Abstract: Willow (Salix spp.) is among the most promising energy crops to be grown on agricultural land and breeding research to increase biomass yield of this perennial crop is performed in Europe and North America. Biomass willows are grown in short rotation and harvests are performed every 3 to 5 years (i.e., at 3- to 5-year cutting cycles) for a period of up to 25 years. However, breeding programs to improve long-term biomass yield are often relying on the results of short-term screening studies performed on juvenile plants. A pre-requisite for successful breeding of perennial energy crops is thus the identification of relevant juvenile plant traits indicative of long-term plant performance under field conditions. In this study a number of juvenile plant traits, measured at various Salix genotypes grown in a short-term experiment were evaluated in terms of their capacity to predict the long-term performance in biomass production after the first and second cutting cycle. The objective was to develop a simple model linking juvenile plant traits such as shoot biomass, total leaf area and leaf nitrogen (N) concentration to the long-term biomass productivity of field-grown plants. A two-component regression model combining juvenile shoot biomass and leaf N concentration provided the highest prediction accuracy (coefficients of determination around 0.8). The model based on two easy-to-measure juvenile plant traits clearly has implications for willow breeding programs. The implications for breeding are discussed in the light of the possibilities and limitations associated with the chosen approach. The original publication is available at www.springer.com.

Abstract: This study assessed the suitability of two deciduous woody perennials (Salix spp. and Populus spp.) and two summer green herbaceous perennials (Phragmites australis and Urtica dioica) for purification of nutrient enriched wastewater. The main hypothesis tested was that species with a particular trait combination of high relative growth rate (RGR), low nutrient productivity (A) and high mean residence time (MRT) of nutrients would be most effective in accumulating nutrients. The nitrogen and phosphorus use efficiency at the whole plant level was analysed. Four treatments comprising two possible phytoremediation substrates (municipal wastewater and landfill leachate) and two control plant nutrition situations (balanced nutrient solution and pure water) were applied in four replications to the four plant species. Generally, all four species studied showed a high RGR and a low P productivity in the balanced nutrient solution treatment, while the opposite (low RGR and high P productivity) was seen in the phytoremediation substrate and pure water treatments. The general conclusion is that if P is present in marginal proportions in the wastewater, a vegetation filter with Phragmites would have an advantage since biomass and nutrient accumulation in Phragmites does not decrease as much during phytoremediation as that in deciduous woody perennials.

Abstract: Future agricultural research will need to increasingly integrate ecological, physiological and molecular methods, in order to understand agricultural crops in situ and their interaction with the environment as well as organisms impacting on their long-term health and productivity ('agricultural eco-genomics'). The need for integration will increasingly implicate on crop breeding strategies for most agricultural systems. In this paper, implications are highlighted for two contrasting areas of agricultural research related to sustainable crop production: first, the possibilities to utilize crop allelopathic activity to suppress weeds as an alternative to chemical weed control; and second the increasing interest to environmentally friendly and sustainable produce perennial energy crops on agricultural land. 'Sustainability' in agriculture is difficult to define unequivocally, but frequently implies the increased utilization of ecological processes. Breeding strategies towards increased utilization of allelopathic crops require initially the integration and verification of allelopathic processes in various agricultural contexts, because there is currently great uncertainty about the predictable operation of allelopathic activity in different ecological contexts. Breeding programs for future biomass crops, most promising are perennials such as Salix, would greatly benefit from the integration of ecological information affecting long-term productivity, e.g., ecophysiological growth determinants at stand level and the biological control of pests. Agricultural ecogenomics could facilitate a compromise between intensive agriculture and the frequently expressed demand for greater sustainability in agriculture. (c) 2008 Elsevier Ltd. All rights reserved.

Abstract: This study assessed the effects of stand structure and fertilisation with wood ash and/or sludge on wood fuel quality of Salix viminalis. The relative proportions of bark and wood in 1-, 2- and 3-year-old shoot populations were determined. The concentrations of essential elements (N, P, K) and heavy metals (Cu, Zn, Cd, Ni) in bark and wood were used to assess the wood fuel quality in harvestable shoot biomass. Controlled field experiments were conducted on two newly harvested commercial short-rotation willow coppice fields. Five treatments were applied: sewage sludge at the maximum legally permitted amount; ash; two sludge-ash mixtures supplying the maximum and twice the maximum permitted sludge-ash amount; and a control receiving mineral nutrients only. The proportion of bark in the willow stands was decreasing with the age of the shoot population. The shoot population with few large stems, compared to that with many small stems, had a lower proportion of element-rich bark in the harvestable shoot biomass, meaning better quality of the wood fuel. overall, wood fuel quality in terms of mineral concentrations was influenced by the age of the shoot population at harvest, stand structure, management practices (e.g. planting density, fertilisation) and site conditions (soil type, element availability). our results imply that harvestable shoot biomass of willows grown as few large stems have better wood fuel quality, compared to harvestable shoot biomass of many small stems. increased length of cutting cycle improves the wood fuel quality. (c) 2008 Elsevier Ltd. All rights reserved.

Abstract: Specific root length (SRL, m g(-1)) 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 CO2, 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 < 0.5 mm, < 1 mm, < 2 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 < 0.5 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 CO2. We suggest that SRL can be used successfully as an indicator of nutrient availability to trees in experimental conditions.

Abstract: Plant canopy optimization models predict that leaf nitrogen (N) distribution in the canopy will parallel the vertical light gradient, and numerous studies with many species have confirmed this prediction. Further, it is predicted that for a given canopy leaf area, a low vertical light extinction coefficient will promote rapid growth. Therefore, the ideal canopy of fast-growing plants should combine high leaf area index with a low light extinction coefficient; the latter being reflected in a flat vertical leaf N gradient throughout the canopy. Based on data from an experimental Salix stand (six varieties) grown on agricultural land in central Sweden, we tested the hypothesis that shoot growth is correlated with vertical leaf N gradient in canopies of hybrid willows bred for biomass production, which could have implications for Salix breeding. Tree improvement research requires screening of growth related traits in large numbers of plants, but assessment of canopy leaf N gradients by chemical analysis is expensive, time-consuming and destructive. An alternative to analytical methods is to estimate leaf N gradients nondestructively with an optical chlorophyll meter (SPAD method). Here we provide a specific calibration for interpreting SPAD data measured in hybrid willows grown in biomass plantations on fertile agri-Cultural land. Based on SPAD measurements, a significant and inverse relationship (r(2) = 0.88) was found between shoot biomass growth and vertical leaf N gradient across canopies of six Salix varieties.

Abstract: Short-rotation willow (Salix sp.) coppice, a commercial crop grown for energy purposes, is used for phytoremediation of landfill leachate in Sweden. However, the leachate's chemical composition can cause plant damage mainly due to high NaCl and NH4 concentrations. A pot experiment was conducted in order to quantify the growth responses of five different willow clones ("78-183", "Jorr", "Loden", "Olof", "Tora") to irrigation with different leachate mixtures (corresponding to 240, 180, and 120 mg C1 1(-1)) and to assess the applicability of leaf length and leaf fluctuating asymmetry as stress indicators. A series of plant traits (shoot, root and leaf dry weight, leaf area, leaf length and leaf fluctuating asymmetry) were measured. The irrigation with leachate resulted in reduced relative growth rates but there were no clear differences between the different concentrations. The clones "Jorr" and "Loden" performed best in terms of differences in relative growth rate between control and leachate treatments. Leaf length appeared to be a useful stress diagnostic tool for use in situ showing a high correlation to growth, whereas fluctuating asymmetry showed no such correlation. Higher N, lower P and higher Na concentrations in plant stems treated with leachate than control plants were observed. (c) 2005 Elsevier Ltd. All rights reserved.

Abstract: Phenotypic correlations and quantitative trait loci (QTL) for important growth traits and a surrogate of intrinsic water-use efficiency (leaf delta C-13) were analysed in a willow pedigree of 92 full-sibling clones grown under two water regimes. The major objective was to examine the genetic basis of the phenotypic correlations. Cuttings of Salix were glasshouse-grown during one growing season. The relative growth rate (RGR) and underlying traits were assessed. QTL analysis was conducted based on an available linkage map for Salix. Leaf area productivity and leaf nitrogen productivity were more important in determining RGR than leaf area ratio and specific leaf area. However, phenotypic correlations among growth traits partly varied between the two environments. QTL were detected for most growth traits, among them many common QTL for different traits. The QTL pattern reflected the phenotypic correlation pattern. None of the QTL for the complex traits was consistent across the different environments. The results demonstrate a genetic basis for phenotypic correlations among growth traits in Salix, and provide evidence for the existence of 'master switches' regulating some of the traits.

Abstract: Growth, biomass allocation and nutrient economy of eight clones of poplar (Populus balsamifera L., P. trichocarpa Hook.) and hybrid poplar (P. trichocarpa Hook. x P. deltoides Bartr.) of north-American origin were studied in a full-factorial pot experiment in Sweden, using the approach of classical growth analysis. The clones were compared in terms of relative growth rate (RGR), biomass production, biomass allocation, nitrogen accumulation and productivity, and ability to withdraw nitrogen from senescing leaves (leaf nitrogen proficiency). The clones differed significantly among treatments by means of several variables, and for some of them also clone x treatment interaction effects were significant. The components of RGR, i.e., leaf area ratio (LAR) and unit leaf rate (ULR), shifted in their importance to explain differences between treatments and genotypes. The results are discussed in the context of growth characteristics particularly important for the selection of clones for multi-purpose poplar plantations grown under the cool-temperate, high-latitude climatic conditions typical for large parts of Sweden. (C) 2005 Elsevier Ltd. All rights reserved.

Abstract: The use of biomass derived from fast-growing trees grown on agricultural land is common practice in parts of Sweden, but not in most other European countries. In the first part of this paper the different conditions for economic use of fast growing trees are analysed in Germany and Sweden. Neither the legal framework nor the natural growing conditions are likely to limit the utilisation of the biomass potential in Germany. Rather, lack of a market for biomass products, poor awareness and little infrastructure for the management of woody biomass plantations are limitations in Germany. The second part offers a possible solution for supporting the market and reducing transaction costs during establishment of the market under German conditions. The proposed solution involves the establishment of 'ecological structures' based on fast-growing trees as a means to compensate for sealed land by roads and buildings. The proposed concept would not only extend the economic potential for producing energy from biomass, but also allow farmers to generate income. The concept can serve both economic and ecological goals. It can reduce municipal budget funding and support the agricultural industry in European countries which need to implement sustainable biomass concepts. (C) 2004 Elsevier Ltd. All rights reserved.

Abstract: The Masha natural bamboo forest was stocked with 8840 trees ha(-1), unifomily distributed with a mean height of 16.8 m. diameter of 7.6 cm and leaf area index (LAI) of 9.7. Age-structure was 13% of < 1 year. 24% of 1-3 years and 63%, of >3 years. Culm contributed 82%, branch 13% and leaf 5% to the 110 t ha(-1) total above ground biomass, while trees mature for harvest (>3 year) made up 73%. The culm component of the mature trees was 60% of the above ground biomass. whereas the biomass of current shoots (<1 year) constituted only 7%. Annual shoot production was 8 t ha(-1) and contained 44, kg ha(-1) of N. 6 kg ha(-1) of P, 122 ka ha(-1) of K and 1 ka ha(-1) of Ca. Significant variations in nutrient concentration of plant tissues were found among age-classes and between seasons. More than 8 t ha(-1) year(-1) plant litter containing 115 kg N, 8 kg P. 56 kg K and 22 kg Ca returned to the soil surface mainly during the humid season. Resorption efficiency of the foliage was 43% for K, 37%, for P and 19% for N. Soil nutrient concentrations declined sharply with soil depth and a large fraction of the total soil nutrient pool was located in the organic layer; the soil was poor in P and K. Management approaches to improve the production efficiency of the forest and the quality of the harvested biomass are discussed. (C) 2004 Elsevier B.V. All rights reserved.

Abstract: Quantitative trait loci (QTL) for growth traits and water-use efficiency have been identified in two water regimes (normal and drought-treated) and for a treatment index. A tetraploid hybrid F-2 population originating from a cross between a Salix dasyclados clone (SW901290) and a Salix viminalis clone ('Jorunn') was used in the study. The growth response of each individual including both above and below ground dry-matter production (i.e. shoot length, shoot diameter, aboveground and root dry weight, internode length, root dry weight/total dry weight, relative growth rate and leaf nitrogen content) was analysed in a replicated block experiment with two water treatments. A composite interval mapping approach was used to estimate number of QTL, the magnitude of the QTL and their position on genetic linkage maps. QTL specific for each treatment and for the treatment index were found, but QTL common across the treatments and the treatment index were also detected. Each QTL explained from 8% to 29% of the phenotypic variation, depending on trait and treatment. Clusters of QTL for different traits were mapped close to each other at several linkage groups, indicating either a common genetic base or tightly linked QTL. Common QTL identified between treatments and treatment index in the complex trait dry weight can be useful tools in the breeding and selection for drought stress tolerance in Salix.Quantitative trait loci (QTL) for growth traits and water-use efficiency have been identified in two water regimes (normal and drought-treated) and for a treatment index. A tetraploid hybrid F-2 population originating from a cross between a Salix dasyclados clone (SW901290) and a Salix viminalis clone ('Jorunn') was used in the study. The growth response of each individual including both above and below ground dry-matter production (i.e. shoot length, shoot diameter, aboveground and root dry weight, internode length, root dry weight/total dry weight, relative growth rate and leaf nitrogen content) was analysed in a replicated block experiment with two water treatments. A composite interval mapping approach was used to estimate number of QTL, the magnitude of the QTL and their position on genetic linkage maps. QTL specific for each treatment and for the treatment index were found, but QTL common across the treatments and the treatment index were also detected. Each QTL explained from 8% to 29% of the phenotypic variation, depending on trait and treatment. Clusters of QTL for different traits were mapped close to each other at several linkage groups, indicating either a common genetic base or tightly linked QTL. Common QTL identified between treatments and treatment index in the complex trait dry weight can be useful tools in the breeding and selection for drought stress tolerance in Salix.

Abstract:

Abstract: The study reports on the salt tolerance of Acacia tortilis compared to A. nilotica. For the investigation, potted seedlings of both species were exposed to three levels of salt treatments (0, 150 and 300 mm NaCl) in a 1-month greenhouse experiment. In terms of biomass growth, both acacias responded similarly to salinity (i.e. insignificant species x treatment interaction) even though A. nilotica appears to be generally more productive than A. tortilis (i.e. significant effect of species). In terms of shoot water status, there was significant variation in the response to increased NaCl salinity between the two acacias. Furthermore, both acacias exposed to salt treatment shed their leaves, although at different time during the experiment. This suggests that both acacias are sensitive to the salt treatment applied here. Further screening tests involving various ssp. and genotypes of both species might be promising to find suitable trees for the afforestation on salt-affected soils in and and semi-arid Africa. (c) 2005 Elsevier Ltd. All rights reserved.

Abstract: Pot experiments are often performed to assess plant physiological traits and relationships among growth traits under controlled environments. However, the reliability of pot studies for predicting the growth and performance of trees in the field has rarely been rigorously assessed. We evaluated the suitability of pot experiments for predicting field performance, measured as shoot biomass production, by investigating determinants of growth in hybrid willows (Salix spp.) grown under various environmental conditions in the field, and by comparing the data with the results from a corresponding pot Study. Biomass production in six hybrid willow clones, bred for use as bio-fuels, was assessed in three field trials located in central and southeastern Sweden throughout the first 3-year cutting cycle. The determinants of biomass productivity, measured as biomass allocation and nitrogen (N) economy, were identified in one of the field trials. Key traits for shoot biomass production in the field were total leaf area and total amount of N; plant N losses by shed leaves were only partly controlled by leaf-litter N concentration. These key traits were also obtained from the pot study and related to shoot biomass production and abscission-leaf N loss in the field. Total leaf area and total N pool of plants grown in pot experiments were good predictors of long-term biomass production in the field, whereas shoot biomass production, specific leaf area and tissue N concentration of pot-grown plants were less suitable as predictors of field performance. Relationships between the key traits and shoot biomass production were clone-specific, indicating the need for analysis of growth traits at the clone level if field performance of trees is to be evaluated based on data from pot studies. Nutrient loss components are important for tree performance in the long term and evaluations of nutrient loss characteristics at the individual-tree level should focus on nutrient pools lost rather than on nutrient concentrations in abscised plant parts.

Abstract: While 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.

Abstract: Variation in in situ growth performance of the mountain birch as indicated by the widths of annual rings was analysed and related mainly to temperature and herbivory using ring width series from five heath forest sites in the Lake Tornetrask area, northern Sweden. Climate explained 48-64% of the variation in age-corrected mean ring width series. In general, the effect of current year July followed by June temperature was most important at all sites. A warm May resulted in wider rings due to an earlier budburst. Short-term (inter-annual) responses to increased temperature were in most cases not reflected into long-term responses (decades). A large proportion of the variation in stem mean ring width was due to variation among stems within trees (81%) in these polycormic trees, while variation among sites was marginal (0.4%). Within trees, main stems grew faster and were more responsive to climate variation than subordinate stems. No effect of insect herbivory on ring width was found at low defoliation levels (less than or equal to12%). At a defoliation level of ca 84% a one-year reduction in stem growth was observed while the growth reduction (ca 50% reduction in ring width) lasted for 4 yr after ca 93% defoliation. After outbreaks resulting in complete defoliation and some stem mortality, ring widths of surviving stems mainly responded with increased growth. Basal sprouts, emerging just after a severe insect outbreak with a high mortality of old stems, grew faster than sprouts occurring during other periods. It is concluded that the mountain birch is well adapted to recover from Epirrita outbreaks; the ability to produce basal sprouts, that can benefit from an existing root system for fast initial growth, is one important mechanism for this.

Abstract: Short rotation forestry (SRF) is regarded as a silvicultural practice employing high-density plantations of fast-growing tree species on fertile land with a rotation period of fewer than 10-12 years. I address the challenges and possibilities of SRF applications under the circumstances of a boreal climate, today as well as after anticipated climate change. The implications of a pronounced winter season for the performance of biomass crops are discussed. Poplars, aspens, and willows are superior in boreal SRF because of their fast growth rate in combination with good cold hardiness. These trees can enrich the coniferous forests of boreal regions and increase biodiversity in open agricultural landscapes of the boreal zone. Further, SRF plantations can serve as tools for the amelioration of environmental problems at local (e.g., phytoremediation) and global (e.g., increased greenhouse effect) scales. The biomass yields achieved in boreal SRF and the appropriate production systems appear do not appear to be principally different from warmer regions, but there are some differences with respect to the importance of fertilization, appropriate spacing, and rotation length. The major barriers for a rapid development of SRF appear not to be climatic, technical, or environmental constraints in many boreal regions.

Abstract: In a greenhouse experiment we examined the effect of willow genotype and irrigation regime (moderate drought and well-watered) on plant growth parameters, foliar nitrogen, and phenolic concentrations, as well as on the preference and performance of the blue leaf beetle, Phratora vulgatissima (L.) (Coleoptera: Chrysomelidae). The 10 vegetatively propagated willow genotypes in the experiments were F2 full-sibling hybrids, originated from a cross between Salix viminalis (L.) (Salicaceae) (high in condensed tannins) and Salix dasyclados (L.) (Salicaceae) (rich in phenolic glycosides). Insect bioassays were conducted on detached leaves in Petri dishes as well as with free-living insects on intact potted plants. The 10-week long irrigation treatments caused statistically significant phenotypic differences in the potted willow saplings. Total biomass was somewhat higher in the well-watered treatment. The root to total biomass ratio was higher in the drought-treatment plants. There was significant genotypic variation in foliar nitrogen concentrations, and they were higher in the drought-treatment plants. There was also a strong genotypic variation in each of the phenolic substances analyzed. Condensed tannins, which accounted for the greatest proportion of total phenolic mass, were higher in the well-watered treatment. There was, however, no difference in levels of the other phenolics (salicylates, cinnamic acid, flavonoids, and chlorogenic acid) between irrigation treatments. The sum of these phenolics was higher in the well-watered treatment. There was a strong variation in P. vulgatissima larval development on different willow genotypes, and larval performance was negatively correlated with levels of salicylates and cinnamic acid. There was, however, no effect of irrigation treatment on larval performance. Phratora vulgatissima preferred to feed on well-watered plants, and we found a preference for oviposition there, but neither feeding nor oviposition site preference was affected by willow genotype. Adult feeding and oviposition preferences were not correlated with larval performance.

Abstract: Floristic diversity in 21 small short-rotation poplar plantations, 0.1 to 13 ha in size and aged 6 to 14 yr, was compared with adjacent agricultural fields of various crop types. The poplars were grown on former agricultural land and most of the study sites were located in southern and central Sweden. Surveys of ground vegetation were carried out by using nested quadrats (plots of 0.25 and 0.016 m(2) size) located along normally five transects running perpendicular to the habitat boundaries. Mean cumulative species number across all sites (all transects and plots included) was around 20 and similar in poplar stands and arable fields. However, the two habitat types shared only 3 to 12 species (mean: 8 species). Mean species number per survey plot was 4.5 in poplar stands compared to 5.3 in arable fields. Classification (TWINSPAN) and Detrended Correspondence Analysis (DCA) showed some floristic separation of the two habitat types. The results were interpreted in a landscape context. Mainly based on the low quantity of species shared by poplar stands and adjacent arable fields, the results support the hypothesis that small-scale poplar plantations increase floristic diversity in landscapes dominated by agriculture. Compared to old-growth mixed deciduous forests in different regions of Sweden, species richness of poplar plantations was similar or lower.

Abstract:

Abstract: Seedling emergence and subsequent survival and growth are vital for natural forest restoration or plantation establishment by means of seeds. Such information is lacking for the African bamboo species. Two experiments were carried out in a greenhouse to evaluate the influence of seed orientation and sowing depth of the lowland bamboo Oxytenanthera abyssinica on seedling emergence, survival and growth. A randomised complete block design was used. Seedling emergence in the seed orientation experiment followed the order embryo-end-up > lay-flat > embryo-end-down. Survival rate after 62 days decreased in the order lay-flat > embryo-end-down > embryo-end-up. Mean seedling height and number of leaves per seedling followed a similar pattern. Seeds sown on top of the soil surface and at 2.5 mm depth achieved faster and higher seedling emergence than those sown at 5 and 10 mm depths. However, mean seedling height and number of leaves per seedling were higher in 5 and 2.5 mm depths than surface and 10 mm depths. There were significant quadratic relationships between sowing depth and seedling height (p = 0.034) as well as number of leaves per seedling (p = 0.032), both peaking around 5 mm soil depth. Lay-flat orientation, which is the most frequent position in broadcast sowing, is recommended at 5 mm sowing depth for the lowland bamboo based on overall performance in seedling emergence, survival and growth. (C) 2002 Elsevier Science Ltd. All rights reserved.

Abstract: 1. The pattern of responses of adult trees of Mountain Birch (Betula pubescens ssp. czerepanovii) to an outbreak of a folivorous insect (Epirrita autumnata) causing approximate to90% defoliation was studied over 8 years in a subarctic area in northern Sweden. 2. Tree recovery was monitored in terms of the numbers, area, mass and nitrogen content of leaves, the numbers of leaf-carrying long and short shoots, and the widths of annual tree rings in stems. 3. The most prominent characteristic of defoliated trees was a threefold increase in the proportion of long shoots 1 year after defoliation. These shoots subsequently produced many new leaf-carrying short shoots, resulting in shoot populations approximate to50% larger than those of control trees. Thereafter the most strongly defoliated trees showed decreasing leaf area. 4. Defoliated trees produced more female catkins but fewer male catkins than control trees. 5. Released apical dominance did not result in any overcompensation in terms of growth or leaf area production. However, it presumably increased the rate of recovery from herbivory. For the most strongly defoliated trees, the benefits from increased production of long shoots was not sustainable.

Abstract: Although poplars are widely grown in short-rotation forestry in many countries, little is known about poplar growth performance in Sweden. In this study, above-ground biomass production was estimated for several hybrid aspen and poplar clones planted at different initial density at five locations across Sweden. Biomass assessments were based on allometric relationships between total above-ground woody dry weight and the diameter at breast height. According to a common harvest practice, tree biomass was partitioned into pulpwood and biomass for energy purposes. The percentage of pulpwood was strongly determined by clone for DBH >10 cm. The mean annual increment ranged from 3.3 Mg ha(-1) yr 1 for balsam poplar in the north to 9.2 Mg ha(-1) yr(-1) for 9-yr-old 'Boelare' in southern Sweden. At the same age, hybrid aspen reached 7.9 Mg ha(-1) yr(-1) . The results suggest that poplars and hybrid aspen are superior as biomass producers compared with tree species commonly grown on agricultural land at these latitudes. The results are discussed in the light of future wood supply for pulpwood and energy purposes in Sweden.

Abstract: Effects of different winter soil temperature on summertime growth rate of individual seedlings of mountain birch (Betula pubescens ssp. czerepanovii) were evaluated in an outdoor pot experiment in subarctic Sweden. The hypothesis tested was that low winter soil temperature could decrease growing-season nutrient uptake capacity and growth rate by injury of the root system; further, the root damage should stimulate growth of replacement roots, which should be reflected by increased summertime biomass allocation to roots. Mountain birch seedlings were exposed to two soil temperatures during wintertime by manipulation of snow depth that accumulated on top of the pots. Minimum soil temperature (hourly recordings) during winter was -1.7degreesC in the "protected" treatment (deep snow cover) and -10.5degreesC in the "exposed" treatment (thin snow cover). The treatments simulated the soil temperature conditions of a local mountain birch forest and heath site, respectively. During June and July, the relative growth rate (R.) and nitrogen accumulation rate (RN) in the exposed treatment were lower compared to the protected treatment. In addition, the seedlings of the exposed treatment allocated a greater proportion of biomass to roots compared to the protected treatment. Thus, leaf growth was reduced in the exposed compared to the protected treatment. We concluded that lower winter temperature in soils can result in stronger root injury that stimulates the growth of replacement roots and reduces nutrient uptake capacity and growth rate during summer. In concert with other possible factors causing root disruption during winter, e.g., frequent freeze-thaw cycles, the mechanism identified here might be important for survival of tree seedlings in subarctic and alpine areas, especially near treelines. The implications are discussed specifically for mountain birch seedling growth and survival in treeless subarctic heath sites and above treelines in northern Europe.

Abstract: Fourteen clones of willow (Salix spp.) were characterised in terms of growth, nitrogen and water-use efficiency under different irrigation and fertilisation treatments. Cuttings of willow clones, some commercially introduced and others new material, were pot-grown outdoors in Central Sweden under four experimental treatments in a full-factorial design. The experiment covered the period from bud-break until leaf abscission and the experimental conditions included two irrigation and two fertilisation regimes. The growth of the clones was evaluated in terms of relative growth rate and total biomass production of whole plants and shoots. Nitrogen (N) economy was studied by means of N productivity, N accumulation and N losses by leaf abscission. Water economy was analysed with respect to intrinsic water-use efficiency (foliar carbon isotope ratio; delta(13)C) and the capacity of leaves to retain water (relative water content). Significant differences between clones were found in nearly all parameters measured and the clones varied in the responses to the experimental treatments (clone x factor interaction effects). Thus, clone ranking often changed depending on the experimental treatment. The results are discussed with respect to clone selection for different willow applications such as biomass production and phytoremediation, and willow growth performance under different water and nutrient availabilities. The growth-physiological characterisation of young willows in the short term (several months) is regarded as a suitable approach for pre-selection of promising clones prior to extensive field evaluation. (C) 2002 Elsevier Science Ltd. All rights reserved.

Abstract: The effects of nitrogen fertilization (100, 200 kg N ha(-1) per year) and soil properties on mycorrhizal formation on Salix viminalis were investigated at three short rotation plantations on Gleysols and Cambisols (Abbachhof (ABB) and Wildeshausen (WIL) in Germany. Ultuna (ULT) in Sweden). During 3 years the ectomycorrhizal colonization, the composition of ectomycorrhizal morphotypes and the VAM spore density in the soil were analyzed. The ectomycorrhizal colonization was significantly altered due to N-fertilization at all sites. The quality and magnitude of the fertilization effects on mycorrhizal formation on Salix viminalis varied due to the soil properties, i.e. soil texture, soil N content and pH. The WIL site was characterized by sandy soil (low pH. high soil N content), whereas the ABB site was characterized by clayey soil (high pH, low N content). The ULT site was characterized by clayey soil (high pH. high N content). In the unfertilized control plots (C), ectomycorrhizal colonization was higher at WIL than at ABB. Fertilization reduced the ectomycorrhizal colonization at WIL but increased it at ABB. The distribution of the ectomycorrhizal morphotypes was very heterogeneous within the treatments, therefore significant differences were rare. Sporocarps were collected at ABB during one growing period. The sporocarps were mostly from saprophytic species, with exception of the ectomycorrhizal species Inocybe glabripes. Significant effects of N-fertilization on VAM spore density were observed at two of the three plantations. The pattern in VAM spore density was similar to the pattern seen for ectomycorrhizal colonization. Thus, VAM spore density was increased by fertilization at ABB (low soil N) and decreased at ULT (high soil N). The soil properties have been shown to modify the effects of fertilization on ectomycorrhizal colonization and VAM spore density. Therefore, in management practice of short rotation plantations, the benefit of N-fertilization should be evaluated keeping secondary effects caused by changed mycorrhizal formations in mind, (C) 2002 Elsevier Science B.V. All rights reserved.

Abstract: This paper (1) describes soil temperature conditions in an area close to the altitudinal and latitudinal distribution Limit of the mountain birch, and (2) evaluates whether soil temperature is likely to impose a limit on nitrogen uptake and subsequent reductions in growth and survival. Soil temperature was monitored at two depths (-5 and -10 cm) during 3 yr in forested and nearby nonforested sites both at low altitude (ca. 400 m) and at forest line (ca. 650 m) in the Tornetrask area, northern Swedish Lapland (68 degrees 21'N, 18 degrees 49'E). The mean summer soil temperature was ca. +6 to +7 degreesC and +5 to +7 degreesC at -5 and -10 cm, respectively. Forest-line soils were on average 0.8 degreesC warmer than valley soils. We found no statistically significant differences in soil temperature between sites colonized by mountain birch and nearby nonforested habitats. Given present soil temperatures, there is thus no reason why (mature) trees should not survive also on alpine and subalpine heaths currently not colonized by birch. However, relating these temperature recordings to experimentally determined relationships between soil temperature and seedling nitrogen (N) uptake, and between N uptake and survival, indicates that the mountain birch seedlings are unlikely to survive their first winter under soil temperature conditions normally prevailing in this area. Our analysis suggests that successful seedling establishment can be expected on disturbed sites when the growing season (15 June to 15 August) mean soil temperature is ca. 12 degreesC or higher. In comparison, growing season soil temperature under undisturbed vegetation (at -5 cm) was on average ca 8.2 degreesC during our 3 yr of soil temperature measurements. Air temperature during the same period was 1.1 degreesC warmer than the average for the last 86 yr (1913-1998, 11.0 versus 9.9 degreesC).

Abstract: The hypothesis that fast-growing breeds of willow (Salix spp.) are more sensitive to nutrient and water stress and less efficient in nutrient- and water-use than slower-growing natural willow clones was tested. Cuttings of a natural clone of S. viminalis L. collected in Sweden (L78183) and a hybrid clone of S. schwerinii E. Wolf. x S. viminalis L. ("Tora") were grown outdoors in pots under various experimental conditions in a full-factorial design. The experimental conditions included three fertilization, two irrigation and two temperature regimes. Classical growth analysis techniques, based on an initial and a final harvest, were used as a screening method, together with calculation of intrinsic water-use efficiency (foliar carbon isotope ratio; delta C-13). In addition, nitrogen-use efficiency was calculated as the product of nitrogen productivity and mean residence time of nitrogen on an annual basis. There were significant differences in plant structural parameters (leaf area ratio, specific leaf area) and water-use efficiency between the clones. Furthermore, several clone x treatment interaction effects on various growth parameters indicated that the clones adapted to specific environments in different ways. "Tora" plants produced up to 25% more shoot biomass than plants of the natural clone in response to high rates of fertilization and irrigation, whereas clone ranking was reversed in most other treatments. The results support the hypothesis that fast-growing hybrids are more sensitive to nutrient and water stress than slower-growing natural clones. The hypothesis that natural clones have higher resource-use efficiency than fast-growing hybrids was supported with respect to water, but not nitrogen.

Abstract: 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.

Abstract: Plants can be expected to utilize different sources of nitrogen with different proportions of N-15 at different times of the year. We expected this to be reflected in a seasonal variation in the natural abundance of plant N-15, and that this pattern would vary among life-forms or species. To test this hypothesis, we studied the delta(15)N of eight different life-forms, selecting two representatives from each of four categories (woody deciduous, woody evergreen, graminoid, and cryptogam life-forms) at two locations having different levels of precipitation, over a six-month period. Sampling was conducted in mid-winter, during snowmelt in May, after leaf emergence, in mid-August, and in September. The sampled species showed a highly significant seasonal pattern in the natural abundance of N-15. Within each species and site, the delta(15)N showed a difference on average of 3.6% (range from 2.1 to 5.3%) between minimum and maximum over the sampling period. In most cases delta(15)N was highest in mid-winter and lowest at start: of the growing season. Most species studied showed some common trends: (i) a decline in delta(15)N from mid-winter to pre-snowmelt (May); (ii) an increase from snowmelt to mid-June (mainly in plants sampled at one site); and (iii) a late-season decline in delta(15)N (August to September). Life-forms differed from each other in term of their pattern of seasonal variation (harvest x life-form interaction) and between sites (sire x life-form interaction). Thus, the outcome of comparisons of natural delta(15)N within and among species or sires depends on the time of year of sampling.

Abstract: The mountain birch (Betula pubescens ssp. czerepanovii) is considered to have originated through introgressive hybridization between B. pubescens and B. nana. It is intermediate between the putative parent species in terms of growth Form and distribution. Consequently, we hypothesized that the mountain birch should have growth characteristics intermediate between the other two birch forms. This hypothesis was tested in an experiment using first-year seedlings. Only in three out of 15 characteristics studied were mountain birch characteristics clearly intermediate between B. pubescens and B. nana. In some casts the mountain birch was most similar to B. pubescens, while in others it resembled B. nana most closely. In certain other respects, B. pubescens and B. nana were more similar to each other than to mountain birch. In three measures of plant productivity, i.e., relative growth rate, leaf area productivity, and plant nitrogen productivity, mountain birch showed the highest values. Cluster analyses of thirteen growth-related characteristics indicate that at a low fertilizer supply, B. pubescens and B. nana are more similar to each other than to the mountain birch. At a high fertilizer supply, mountain birch was more similar to B. pubescens. The results indicate that the growth characteristics of mountain birch seedlings are not inherited from its two "parent" species in any simple way.

Abstract: Seedlings of mountain birch (Betula pubescens ssp. czerepanovii), a subarctic tree, mainly survive and establish in early-successional patches with low vegetation cover. In particular, during the first years after seed germination, a rapid seedling growth rate is important for winter survival. Seedling growth rate is controlled by plant nitrogen (N) concentration. On a year-round perspective, the N concentration is influenced by N uptake rate during both summer and winter and by N loss during autumn. The aim of the present study was to evaluate the effects of autumn N loss and winter N uptake for seedling growth during summer. The study used young seedlings growing in situ in northern Sweden. Since the growth rate of whole plants cannot be measured in situ, it was estimated using a simple, empirical seedling growth model. The model was based on data from controlled experiments and validated using growth data from a field study. The field study included sequential seedling harvests which were carried out at two sites differing in altitude, from autumn 1994 until autumn 1996. The seedling growth model was used to simulate the effects on growth rate of autumn N losses and winter N uptake. It was found that a decrease in the amount of N lost in autumn and an increase in the amount of N taken up during winter could enhance the growth rate of mountain birch seedlings by the same order of magnitude as an increase in growing season soil temperature by 1 to 2 K.

Abstract: 1. The effects of previous-year environment on current growth response were tested in seedlings of Mountain Birch (Betula pubescens Ehrh. ssp. czerepanovii [Orlova] Hamet-Ahti), a subarctic tree species with indeterminate shoot growth. 2. Mountain Birch seedlings were pot-grown outdoors in subarctic Sweden for 2 years and showed a clear delay in growth response when fertilization and temperature were reduced after the first year. The seedlings were grown under four experimental treatments (two temperatures and two nutrient availabilities) in 1994, and under low-temperature/low-nutrient conditions in 1995. 3. When nutrient supply and/or temperature was reduced in 1995 compared to 1994, the seedlings maintained the high relative growth rates (RGR) of the previous growing season, although the internal plant N accumulation rate was lower than in 1994. This resulted in decreasing plant N concentration (PNC), and a poor relationship between RGR and PNC during 1995. The high RGR in 1995 was achieved in response to phenotypic adjustments (e.g. number of foliar buds) to a more favourable environment in the past, and by dilution of the internal nutrient storage. 4. The effects of delayed responses found in this study indicate problems for the interpretation of results from growth studies performed under any climate with great year-to-year variability, such as the subarctic, because a delay in growth response could distort the relationships between plant growth, resource availability and climate. Predictions of current growth are therefore meaningless if the previous history of the plants is not taken into account.

Abstract: Objectives-Organic acid anhydrides are potential sensitisers and cause occupational airway diseases. In an intervention study the efficacy of measures of hygiene at the workplace and possible selection bias were investigated. Methods-A first investigation with 110 workers exposed to hexahydrophthalic acid anhydride (HHPA) and methyltetrahydrophthalic acid anhydride (MTHPA) was carried out in July 1991. The results (skin prick test, specific serum IgE) showed that 20 people were sensitised, and in a challenge test the clinical relevance of the sensitisation was confirmed in six subjects. In December 1991, the hygiene conditions at the plant were improved. In November 1995 a second investigation of 84 people was performed (anamnesis, skin prick test, specific IgE, spirometry, and ambient and biological monitoring). The 27 people who had left the plant in the meantime were asked their reasons for leaving. Results-The relative risk of people sensitised in 1991 of leaving the plant between 1991 and 1995 was 2.6 (95% confidence interval (95% CI) 1.4 to 4.9) compared with people without any sign of sensitisation. The percentage of people identified as sensitised in 1991, who were still working at the plant and came to the second investigation, was higher than for people without evidence of sensitisation (10/10 v 47/73; p<0.05). In all the 10 sensitised people in 1991 the findings of the first investigation were confirmed in 1995. The rate of sensitisation in 1995 was 21%. None of the six people employed after 1991 showed evidence of sensitisation. Of the six people with clinically relevant sensitisation confirmed by a challenge test in 1991, five were still at their workplace. From 1991 they were only exposed to MTHPA at a reduced concentration (<0.5-36 mu g/m(3) in 1995). All of them reported fewer symptoms than in 1991. No signs of bronchial obstruction were detected by spirometry at the workplace. Conclusions-In cross sectional studies there is a selection bias with a risk of underestimating the incidence of allergic diseases. The results further suggest that the improved hygiene conditions probably had a positive effect on the symptoms in sensitised people.

Abstract: High- and low-altitude ecotypes of mountain birch (Betula pubescens ssp, czerepanovii) showed clear differences in their responses to various experimental conditions, including two temperature regimes and four fertilisation rates. There was, however, no simple way to characterise the elevational ecotypes in terms of relative growth rate, nitrogen (N) productivity or root N uptake rate. The leaf N concentration was generally higher in the high-altitude seedlings than in the low-altitude seedlings. At low temperature, high-altitude mountain birch maintained a relatively high growth rate by combining high root N uptake rate and high leaf N concentration with high N productivity. An increase in temperature and/or fertiliser rate resulted in a marginal increase in N productivity in the high-altitude seedlings but resulted in a strong increase in N productivity in the low-altitude seedlings. In parallel, increased temperature resulted in a pronounced decrease in leaf N concentration only in the low-altitude seedlings. Our results suggest that the weak growth response to increased temperature in high-altitude mountain birch is functionally related to high leaf N concentration. The high leaf N concentration of high-altitude mountain birch is genetically determined and has an adaptive value in a cold environment, This suggests that there is a trade-off between high N productivity at low temperature and a strong response of N productivity to temperature.

Abstract: 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.

Abstract: 1 Seedlings of mountain birch Betula pubescens ssp. czerepanovii were grown outdoors, under different environmental conditions, during their first growing season at Abisko, northern Sweden. Winter survival of seedlings was studied in relation to their growth and nitrogen (N) acquisition rate during the previous growing season. 2 Effects of fertilization and soil temperature on seedling growth and N acquisition were analysed in a factorial experiment including seven fertilizer levels and two temperature treatments. Effects of shading and neighbours (B. p. ssp. czerepanovii and Empetrum hermaphroditum) on seedling growth and N economy were evaluated in another experiment including five different neighbour interaction treatments. 3 An increase in either soil temperature or fertilization rate caused the birch seedlings to take up more N and grow faster. The relative growth rate and rate of N accumulation during the 12-week growing season were closely related to winter survival: at a relative growth rate of 2.0% and 3.8% day(-1), the winter survival of mountain birch seedlings was estimated to be 5% and 95%, respectively. This range corresponded to a relative N accumulation rate between 2.4% and 4.3% day(-1) 4 The relative N accumulation rate was clearly reduced by shading and by the plant-plant interactions studied. The effects of shading and birch neighbours but not of Empetrum on the rate of N accumulation could be explained by lower soil temperature. 5 Nutrient supply, soil temperature, vegetation shade and, presumably, allelopathy affect the N acquisition of first-year mountain birch seedlings, and thus also influence their winter survival. Soil temperature might be the major influence on the survival rate, due to its strong influence on the root N uptake rate.

Abstract: Growth patterns and nitrogen economy were studied in pot-grown seedlings of mountain birch subjected to different ultraviolet radiation under both laboratory and outdoor conditions at Abisko in northern Sweden. In the laboratory, nutrient supply, temperature, humidity, ultraviolet radiation-A (UV-A, 320-400 nm) and B (UV-B, 280-320 nm) were controlled, while photosynthetically active radiation (PAR, 400-700 nm) and photoperiod varied naturally. Under outdoor conditions nutrient supply was controlled, and the irradiation treatments were ambient and above-ambient W-B using additional fluorescent lamps. Mountain birch nitrogen economy was affected by increased ultraviolet radiation, as reflected by a changed relationship between plant growth and plant nitrogen both in the laboratory and outdoors. In the laboratory enhanced UV-A decreased leaf area per unit plant biomass (leaf area ratio) but increased biomass productivity, both per unit leaf area (leaf area productivity) and per unit leaf nitrogen (leaf nitrogen productivity). Low levels of W-B affected growth patterns and nitrogen economy in a similar way to enhanced W-A. High levels of W-B clearly decreased relative growth rate and nitrogen productivity, as leaf area ratio, leaf area productivity and leaf nitrogen productivity were all decreased. Under outdoor conditions above-ambient levels of W-B did;not alter growth or biomass allocation traits of the seedlings, whilst nitrogen productivity was increased. Mountain birch seedlings originating from different mother trees varied significantly in their responses to different ultraviolet radiation.

Abstract: The seasonal course of the availability of nitrogen (ammonium and nitrate) and phosphate in soils of some mountain birch woods in subarctic Sweden was measured in situ by using ion exchange membranes. One meadow system and two heath systems (differing in moisture availability) were selected for the study, and ion exchange membranes were buried in soils for between 4 and 37 wk during 1 yr. Annual nitrogen availability increased in the order dry heath < moist heath < meadow system, while phosphate availability was lowest in the meadow system. There were clear seasonal patterns of nitrogen and phosphorus availabilities, with a maximum during the, early (nitrogen) and late growing season (phosphorus) and a minimum during the winter season. Nitrate was detected in the meadow and in the moist heath system. The relative availabilities of nitrogen and phosphorus varied considerably among the study sites. Early-season nitrogen availability corresponded well with atmospheric nitrogen input by snow deposition during the winter season. Soil hydrology was concluded to exert stronger influence on nutrient availability than soil temperature in these woodlands. Nitrogen uptake rates in experimentally grown seedlings of mountain birch usually exceeded adsorption rates to membranes, whilst seedling phosphorus uptake was estimated to be similar to membrane phosphate adsorption.

Abstract:

Abstract: Five evergreen and four deciduous species of woody plants were experimentally defoliated to address the question of whether the relative importance of resource resorption from leaves to support new growth varies between these life-forms or whether the species show individualistic response patterns. The deciduous and evergreen species were defoliated before leaf senescence in autumn and before resource mobilisation in early summer, respectively. Responses were measured in terms of growth emerging the season following defoliation. Averaged across all species, defoliation had general effects on growth and nutrient status of new shoots. Shoot, leaf and stem weights, leaf area, leaf dry weight per unit area, shoot dry weight per unit phosphorus (P) and shoot nitrogen (N) and P pool sizes were reduced in defoliated shoots. Shoot P concentrations in defoliated plants were increased in comparison with controls, leaf area per unit shoot dry weight increased, particularly in evergreens. Leaf number, leaf N and P per unit leaf area, shoot N concentrations and shoot dry weight per unit N were not significantly affected by the treatment but some of these parameters showed large variation among species. However, there were no significant life-form specific responses of the parameters measured. In contrast, marked differences were found among species in their response to defoliation. The resorption of dry matter, N and P from old leaves to support new growth appears to be equally important in deciduous and evergreen woody species despite morphological, physiological and phenological differences between these life-forms. The magnitude of growth depression in response to defoliation across all species was not correlated to the amount of dry matter or nutrients removed, to the species' resorption efficiency or to the potential benefit of resorption in terms of future growth. Rather the species responded to defoliation in an individualistic way.

Abstract: Intra-specific relationships between growth traits and nitrogen economy were studied for seedlings of mountain birch (Betula pubescens ssp. totuosa) originating from three climatically different regions within the northern forest margin in subarctic Fennoscandia. The experiment was carried out using potted seedlings grown in Abisko, northern Sweden, through two complete growing seasons. The seedlings were grown in peat and assigned to two different temperature and fertilization treatments according to a factorial design. The treatments were ambient and ca. 2.5 degrees C elevated temperature (passive greenhouse), and soil nutrient fertilization corresponding to 1 and 10 g nitrogen m(-2) season(-1). Significant differences among provenances were found In almost all growth and nitrogen economy traits studied The differences in growth rate, which reached 47%, were explained primarily by leaf weight ratio (biomass allocation to leaves) and secondly by leaf area productivity (biomass production per unit leaf area). Both the nitrogen productivity and residence time of nitrogen (with respect to aboveground nitrogen losses) varied between provenances: no significant relationship was found between these two traits. Differences in residence time of nitrogen were more related to biomass loss ratio (leaf biomass losses per unit total plant biomass production) than to nitrogen concentration of abscised leaves. The intra-specific variation in nitrogen economy between mountain birch provenances is discussed with respect to the possibility of significant genotype x environment interaction.

Abstract: Patterns of growth and nitrogen utilization in seedlings of mountain birch (Betula pubescens ssp, tortuosa) were monitored by growth analysis over two growing seasons in Abisko, northern Sweden. The seedlings were grown in pots with pear, which were placed outdoors under a clear roof (natural light and temperature) or in a greenhouse (natural light and increased temperature). The plants were exposed to two nutrient (1 and 10 g nitrogen m(-2) season(-1)) and two temperature regimes (ambient and ca 2.5 degrees C above ambient) in a factorial design. The climatic conditions were rather different during the two years of the study, which clearly affected the growth and nitrogen economy of the birch seedlings. Nutrient supply was the main growth-limiting factor during 1992, which resulted in high root growth and a high leaf area per unit plant biomass (leaf area ratio). In contrast, during 1993 temperature was the main growth-limiting factor, which resulted in high leaf growth and a high biomass production per unit leaf area (leaf area productivity). Relative growth rate was affected by temperature early in the season (early to late June), and by plant nitrogen concentration during mid-season (late June to mid August). There were indications of substantial nutrient uptake by the birch seedlings during the winter season (September to May). The inter-annual variations in growth and nitrogen economy are interpreted as consequences of physiological adjustments to the different climatic conditions during the two growing seasons studied. The role of phenotypic plasticity in determining plant performance in a fluctuating environment is discussed, particularly in relation to the variable environmental conditions typical of the natural range of the mountain birch in subarctic Fennoscandia.