Ecological, biological balances and conservation

0961-9534(95~

Biomussand Bioenergy, Vol. 9, Nos l-5, pp.107-l 16,1995 Elsf&fscienccJ.&d

PrintcdillGrcatBritrin

0961-9534i95$9.50+ 0.00

ECOLOGICAL, BIOLOGICAL BALANCES AND CONSERVATION K L PERTTU Swedish University of Agricultural Sciences, Department of Ecology and Environmental Research, Section of Biogeophysics, P.O.Box 7072, S-75007 Uppsala, Sweden

ABSTRACT The scientific work within the activity “ecological/biological balances and conservation” is summarised in this report. The aims of the activity during its existence between 1992 and 1994 have been to: i) arrange a workshop and publish the presentations on the environmental aspects of energy forest cultivations, ii) perform joint scientific work together with the activity group on “biological disposal of wastewaters and sludges”, that is closely related to environmental problems, and iii) produce ecological guidelines concerning energy forestry, suitable for advisers and farmers dealing with bioenergy problems. The most important results from the workshop were the environmental benefits from energy forestry when compared with intensive agriculture and forestry. Energy forestry has positive influence on the carbon balances, nutrient recycling, and soil sustainability. The effects are also positive on the natural flora and fauna, which in most cases are enriched when compared with agricultural crops. From the joint efforts of the two activities the main result was a study tour, conference and workshop, concentrating on biological purification systems. The most promising system seems to be the vegetation filters of short rotation coppice. The report on ecological guidelines contains a number of ideas and recommendations for establishment, management and harvesting of energy forests in an environmentally acceptable way. It gives also advice on how to locate the stands to minimise the risk of nutrient leakage from arable land. KEYWORDS

Biodiversity, energy forestry, environmental aspects, nature preservation, short rotation coppice, vegetation filter,

BACKGROUND

AND INTRODUCTION

When the ideas for utilisation of fast-growing, short rotation forests for energy purposes (energy forests) were put forward after the oil crises in the seventies, there was a lack of consideration for nature preservation in this context was obvious. It was, of course, satisfactorily noted that a reduced oil consumption in the world was positive for the environment as a whole, but the sometimes negative impact on, e.g. biodiversity, was at that time neglected. Biodiversity means existence of life in different forms in a certain area, and the concept is often used to describe the number of species and individuals of different species. During the eighties, however, land use for various purposes was increasingly discussed. It was also more and more obvious that agriculture in most western countries was producing a surplus of products, which had to be sold below cost price on the world market. In Sweden, this resulted in a change from willow energy

107

108

K. L. PERTTU

forestry on marginal land (peat land and abandoned farmland) to agricultural land in use. The benefit was twofold, namely reduction in production of agricultural crops and higher production of wood for energy purposes. The discussions on land use strategies from the environmental point of view were, therefore, less intensive for a period, because most agricultural crops are typical monocultures, and cultivation of energy forests are less negative. However, when the areas with willows became more extensive, the requirements with respect to other environmental factors, such as biodiversity, were stated. Within the IEA/BA Task VIII, for example, an activity was started on “ecological/biological balances and conservation” with Sweden as the activity leader. During the two first years, Dr. Lena Gustafsson, Swedish University of Agricultural Sciences, Uppsala, was the activity leader, and from January 1 1994, Dr. Kurth Perttu replaced her for the rest of the period. The general aim of the activity was to discuss and give advice on how to handle the very delicate problems with ecological balances as well as the preservation of nature when growing short rotation energy forests. More specifically it can be divided into three goals, namely: Arranging an open workshop concerning the environmental aspects when growing energy forestry. Acting jointly with the IEA/BA activity group on “biological disposal of wastewaters and sludges”, which is closely related to different environmental problems, including balances and preservation. Producing a publication with ecological guidelines on energy forestry adjusted specially for practical use by advisers and farmers.

SUMMARY OF THE UPPSALA WORKSHOP An open workshop was arranged in Sweden on “environmental aspects of energy forest cultivation”. The proceedings were published in a special issue of the ‘Biomass and Bioenergy’.’ This report contains 17 individual papers presented at the meeting. Quotations from the foreword confirm that ‘I.. there are environmental benefits to be made from energy forestry when comparisons are made with intensive agricultural and forestry systems. Energy forests have advantages with regard to carbon balances, nutrient cycling and soil sustainability. The eflects on landscape and biodiversity are more complex. For instance, the natural flora and fauna are enriched when compared with agricultural crops, but may be deteriorated if seminatural deciduous woodlands or unfertilised grazing lands are replaced. ” This opinion is strengthened and underlined in the guidelines (see below). Short summaries are presented of some of the contributions below. An environmental analysis of energy forestry by Stjernquis2 pointed out that two types of sustainability are necessary, namely a long term one of biomass productivity, and an overall one of the ecosystems under present and future climatic conditions. Her conclusions are favourable to energy forestry when some criteria are met, e.g. not forcing the biomass production system to maximum production, minimising the nutrient leakage, adapting the stands to the landscape, and taking measures to retain and improve various biotopes. Energy and environment is very closely related, and this becomes very obvious when discussing, for instance, energy consumption and climate. In this respects the discussion concerns the carbon cycle in the soil (sea) - vegetation - atmosphere - system. Here, carbon plays a key role

Ecological, biological balances and conservation

109

both as a greenhouse gas and as an energy resource. In the paper by Hall and Hous~,~ biomass is discussed from the viewpoint of being a carbon storage or a substitute for fossil fuel. Their conclusions read: “Of the major alternatives to reduce atmospheric CO2 levels, forestry options are among the most promising and environmentally acceptable”. The requirement of land for producing energy from biomass to substitute for fossil fuels is for developed countries about 15 % of the land area while it is only about 2 % for the developing world with a worldwide average of 10 %. According to the authors, there is sufficient land available for biomass production that it would significantly impact on the atmospheric carbon content.3 Biodiversity is to a large extent affected by energy forest plantations. If the influence is positive or negative depends at least partly on the type of original vegetation replaced by the energy crop. In most cases, alteration of traditional farmcrops to broadleaf energy forests gives a positive effect on the habitat for plants and animals. Christian et al4 have studied how birds and small mammals were effected by change of habitat to energy forests. Hybrid poplar plantations replacing agricultural cropland have obviously very little negative impacts on the abundance and diversity of birds and mammals. The authors conclude, however, that there are very few studies on ecological interactions and that more work is needed in order to incorporate perspectives not only at the level of the plantation itself but also at the level of the landscape and region.4 A bird fauna study was performed in willow plantations in southern Sweden.’ The stands investigated were bush-formed Salix viminalis, and these type of crops were favourable for birds adapted to bushy habitats. The author found that a change from traditional agricultural crops to willow energy plantations was positive for 13 and negative for 4 species of birds. He concludes that a probable change of lo-20 % of completely open farmland to energy forestry might be an optimal mixture for the bird fauna.’ In poplar and willow plantations on former arable land in southern and central Germany, Makeschin has performed studies of the effects on soils.6 The physical properties were positively influenced because of less use of heavy machinery. The nitrate in soil water can be reduced provided that the energy plantations are fertilised according to the crop requirement. This is also one of the most important facts underlined in the guidelines (see below). Soil organisms, like decomposers, increase in tree plantations, and the fauna is generally richer there than in arable soils.6 The water and the nutrient cycling is closely related to each other. Because the amount of water in large plantations is controlled by the precipitation (with considerable and unforeseen variations), the application of fertilisers should follow the crop requirement in order to avoid undesirable leakages. Rijtema and Vries have simulated the water balances and the nitrogen requirement, uptake and leakage in different crops in The Netherlands.’ The results from the computer simulations indicate that the precipitation excess (i.e. the water draining from the root zone expressed as the difference between precipitation and evapotranspiration) and the nitrogen leakage is considerably less from a forest ecosystem compared to an agricultural ecosystem. For example, if the nitrogen leakage from a grass field on clay soil is denoted with 100, the corresponding leakage from poplar is 10 and from willow 15. Similarly, the leakage from potatoes is 71 and from cereals 45. The effect of the different crops on the precipitation excess was less pronounced.’

110

K. L. PERTTU

SUMMARY OF JOINT EFFORTS OF TWO IEA/BA ACTIVITIES Because there are many relations between the two IEA/BA activity groups on “ecological/biological balances and conservation” and “biological disposal of wastewaters and sludges”, respectively, it was natural to seek for points in common. As a result of this, a combined study tour and conference was arranged in Sweden in June 1994 on development and use of vegetation filters. Such research is also performed in Poland and, therefore, the arrangements included a joint Swedish-Polish workshop. The proceedings of the whole meeting have been published in a report at the Agricultural University.* Until very recently wastewaters and sludges have been considered in many countries mainly as waste products which society has to get rid of in the easiest way and at the lowest cost. In particular, the content of nitrogen and phosphorus in wastewaters and sludges has caused problems in treatment and deposition. Because nutrient leakage is a major threat to many water ecosystems, it is logical to seek systems for treatment of wastewaters and sludges, in which the plant nutrients can be used for production of biomass fuels, at the same time as their harmful effects are eliminated. Such systems, also named ‘vegetation filters’, have already been shown to be of great use both economically and environmentally. The two-day study tour comprised visits to establishments using willow vegetation filters (Sulix spp.) for purification of landfill leakage water, municipal wastewater, nitrogen enriched groundwater, etc. These practical establishments, briefly described in the proceedings, involve purification of municipal wastewater,‘-” landfill leakage water,12 and nutrient rich groundwater. ’3 A visit was made at the energy company in brebro, the largest and most complete heat and power plant for biofuel in Sweden, and information was also gathered from Agrobransle which is the main commercial company for energy forestry.14 During the one-day conference and the two-day workshop in Uppsala, many high-quality papers were presented and their results critically discussed. The oral and poster presentations, published in the proceedings,8 deal with the state of the art, the vegetation filter experiments using wastewaters and sludges, the heavy metal tolerance and uptake, the restoration of soils, and some theoretical considerations. In one of the state of the art papers, Heding” briefly introduced the activity “biological disposal of wastewaters and sludges”. In several other papers, the present state of the art in Sweden, Estonia and Poland were presented.‘6-18 Also the Swedish energy forestry programme as a whole was illuminated.‘9 Interesting results from municipal wastewater and sewage sludge experiments in Poland, United Kingdom and Denmark were presented in a number of papers, some of which are referred to here. The Polish experiences of using vegetation filter systems are actually more than 100 years old, bu\o_;ere are also recently started, ongoing experiments with preliminary results of interest. The Briti;$Fperiments on use of sludges in sho$ rotation coppice were presented in two separate papers, ’ and the Danish experiments in one. All the time when dealing with wastewaters and sewage sludges, one has to be aware of the noneligible compounds in them. The most common are various heavy metals. If using vegetation filter purification, it is necessary to know the tolerance of and uptake by the vegetation used and, therefore, a considerable part of the meeting was devoted to this matter. The presentations involved metal tolerance of and metal uptake by different plants.27”2 Another vegetation filter approach is to use the plants for covering and/or restoration of areas with contaminated soils. Some of the willow clones seem to be able to tolerate high amounts of metals in soils, others are efficient in taking up these metals and, thus, possible to use for

Ecological, biologicalbalancesand conservation

111

restoration of soils.28 In Sweden, some willow clones have shown a large capacity to take up cadmium from arable farmland.30’33 Willows are also used in Belgium as a cover on wet substrates from dredging of inland waterways.34

GUIDELINES

FOR NATURE PRESERVATION

AND ENERGY FORESTRY

Cultivation of short rotation energy forests is one of the most practical methods to meet the requirements for renewable energy in many countries. It is, however, of the greatest importance to establish, manage and harvest the plantations in an ecologically sound way, using all the possible knowledge available. To fulfil the specific aims of the activity, an illustrated report with guidelines has recently been published.35 This report provides hints and ideas for farmers and advisers on how to grow energy forests under consideration of various environmental aspects. The aim is that the ideas presented here are applicable for energy forestry not only in Sweden but also in other countries. The preservation of the biodiversity has become one of the main environmental issues, and has also been included in the legislation regarding traditional agriculture and forestry. This can be justified in many ways, not at least from the viewpoint that there is an obligation to save species in a natural environment as a gene bank in order to allow the different species to develop their own strategies of survival under natural competition.35 There is a pronounced deficiency of many habitats in the present agricultural landscape. This is a consequence of the large changes in the agricultural practice that has occurred mainly after the second world war. Large areas of meadows and pasture land have been transformed into arable land or planted with coniferous forests. Chemical weed control and use of artificial (inorganic) fertilisers have to a large extent affected the flora (and fauna) of the landscape. In the agricultural landscape of today, the conditions for many species to survive are harsh or nonexistent. Short rotation energy forestry encompasses the chain from establishment, management, and harvesting of the broadleaved trees to utilisation of the wood produced. In Sweden, almost exclusively different types of willows (S&x spp.) are used. They are generally fertilised to ields, but municipal wastewaters and sewage sludges can be a good obtain high Y alternative.8’36’ ’ Other broadleaf tree species like alder, birch, poplar and hybrid aspen can similarly be used in energy forestry. Among these, alder is special in the sense that it is a nitrogen fixing tree and, thus, less dependent on nitrogen fertilisation. Municipal sludges, normally with an unbalanced ratio between nitrogen and phosphorus, are well suited as fertiliser in alder stands, provided that the doses are related to the phosphorus requirement of the plant.35 Even in a rather rationalised agricultural landscape, there are small “islands” where remains of once vital populations of plants and animals have survived. These islands can be cairns, open ditches, edges of forest, etc. Such small habitats are of the greatest importance and should be preserved, and they must not be shaded by too closely established energy forest stands. Certain areas, such as old meadows, pastures, paddocks, and wetlands with their high value as far as the special flora and fauna is concerned, should not be used for energy forestry at al1.35 It is important to fit the cultivation to the type of landscape, which in many aspects coincides with preservation considerations. One rule is to restrict planting of stands too close to the roads, except if they are specially designed, for example, as wind breaks. Another general rule is to

K. L. &Rl-l-IJ

112

avoid cutting off scenic views in the landscape. Cultivations in the lower part of the landscape usually cause little harm from the scenic viewpoint, but might be unsuitable because of an increased risk for frost damage. An artificial impression of symmetry can be avoided by allowing the corners to be more rounded and the edges be curved. It is also recommended to leave some shrubs and bushes of other species along the fringes in order to smoothen the view at the same time as they are attractive to birds, insects, and smaller animals.35 Generally, variation in the landscape favours biodiversity. Compared to most agricultural crops, short rotation energy forests show a larger variation, and it is possible to further increase it in different ways. Instead of planting one large stand, it is more favourable to design a cultivation consisting of several smaller stands of various shapes and ages, harvested during different years.35 The flora of mature willow stands mainly consists of ordinary weeds but sometimes rare herbs are found. Some of these rare species spread slowly and it will take a considerable time for them to colonise a willow stand and, therefore, they are favoured by relatively older stands. On the other hand, the short period between harvests is positive for such herbs which are less tolerant to shading. Willow forests have shown a large potential when used for purification of contaminated waters and soils (see also above).36’37 Stands located along watercourses can act as catchcrops taking up nutrients from surrounding arable land that otherwise would leach into the streams. The same principle can be applied for purification of other types of contaminated waters, e.g. domestic wastewater and leakage water from landfills. Some willow clones are also efficient in taking up and accumulating heavy metals, e.g. cadmium (see above). This can pose threats as well as possibilities. During the 20th century, the soil cadmium content of arable land has been built up mainly by application of commercial phosphorus fertilisers. Certain willow clones could be used in order to purify arable land and other heavy metal contaminated soils. Weed control should be adjusted to the need and prerequisites. Mechanical weed control implies repeated efforts using machinery and equipment and is, therefore, an energy-intensive method and also negatively affecting soil compaction. Chemical control, on the other hand, implies a risk of negative impacts on other species than the weeds and a risk of leakage of chemicals into waters. It is important not to overdo the weed control, no matter what is used.35 Application of fertilisers is negative both for the largest when the foliage nutrients is recirculated leaves3’

should also be adjusted to the stand development. Leakage of nutrients environment and for the farmers economy. The nutrient requirement is of the stand is building up and the canopy is closing. A major part of the already after 2-3 years because of the high decomposition rate of such

DISCUSSION AND CONCLUSIONS Although energy forestry research has been carried out for 20 years now, there are still many problems to be solved. Introduction of new crops takes time, even if corresponding crops in agriculture normally require more time. During the whole period of energy forestry research, the impact on the environment was regarded positive, but not until recent years has this been concretely manifested. The guidelines3’ now available, are a first attempt to give practical advice (without being rigorous) on how to prepare, establish, manage and harvest energy forests with as low negative impact on the environment as possible. Of course, the recommendations are founded on the previous findings, not at least on the results from the two meetings in Sweden 1993 and 1994, respectively.

Ecological,biologicalbalancesandconservation

113

During the last ten years, energy forest plantations in Sweden have been established on farmland, thus replacing traditional agricultural crops. In fact, this has had positive influences on the biodiversity and the nutrient capture at the fields concerned, as well as on the carbon circulation in the soil-plant-atmosphere system as a whole. One of the main conclusions to be drawn from the presentations and discussions during the meetings is that the research activities concerning short rotation energy forestry and environmental aspects must be intensified in most countries. This is particularly important as far as utilisation of different waste products is concerned, if their negative impact can be neutralised in an ecologically sound way and their positive values at the same time exploited with acceptable economic benefits. This research must not only take into account the basic biological, technical and economical aspects, but also opinions and expectations of the society.

REFERENCES 1. 2.

3.

4.

5.

6.

7.

8.

9.

10.

L. Gustafsson ed., Environmental Aspects of Energy Forest Cultivation. Biomass and Bioenergy, Vol. 6, No. l/2, 158 pp. 1994. I. Stjemquist, An integrated environmental analysis of short rotation forests as a biomass resource. In: Environmental Aspects of Energy Forest Cultivation (L. Gustafsson ed.) Biomass and Bioenergy, Vol. 6, No. l/2,3-10. 1994. D.O. Hall and J.I. House, Trees and biomass energy: Carbon storage and/or fossil fuel substitution. In: Environmental Aspects of Energy Forest Cultivation (L. Gustafsson ed.) Biomass and Bioenergy, Vol. 6, No. l/2, 1l-30. 1994. D.P. Christian, G.J. Niemi, J.M. Hanowski and P. Collins, Perspectives on biomass energy tree plantations and changes in habitat for biological organisms. In: Environmental Aspects of Energy Forest Cultivation (L. Gustafsson ed.) Biomass and Bioenergy, Vol. 6, No. l/2,31-39. 1994. G. Giiransson, Bird fauna of cultivated energy shrub forests at different heights. In: Environmental Aspects of Energy Forest Cultivation (L. Gustafsson ed.) Biomass and Bioenergy, Vol. 6, No. l/2,49-52. 1994. F. Makeschin, Effects of energy forestry on soils. In: Environmental Aspects of Energy Forest Cultivation (L. Gustafsson ed.) Biomass and Bioenergy, Vol. 6, No. l/2, 63-79. 1994. P.E. Rijtema and W. de Vries, Differences in precipitation excess and nitrogen leaching from agricultural lands and forest plantations. In: Environmental Aspects of Energy Forest Cultivation (L. Gustafsson ed.) Biomass and Bioenergy, Vol. 6, No. l/2, 103-l 13. 1994. P. Aronsson and K.L. Perttu eds., Willow vegetation filters for municipal wastewaters and sludges: A biological purification system. Swedish University of Agricultural Sciences, Section of Short Rotation Forestry, Report 50,230 pp.. 1994. K. Hasselgren, Kagerod wastewater recycling experiment. In: Willow vegetation filters for municipal wastewaters and sludges: A biological pur$cation system (P. Aronsson and K.L. Perttu eds.) Swedish University of Agricultural Sciences, Section of Short Rotation Forestry, Report 50,207. 1994. K.L. Pert&, Wastewater treatment at &ter$ing, Giitene using willow vegetation filters. In: Willow vegetation filters for municipal wastewaters and sludges: A biological purification system (P. Aronsson and K.L. Perttu eds.) Swedish University of Agricultural Sciences, Section of Short Rotation Forestry, Report 50,209-2 10. 1994.

114

11.

12.

K.L. PERTllJ P. Aronsson and K.L. Pert& A complete system for wastewater treatment using vegetation filters. In: Willow vegetationJilters for municipal wastewaters and sludges: A biological puriJcation system (P. Aronsson and K.L. Perttu eds.) Swedish University of Agricultural Sciences, Section of Short Rotation Forestry, Report 50,2 1 l-21 3. 1994. K. Hasselgren, Landfill leachate treatment in energy forest plantations. In: Willow

vegetation filters for municipal wastewaters and sludges: A biological purljkation system (P. Aronsson and K.L. Perttu eds.) Swedish University of Agricultural Sciences, 13.

Section of Short Rotation Forestry, Report 50,2 15-2 17. 1994. S. Elowson and L. Christersson, Purification of groundwater using biological filters. In:

Willow vegetation filters for municipal wastewaters and sludges: A biological purzjkation system (P. Aronsson and K.L. Perttu eds.) Swedish University of 14.

15.

16.

17.

18.

19.

20.

21.

Agricultural Sciences, Section of Short Rotation Forestry, Report 50,2 19-223. 1994. K.L. Perttu, Agrobransle and ijrebro Energi: Two companies of importance for biofuel utilization. In: Willow vegetation Jilters for municipal wastewaters and sludges: A biological purzjkation system (P. Aronsson and K.L. Perttu eds.) Swedish University of Agricultural Sciences, Section of Short Rotation Forestry, Report 50,225-227. 1994. N. Heding, Biological disposal of wastewaters and sludges - the history of a young activity. In: Willow vegetation jilters for municipal wastewaters and sludges: A biological puriJcation system (P. Aronsson and K.L. Perttu eds.) Swedish University of Agricultural Sciences, Section of Short Rotation Forestry, Report 50, 9. 1994. A. Finnson, A. Peters, A. Lind and 0. Palm, Use of the resources in municipal wastewater - a comprehensive view. In: Willow vegetation filters for municipal wastewaters and sludges: A biological pur$cation system (P. Aronsson and K.L. Perttu eds.) Swedish University of Agricultural Sciences, Section of Short Rotation Forestry, Report 50, 11-17. 1994. A. Koppel, Energy forests in Estonia. Situation in 1994. In: Willow vegetationfilters for municipal wastewaters and sludges: A biological purification system (P. Aronsson and K.L. Perttu eds.) Swedish University of Agricultural Sciences, Section of Short Rotation Forestry, Report 50,23-26. 1994. H. Obarska-Pempkowiak, Present state of the art in Poland concerning vegetation filters. In: Willow vegetation filters for municipal wastewaters and sludges: A biological purzjication system (P. Aronsson and K.L. Perttu eds.) Swedish University of Agricultural Sciences, Section of Short Rotation Forestry, Report 50, 27-34. 1994. L. Sennerby-Forsse, The Swedish energy forestry programme. In: Willow vegetation filters for municipal wastewaters and sludges: A biological purification system (P. Aronsson and K.L. Perttu eds.) Swedish University of Agricultural Sciences, Section of Short Rotation Forestry, Report 50, 19-22. 1994. H. Obarska-Pempkowiak. Application of surface flow wetlands for treatment of municipal wastewater - two full-scale systems. In: Willow vegetation filters for municipal wastewaters and sludges: A biological purification system (P. Aronsson and K.L. Perttu eds.) Swedish University of Agricultural Sciences, Section of Short Rotation Forestry, Report 50,49-58. 1994. J. Kutera and M. Soroko, The use and treatment of wastewater in willow and poplar plantations. In: Willow vegetation jilters for municipal wastewaters and sludges: A biological purification system (P. Aronsson and K.L. Perttu eds.) Swedish University of Agricultural Sciences, Section of Short Rotation Forestry, Report 50, 37-47. 1994.

Ecological,biologicalbalancesand conservation 22.

115

H. Obarska-Pempkowiak and M. Mierzejewski, Water quality improvement by hydrobiological methods in streams flowing into the Gulf of Gdansk. In: Willow vegetation filters for municipal wastewaters and sludges: A biological purijcation system (P. Aronsson and K.L. Perttu eds.) Swedish University of Agricultural Sciences,

23.

24.

Section of Short Rotation Forestry, Report 50,69-78. 1994. H. Obarska-Pempkowiak, Application of willow and reed vegetation filters for protection of a stream passing through a zoo. In: Willow vegetation filters for municipal wastewaters and sludges: A biological purijkation system (P, Aronsson and K.L. Perttu eds.) Swedish University of Agricultural Sciences, Section of Short Rotation Forestry, Report 50,59-67. 1994. D. Riddell-Black, Sewage sludge as a fertiliser for short rotation energy coppice. In: Willow vegetation filters for municipal wastewaters and sludges: A biological purification system (P. Aronsson and K.L. Perttu eds.) Swedish University of

25.

26.

27.

28.

29.

Agricultural Sciences, Section of Short Rotation Forestry, Report 50,91-99. 1994. R.W. Hodson, F.M. Slater and P.F. Randerson, Effects of digested sewage sludge on short rotation coppice in UK. In: Willowvegetationfilters for municipal wastewaters and sludges: A biological purification system (P. Aronsson and K.L. Perttu eds.) Swedish University of Agricultural Sciences, Section of Short Rotation Forestry, Report 50, 113118.1994. K.H. Nielsen, Sludge fertilization in willow plantations. In: Willow VegetationJilters for municipal wastewaters and sludges: A biological purijkation system (P. Aronsson and K.L. Perttu eds.) Swedish University of Agricultural Sciences, Section of Short Rotation Forestry, Report 50, 101-l 11. 1994. N.M. Dickinson, T. Punshon, R.B. Hodkinson and N.W. Lepp, Metal tolerance and accumulation in willows. In: Willow vegetation filters for municipal wastewaters and sludges: A biological purijkation system (P. Aronsson and K.L. Perttu eds.) Swedish University of Agricultural Sciences, Section of Short Rotation Forestry, Report 50, 121127. 1994. T. Landberg and M. Greger, Can heavy metal tolerant clones of Salk be used as vegetation filters on heavy metal contaminated land? In: Willow vegetation filters for municipal wastewaters and sludges: A biological purification system (P. Aronsson and K.L. Perttu eds.) Swedish University of Agricultural Sciences, Section of Short Rotation Forestry, Report 50, 133-144. 1994. D. Riddell-Black, Heavy metal uptake by fast growing willow species. In: Willow vegetation jilters for municipal wastewaters and sludges: A biological purification system (P. Aronsson and K.L. Perttu eds.) Swedish University of Agricultural Sciences,

30.

31.

Section of Short Rotation Forestry, Report 50, 145-151. 1994. G. &tman, Cadmium in Salk - a study of the capacity of Salk to remove cadmium from arable soils. In: Willow vegetation Jilters for municipal wastewaters and sludges: A biological purijkation system (P. Aronsson and K.L. Perttu eds.) Swedish University of Agricultural Sciences, Section of Short Rotation Forestry, Report 50, 153-155. 1994. A. G&ansson and S. Philippot, The use of fast growing trees as ‘Metal-collectors’. In: Willow vegetation jilters for municipal wastewaters and sludges: A biological purification system (P. Aronsson and K.L. Perttu eds.) Swedish University of

32.

Agricultural Sciences, Section of Short Rotation Forestry, Report 50, 129-132. 1994. M. Labrecque, T.I. Teodorescu and S. Daigle, Effect of sludge application on early development of two Salix species: productivity and heavy metals in plants and soil solutions. In: Willow vegetation Jilters for municipal wastewaters and sludges: A biological purifzcation system (P. Aronsson and K.L. Perttu eds.) Swedish University of Agricultural Sciences, Section of Short Rotation Forestry, Report 50, 157-165. 1994.

116

33.

34.

35. 36. 37.

K.L.PERl-l’U S.-O. Ericson, Salix can remove cadmium from arable land - technical and infrastructural implications. In: Willow vegetation filters for municipal wastewaters and sludges: A biological purzjkation system (P. Aronsson and K.L. Perttu eds.) Swedish University of Agricultural Sciences, Section of Short Rotation Forestry, Report 50, 173-174. 1994. B. De Vos, Using the SALIMAT technique to establish a willow vegetation cover on wet substrates. In: Willow vegetation filters for municipal wastewaters and sludges: A biological purzjication system (P. Aronsson and K.L. Perttu eds.) Swedish University of Agricultural Sciences, Section of Short Rotation Forestry, Report 50, 175-182. 1994. Anon., Nature preservation and energy forestry - Guidelines. Swedish University of Agricultural Sciences. Printed by Wikstroms Uppsala, Sweden, 16 pp. 1995. K.L. Perttu, Biomass production and nutrient removal from municipal wastes using willow vegetation filters. J of Sustainable Forestry, Vol. l(3), 57-70. 1993. K.L Perttu, Purification of sludges and wastewaters using willow vegetation filters. In: Proc. of Seminar on Sustainable Utilization of Wastewater Sludge, 27 Sep. I994 at Amsterdam l&41.ISWA General Secretariat, Bremerholm 1, Copenhagen, Denmark, 91102. 1994.