, on the other hand, was not litter specific and explained <0.5% of the variation for Green tea and 5% for Rooibos tea, and was of significance only under unfavorable decomposition conditions (i.e. xeric versus mesic environments). When the data were aggregated at the biome scale, climate played a significant role on decomposition of both litter types (explaining 64% of the variation for Green tea and 72% for Rooibos tea). No significant effect of land-use on early stage litter decomposition was noted within the temperate biome. Our results indicate that multiple drivers are affecting early stage litter mass loss with litter quality being dominant. In order to be able to quantify the relative importance of the different drivers over time, long-term studies combined with experimental trials are needed.

The increasing demand for biomass for the production of bioenergy is generating land-use conflicts. These conflicts might be solved through spatial segregation of food/feed and energy producing areas by continuing producing food on established and productive agricultural land while growing dedicated energy crops on so called "surplus" land. Ambiguity in the definition and characterization of surplus land as well as uncertainty in assessments of land availability and of future bioenergy potentials is causing confusion about the prospects and the environmental and socio-economic implications of bioenergy development in those areas. The high level of uncertainty is due to environmental, economic and social constraints not yet taken into account and to the potentials offered by those novel crops and their production methods not being fully exploited. This paper provides a scientific background in support of a reassessment of land available for bioenergy production by clarifying the terminology, identifying constraints and options for ReseARCh ARtiCle BioRisk A peer-reviewed open-access journalJens Dauber et al. / BioRisk 7: 5-50 (2012) 6 an efficient bioenergy-use of surplus land and providing policy recommendations for resolving conflicting land-use demands. A serious approach to factoring in the constraints, combined with creativity in utilizing the options provided, in our opinion, would lead to a more sustainable and efficient development of the bioenergy sector. Unless the sustainability challenge is mastered, the interdependent policy objectives of mitigating climate change, obtaining independence from fossil fuels, feeding and fuelling a growing human world population and maintaining biodiversity and ecosystem services will not be met. Despite the advanced developments of bioenergy, we still see regional solutions for designing and establishing sustainable bioenergy production systems with optimized production resulting in social, economic and ecological benefits. Where bioenergy production has been identified as the most suitable option to overcome the given problems of energy security and climate change mitigation, we need to determine which bioenergy cultivation systems are most suitable for the respective types of surplus land, by taking into account issues such as yields, inputs and costs, as well as potential environmental and socio-economic impacts.

Summary• Growth and nitrogen (N) economy of mountain birch are reported here in response to temperature change. Mechanisms of temperature effects on plant growth in temperate -arctic regions are discussed in the light of decreasing growth rates and increasing leaf-N contents along altitudinal and latitudinal temperature gradients.• Mountain birch ( Betula pubescens ssp. czerepanovii) seedlings were grown at two soil temperatures, air temperatures and nutrient concentrations in a full-factorial experiment during one growing season in northern Sweden.• Changes in air and soil temperature affected aboveground growth more than belowground growth. An increase in air temperature increased leaf area ratio and plant-N productivity while decreasing plant-N concentration and leaf-N content. A change in soil temperature affected root-N uptake rate and plant-N concentration, similar to the effect of a change in nutrient supply. Air and soil temperature had interactive effects on growth rate, N productivity and leaf-N content.• The results indicate that increasing leaf-N content with increasing altitude and latitude is not only a passive consequence of weaker N dilution by reduced growth, but also a physiological acclimation to lower air temperature.

This review summarises the knowledge about the ecology, breeding and management of hybrid aspen (Populus)wettsteinii Hämet-Ahti 0P. 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.