Composition, Stand Structure, and Biomass Estimates of “Willow Rings” on the Canadian Prairies

Many small wetlands are scattered across the Canadian Prairies. These wetlands are often surrounded by a ring of phreatophytic shrubs, of which willows are the dominant genus. Even though many of these willow rings are currently degraded, due to the lack of disturbance factors such as prairie wild fires and bison herds, they are still of great value as a biomass resource without carbon debt. Our study had the objectives to (1) study the distribution of five native willows (Salix bebbiana, Salix discolor, Salix eriocephala, Salix interior, and Salix petiolaris) in 12 willow wetland communities in relation to a moisture gradient and (2)examine the effects of age of the willow ring on biomass potential, stand structure, and species distribution. Annual biomass production ranged from 1.9 to 16.2 odt ha^?1 year^?1 for the 12 sites that were between 9 and 34 years old. Current standing biomass for the 12 willow rings with a total area of 1.5 ha was estimated at 239.2 odt. With age, stand structure changed from multiple small-diameter stems to fewer large-diameter stems, and at the same time the species distribution shifted from one dominated by S. petiolaris to one being dominated by S. discolor. Additionally, the sympatric willows showed a distribution along a moisture gradient, with S. eriocephala, S. interior, and S. petiolaris in moist locations and S. bebbiana and S. discolor in drier locations.     

Establishment and Early Growth of Willow at Different Levels of Weed Competition and Nitrogen Fertilization

To evaluate the effects of weed competition and nitrogen fertilization on the early growth performance of willow, cuttings of the clone Tora (Salix schwerinii x S. viminalis) were planted in buckets together with model weeds (spring barley or white mustard) sown 15, 26, and 30 days after willow planting. The buckets were fertilized with 30 or 90 kg N ha^?1. Willow with weeds sown after 15 days produced less biomass and smaller leaf area and had a lower maximum shoot height compared to willow planted without weeds and willow with weeds sown after 26 or 30 days. Fertilization with 90 kg N ha^?1 gave higher willow biomass production in willow with weeds sown after 26 or 30 days. Type of model weed had no effect on willow performance. Weed biomass and maximum shoot height were higher in weeds planted without willows compared to the willow-weed mixtures. A high nitrogen level gave more weed biomass when planted without willows and in the willow-weed mixture with weeds sown after 15 days. We conclude that for the given high density of willow, competition from weeds emerging soon after willow planting had strong effect on early production. Furthermore, if there is a risk of weed infestation, fertilization should be delayed.     

Melampsora rust species on biomass willows in central and north-eastern Germany

Melampsora willow rusts are the most important fungal pathogens in short rotation coppices of biomass willows. In the past, breeding programmes for rust resistant biomass willows concentrated on the distinction of races within the forma specialis Melampsora larici-epitea f. sp. larici-epitea typica that colonized Salix viminalis and related clones. In a new breeding program that is based on a wider range of willow species it is necessary to identify further Melampsora species and formae specialis that are pathogens of willow species other than S. viminalis. Therefore, three stock collections with Salix daphnoides, Salix purpurea, and other shrub willow species (including S. viminalis) species were sampled in north-eastern Germany. A fourth stock collection in central Germany contributed rusts of tree willows (Salix fragilis and Salix alba) and the large shrub Salix caprea. Out of 156 rust samples, 149 were successfully sequenced for ITS rDNA. A phylogenetic analysis combining Neighbour-Joining, Maximum-Likelihood and Bayesian analysis revealed six species: Melampsora ribesii-purpureae, Melampsora allii-salicis-albae, Melampsora sp. aff. allii-fragilis, Melampsora larici-pentandrae, Melampsora larici-caprearum, and Melampsora larici-epitea. The first four species were found exclusively on the expected hosts. Melampsora larici-caprearum had a wider host range comprising S. caprea and S. viminalis hybrids. Melampsora larici-epitea can be further differentiated into two formae speciales. The forma specialis larici-epitea typica (59 samples) colonized Salix viminalis clones, Salix purpurea, Salix×dasyclados, and Salix×aquatica. In contrast to this relatively broad host range, f. sp. larici-daphnoides (65 samples) was found exclusively on Salix daphnoides. With the distinction and identification of the rust species/formae speciales it is now possible to test for race-specific resistances in a more targeted manner within the determined pairings of rust and willow species.     

Response of Organ Structure and Physiology to Autotetraploidization in Early Development of Energy Willow Salix viminalis

The biomass productivity of the energy willow Salix viminalis as a short-rotation woody crop depends on organ structure and functions that are under the control of genome size. Colchicine treatment of axillary buds resulted in a set of autotetraploid S. viminalis var. Energo genotypes (polyploid Energo [PP-E]; 2n = 4x = 76) with variation in the green pixel-based shoot surface area. In cases where increased shoot biomass was observed, it was primarily derived from larger leaf size and wider stem diameter. Autotetraploidy slowed primary growth and increased shoot diameter (a parameter of secondary growth). The duplicated genome size enlarged bark and wood layers in twigs sampled in the field. The PP-E plants developed wider leaves with thicker midrib and enlarged palisade parenchyma cells. Autotetraploid leaves contained significantly increased amounts of active gibberellins, cytokinins, salicylic acid, and jasmonate compared with diploid individuals. Greater net photosynthetic CO₂ uptake was detected in leaves of PP-E plants with increased chlorophyll and carotenoid contents. Improved photosynthetic functions in tetraploids were also shown by more efficient electron transport rates of photosystems I and II. Autotetraploidization increased the biomass of the root system of PP-E plants relative to diploids. Sections of tetraploid roots showed thickening with enlarged cortex cells. Elevated amounts of indole acetic acid, active cytokinins, active gibberellin, and salicylic acid were detected in the root tips of these plants. The presented variation in traits of tetraploid willow genotypes provides a basis to use autopolyploidization as a chromosome engineering technique to alter the organ development of energy plants in order to improve biomass productivity.Energy security and climate change as global problems urge increased efforts to use plants as renewable energy sources both for power generation and transportation fuel production. Selected wood species, such as willows (Salix spp.), can be cultivated as short-rotation coppice for the rapid accumulation of biomass and reduction of CO₂ emission. Coppicing reinvigorates shoot growth, resulting in a special woody plant life cycle that differs from natural tree development, which takes decades. In this cultivation system, small stem cuttings are planted at high densities (15,000–25,000 ha⁻¹). In the soil, these dormant wood cuttings first produce roots and shoots that emerge from reactivated buds. During the first year, the growing shoots mature to woody stems. In the winter, these stems are cut back, and in the following spring, the cut stumps develop multiple shoots. The short-rotation coppice plantations are characterized by a very short, 2- to 3-year rotation, and the most productive varieties can produce up to 15 tons of oven-dried wood per hectare per year (Cunniff and Cerasuolo, 2011). The high-density willow plantations can also be efficiently used for heavy metal or organic phytoremediation, as reviewed by Marmiroli et al. (2011).The biomass productivity of shrub willows is largely dependent on coppicing capability, early vigorous growth, shoot growth rate and final stem height, root system size, photosynthetic efficiency, formation and composition of woody stems, water and nutrient use, as well as abiotic and biotic stress tolerance. Genetic improvement of all these traits can be based on broad natural genetic resources represented by more than 400 species in the genus Salix. More than 200 species have hybrid origins, and ploidy levels vary from diploid up to dodecaploid (Suda and Argus, 1968; Newsholme, 1992). In addition to molecular marker-assisted clone selection, intraspecific and interspecific crosses have been shown to further extend genetic variability in breeding programs for biomass yield (Karp et al., 2011).During natural diversification and artificial crossings of Salix spp., the willow genomes frequently undergo polyploidization, resulting in triploid or tetraploid allopolyploids. In triploid hybrids, both heterosis and ploidy can contribute to the improved biomass yield (Serapiglia et al., 2014). While the alloploid triploids have attracted considerable attention in willow improvement, the potentials of autotetraploid willow genotypes have not been exploited so far. As shown for other short-rotation wood species (poplar [Populus spp.], black locust [Robinia pseudoacacia], Paulownia spp., and birch [Betula spp.]), doubling the chromosome set by colchicine treatment can cause significant changes in organ morphology or growth parameters (Tang et al., 2010; Cai and Kang, 2011; Harbard et al., 2012; Mu et al., 2012; Wang et al., 2013a, 2013b). In several polyploidization protocols, the in vitro cultured tissues are exposed to different doses of colchicine or other inhibitors of mitotic microtubule function, and plantlets are differentiated from polyploid somatic cells (Tang et al., 2010; Cai and Kang, 2011). Alternatively, seeds or apical meristems of germinating seedlings can be treated with a colchicine solution (Harbard et al., 2012). Allotetraploids of poplar were produced by zygotic chromosome doubling that was induced by colchicine and high-temperature treatment (Wang et al., 2013a).Since tetraploid willow plants with 2n = 4x = 76 chromosomes are expected to represent novel genetic variability, especially for organ development and physiological parameters, a polyploidization project was initiated that was based on a highly productive diploid energy willow (S. viminalis var. Energo). Colchicine treatment of reactivated axillary buds of the in vitro-grown energy willow plantlets resulted in autotetraploid shoots and, subsequently, plants. For comparison of diploid and tetraploid variants of willow plants, digital imaging of green organs and roots was used for phenotyping. Among the tetraploid lines, genotypes were identified with improved biomass production, better photosynthetic parameters, and altered organ structure and hormone composition. The new tetraploid willow variants produced can serve as a unique experimental material to uncover key factors in biomass production in this short-rotation energy plant. In the future, these plants can also serve as crossing partners of diploid lines for the production of novel triploid energy willow genotypes.     

The use of non-dormant rods as planting material: A new approach to establishing willow for environmental applications

In this trial we tested a new planting technique for willows used in environmental applications such as green structures in urban settings. Three commercial willow clones were planted in spring 2008 at different periods as non-dormant woody rods. Comparing the main growth parameters at the end of the season, we found that planting date may affect plant establishment depending on the clone. Based on our results, the use of non-dormant willow material collected and planted in spring is possible in our environment with several different clones. Best results were obtained with Salix viminalis (Sv 5027), which can be planted with a delay of about one month without showing any significant decrease in biomass production or survival rate.     

Effects of Cutting Phenology (Non-dormant Versus Dormant) on Early Growth Performance of Three Willow Clones Grown Under Different Weed Treatments and Planting Dates

To assess the effects of cutting phenology on early growth performance of three willow clones grown under different weed treatments and planting dates, freshly harvested (non-dormant) and cold-stored (dormant) cuttings from willow clone Tora, Jorr, and Olof were planted in bucket experiment outdoors in central Sweden on five planting dates (May–June 2013) with or without a model weed (spring barley). Non-dormant cuttings sprouted faster than dormant cuttings when planted early in the season. For cuttings planted later in the season, bud sprouting was affected only by willow clone. Aboveground biomass production was affected by cutting phenology, planting date, clone, and weed treatment. When planted on May 3 and May 10, biomass produced from non-dormant and dormant cuttings did not differ, while willows grown from dormant cuttings produced 59% more aboveground biomass than willows grown from non-dormant cuttings when planted on May 24–June 16. Tora produced on average 12% more biomass than Jorr and Olof, and weed competition reduced aboveground biomass production on average with 36%. The ability of willow to suppress weeds (WSA) was 26 (non-dormant cuttings) and 12% (dormant cuttings) higher for willows planted on May 3 compared with WSA of willows grown from cuttings planted later in the season. The ability to tolerate competition from weeds (WT) was 51 and 52% lower for willows grown from non-dormant and dormant cuttings planted late in the season compared with WT of willows planted earlier in the season. We conclude that planting with long-term cold storage of willow cuttings can be replaced with planting freshly harvested cuttings when planting is performed in early season, and that weed competition strongly reduces biomass production. Weed control during the establishment phase is crucial in order to maximize willow biomass production.     

Effects of Propagule Phenology (Non-Dormant Versus Dormant) and Planting System (Vertical Versus Horizontal) on Growth Performance of Willow Clones Grown Under Different Weeding Regimes

To assess the effects of propagule phenology and planting system on growth performance of three willow clones grown under different weeding regimes, a field experiment was performed in central Sweden 2014–2016. Freshly harvested (non-dormant) and cold-stored (dormant) cuttings (planted vertically) and billets (planted horizontally) from willow clones Tordis, Tora, and Jorr were planted in weeded and in unweeded plots. Sprouting was significantly higher for willows grown from non-dormant (74%) than dormant (58%) propagules and for cuttings (84%) compared with billets (42%). Survival was higher for willows from non-dormant propagules in weeded (71%) compared with unweeded (63%) plots, willows from dormant propagules in weeded (72%) compared with unweeded (60%) plots, and for willows from cuttings (93%) compared with billets (39%). During 2014–2016, aboveground biomass production was significantly higher for willows from cuttings (11.71 t DW ha^?1) than from billets (6.13 t DW ha^?1), grown in weeded (15.29 t DW ha^?1) than in unweeded (2.55 t DW ha^?1) plots, and differed significantly among willow clones (11.48, 9.27, and 6.01 t DW ha^?1 for Tordis, Tora, and Jorr, respectively). In this study, (i) planting with cold-stored and freshly harvested willow propagules was equally successful and therefore cold storage could be potentially avoided and replaced with planting of freshly harvested propagules in early spring; however, (ii) in terms of measured growth performance parameters, willows grown from cuttings performed better than grown from billets; and (iii) weed competition significantly reduced survival and aboveground biomass production, confirming that weed control during establishment of willow is crucial.     

Correspondence of ectomycorrhizal diversity and colonisation of willows (Salix spp.) grown in short rotation coppice on arable sites and adjacent natural stands

Willows (Salix spp.) are mycorrhizal tree species sometimes cultivated as short rotation coppice (SRC) on arable sites for energy purposes; they are also among the earliest plants colonising primary successional sites in natural stands. The objective of this study was to analyse the degree of colonisation and diversity of ectomycorrhizal (EM) communities on willows grown as SRC in arable soils and their adjacent natural or naturalized stands. Arable sites usually lack ectomycorrhizal host plants before the establishment of SRC, and adjacent natural or naturalized willow stands were hypothesized to be a leading source of ectomycorrhizal inoculum for the SRC. Three test sites including SRC stands (Salix viminalis, Salix dasyclados, and Salix schwerinii) and adjacent natural or naturalized (Salix caprea, Salix fragilis, and Salix?×?mollissima) stands in central Sweden were investigated on EM colonisation and morphotypes, and the fungal partners of 36 of the total 49 EM fungi morphotypes were identified using molecular tools. The frequency of mycorrhizas in the natural/naturalized stands was higher (two sites) or lower (one site) than in the corresponding cultivated stands. Correspondence analysis revealed that some EM taxa (e.g. Agaricales) were mostly associated with cultivated willows, while others (e.g. Thelephorales) were mostly found in natural/naturalized stands. In conclusion, we found strong effects of sites and willow genotype on EM fungi formation, but poor correspondence between the EM fungi abundance and diversity in SRC and their adjacent natural/naturalized stands. The underlying mechanism might be selective promotion of some EM fungi species by more effective spore dispersal.     

Quantitative Genetics of Traits Indicative of Biomass Production and Heterosis in 34 Full-sib F1 Salix eriocephala Families

This project examined the heritability of traits that affect biomass production of Salix eriocephala, a shrub willow native to North America and an essential species in the breeding of bioenergy crop varieties. Using an incomplete factorial design, seven females and eight males were crossed to produce 34 full-sib F₁ families. Five to 12 entries per family were planted in four-plant plots in a randomized complete block design on two sites. Melampsora rust incidence was scored in the fall of the first growing season (prior to coppice). Height of the tallest stem, cross-sectional stem area per stool, and number of stems per stool were recorded in the winter after the first growing season post-coppice. Plants were harvested 3 years post-coppice and biomass yield was determined. A large percentage of the total variance was additive for all of the traits studied and heritability estimates were low to moderate, suggesting that phenotypic expression of these traits is predictable and can be improved through breeding. Based on yield 3 years after coppice, 29 of the 34 families displayed midparent heterosis, ranging from 1–115%, for the composite trait—biomass yield, strongly indicating that offspring often perform better than their parents in this population. This study will assist in selecting parents which may produce superior progeny in the breeding program.     

Variation in the uptake of Arsenic, Cadmium, Lead, and Zinc by different species of willows Salix spp. grown in contaminated soils

The experiment assessed the variability of in seven clones of willow plants of high biomass production (Salix smithiana S-218, Salix smithiana S-150, Salix viminalis S-519, Salix alba S-464, Salix ’Pyramidalis’ S-141, Salix dasyclados S-406, Salix rubens S-391). They were planted in a pots for three vegetation periods in three soils differing in the total content of risk elements. Comparing the calculated relative decrease of total metal contents in soils, the phytoextraction potential of willows was obtained for cadmium (Cd) and zinc (Zn), moderately contaminated Cambisol and uncontaminated Chernozem, where aboveground biomass removed about 30% Cd and 5% Zn of the total element content, respectively. The clones showed variability in removing Cd and Zn, depending on soil type and contamination level: S. smithiana (S-150) and S. rubens (S-391) demonstrated the highest phytoextraction effect for Cd and Zn. For lead (Pb) and arsenic (As), the ability to accumulate the aboveground biomass of willows was found to be negligible in both soils. The results confirmed that willow plants show promising results for several elements, mainly for mobile ones like cadmium and zinc in moderate levels of contamination. The differences in accumulation among the clones seemed to be affected more by the properties of clones, not by the soil element concentrations or soil properties. However, confirmation and verification of the results in field conditions as well as more detailed investigation of the mechanisms of cadmium uptake in rhizosphere of willow plants will be determined by further research.     

Nutritional imbalance caused by nitrogen excess is correlated with the occurrence of watermark disease in white willow

Willows in the Flemish region are threatened by the watermark disease, a wood disease caused by the bacterium Brenneria salicis (Hauben et al. Syst Appl Microbiol 21:384–397, 1998). Affected trees are observed to grow at specific sites which seem more sensitive to periodical watermark symptom development than other sites. In this study we aim at revealing abiotic site factors that could play a role in triggering the watermark disease. During three consecutive years we analyzed stands of young isogenic willows (Salix alba L. cv. Lichtenvoorde), which were planted on four different sites in an agricultural environment. These test sites were chosen, based on the occurrence/absence of watermark disease in the old willows that grow on these sites. Two sites had old willow pollards with clear watermark symptoms, and on the two other sites the old willows were unaffected. Differences in abiotic characteristics between the sites were analysed and their effects on the nutrient status in willow were measured in the isogenic willows that were newly planted. Brenneria salicis-specific multiplex PCR revealed the presence of this pathogenic bacterium in both the old and the freshly planted young willows. At each site, the physical and chemical characteristics of soil and groundwater were investigated; the young tree growth was monitored and nutritional status of the willow plantings was assessed by foliar analysis. Physical soil properties including soil type and profile water availability, and chemical soil analysis were unable to explain differences between healthy and affected sites. Diseased sites, however, had significantly higher N concentrations and traces of herbicides in the groundwater. In young trees, foliar levels of N and S were always greater at the affected sites, while P contents were lower. Dry mass based N concentration in the leaves of the newly planted trees increased systematically over the years, but more at affected than at healthy sites. Also, chlorophyll content was significantly greater at the affected sites. Growth rate of the young isogenic trees was twice as fast at the diseased sites. The presented data suggest that application of fertilisers in agriculture causes an imbalance in the nutritional status of Salix alba L. Excess nitrogen makes this species fast-growing, reflected by an average foliar N:P ratio >10, and more susceptible for watermark disease, indicated by an average foliar N:Ca ratio exceeding 1.5.     

Interactive effects of biochar and an organic dust suppressant for revegetation and erosion control with herbaceous seed mixtures and willow cuttings

Land reclamation efforts on “non‐soil” substrates frequently involve additions of organic material coupled with soil‐stabilizing dust suppressants to control erosion, but formal experiments, particularly of interactive effects, are lacking. We examined the effects of Entac?, a tall oil pitch emulsion, and pyrolyzed wood waste‐derived biochar on early plant establishment, growth, and survivorship, in 2 factorial greenhouse experiments: 1 using a seed mix widely used for roadside erosion control (the grasses Festuca rubra, Poa pratensis, Lolium perenne, and white clover [Trifolium repens]) and 1 using rooted cuttings of sand bar willow (Salix exigua). Biochar additions of 5, 10, and 20 t/ha enhanced biomass of clover by 14–250%, but had no significant effects on grasses. Entac additions inhibited early establishment and growth of both clover and grasses at all biochar treatment levels. Entac plus biochar treatments significantly enhanced survivorship of willows following a drought treatment in a greenhouse trial. In a field trial, Entac treatments had positive effects on willow diameter growth. Collectively, these results suggest that better soil moisture retention is the principal mechanism responsible for enhanced survivorship and growth of planted willows treated with Entac and biochar on sand substrates. Given Entac's inhibitive effects on early establishment of herbaceous species, and the abilities of Entac and biochar in combination to mitigate drought stress, these treatments may be particularly well suited for ecological restoration efforts using planted tree stock on coarse‐textured substrates and where exposure and ingress of non‐native herbaceous species are of concern.     

Seasonal dynamics of dry weight, growth rate and root/shoot ratio in different aged seedlings of Salix caprea

Willows (e.g. Salix caprea L.) are deciduous and richly branched shrubs or small trees. Salix caprea shows a high adaptability to different habitat conditions. One way of evaluating this adaptability is to measure willow biomass and production. Young plants of S. caprea were sampled from the bottom of an artificial lagoon in which sediments removed from the local Vajgar fishpond were deposited. The bottom of the lagoon was overgrown by vegetation dominated by seedlings of the willows S. caprea and S. aurita. Willows grew in the lagoon at average density of 58 plants per m². The biomass production and growth of S. caprea were determined for 15 samples (collected from 315 individuals) during the growing season. Annual net dry matter production in the whole community was estimated for 2.7 kg m⁻². Willows are generally considered to be fast-growing plants. The highest RGR of willows recorded by us was 0.03 to 0.04 g g⁻¹ d⁻¹ both in the stems and roots. This value was often recorded from July to August.     

Heavy Metal Uptake by Willow Clones from Sewage Sludge-Treated Soil: The Potential for Phytoremediation

Willow (Salix spp.) has shown potential for use in the phytoremediation of soil contaminated with heavy metals. In particular, it can be grown in short rotation coppice systems to produce biomass that can be used for energy production. Twenty different species or varieties of willow, grown over 2 years (1995 to 1997) on a soil that was highly contaminated with heavy metals due to long-term sewage sludge disposal, showed considerable variation in survival, biomass production and metal uptake. The willows could be divided into two groups after the first harvest. One group had relatively low Ni and Cu in the bark and high Cd and Zn in the wood, with a good survival rate and biomass production. This group partitioned Cu, Cd, and Zn into the wood tissue from the bark, whereas Ni was excluded. The second group had relatively high Ni and Cu in the bark and low Cd and Zn in the wood and performed poorly in terms of survival and biomass production. Of the 20 types of willow used, 11 showed potential for use in phytoremediation, combining good survival and biomass production with high metal uptake. Of the others, 2 failed to survive until the second harvest and the other 7 had very poor survival rates.     

Are Vegetative Reproduction Capacities the Cause of Widespread Invasion of Eurasian Salicaceae in Patagonian River Landscapes?

In recent decades, invasive willows and poplars (Salicaceae) have built dense floodplain forests along most of the rivers in Patagonia, Argentina. These invasion processes may affect Salix humboldtiana as the only native floodplain tree species in this region. It is assumed, that the property to reproduce vegetatively can play an important role in the establishment of invasive species in their new range. Thus, in order to contribute to a better understanding of willow and poplar invasions in riparian systems and to assess the potential impacts on S. humboldtiana the vegetative reproduction capacities of native and invasive Salicaceae were analysed. In a greenhouse experiment, we studied cutting survival and growth performance of the three most dominant invasive Salicaceae of the Patagonian Río Negro region (two Salix hybrids and Populus spec.), as well as S. humboldtiana, taking into account three different moisture and two different soil conditions. In a subsequent experiment, the shoot and root biomass of cuttings from the former experiment were removed and the bare cuttings were replanted to test their ability to re-sprout. The two invasive willow hybrids performed much better than S. humboldtiana and Populus spec. under all treatment combinations and tended to re-sprout more successfully after repeated biomass loss. Taking into account the ecology of vegetative and generative recruits of floodplain willows, the results indicate that the more vigorous vegetative reproduction capacity can be a crucial property for the success of invasive willow hybrids in Patagonia being a potential threat for S. humboldtiana.     

METAL UPTAKE AND ALLOCATION IN TREES GROWN ON CONTAMINATED LAND: IMPLICATIONS FOR BIOMASS PRODUCTION

Phytostabilization aims to reduce environmental and health risks arising from contaminated soil. To be economically attractive, plants used for phytostabilization should produce valuable biomass. This study investigated the biomass production and metal allocation to foliage and wood of willow (Salix viminalis L.), poplar (Populus monviso), birch (Betula pendula), and oak (Quercus robur) on five different soils contaminated with trace elements (TE), with varying high concentrations of Cu, Zn, Cd, and Pb as well as an uncontaminated control soil. In the treatment soils, the biomass was reduced in all species except oak. There was a significant negative correlation between biomass and foliar Cd and Zn concentrations, reaching up to 15 mg Cd kg^?1 and 2000 mg Zn kg ^?1 in willow leaves. Lead was the only TE with higher wood than foliage concentrations. The highest Pb accumulation occurred in birch with up to 135 mg kg ^?1 in wood and 78 mg kg ^?1 in foliage. Birch could be suitable for phytostabilization of soils with high Cd and Zn but low Pb concentrations, while poplars and willows could be used to stabilise soils with high Cu and Pb and low Zn and Cd concentrations.     

Estimating root biomass in Salix viminalis × Salix schwerinii cultivar “Olof” using the electrical capacitance method

Abstract

Non‐destructive assessment of root systems is important in order to understand and optimise the potential of resource capture and allocation by the plant. We studied the relationships between electrical capacitance (EC) and the below‐ and above‐ground biomass of willows. Cuttings of Salix viminalis × Salix schwerinii cv. Olof were maintained in pots and root development was followed up using a portable capacitance meter over the course of 2.5 months. Pot observations were compared with excavation of two‐year‐old established trees. A strong significant linear relationship (R ² = 0.81, p < 0.001) was obtained between EC and root biomass (dry weight [DW]) for the pot experiment. EC also showed good correlations with stem and leaf biomass, as well as with stem height. In the excavated willow trees, there was a strong logarithmic relationship between EC and root biomass (R ² = 0.66, p < 0.001). These results suggest that EC is a good estimator of below‐ground biomass in willow and may become useful in screening varieties for differences in root biomass traits, especially in distinguishing below‐ground resource allocation at an early stage.     

The potential of the decay fungus Chondrostereum purpureum in the biocontrol of broadleaved tree species

The ability of the decay fungus Chondrostereum purpureum to grow within birch (Betula pendula and Betula pubescens), aspen (Populus tremula), rowan (Sorbus aucuparia) and willow (Salix caprea) stumps or root systems is unknown, although such information would be crucial in understanding its impact on the vegetative growth of broadleaved trees, their ecology and potential biocontrol. Saplings of these tree species were cut and inoculated with the fungus. After 3 months, 47, 14, 0 and 0% of birch, aspen, rowan and willow stumps, respectively, were dead, and C. purpureum was frequently present within the stumps of all species. In more than half of the birch stumps investigated, the fungus had penetrated into the roots unlike in the other tree species. Our results indicate that C. purpureum can utilize the woody material of birch better than that of other species, and that penetration into the roots is needed to kill the host.     

Taming the Beast: Managing Hydrology to Control Carolina Willow (Salix caroliniana) Seedlings and Cuttings

Historically, wetlands along the St. Johns River, Florida, were dominated by herbaceous marshes. However, in the last 50 years many areas transformed to shrub‐dominated wetlands, at the same time a system of levees and canals was constructed to control flooding. We tested the role of water management in controlling Carolina willow (Salix caroliniana), a native shrub that accounts for most of this shift. We assessed survival and growth of seedlings and cuttings on four artificial islands. We planted willow seedlings and cuttings at the spring waterline and at three higher levels (+17.5, +35, and +50 cm) and evaluated their responses to natural hydrologic fluctuations. Overall, seedlings had lower survival than cuttings. Highest mortality occurred during summer floods and willows greater than 50 cm above marsh surface had the highest survivorship. Surviving seedlings attained similar height and biomass among elevations, but the cuttings had greater stem diameter, stem height, and biomass at higher elevations. In the second experiment, we planted seedlings and short (25 cm) and tall (50 cm) cuttings at the waterline and at three higher levels (+25, +35, and +50 cm) in artificial ponds with controlled water levels. Before flooding, seedlings at the highest elevation suffered some mortality due to desiccation, but after flooding, they had the highest survival. Elevation did not affect cutting survival, but those at the lowest elevation had the greatest height and biomass. Hydrologic manipulation can be a powerful tool to control willow establishment. However, its success depends on timely and prolonged inundation or water drawdown .     

Correlations of expression of cell wall biosynthesis genes with variation in biomass composition in shrub willow (Salix spp.) biomass crops

We have measured significant genetically determined variation in biomass composition among breeding populations of shrub willow, a biomass feedstock crop. This project was aimed to ask whether patterns of cell wall gene expression can be correlated with genetic variation in biomass composition at harvest, in order to develop assays of early differences in gene expression as indicators of harvestable biomass chemical composition and potentially reduce the time of selection for new willow genotypes. Previous studies have demonstrated that manipulation of expression of cell wall biosynthetic genes results in altered biomass chemical composition. We analyzed genes encoding enzymes involved in lignin biosynthesis and carbohydrate active enzymes selected based on their functional characterization and conservation in Populus trichocarpa and Arabidopsis thaliana. Fragments of 20 genes were cloned from young stem cDNA of Salix sachalinensis and Salix miyabeana. Expression profiling in willow stem apical tissue and developing stem tissue was performed for each isolated gene using probe-based quantitative real-time PCR. Two willow parental genotypes and six progeny within a hybrid family were selected for analysis, and significant differences in expression among the individuals and between tissue types were observed for most of the genes. Significant correlations between patterns of gene expression and variation in the biomass chemical composition of those genotypes provide insight into the genetic regulation of lignocellulosic deposition in this important bioenergy crop and could be utilized as a tool for early selection of new genotypes.