IEA End of Task Report Short Rotation Forestry Production Systems 1995-1997 |
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Stig Ledin, Swedish University of Agricultural Sciences, Department of Soil Sciences, P.O. Box 7014, SE-750 07 Uppsala, Sweden Lisa Sennerby-Forsse, The Swedish Forestry Research Institute, SkogForsk, Dag Hammarskjölds väg 36 A, Glunten, SE-751 83 Uppsala, Sweden Lynn Wright, Oak Ridge National Laboratory, Environmental Sciences Division, P.O. Box 2008, Oak Ridge, Tennessee, U.S.A. |
One objective was to tackle non-technical barriers and facilitate the operational application of short rotation biomass systems. It was found that the level of concern about non-technical barriers differed greatly among the participant countries. Countries such as Canada and the U.S. were most concerned about the differential costs between fossil fuels and biomass fuels and the lack of energy policy equalising the playing field. Countries like Sweden, Denmark and UK were more concerned about technology transfer, incentives for getting long-term commitments from biomass fuel suppliers, financing mechanisms, and environmental issues. The reason for the different types of non-technical barriers affecting different countries can be seen partly in a comparison of energy prices for gasoline and electricity for industry and households. These comparisons show that some countries have tax policies which partly level the playing field among energy technologies while others do not. It could be concluded that all barriers could be grouped under environment, cost and policy. Environmental barriers could likely be resolved through education of public and cost could be dealt with if policies were stable, thus the major barriers are policy related. The conclusion is that government intervention is required to facilitate the commercialisation of biomass energy and that new policy tools may be needed.
Other objectives were to update the srf production systems handbook and to complete and update the computerised version. One new hard copy edition of the Handbook on How to Grow Short Rotation Forests was produced during the period. The computerised version of the Handbook has been extended with Robinia pseudoacacia and Eucalyptus sections. This version can be downloaded from the World Wide Web. The user can instantly get access to the content by country or by topic. There is a search function, which gives the user access to any existing aspect in the book within a couple of seconds. In addition a CD-ROM with the handbook content, extended with pictures of e.g. harvesting machines plus short video sequences showing different steps in managing operations has been produced and is accessible. Full scale plants for post treatment of sewage water with the aim of improved nitrogen reduction has been built at several sites in southern Sweden. Willows are usually efficient in taking up nutrients, including nitrogen, but when used as vegetation filters it is important to assess the actual ability of a stand to take up N. This ability varies considerably with the development stage of the plantation and also with site conditions. In a specific trial it was found that sludge applications only gave small increases of heavy metals in the soil. Leaching of nitrate appears, instead, to be the largest restricting factor for the size of the dose. Ash additions gave equal Salix stem mass as commercial fertilisers did. A lower uptake of P in ash treatments than in treatments receiving P fertiliser indicated that the short term availability of P in ash could be 80% of that in P fertiliser. The liming effect of ash was similar to that of slaked lime on a CaO-equivalent basis. Due to increased pH in the lime and ash treatments, Mn and Cd mass in stems decreased. In the coppice research one study showed that the high-N plants had lower amounts of starch in the tissue than the low-N plants. The major storage locations for starch were in roots, stems and stool in this order. In these organs most of the starch was found in cortex cells in the bark followed by phloem cells and also to some extent in the xylem. The seasonal variation showed at what time the storages were built up and when reserve products were utilised for the regrowth and resprouting. Also the effect of different harvesting technique was studied. It was shown that significant damage to stools at harvesting remained throughout the 3-year period of investigation. Surprisingly enough the stools with extremely severe damage still showed a high ability to sprout, although at a lower level than less damaged stools. Studies have also indicated significant variation in sprouting ability between clones of S viminalis, probably mainly due to genotypic differences.
Keywords: bioenergy barriers; waste water vegetation filters; harvesting biofuels; coppicing
The overall aim has been to exchange information on the practical development of short rotation forestry to enable the rapid deployment of cost-effective systems in the participating countries. The objectives for the period were (a) to tackle non-technical barriers and facilitate the operational application of short rotation biomass systems; (b) to update the srf production systems handbook; (c) to complete and update the computerised handbook; (d) to investigate other woody crops, such as black locust, for their potential as producers of biomass for energy; (e) to exchange information on the mechanisation of srf production; (f) to exchange information and conduct co-operative R&D on the disposal of sewage sludge and waste waters on srf; and also (g) on the interaction between coppice production systems and mechanisation
The past more than 20 years of R&D in coppice production systems have resulted in a substantial body of knowledge concerning the behaviour and responses of coppiced deciduous trees, mainly willows. The efforts in this field have spread and today several research groups all over the world are engaged in studies of importance for the development of woody crops production systems. A substantial part of the international co-operation has been undertaken within the IEA in the form of activities within TASK areas. The immediate previous programme during 1992-94, TASK VIII, included the activity ”Growth processes” in which coppicing ability was examined to some extent (Sennerby-Forsse 1995). It was clear from these studies, that maximising coppice production is more complicated than was first believed and we must emphasise the need to better understand whole-plant responses to different growth conditions, harvesting and to which extent the coppicing ability between clones is genetically determined or influenced. There is thus a multitude of factors influencing the performance of the coppice systems and more knowledge about these factors and their interactions is needed. The countries with national research programmes in this area often have their own preferred species, which might be Salix, Populus, Robinia, Eucalyptus, Betula, Alder etc. However, national information exchange and research co-operation is usually favourable since on the level of more basic studies many problems and results are applicable to several species.
Most countries involved in the International Energy Agency´s Bioenergy Agreement are dealing with a variety of non-technical barriers to bioenergy commercial expansion as well as some technical barriers. It was found that the level of concern about non-technical barriers differed greatly among the participant countries. Countries such as Canada and the U.S. were most concerned about the differential costs between fossil fuels and biomass fuels and the lack of energy policy equalising the playing field. Countries like Sweden, Denmark and UK were more concerned about technology transfer, incentives for getting long-term commitments from biomass fuel suppliers, financing mechanisms, and environmental issues. The reason for the different types of non-technical barriers affecting different countries can be seen partly in a comparison of energy prices for gasoline and electricity for industry and households. These comparisons show that some countries have tax policies, which partly level the playing field among energy technologies while others do not. It could be concluded that all barriers could be grouped under environment, cost and policy. Environmental barriers could likely be resolved through education of public and cost could be dealt with if policies were stable, thus the major barriers are policy related. The conclusion is that government intervention is required to facilitate the commercialisation of biomass energy and that new policy tools may be needed.
The non-technical barriers have been treated at two workshops during the period and participants perceived that barriers to the production of heat and electricity had very different issues and thus they are summarised separately below. Barriers to the production of liquid transportation fuels were not directly discussed since few of the participants were from countries with active efforts to develop liquid fuels from lignocellulosic energy crops.
Use of biomass for district heating is already occurring in many of the participating countries. Thus the discussion addressed the barriers to further expansion of that technology and with a focus on local issues. The barriers expressed were as follows:
Not all barriers to district heating can be overcome easily. For example, towns that are poorly designed for district heating will not likely be redesigned in the foreseeable future. However, for many of the barriers, the solutions are largely suggested by the definition of the problem.
Suggestions for solving district-heating barriers were as follows:
It is apparent from the above that project developers of community district heating systems need to be prepared to educate the consumer and work the political systems within the community. Consumers and politicians will need to be convinced of the environmental or local economic advantages and in some cases be prepared to pay higher prices for their heating. This is actually occurring in some countries, e. g Austria.
The barriers to biomass electric exist at the international, national, and local levels. The barriers seem to fall into five major areas including unfavourable policies, difficult competition (cost) social/environmental perceptions, crop supply uncertainty, and technological risks.
The policy barriers identified generally lead to the conclusion that there is a non-level playing field among competing technologies. Specific examples included the following:
Comments about difficult competition/costs included the following:
There are social and environmental perceptions, which are creating barriers to biomass energy project development. Examples discussed include the following:
One of the major concerns of entrepreneurs involved in project development is uncertainty about the reliability of biomass fuel supplies. The issues are expressed by the following questions:
Risks associated with using new conversion technologies are reflected in the reluctance of the financial community to back project development. Comments on this topic included the following:
The solutions proposed focused primarily on policy and education actions that would have to be undertaken at the federal or state levels and to a limited extent actions that could be taken by the private sector for a profit motive.
Policy actions recommended included the following:
Education activities suggested that could make a difference in reducing non-technical barriers included:
Government research and development program management activities suggested that could make a difference include:
One new hard copy edition of the Handbook on How to Grow Short Rotation Forests (Ledin and Willebrand, 1996) was produced during the period. The computerised version of the Handbook has been extended with Robinia pseudoacacia and Eucalyptus sections. This version can be downloaded from the World Wide Web. The user can instantly get access to the content by country or by topic. There is a search function, which gives the user access to any existing aspect in the book within a couple of seconds. In addition a CD-ROM with the handbook content, extended with pictures of e.g. harvesting machines plus short video sequences showing different steps in managing operations has been produced and is accessible.
One project on the cleaning of sewage water in energy forests and contamination aspects is running with financial support from NUTEK. The project has been running for one year. For large-scale application of sewage water a technique has been developed This technique is equivalent to controlled flooding. In the field the technique used gives points of application of the sewage water, leading to “islands” of effects in the plantation. The difference between spots is anticipated to decrease with time as the roots compensate by growing laterally in the soil. Parallel to the field study there is lysimeter investigations going on, looking into N-leakage and transport of viruses in the soil profile (Aronsson, 1998 personal com.).
In the lysimeters the plants are large in relation to available soil volume, implying that root density is higher than if plants were growing in the field. The soil is “undisturbed” cores of soil, sampled with specially designed equipment attached to a tractor (a large boring cylinder). The applied liquid fertilisers have a content of nutrients similar to sewage water. The application of nutrient solution has been going on from July to November. 15N has been used to follow the nitrogen in the system. A significant retention of nitrogen in the system has been observed also after the leaves had been shed. A preliminary conclusion is that denitrification occurs. There is plenty of carbon in the soil originating from turnover of the fine roots. Simultaneously a fair amount of nitrate-nitrogen exists in the soil and the application of the nutrients in water solution leads to anaerobic conditions prevailing intermittently or most of the time. The bacteria responsible for the denitrification are facultative bacteria, using oxygen in the soil air when available and nitrate at reducing conditions in the soil. The large amount of turned over fine roots supply the bacteria with a rich source of carbon for energy. The nitrogen will end up as N2 or N2O which eventually ends up in the above ground atmosphere (Aronsson, 1998 personal com.).
In a co-operation project between the Department of Short Rotation Forestry, SLU and the Swedish Institute for Infectious Disease Control a pilot trial, using lysimeters, studies of virus movement in the soil has been performed. The virus particles used were in fact bacteriophages with sizes comparable to the size of virus. It was shown that these particles could move through the soil profile down to drainage depth (90 cm) within a few hours. There were indications that a dense root system may function as a filter also for particles with the size of virus. It seems that very little research has been done up till now regarding transport of small particles in the soil water and interaction with plants (Aronsson, 1998 personal com.).
Full scale plants for post treatment of sewage water with the aim of improved nitrogen reduction has been built at several sites in southern Sweden. Thus plantations have been established at Kågeröd, Svalöv municipality (13 ha, see below); Bromölla(~11 ha) and Åstorp (similar area). A similar concept is used for the cleaning of leakage water from municipality dumps by recycling this water via a willow plantation. That kind of leakage water contains more “aggressive” substances like salt and different organic compounds. This kind of treatment facilities has been built at several sites (Rönneholm at Eslöv, Laholm,) and other communities are planning to build trial sites or full scale facilities (Luleå, Nyköping, Göteborg). Similar activities are found, at least, in the Baltic States, in Poland (Wroclaw) and in USA (Aronsson, 1998 personal com.).
Until recently, treatment of municipal wastewater in Sweden has mainly consisted of removal of phosphorus and easily biodegradable organic substances. Most of the nitrogen has been released in the water phase to the recipients. The recently introduced legal requirements to remove also nitrogen have encouraged the recycling approach, meaning that, before chemical phosphorus precipitation, municipal wastewater can be used as an adequate nutrient resource for biomass production (Rosenqvist et al., 1997). Tests have clearly indicated that application of municipal wastewater in Salix plantations could replace a large part of the conventional wastewater treatment otherwise needed due to natural purification processes in the soil-plant system (Hasselgren, 1998). In the village of Kågeröd, Svalöv municipality, in the south of Sweden, pre-treated wastewater was used for irrigation and fertilisation of a 13 ha Salix plantation. The design allows the total wastewater from 1500 citizens during May-October, about 150 000 m3, to be used and treated in the energy willow plantation. Wastewater is applied via PE-tubes in modules with different spacing for assessments of the willow growth due to different watering and fertilising effects. The willow plants developed better in parcels applied with wastewater than in control parcels. Irrigated willows with 3-year-old stems on 4-year-old roots yielded between 22 and 37 tonnes of dry matter per hectare or four times more than control plants (Hasselgren, 1996). Measurements are being made concerning biomass production and uptake of nutrients and heavy metals in harvested willow stems. The groundwater quality is monitored in terms of possible influences of nutrients and metals.
Municipal sludge as manure in a Salix plantation has been tried at Brunnby, Västerås in Sweden (Diedrichs and Bramryd, 1995). Two different types of sludge, normal mixed sludge (biologically and chemically processed) from Västerås sewage plant, and limed mixed sludge from Norberg, were spread out in different rates in connection with planting. The doses were 5 and 25 tonnes of dry matter per ha. The highest average growth of three production years was obtained in the plot which had been given fertilisers (60 kg N year two and year three) 10.4 tonnes stem wood per year. This was closely followed by the treatment with 25 tonnes dry matter of Västerås sludge that gave 10.3 tonnes DM/ha and year.
The N-content in the soil, in the form of nitrate, increased most with the 25 tonnes dose of mixed sludge from Västerås and increased almost two-fold in the topmost 30 cm of the soil. The increase in deeper soil layers was much lower.
Regarding uptake of nutrients in the Salix plants at the first harvest, the large doses of sludge resulted in high uptake of P. Increased N-levels in both leaves and stems were found foremost after the large dose of Västeås sludge and after fertiliser. These treatments also gave the best growth. Despite Salix having relatively good uptake ability for heavy metals, the sludge applications have not generally given any significant increases of heavy metals in stems and leaves. Sludge applications only gave small increases of heavy metals in the soil. Leaching of nitrate appears, instead, to be the largest restricting factor for the size of the dose Sweden (Diedrichs and Bramryd, 1995).
Sander (1998) studied the variable properties and the effects of applications on vegetation of bioash. In a field study on arable land, straw ash, Salix ash and ash from conventional deciduous forests were used as amendments in willow plantations during the first rotation. It was shown in a study of 7 heating plants that the composition of ash depended on plant species burned (e. g., wheat, rape, Salix), ash fraction (bottom ash, fly ash) and heating plant. Compared with bottom ash, Zn, Pb, and Cd concentrations in fly ash were 10-90 times higher. The Cd/P-ratio of straw bottom ash was similar to that of P fertilisers (30 mg Cd kg-1 P).
Ash additions gave equal Salix stem mass as commercial fertilisers did. A lower uptake of P in ash treatments than in treatments receiving P fertiliser indicated that the short term availability of P in ash could be 80% of that in P fertiliser. The liming effect of ash was similar to that of slaked lime on a CaO-equivalent basis. Due to increased pH in the lime and ash treatments, Mn and Cd mass in stems decreased. Net accumulation in the topsoil of Cu, Mn, and Ni occurred in all the ash treatments, Zn and Cd were net accumulated in the wood ash treatments and net removed in all other treatments (Sander, 1998).
Some known facts about coppicing: The understanding concerning coppicing processes and responses in different tree species is far from complete. An attempt to summarise the knowledge available has been done (Sennerby-Forsse et al., 1992). Studies have shown that coppicing (i.e. harvesting) of certain plant parts cause reinvigoration and in some species even accelerate growth towards the theoretical maximum. The reasons for the rapid growth of regenerating shoots are probably several, such as the proximity of roots to shoots, a large parental root system, structural and physiological factors etc. Dynamics and interactions of growth regulators, water, nutrients including storage products makes it difficult to single out specific factors as responsible for certain responses. We know that different tree species, such as birch and willows may have completely different resprouting strategies (Sennerby-Forsse et al., 1992). There is also evidence of a large variation in resprouting ability among clones of the same species (Rönnberg-Wästljung et al., 1994). Some trees develop new shoots from the roots, some from roots and stumps and some species only respond to coppicing by stump shoots. A large number of studies have revealed that harvesting of coppice plantations should be done during dormancy, due to a number of physiological and morphological reasons. The nutrient reserves for the new growth should be at maximum level in the remaining tissues (i.e. roots and stool), the morphological prerequisites such as the differentiation stages of the buds giving rise to the next shoot generation, should be optimal (Sennerby-Forsse and Zsuffa, 1995). The carbohydrate reserves in the roots are believed to be especially important for regrowth, whereas little is known about the nitrogen sources for the regrowth processes (Kramer and Kozlowski, 1979, von Fircks and Sennerby-Forsse 1998). The sustainability of coppice plantations is a key-factor to economical and ecological aspects of bimass production (Sennerby-Forsse and Christersson, 1995). The ability to keep up a high production level after several harvests varies a lot between species. Species which are sensitive to more than one harvest, i.e. are likely to decrease production are for example Betula alba and some Populus species, while Salix viminalis, S dasyclados and others have a high ability to withstand repeated harvests and tend to increase production over time. The optimal rotation length i.e. the time between two harvests differs between species and also depends on the intensity of the management procedures. It might vary between 5 years (Salix) up to 15-20 years ( Populus, R pseudoaccacia) (Sennerby-Forsse et al., 1992). When it comes to silviculture and management regimes of coppice plantations we know a good deal about fertilization and the influence of different abiotic factors on growth (Alriksson and Ledin, 1997). However, the knowledge concerning the harvesting methods and their influence on resprouting is more limited. Population dynamics in dense stands due to stool morphology and spacing have been studied (Verwijst 1991) and was part of the former IEA-activity ”Growth processes”. We also think that there is a substantial genotypic variation and also phenotypic responses to different abiotic factors (Rönnberg-Wästljung et al., 1994).
A Salix stool consists of the above ground stems, the cutting and the root system. The shoots originate from axillary buds located in leaf axils on the stem. The buds consists of one main bud and two lateral buds. Sometimes only the main bud will sprout and sometimes all three buds sprouts with some time lag between them.
The root systems originate from preformed root primordia in the stems. They develop into roots and differentiate into morphologically and functionally different roots. Coarse roots are woody, have secondary growth and may live for several years. They anchor the plants, and are important for transport of water and nutrients and for storage of nutrients. Fine roots have an annual turnover and are responsible for most of the uptake of water and nutrients in woody plants (Rytter, 1997). Unfortunately, we have far from enough of detailed information about the roots in coppice systems and studies are underway to highlight some of the questions concerning the role of roots in coppicing processes.
Growth dynamics in coppiced Salix follows the general pattern of woody plants adapted to temperate conditions. However, in coppiced stools the stems are removed at regular intervals. In terms of survival strategy the harvest of the stools can be compared to browsing, pest attack, fire or other natural disasters. Members of the Salicaceae family are pioneer species with a high ability to utilise the available resources, often during unfavourable conditions, and have adapted to natural disasters. In some species, such as Salix viminalis, S dasyclados, S eriocephala, this ability is utilised in short rotation forest plantations with rotations cycles of 3-5 years. Besides genetic characteristics of the plants, environmental factors such as water and nutrient availability play an important role for growth patterns in Salix. Several studies have shown that a high N-content in the plants enhances the accumulation of nutrient reserves, delay the growth cessation and promotes early-season growth (Sennerby-Forsse and von Fircks, 1987; Ericson and xx 19??; Bollmark et al,. in press)?.
New IEA-programme: From the state of the art in 1994 we were able to define several areas where more research was needed in order to increase the understanding of coppice behaviour. It was agreed that the work would focus on the interaction of ecophysiological factors with cultural methods and harvesting as it affects coppicing sustainability. Studies have been undertaken in the following areas: i) accumulation and mobilisation of root reserves in coppice growth systems, ii) genetic variation of resprouting ability among clones of Salix viminalis and iii) effects of different harvesting techniques on stool survival and resprouting in Salix coppice plantations.
Summary of the coppicing state of the art: In an attempt to summarise our knowledge concerning some factors and processes involved in coppice, we will follow a Salix stool from one autumn to the next and describe the physiological processes involved during the different growing seasons. Most of the information originates from studies of S viminalis and S dasyclados. The growing season starts in April/May and growth generally ceases in September/ October. During the spring, the average temperature is the most important factor for growth start, while in the autumn, the photoperiod determine when growth will be halted. Of course, temperature is also important, i.e. after the first frost (usually in September under Swedish conditions) no more growth occur.
Autumn: During early autumn growth slows down, i.e. cell divisions (mitosis) becomes less frequent, while elongation and differentiation/maturation processes continue. Towards the end of the growing season the apex of the shoots develops an abscission zone. A similar process cuts of the leaves from the stem leaving the buds for next years growth. Before the shedding of the leaves, carbohydrates were transported from the leaves, to the stem and further down to the root system. The accumulation of starch was shown to coincide with the cessation of primary growth (Bollmark et al. 1998). The cambial activity, responsible for the production of xylem and phloem cells decreases leaving mature tissue ready for the dormancy period (Sennerby-Forsse 1987). The living parenchyma cells changes in ultrastructure and become filled with nutrients such as starch, protein bodies and spherosomes (lipid bodies). At the same time, the winter hardiness develops in the plants. In the roots, growth continues somewhat longer (Rytter 1998). It has been reported that roots may grow at temperatures between +2 - + 35oC (Hermann 1977). The standing fine-root number decreases during autumn and winter compared to summer and the magnitude of growth and decay is lower during this time of the year (Rytter 1997). The allocation of reserve nutrients to roots during the autumn have shown roots to be prominent in carbohydrate storage, while nitrogen is found in all perennial organs (Bollmark et al. 1998).
Winter: During the winter the plants are in a dormant stage. No new growth occurs and the ultrastructure of cells is consistent to a high degree. The reserve nutrients are found in all perennial organs and the allocation pattern shows that the major storage location for carbohydrates and starch are in the roots. Starch accumulates mainly in phloem and cortical cells in both roots and shoots (von Fircks and Sennerby-Forsse 1998). Nitrogen and lipids are other important products, stored as organells (protein bodies, spherosomes) in storage cells (Sennerby-Forsse 1987, 1989). This is the time when harvest should take place, the plants being well prepared with nutrients to initiate and start new growth. The cuttings produced from young stems, harvested in a dormant condition, will thus contain enough energy for the early phases of development of the preformed root primordia to roots and of the buds to shoots.
Spring: When the average daily temperature reaches the levels needed for breaking of the dormancy, cellular processes start to occur. The nutrient reserves in the storage cells are transformed and retranslocated to those parts where they are needed. It has been shown that in coppice willows, fine-root growth starts well before bud-break, in some cases up more than 6 weeks prior to shoot growth (Rytter and Hansson 1996). The amount of root reserves used locally by the roots as opposed to transportation of stored nutrients to the stems is not well known. At the moment we are carrying out studies to learn more about the strategy of the plants when it comes to using internal nutrients in combination with exogenous nutrients taken up by newly formed roots from the soil (Bollmark et al, in prep). We have been able to show that, in controlled conditions, root cells of decapitated plants, were more depleted of starch at the time of resprouting than root cells of intact plants, supporting the hypothesis that starch reserves in the roots are important during the early phase of resprouting in coppice systems (von Fircks and Sennerby-Forsse 1998).
In an intact stool, i.e. not harvested during the last winter, the first visible change would be the swelling of vegetative buds and the flowering. The willows usually flower 3-4 weeks before bud burst and are important food sources for bees and other insects. The swelling of buds is believed to be the result of local nutrient reserves being used. This hypothesis is supported by the finding that the amount of starch and protein bodies significantly decreased in stem storage cells during the time of bud growth initiation (Sennerby-Forsse 1987). The newly formed leaves of the shoots are able to start photosynthesis at a very early stage of development (Bollmark et al,. in prep) and they transport carbohydrates to different parts of the shoot depending on their stage of maturity (Bollmark et al., in prep). Each bud contain a certain number of leaf primordia that was developed before dormancy. These leaves/internodes are referred to as ”predetermined growth”. During the growing season, new internodes are initiated and developed, and are referred to as ”free growth”. The ability of free growth is supposed to be a strategy of the plant to utilise existing resources. This ability is extremely well developed in the coppice willows during their younger ages (1-2 years) and decreases with age.
In the cambial zone, phloem cells are first formed, supporting the finding that young leaves export some of their carbohydrate to other members of the ”cohort”. Phloem formation is followed by the mitosis of new xylem cells early in the summer (Sennerby-Forsse 1987). The growth activity increases and growth reaches a peak in the middle of the summer.
Summer: During early summer growth accelerates and the leaf area is built up through rapid shoot growth. The root system is now favouring the transport of water and nutrients to the stems and photosynthesis rate is high. New cells are formed, by the primary meristems as well as by the cambium. During the same time the shoot tip can increase in length up to 7 cm per day! The leaf area is rapidly building up and leaves started to shed from the lower most part of the stems, mainly due to competiton from light. However, new leaves are formed at a higher rate, which results in increasing LAI. The length growth is dependent on temperature and tends to be highest during the first part of the summer (June). During this time the shoot can increase in length up to 7 cm per day (Brunkener 1988). The cambial activity reaches it’s maximum during July at which time the cell formation rate in the cambial zone reaches its peak with more than 18-24 cells in the division zone.
Cutting - the start of a new stool: The cutting comes complete with it’s own lunch bag. If in good condition at harvest, it contains enough energy for the development of root primordia into functional roots, able to take up and transport water and nutrients. The buds get their first energy from nearby stored nutrients (phloem and cortex cells in the stem) and after sprouting the young leaves are able to start photosynthesis at a very early stage (Bollmark el al, in prep). The moisture content of the cutting is a limiting factor during the sensitive first stages of root development. Cuttings therefore must not dry out, and the soil should be kept moist during the initial stages of growth (Telenius and Sennerby-Forsse 1989). The first spring and summer are important for the young plants to get a successful establishment and it is usually recommended to irrigate the plantations at this stage. It is a good investment for future growth. As soon as the root system is developed, the most important prerequisite for a high production of stem wood is fulfilled.
Accumulation and mobilisation of root reserves in coppice growth systems: The studies undertaken within the frame of the IEA-programme was focused on i) the role of carbohydrates during resprouting in coppice systems and ii) the effects of N-availability on starch reserves in roots and shoots of S viminalis plants. The results will help to understand better how reserve nutrients are involved in resprouting and new knowledge will lead to improved fertilisation recommendations to growers.
The student exchange with Rhinelander laboratories (Sennerby-Forsse 1995) in the former IEA-activity ”Growth processes” was focused on carbon- and nutrient retranslocation in coppice systems. The results of the study are presently in preparation for publishing and will be part of a PhD-thesis (Bollmark et al., in prep.) The study included the use of 14C labelling techniques to study the carbohydrate dynamics in intact and coppiced Salix plants. This study showed that expanding leaves retain most of their fixed carbon, whereas non-expanding leaves export nearly all of their fixed carbon, to sustain growth in other parts of the plant. Further analysis showed that there is a certain pattern of 14C retranslocation to different parts of the plant which is closely related to the leaf expansion rate
The other study aimed at assessing the exact location of starch inside the plants during a growing season. We used chemical analysis combined with light microscopy and image analysis and the results are recently published (von Fircks and Sennerby-Forsse, 1998) The study showed that the high-N plants had lower amounts of starch in the tissues than the low-N plants. The major storage locations for starch were in roots, stems and stool in this order. In these organs, most of the starch was found in cortex cells in the bark followed by phloem cells and also to some extent in the xylem. The seasonal variation showed at what time the storages were built up and when reserve products were utilised for the regrowth and resprouting.
Genetic variation of resprouting ability among clones of Salix viminalis: The main aim in this still ongoing project is to assess the genetic variation of resprouting ability within the species Salix viminalis. The plant material consists of 480 well-defined clones (8 replications) on two different sites. Clones were previously harvested and measured during 1988, -91,-93 and during 1997. Growth data and number of shoots/stool were gathered as well as more detailed measurements of shoot dynamics. The goal is to define and classify genotypes in terms of morphology, stool form and productivity. Also the variation of productivity and stool development over time will be described. Preliminary results from this study show a large phenotypic variation in number of shoots/stool which exists still after four subsequent harvests (i.e. each 3rd year). Furthermore, the two sites differ in that one site is clay soil and the other is sandy soil. It is evident from this study that the sandy soil allowed a faster establishment of the plants and better growth during the first years. However, after a longer establishment phase the plants on the clay soil increased production and outgrew the plants (same clones) on sandy soil. This study will be finalised during 1998 and results published internationally (Rönnberg-Wästljung et al., in prep). Hopefully, results will improve basis for selection and increase understanding of stool behaviour and production development over time.
Effects of different harvesting techniques on stool survival and resprouting in Salix coppice plantations: Different harvesting techniques and machinery for Salix coppice plantations have been developed. It is important that besides the economical and technical prestanda of the machinery also the biology of the plants is considered. In order to maintain healthy plants for a sustainable production the damage caused by the harvesters must be kept at a minimum. The main cause for damage is the type of cutting device used, the speed of the cutting and the height of the cut and finally the age of the stools. In order to examine the type of damage as well as the degree of damage done to the plants four different harvesting equipments were investigated.
The study was carried out in two different plantations, with different climatic and soil conditions. Both plantations were established in 1987. Different clones of S viminalis and S dasyclados were included in the study (Sennerby-Forsse, 1994). Altogether four different harvesting machines were studied and as control was used harvesting with a manual brush-saw. The harvesters included in the study were: Austoft, Claas, ESM and Fröbbesta. The measurements made included i) stool morphology at the time of harvesting, ii) type of damage, iii) amount of damage, iv) survival rate and v) resprouting and production.
The study shows that significant damage to stools at harvesting remained throughout the three-year period of investigation. Surprisingly enough the stools with extremely severe damage still showed a high ability to resprout, although at a lower level than less damaged stools. The differences found in this study is equivalent to more than 10 tonnes of DM per hectare and year (Sennerby-Forsse, in prep).
Sennerby-Forsse, 1994. Different harvesting techniques and their influence on stool survival and resprouting in Salix coppice plantations. Proceedings of the 8th European Conference on Biomass for Energy, Environment, Agriculture and Industry. Vienna, Austria, Ocotober 3-5. 1994.
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Bollmark, L., Sennerby-Forsse, L., Dickson, D., Coleman, M,. and Isebrands, J. Carbon sink-source function as related to leaf development in young Salix viminalis L. plants. (In prep.)
von Fircks, Y., and Sennerby-Forsse, L. Seasonal fluctuation of starch in root and stem tissues of coppiced Salix viminalis L. plants grown at two nitrogen regimes”. Tree Physiology. (In press).
Rönnberg.Wästljung, AC., Werwijst, T., and Sennerby-Forsse, L. Genetic variation of resprouting ability in Salix viminalis. (In prep.)
Sennerby-Forsse, L. Harvesting Salix coppice plantations - effects on stool survival and production. (In prep.).