Effects of mechanical weed control or cover crop on the growth and economic viability of two short-rotation willow cultivars

Willow grown as short rotation coppice (SRC) is sensitive to weed competition, so herbicide treatment combined with mechanical weed control is recommended when establishing a plantation. This study compares that practice with two mechanical, row crop cultivator (RC) and row crop cultivator with torsion weeder (RCT), and two cultural treatments, cover crop (CC) and cut cover crop (CCC), to control weeds. Willow responses to these treatments were compared during the first three years after planting using two cultivars, Gudrun and Tordis with broad and narrow leaves, respectively. At harvest, the RCT treatment had produced 27% more biomass than the RC treatment (13.9 vs 11.0 Mg ha⁻¹ dry matter) and approximately three times more than the cultural treatments. However, the standard control treatment, herbicides and row crop cultivator (HRC), produced more than all other treatments (17.3 Mg ha⁻¹ dry matter). The two cultural treatments had higher plant mortality (CC 26.2% and CCC 32.8%) than the other treatments (HRC 2.7%, RC 7.0% and RCT 7.0%) after the first harvest cycle. No interaction between cultivar and treatment was found for willow shoot biomass, weed biomass or plant mortality. Overall, however, Gudrun had lower plant mortality and less weed biomass after the first harvest cycle than Tordis. All treatment and cultivar combinations gave positive financial annual returns when the whole life-span of the plantation was considered. This study suggests that without using herbicides, it is possible to establish a willow SRC plantation that produce enough to be economically viable.     

Alternative planting method for short rotation coppice with poplar and willow

A reduction in energetic and economic costs is key to the sustainable development of Short Rotation Coppices (SRC) for energy purposes. During their cultivation, the highest costs are incurred during the planting and harvesting phases. A new planting method, which involves the horizontal disposition of stems or cuttings 120 cm long, at a depth of 5–10 cm, could provide energy and cost savings during planting. The results of three experimental plots in Casale M. to (AL), one in Cannara (PG) and one in Chioggia (VE) are shown: horizontal stems and long cuttings were able to produce from 1 to 5 sprouts per meter (sp m⁻¹), depending on the genotype and environmental conditions. Willow was able to produce on average from 2.1 to 4.8 sp m⁻¹ and between poplars, the species P. ×canadensis produced more sprouts that Populus deltoides (3.9 sp m⁻¹ compared with 1.9 sp m⁻¹). Yields reached a maximum in a Casale M. to trial with 12.7 oven dry tons per hectare (Odt ha⁻¹) for poplar ‘Orion’ and 12.3 Odt ha⁻¹ for willow ‘Levante’ at the end of first year. The variability of sprouts production and growth of trees makes this method suitable for SRC or stool-beds.     

Interaction of willow (Salix) clones grown in polyclonal stands in short rotation coppice

Stool survival of Salix spp. clones is reduced when growing in polyclonal stands as compared to monoclonal stands. Stools growing in polyclonal stands show increased vigour with compensation of weak or dead stools from adjacent stools. In monoclonal stands the stools tend to be less vigorous but there is also less variation between stools. When a clone, which has become highly susceptible to rust infection, is included in the mixture, e.g. Salix burjatica Korso, it will die out completely within two years.     

The influence of soil and coppice cycle on the rooting habit of short rotation poplar and willow coppice

The increased demand for renewable energy sources has led to large areas of former agricultural land being proposed for short rotation coppice (SRC) establishment. Concerns expressed over the potential impacts of tree roots on buried archaeological evidence led to a study into the rooting habit of SRC. Roots were exposed in trenches dug within a variety of willow and poplar clonal stands grown on brown earth, pelosol, ground-water gley and surface-water gley soils. Root depths and diameters were recorded in each of the 33 trenches. In total, over 18,000 roots were measured on 264 coppice stools. The rotation length, species and stool location within a block were all found to influence the maximum size of root produced. Soil type had some influence on the root number and depth, but the pattern of root distribution down the soil profile was similar for both species.     

Early rotation short rotation willow coppice as a winter food resource for birds

Unlike a century ago when mixed arable/pastoral farms prevailed, the current Welsh pastoral landscape lacks arable crops for arable weed seeds as a winter food resource for granivorous birds. Biomass crops such as short rotation willow coppice (SRC) where Salix cultivars are grown at high density (10 000-40 000 per ha) and each plot harvested on a usually three year rotation may help redress this loss. SRC, certainly in its establishment phase, and, if suitably managed, in early post-harvest rotation, offers significant winter seed resource which within one resource, such as spear thistle (Cirsium vulgare), may change its bird availability depending whether it is upright when used by finches or fallen when utilised by ground feeding thrushes. If this crop becomes widespread in the landscape and it is managed in an environmentally friendly way, it will provide a food resource and possibly habitat “stepping stones” in countryside currently barren of such features.     

Productivity,stand dynamics and the selection effect in a mixed willow clone short rotation coppice plantation

Short Rotation Coppice (SRC) is a promising method of biomass production for energy purposes, but there have been concerns about the low diversity of these stands, including the risk for pest outbreaks. Mixing different clones has been advised as a way to tackle this problem and improve yields through positive diversity effects. Recent research into the relationship between biodiversity and ecosystem functioning supports these recommendations, but also raises worries about mechanisms that may confound results, such as a selection effect due to dominant clones outcompeting weaker ones. However, the few available studies on diversity effects in SRC plantations did not allow the disentangling of the mechanisms at play.We used data from an experimental SRC site, which incorporated three Swedish Salix clones in a row-based mixing design. Productivity was expected to be greater in mixtures and we attempted to elucidate whether this was due to a complementarity or a selection effect. We found that complementarity effects were generally larger than selection effects, yet the total diversity effect on yield was not significant. Leaf surface temperature measurements indicated that drought stress was unlikely to be the factor underlying this distorted diversity productivity relationship. We also found that a less-productive clone (Gudrun) had a different stem biomass distribution when in monoculture, which may have repercussions for the quality of the harvested product and points at a so far less recognized potential benefit of mixing.     

Productivity model and reference diagram for short rotation biomass crops of poplar grown in Mediterranean environments

A Reference Diagram (RD) was constructed for first rotations of the Euroamerican poplar ‘I-214’ grown as short rotation coppice (SRC). Data from 144 plots, established in eleven sites in Mediterranean environments, were used to develop the model. The density at establishment of the plantations ranged between 6666 and 33,333 stools ha⁻¹, covering the usual densities ranges used in short rotation forestry (SRF). The RD was based on a density-independent mortality model that relates the density of living stools to the average height of dominant shoot and the initial plantation density, and it includes a system of two simultaneously fitted equations relating a) quadratic mean basal diameter of dominant shoots to the average height of dominant shoot and the final density, and b) total above-ground woody dry biomass to quadratic mean basal diameter and final density. The isolines in the RD represented mortality, quadratic mean basal diameter of dominant shoots and total above-ground woody dry biomass at the end of a first rotation of three years. The final yield in terms of biomass ranged from 1 to 85 Mg dm ha⁻¹. The RD enables rapid and straightforward comparison of different situations, both at planting and at harvesting, and is a useful tool, based on a wide range of empirical data, for management and decision making regarding short rotation poplar crops.     

Influence of particle size and moisture content on tendency to bridge in biofuels made from willow shoots

Different systems for harvesting and storage of wood fuel from willow shoots give fuels with different particle size distribution, particle shape and moisture content. These factors influence the tendency for wood fuel particles to form a stable bridge over openings, which prevents the feeding of the fuel. The influence of cutting and storage method on the tendency to bridge was studied for chips and chunks made from 3 to 5 year old willow shoots harvested in January and December. Shoots were cut with four different machines to produce five fuel assortments with nominal particle length from 28 to 200 mm, and stored outdoors, in central Jutland, Denmark, in 160 m³ loose volume piles. Some piles were uncovered, some covered with plastic and two were sealed in an airtight silage plastic film enclosure. The bridging tendency was measured at the end of May and September by determination of how wide a “bridge” of fuel over a slot opening could be before it collapsed. With a 500 mm thick layer of fuel above the slot opening, the bridge width varied between 58 mm for the small chips and 977 mm for the large chunks. Most of the variation was due to two fuel properties, the proportion of particles longer than 100 mm and the moisture content of the fuel.     

Development and test of a simplified method to calculate dry matter loss during open-air storage of poplar wood chips by analysing ash contents

Short rotation coppice (SRC) in agriculture can be established successfully, only if the entire process chain is economically competitive. Despite the substantial dry matter loss (up to 25%) occurring during open-air storage of wood chips, it is the most applied storage technique. Since the particle size of wood chips plays an important role in storage and drying processes, two storage piles (>500 m³) with fine and coarse wood chips were investigated comprehensively under the weather conditions of North-East-Germany over a period of 9 months.The objective of this experiment was to develop a simplified method to calculate dry matter loss by determining the ash content and to compare the results with the conventional sample bag method. The new method delivered statistically sound results, particularly when ash contents were calculated from separated bark and wood samples instead of from wood chip samples. Furthermore, the significant and consistent increase in ash content of the bark samples during storage suggests, that the development of a model solely based on ash content of separated bark could be advantageous in terms of simplified yet reliable determination of dry matter losses. Since the ash content of wood chips is largely governed by the bark content, exact data for the proportion of bark is required, which varies between 18 and 35% depending on tree size represented by the stem diameter at cutting height (SDCH). Moreover, it was found that the fuel quality depends more on the SDCH of the SRC-harvest than on the produced wood chip format.     

A methodology for assessing gamebird use of short rotation coppice

Gamebirds provide a considerable incentive to plant and manage new woodlands. The supplementary income thus generated can improve their overall value. Other studies suggest that Short Rotation Coppice (SRC) crops may provide an attractive habitat for some gamebird species and could increase the value of the crop. To investigate this, the use by pheasants and partridge of different types of SRC at one site was assessed.

A method using sand quadrats to record gamebird footprints and hence relative use was developed. Pheasant footprints were recorded more frequently in the quadrats alongside the willow coppice, whereas partridge footprints occurred more frequently alongside the poplar coppice. Footprint analysis offers a reliable method of investigating how gamebirds use different types of SRC within small areas. The results are considered in terms of the placement of short rotation coppice plantations in the landscape to improve their ecological design.     

Quantity and quality of harvestable biomass from Populus short rotation coppice for solid fuel use—a review of the physiological basis and management influences

Woody biomass from poplar and aspen (Populus sp.) short rotation coppice (SRC) has good combustion properties compared to non wood solid bio fuels and fossil solid fuels. This review compiles and discusses relevant literature on fuel quality and yield for Central European conditions. The most problematic quality parameter of woody biomass from Populus SRC is its high water content at harvest time (55–60%). Storing unchipped material on the field during summer is an efficient tool to lower it. In order to control other quality parameters—mainly nitrogen (N), potassium (K) and heavy metal contents but also yield—one has to take into account the physiological background of SRC. Important features are species/clone, age of sprouts, growth pattern, site and nutrient cycling. Maximum mean annual increment (MAI) occurs later than in willows. Therefore rotations should be longer than in willow: at least 6–7 years for poplars and,—due to differences in growth pattern,—10 to 12 years for aspen. Both results in MAIs of 10–12 o.d.t.ha⁻¹ yr⁻¹ and reduced nutrient concentrations due to a lower share of branches and twigs in the harvested biomass. However, with elongated rotations costs rise because yet no automated (and thus cheap) harvest methods for large stem diameters were developed. Although sometimes ignored poplars are demanding concerning site characteristics. Basic requirements are good water (minimum 350 mm rainfall during growing season) and nutrient supplies, deep soils and favourable climatic conditions (average air temperature between June and September at least 14°C). Only aspen are partly suited for poorer conditions. For Populus-SRC in general weed control during establishing phase is essential.     

Energetic and economic evaluation of a poplar cultivation for the biomass production in Italy

The cultivation of crops for biomass production on good soils allows to reduce surplus production of food crops and increase the sustainability of energy production from the environmental point of view. The short rotation forestry (SRF), is only at a preliminary study level in Italy but, is already a reality in North Europe where was already developed an high planting density (6000–8000 cuttings ha^-1) technique and a whole mechanization of plantation and biomass harvest.On the basis of this cultivation technique, it was realized as an energetic and economic evaluation of a poplar SRF in Northern Italy. In detail, they were considered data of poplar growth in a plantation for the production of two-year whips in Western Po Valley considering SRF duration of 8 years and a biomass (20 t ha⁻¹ D.M.) harvest every 2 years. Indeed it was assumed to operate on a plantation in production (12.5% of the surface replanted every year) with a spacing 3.00 × 0.4 m (6700 cutting per hectare) that allows the use of conventional tractors.In this computing system it was pointed out a ratio between output and input energy of 13 and a cost of 80 € t⁻¹ of D.M. Nevertheless a positive energetic balance, the economic sustainability of poplar SRF depends, due to the present monopolistic energy management in the same countries, on political choices of chip price or public subventions to the producers.     

Identification of temperature and moisture content fields using a combined neural network and clustering method approach

Studies on the dynamics of temperature and moisture content distributions in porous soils have provided important insight on their effect on the building hygrothermal behavior, where the interaction between both building and soil can contribute to reduce building thermal gains or looses. Hygrothermal aspects can be related to many attributes such as energy consumption, occupants' thermal comfort and health, and material deterioration. Recently, a great variety of mathematical models to predict thermal and moisture content profiles in porous media have been presented in the literature. Most of those models are based on analysis of multilayer measurements or on Fourier analysis. The development and validation of such mathematical models facilitate the understanding of heat and moisture flows at different soil depths. In this research, a radial basis function neural network (RBF-NN) approach, combined with Gath–Geva clustering method in order to predict the temperature and moisture content profiles in soils, has been presented. A set of data obtained from the computation of the coupled heat and moisture transfer in porous soils for the Curitiba city (Paraná State, Brazil) weather data file has been used by the RBF-NN modeling method. Simulation results indicate the potentialities of the RBF-NNs to learn, for the one step ahead identification, the behavior of temperature and moisture content profiles in the media at various depths.     

Economics of poplar short rotation coppice plantations on marginal land in Germany

Although there is a need for biomass and a potential for short rotation coppice (SRC), farmers hesitate to establish SRC, even on marginal agricultural land on which annual crops show low productivity. Probably the most important factor explaining this reluctance might be the uncertain economic prospects of the cultivation of SRC. Therefore, the aim of this study is to analyse the economy of a typical SRC supply chain by calculating the annuities which can be expected by German farmers who establish SRC on their marginal land.The result shows that the yearly annuity of a 20-year SRC cultivation is about 70 € y⁻¹ ha⁻¹ when poplar SRC is harvested every 4 years with a forage harvester (one-step system). The result includes the establishment, cultivation and transport of the fresh wood chips to a plant 50 km away. However, this result is not competitive with the result of annual crops (226–462 € y⁻¹ ha⁻¹) and is also lower than the CAP subsidy payments that farmers receive from the EU (300 € y⁻¹ ha⁻¹). To achieve higher annuities, four options were analysed possibly leading either to higher biomass yields or to higher market prices (extension of rotation cycle, implementation of irrigation, technical drying of fresh wood chips, using a two-step harvesting system). The implementation of drip irrigation to increase biomass yield turned out to be uneconomic. An extension of the rotation cycle from 4 to 5 years can be recommended as it leads to an annuity of 255 € y⁻¹ ha⁻¹ (instead of 69 € y⁻¹ ha⁻¹). Results also show that the technical drying of chips using (cheap) surplus heat can be very profitable if the added value is reflected in higher market prices. Furthermore, it is shown that the use of an alternative two-step harvesting system with natural interim drying of the rods can be an attractive option for farmers to increase the annuity of their SRC.     

Characterising willows for biomass and phytoremediation: growth, nitrogen and water use of 14 willow clones under different irrigation and fertilisation regimes

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; δ¹³C) 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 × 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.     

Production and quality analysis of pellets manufactured from five potential energy crops in the Northern Region of Costa Rica

Modifications to a pellet manufacturing process must be made based on the characteristics of raw material used. The purpose of this work was to determine the alternations required to a wood pellet manufacturing process and the quality of the pellets produced using this process from five energy crops. Quality measurements include: the caloric value, the loss of moisture content in each production stage, the efficiency index of particle-pellet, ash content and quality as defined using the quantity of cracks and the transversal density and longitudinal density determined using X-ray radiography. The crops analyzed were rhizomatous plants, with caloric values ranging between 17.1 and 20.3 MJ kg⁻¹. This work determined that it was possible to produce pellets with Gynerium sagittatum and Phyllostachys aurea using the same production process for wood; however, Arundo donax and Pennisetum purpureum needed pre-air-drying and the Sorghum bicolor required mechanical dewatering before drying. A. donax, P. purpureum and G. sagittatum provided the highest efficiency index. When evaluating the pellet quality P. aurea and G. sagittatum hard a large quantity of cracks, unlike A. donax, P. purpureum and S. bicolor. The transversal and longitudinal pellet density varied from 1129 to 1294 kg m⁻³. The highest values of bulk density were obtained in A. donax and P. purpureum, followed by G. sagittatum and P. aurea, and the lowest bulk density was obtained in S. bicolor. Althogh out, some species produced cracks and high ash content, this work demonstrated that it is possible to produce pellets with moderate quality.     

Effect of fertilization on the biomass production of coppiced mixed birch and willow stands on a cut-away peatland

This investigation deals with the biomass production of coppiced mixed birch and willow stands growing on a peat cut-away area at Aitoneva, Kihniö (62°12′N, 23°18′E), Finland. The 16-year-old stands were harvested and left to coppice for 14 years, fertilization experiment (control, PK and wood ash fertilization) with three replications was established after the clear cutting. The mother stands before clear cutting (11,000 stems per hectare on average) were dominated by silver birch (69% of the stem number). After clear cutting the number of stems rose 7-fold to 75 500 stems ha⁻¹. Now the stands were dominated by Betula pubescens with 46% out of the total stem number, the share of B. pendula being only 25% and with Salix spp 29%. At the age of 14 years self-thinning had decreased the stem number to 12,800 stems ha⁻¹. The leafless biomass production of the coppiced stands on the control plots was lower than that of the seed originated mother stands had been during the previous rotation. However, with fertilization the 14-year-old coppiced stands reached the same total production as the 16-year-old mother stands had reached. The foliar phosphorus concentrations showed a severe phosphorus deficiency on the controls. Fertilization increased biomass of the stands considerably. After 14 growing seasons the above-ground leafless dry-mass of the fertilized stands was 61.8 (PK-fertilization) and 61.4 t ha⁻¹ (Ash) and that of the control stands 37.6 t ha⁻¹. A single PK fertilizer application had increased the standing biomass by 24 t ha⁻¹ (64%). Even though fertilization increased biomass production it did not increase nutrient concentrations of wood and bark, but rather decreased them. Fertilization decreased the nitrogen concentrations of leafless above-ground biomass.     

An easy parameter estimation procedure for modeling a HT-PEMFC

Fuel cells are a very complex system in which many phenomena of different nature occur simultaneously and within a small space, so a truthful measurement of some variables is not feasible using state-of-the-art technology. If a deep knowledge of the unit is desired, modeling can be of great help when it is properly used as it is possible to calculate the value of the variables of interest by adjusting experimental data. However, when models are complicated, it is not trivial to identify in which way a certain parameter alters the model results and then it is necessary to resort to sensitivity analysis before approaching an adequate parameter estimation procedure. In this work, a parameter estimation procedure has been proposed with the results obtained from the sensitivity analysis applied to a high temperature proton exchange membrane fuel cell (HT-PEMFC) model. The procedure has been demonstrated to be straightforwardly applicable as well as effective, which makes it suitable to be used as engineering tool to obtain a first consistent value of the parameters that define the main characteristics of a HT-PEMFC.     

Open-air storage of fine and coarse wood chips of poplar from short rotation coppice in covered piles

The cultivation of short rotation coppices (SRC) on agricultural land represents an economically and environmentally promising option for sustainable provision of bioenergy. Not only the further development of efficient harvesting machinery, but also the development of harvest-optimised storage systems are necessary to implement cost-efficient cultivation and use strategies for SRC in practice. The storage of fine wood chips from poplar harvest with a forage harvester results in high dry matter losses of up to 25%. Tractor-mounted mower-chippers can harvest coarse wood chips that might possess more favourable storage and drying properties. The main objective of the current research project was to develop and perform a storage experiment in which the storage behaviour of fine and coarse wood chips could be examined and compared in detail over a period of nine months. In this experiment two covered storage piles (height 3.5 m), with over 500 m³ fine and coarse wood chips respectively, were examined under practice scale conditions in Germany. After nine months of storage the fine chips in the core of the storage pile had dried to a moisture content of 34% with dry matter losses of 22%. Coarse chips, on the other hand, achieved a moisture content of 29% and dry matter losses of 21% in the same period. The maximum moisture content of 40% required by heating plants in practice is achieved by fine chips after 6.5 months and by the coarse chips already after 3.5 months.     

Analysis of moisture flow in porous materials using microfocus X-ray radiography

Microfocus X-ray projection is presented as an accurate experimental technique for measuring moisture transport processes in porous materials. The technique has evolved from the better-known medical technique. Compared to classical γ-ray and NMR-techniques it allows an instant and accurate determination of the two-dimensional transient moisture content profiles. A calibration procedure is presented to optimise the resolution in moisture content by choosing an adequate sample thickness in relation to the energy of the X-ray source. To illustrate the capabilities of the X-ray projection method, examples of two-dimensional liquid water uptake in homogeneous, heterogeneous, micro- and macro-fractured materials are presented.