Hybrid Aspen and Poplars in the Baltic Sea Region and Iceland — RESULTS from a QUESTIONNAIRE and a LITERATURE REVIEW

ARBETSRAPPORT 999-2019 Hybrid aspen and poplars in the Baltic Sea region and Iceland — RESULTS FROM A QUESTIONNAIRE AND A LITERATURE REVIEW

Hybridasp och övrig poppel i länder i Östersjöregionen och Island ‒ Resultat från en enkät och en litteraturgenomgång

FOTO: LARS RYTTER/SKOGFORSK

Lars-Göran Stener Lars Rytter Egbert Beuker Hardi Tullus 1 Reimo Lutter Sammanfattning Plantering och odling av hybridasp och andra poppelarter är aktuella alternativ till traditionella trädslag för att öka produktionen av förnybara råvaror för energiändamål. Det är främst deras höga produktionspotential i kombination med en kort omloppstid som gör dem intressanta. De resurser som lagts ned på att förbättra produktionen, genom bl.a. skogsträdsförädling och skötsel, har varierat mellan de olika länderna inom Östersjöregionen och arbetet har så här långt till stor del gjorts på nationell nivå.

Under 2017 startade ett projekt, finansierat av Intereg, Baltic Sea Region, med syfte att ta fram idéer för utveckling och samverkan kring hybridasp och poppelodlingar i Östersjöområdet (Danmark, Estland, Lettland, Litauen, Finland, Sverige) och Island. I projektet ingick bland annat en enkät som skickades ut till berörda myndigheter, organisationer och företag samt en litteraturgenomgång av relevant forskning de senaste 20 åren i de aktuella länderna för prioritering av forskningsområden.

De viktigaste slutsatserna från enkäten var att det finns goda förutsättningar för att starta upp samarbetsprojekt. Det finansiella stödet för den här typen av forskning är begränsat, vilket understryker vikten av samarbete mellan länderna. Som exempel på samarbetsområden nämns skogsträdsförädling som skulle kunna bli mer kostnadseffektiv och därmed leda till att man snabbare kan få fram ett bättre odlingsmaterial i respektive land. Andra områden som prioriterades högt var skogsskötsel med fokus på etablering, olika skötselregimer, viltskador och kostnads/intäktsanalyser.

Av enkäten framgick också att statliga bidrag och lagar/rekommendationer som reglerar användningen av hybridasp och poppel varierar både över tiden och mellan de olika länderna, och det får en stor inverkan på framtida odlingar. Därför är det viktigt att myndigheter och organisationer ges bra information som stöd inför kommande beslut.

Litteraruröversikten visade att det finns relativt många studier som behandlar skötsel, produktion och skogsträdsförädling i Österjöområdet. Däremot var de få referenser som hade anknytning till de ekonomiska aspekterna, vilket man bör ta fasta på framöver.

Skogforsk, Uppsala Science Park, 751 83 Uppsala [email protected] skogforsk.se ©Skogforsk 2019 ISSN 1404-305X

2 Preface Many countries are adopting targets for becoming carbon-neutral within a near future. One option to meet this target is to expand the energy supply from both forest and agricultural land by using high-productive forest systems and energy crops. High yield combined with short rotations have led to an increased interest in using fast-growing species such as hybrid aspen and poplars in the Baltic sea region and Iceland.

This report includes results from a questionnaire and a literature review and gives information about political and socio-economic regulations aiming at finding major gaps in knowledge and to define and select essential research topics for hybrid aspen and poplar species in the coming years.

The report was performed with financial support from the Seed Money project #S017 Wood Biomass Production in Medium Rotation Plantations with Hybrid Aspen and Poplars (MedRoPlan) of Interreg Baltic Sea Region.

Ekebo, Svalöv December 10, 2018

Lars-Göran Stener, Skogforsk, Ekebo, Sweden Lars Rytter, Skogforsk, Ekebo, Sweden Egbert Beuker, LUKE, Natural Resources Institute Finland, Finland Hardi Tullus, Estonian University of Life Sciences, Estonia Reimo Lutter, Estonian University of Life Sciences, Estonia

3 Contents Sammanfattning...... 2 Preface...... 3 Summary...... 5 Introduction...... 7 Questionnaire...... 8 Method...... 8 Results and discussion...... 8 Political and socio-economic regulations...... 11 International agreements and climate change...... 11 Other relevant political documents...... 11 Baltic sea countries – Woody biomass for energy consumption...... 11 Political barriers and subsidies for medium rotation hybrid aspen and poplar plantations...... 12 Literature review...... 13 General...... 13 Breeding and genetics...... 15 Mangement...... 15 Growth and yield...... 15 Nutrients...... 16 Carbon...... 16 Biotic and abiotic risks...... 17 Biodiversity...... 17 Wood properties...... 18 Economics...... 18 Phytoremediation...... 18 Climate change...... 18 Projects in progress...... 19 Estonia...... 19 Finland...... 19 Sweden...... 20 Major conclusions...... 21 Appendix 1. Questionnaire − cultivation of hybrid aspen and poplar...... 22 Appendix 2. Questionnaire −breeding activities for hybrid aspen and poplar...... 24 Appendix 3. Questionnaire − regulations of hybrid aspen and poplar...... 26 Appendix 4. Questionnaire − subsidies of hybrid aspen and poplar...... 26 Appendix 5. Information about the participants and the distribution on their involvement in the different parts of the questionnaire...... 27 Appendix 6. Results from the questionnaire regarding general cultivation of hybrid aspen and poplar...... 28 Appendix 7. Results from the questionnaireregarding breeding of hybrid aspen and poplar...... 30 Appendix 8. References in the litteratur review...... 33

4 Summary Planting and cultivation of hybrid aspen and other Poplar species are alternatives to more traditional species, as a way to increase the sustainable biomass production for energy supply. The resources put into improvement of yield by for instance tree breeding and silviculture, for poplar species have varied among the different countries in the Baltic region and have mainly been performed at a national scale. Putting the knowledge obtained from each country together, will improve the overall knowledge and a cooperation among countries will make poplar research more cost efficient.

During the period 2017-2018, the MedRoPlan seed-money project was carried out by financial support from Interreg, Baltic Sea Region. The main aim was to work out ideas for development and improvement of wood biomass production by medium rotation forestry using hybrid aspen and other poplar species as a cooperation project among countries within the Baltic Sea Region and Iceland. The project included a questionnaire to different stakeholders and a literature review aiming at finding major gaps in knowledge and to define and select essential research topics for hybrid aspen and poplar species in the coming years.

Results from the questionnaire indicated that the presumptions for initiating joint projects about Populus species among the countries in the Baltic Sea area and Iceland are good. The limited financial support for poplar research was found to be a problem. This emphasizes the importance of cooperation among countries. Topics like tree breeding and silvicultural management with focus on establishment, management, damage by wild game and economy were mentioned in the first place.

The literature review showed that there are quite many studies in the Baltic region, relating to management, yield and breeding. However, few are dealing with the economic aspects, which should be taken into consideration the coming years.

5 FOTO: LARS RYTTER/SKOGFORSK LARS FOTO:

6 Introduction Recent discussions about societies becoming more bio-economically based in the future, has led to an increased interest in using fast-growing species such as hybrid aspen and poplars. Populus species have a great potential to provide a significant share of the wood based biomass. For instance, biomass production has been estimated to around 10 tons DM per hectare and year on former agricultural land for selected hybrid aspen clones in southern Sweden with a rotation of 20-25 years. Research on poplars indicates an even higher yield. Besides, for energy purposes, the wood can be used for a variety of products, such as timber, matches and pulp. There is also a potential for new products such as bio- plastics, biocarbon fibres and biorefineries. In Iceland wood biomass from locally grown poplars provides a green carbon alternative to fossil coal used in the production process of silicium metal. The fast growth of Populus species at young ages also give possibilities for utilisation in shelterbelts, phytoremediation and reclamation of degraded land.

Due to the interest in poplar research at national levels, each country in the Baltic Sea Region has gained a certain level of knowledge. However, so far cooperation among countries in these matters have been quite limited. Putting the knowledge obtained from each country together, will allow for an interpretation on a larger, transnational scale, where different environmental conditions of importance for establishment and growth are covered. Beside an improvement in knowledge, a cooperation among countries will make poplar research more cost efficient.

During the period 2017-2018, the MedRoPlan seed-money project was carried out by financial support from Interreg, Baltic Sea Region. The aim was to work out ideas for development and improvement of wood biomass production by medium rotation forestry using hybrid aspen and other poplar species as a cooperation project among countries within the Baltic Sea Region and Iceland. In order to create incentives for establishment of new fast-growing cultivations, the project was targeted towards different stakeholder groups having interest in different aspects/needs of hybrid aspen and poplar in the Baltic Sea Region. The groups and their aspects/needs were: 1. Tree breeders and nursery staff: Build-up of test populations of hybrid aspen and poplar clones for selection of new material as alternatives to high yielding clones used commercially today. 2. Researchers (in general): Establishment of field tests for research in genetics, management and wood quality and construction of an extensive database including data and results from previous established tests. 3. Forest managers, land owners and forest consultants: Build-up of cost-efficient growth models and management guidelines for cultivation of hybrid aspen and poplars, including risk management. 4. Forest industries and energy pro- ducers: Need of high-quality biomass material for specific (novel) end products and energy production. 5. Policy makers and the general public: Economic and social aspects of short rotation forestry with hybrid aspen and poplars. This project is in line with the implementation of the EU strategy aiming at an increased capacity for transnational cooperation for countries within the Baltic Sea Region. The report includes results from a questionnaire and a literature review and also gives information about political and socio-economic regulations, aiming at finding major gaps in knowledge and to define and select essential research topics for hybrid aspen and poplar species in the coming years.

7 Questionnaire METHOD A questionnaire was prepared, asking for information on the state of play with respect to a) forest management, b) tree breeding activities, c) political and socio-economic regulations (subsidies, restrictions etc.). The final questionnaire (App. 1-4) was sent in November 2017 to various stakeholders dealing with these questions in the different countries in the Baltic Sea Region and in Iceland. Each respondent represented an organisation (private estate etc…) within a specific country and the answers were written from this perspective.

A scale from 0 to 5 was used for each question where 0 = no or nothing … 5 = very much (or similar), i.e. the higher score the more positive or important attitude. In the evaluation, overall mean scores as well as mean scores for each country and type of organisation was calculated for each question.

FOTO: LARS RYTTER/SKOGFORSK

RESULTS AND DISCUSSION Participants Out of the totally 39 number of questionnaires sent out, we got 26 answers where Denmark, Iceland and Latvia were each represented by one respondent, Lithuania by two, Finland by three, Estonia by six and Sweden by 12, which in a way may indicate differences in interest in hybrid aspen and poplars among the countries (App. 5). The answers of the general questions were represented by five major types of organisations (Companies, Major estates, Research organisations, Nurseries and Other). The questions regarding breeding, regulations and subsidies were only sent to specific people with good expertise on such topics (App. 5).

8 General questions Mean scores for each question was calculated for hybrid aspen and other poplars species based on the individual answers within each country and organisation respectively (App. 6). The main results are here commented upon.

Question 1: What is the level of interest for cultivation of hybrid aspen or poplars in your country? The level of interest in cultivation for hybrid aspen and poplar was in general moderate. Hybrid aspen showed in relative terms highest interest in Sweden and poplar got the highest scores in Denmark, Iceland and Latvia. The respondents from the research organisations seemed to be more interested than the others.

Question 2: What land type is /will be of concern? Cultivation on agricultural land was generally of higher concern than on forest land except in Latvia and Finland.

Question 3: Main objective of cultivation? The main intention with cultivation varied among countries and this is most probably related to differences in the basic industrial use of the wood. Thus, as expected, the scores for the use of biomass for energy was higher for poplar than for hybrid aspen, while the opposite was found for production to the mechanical wood industry.

Question 4: What are the main challenges/problems? The main challenges/problems varied greatly among countries, but high establishment costs had high mean scores for hybrid aspen and damage by wild game had high mean scores both for hybrid aspen and poplar. In addition, poplar had high scores for uncertain future market and poor knowledge of the production on different sites.

Questions 5-7: Is research and development in 5) breeding, 6) silviculture/management and 7) wood quality relevant for hybrid aspen and poplar? The relevance of research and development had generally rather high scores for all three activities (breeding, silviculture/management and wood quality). Growth/survival and climatic adaptation was stressed for breeding, while economy and optimal management was pointed out for silviculture research.

Question 8: Are there other factors of importance such as social values, nature conservation issues, effects on soil and water? The mean scores for “other factors of importance” varied greatly among countries but the overall means were intermediate, indicating that these factors should not be neglected.

Question 9: What are the reasons for limited research activities for these species presently in your country? Limited resources, fundings and insufficient support from the forestry sector and policy makers were the main reasons for limited research activities.

Question 10: Would you or your organisation be interested in joining applications for funding for international projects on wood biomass production with hybrid aspen and poplars in Northern Europe? The response for joint applications of new biomass production projects was much related to the type of organisation. Major estates and research organisations gave high scores, while the scores for the others were quite low. This seems logic, since it is the first two types of organisations that mainly are involved in these activities.

9 Breeding Individual scores for some of the questions asked regarding breeding of hybrid aspen and poplar is presented in Appendix 7 and is here commented upon.

Question 1: Previous and present breeding activities for hybrid aspen or other poplars? The overall breeding activities are generally low. Presently, intermediate breeding activities of hybrid aspen is carried out in Lithuania, Finland and Sweden and to some extent in Latvia, while poplar is only of concern in Lithuania and Sweden. Previously, intensive hybrid aspen breeding work was apart from Sweden and Finland also performed in Latvia, while intensive poplar breeding was carried out in Iceland with some activities also in Finland.

Question 2: Interest in starting up new activities aiming at selection of well performing hybrid aspen/poplars? There seems to be a great interest in starting up new breeding activities in all countries except for hybrid aspen in Iceland and poplar in Latvia. There is also a general high interest for starting up joint breeding activities.

Question 3: What are the target traits for selection/breeding? The most essential breeding traits for both aspen and poplar are growth/survival, climatic adaptation, while stem quality and resistance to pathogens seem to be of intermediate importance. Insect resistance is generally of very small priority, probably because this has so far not caused any serious problems.

Regarding questions on available test material for hybrid aspen, there are 35 – 150 clones in each of Finland, Latvia, Lithuania and Sweden where around in total 100 clones may be relevant for joint field testing. These countries were also positive to exchange this material among countries for such tests. There exists genotypically selected clones of hybrid aspen in all countries except in Denmark, Estonia and Iceland and the number within the countries varies between 10 and 20 clones. The knowledge of genetic parameters, which are of importance for effective selection and breeding, seem to be rather good. The mean score was 3.2 in the countries where breeding activities have been performed.

For poplars, genetic material is available in Finland, Iceland, Lithuania and Sweden where the two Swedish organisations STT and SLU have the major part (up to 300 clones). Most clones consist of P. tricocharpa, but there are also clones of P. maximowiczi, P.nigra, P. deltoides and their hybrids. Several of the clones could be relevant for joint field tests and the organisations seem, in general, to agree on making them available. Genotypically selected clones of poplar are used in Denmark, Iceland, Lithuania and Sweden and the number varies between 2 and 15 clones. The knowledge of genetic parameters is less compared to hybrid aspen (mean score of 2.3).

10 Political and socio-economic regulations INTERNATIONAL AGREEMENTS AND CLIMATE CHANGE The Kyoto protocol (1997) and the more recently Paris Agreement (2015) under the United Nations Framework Convention on Climate Change (1992) are agreements expected to reduce greenhouse gas emissions and mitigate global warming. The bio- economic goals of the European Union are targeted towards more resource-efficient, low-emission and sustainable economy (Updated Bioeconomy Strategy, A sustainable bioeconomy for Europe: strengthening the connection between economy, society and the environment. 2018). At the same time, biodiversity and environmental protection must be ensured. The Nordic countries have set a target to be carbon neutral in 2050. Forestry and woody biomass plays an important role in achieving such goals for the Nordic and Baltic countries.

OTHER RELEVANT POLITICAL DOCUMENTS • European Commission (2009) DIRECTIVE 2009/28/EC: The promotion of the use of energy from renewable sources and amending and subsequently repealing Directives 2001/77/EC and 2003/30/EC.

• European Commission (2011) Energy Roadmap 2050. Communication From The Commission To The European Parlament, The Council, The European Economic and Social Committee And The Committee Of The Regions. Brussels, 20 p.

• European Parliament (2015) A new EU Forest Strategy: for forests and the forest-based sector’ (2014/2223(INI)).

• International Energy Agency (2016) Nordic Energy Technology Perspectives 2016: Cities, flexibility and pathways to carbon-neutrality. OECD, France; Nordic Energy Research, Norway, 265 p.

• IPCC (2014) Climate Change 2014. Synthesis Report. Contribution of Working Groups I, II and III to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change [Core Writing Team, R.K. Pachauri and L.A. Meyer (eds.)]. IPCC, Geneva, Switzerland, 151 pp.

BALTIC SEA COUNTRIES ‒ WOODY BIOMASS FOR ENERGY CONSUMPTION Countries around the Baltic Sea are characterized with a high share of forests in relation to the total land area and a relatively intensive forest management. On the other hand, forest production of native tree species like spruce, pine and birch is rather low compared to forests in more southern countries. For example, the rotation cycle in northern Europe can vary from 40 to more than 120 years. At the same time, Nordic and Baltic forests are assumed to meet other functions like biodiversity and environmental protection. The increasing demand of woody biomass by the energy sector as well as by the forest industry will increase the need for alternative wood sources. Using the novel forest management called “short rotation forestry” with fast-growing tree species is one promising alternative. The new European Union Forest Strategy (2015) emphasizes that short rotation forestry could provide sustainable wooden biomass in the near future.

11 A relatively high share of the gross final energy consumption in countries around the Baltic Sea (Sweden, Finland, Latvia, Estonia, Lithuania and Denmark) comes from renewable energy resources. For example, all these countries, except Latvia, have already achieved their renewable energy goal for EU Member States 2020. Woody biomass con- stitutes more than 50% of the renewable energy sources in the Baltic Sea countries, being over 90% for example in Estonia. Thus, woody biomass plays an important role in the Baltic Sea countries and indicates a high dependence of forest resources to the energy sector. Moreover, the Baltic and Nordic countries have modern forest and wood processing industries that is oriented to improvement of the value of solid timber by producing more advanced wood products.

Even though all the Baltic Sea countries are characterized by a relatively high forest cover, still a surprisingly high share of former agricultural land is not used. The latest estimations indicate that about 1.8 to 2.6 Mha of abandoned agricultural land may be implemen- ted for energy and biomass production in the Nordic and Baltic countries. Using such land for intensive woody biomass production with non-native species is one alternative to satisfy the demands from society and forest industry. Hybrid aspen and poplars are probably the most promising species alternatives, since they have a large growth potential at short rotations.

POLITICAL BARRIERS AND SUBSIDIES FOR MEDIUM ROTATION HYBRID ASPEN AND POPLAR PLANTATIONS The results from the questionnaire carried out, indicated several political barriers that may be obstacles for establishment of new plantations of hybrid aspen and poplars. The regulations differ among the countries and are here shortly commented upon.

Legislation and environmental regulations such as certifications must be considered. For instance, the limitations of FSC certification regarding the utilized area of non-native tree species reduce the interests of forest companies to expand their forest portfolio with hybrid aspen and poplar plantations. In general, up to 5% of the forest property can be used for non-native species according to FSC. However, this restriction can be modified within each country.

In addition, country specific silvicultural regulations made by the states, is another reg- ulating factor of using non-native species on forestland. The limitations are often more restricted to poplars compared to hybrid aspen. For example, in Finland and Estonia, hybrid aspen can be used on forestland, while poplars are not allowed. In Sweden, Latvia and Lithuania, poplars can be planted on forestland. The use of a single hybrid aspen or poplar clone is allowed by the forestry acts in all countries, while a clonal mixture cannot be used in Finland.

Subsidies is a factor having a large impact on the interest in establishing plantations of hybrid aspen and poplar. Subsidies vary over time and among countries. At present, there are subsidies for establishment of hybrid aspen in Latvia, Lithuania and Sweden and for poplar in Iceland, Latvia and Sweden. The subsidies in Sweden are only given for crops on agricultural land and the rotations must not be longer than 20 years. There are no subsidies in Finland and Estonia. In Estonia, subsidies can only be applied to plantations of short-rotation willows, which are considered to be an agricultural crop.

12 Literature review GENERAL A review was made of recent (year 2000 or later) publications (mainly peer reviewed, also incl. doctoral dissertations), published in English, relating to hybrid aspen and poplar cultivations in the Nordic and Baltic countries. In total 118 publications (Appendix 8) were identified on 1) breeding and genetics, 2) management, 3) production and yield, 4) nutrients, 5) biotic and abiotic risks (incl. climate change), 6) wood properties and use, 7) economics, 8) phytoremediation and 9) others.

Of course, several publications deal with more than one topic, but a major topic was identified for each of them. Publications purely on the biology or ecology of aspen or poplars were not included. In addition to these, there are many publications in national languages of the Nordic and Baltic countries. Most publications were found in Sweden. In general, it seems that the publications written in English, are directed towards the scientific community, whereas the publications in national languages are aimed for the practical forest sector.

Almost half (52) of the recent publications are from Sweden, 34 from Finland, 24 from Estonia, 7 from Latvia and 1 from Denmark. There were no publications found from Norway or Lithuania. Management (25) and production and yield (22) are the main topics of the publications, followed by wood properties (17) and breeding (12). Surprisingly there were only 2 publications on economics and 4 on biotic ad abiotic risks.

Number of publications

Figure 1. Number of publications per year

13 Number of publications

Figure 2. Number of publications per country

Number of publications

Figure 3. Number of publications per item

A short summary is presented below for each topic. This is based on the publications found but also information from country presentations given at a HealGenCAR workshop on hybrid aspen and poplar breeding and cultivation held in Helsinki 7-8 February 2017, was used.

14 BREEDING AND GENETICS Some breeding activities have been carried out with hybrid aspen in all the Nordic and Baltic countries. The first hybrid crossings were made in Sweden in the late 1930’s. In the other countries hybrid aspen breeding started in the 1950’s or 1960’s, in Estonia not until the 1980’s. In the beginning the main goal was focused on yield, but later other selection traits, such as vitality (resistance), propagation ability, stem straightness and branchiness, were included. More recently, clones are also tested for wood quality characteristics. The hybrid material produced during the periods mentioned above, have been used for selection of well-performing clones for use in Sweden, Finland, Estonia and Lithuania respectively. New crossings (the recent 20 years) followed by new field tests have mainly been performed in Finland.

Breeding of other poplars has been very limited in the Nordic and Baltic countries. Minor activities have been carried out in Sweden where clones selected in other European or North American improvement programmes have been tested in field trials. Greatest interest is placed on single species or hybrids of P. trichocarpa and P. maximowiczii. The best clones, according to results from the field tests, have been selected for commercial use.

MANAGEMENT The way of management depends on the purpose of the plantation. For pulpwood or raw material for the mechanical wood industry, the rotation cycle is 20-30 years, while for production of energy wood, rotation periods down to 4 years have been tested. The plantations are established with a rather wide spacing, generally using 1000-1500 seedlings per hectare. With hybrid aspen, after the clear cut of the first plantation, a second stand can be grown from root suckers. Root suckers usually grow very dense, and the new stand can be raised for energy biomass in short rotation, large size trees, or a combi- nation of the two. Different management practices for the root sucker stands have been described and shown an early flexibility without losing productivity.

GROWTH AND YIELD Poplar plantations in general have been shown to have a very high biomass production potential. In hybrid aspen and poplars, a mean annual increment of up to 25 m³ of stem wood per hectare and year during a rotation of around 25 years appears to be realistic on fertile sites in southern Sweden, when using the best available selected clonal material. This corresponds to a dry weight of stem biomass of 8.2 tons of dry matter (DM) per hectare and year and to about 10 tons DM per hectare and year if branch biomass is included. Similar production levels have been reported at two Danish sites but already after only 13 years, i.e. far earlier than the expected optimal rotation age. Root sucker stands of hybrid aspen have even a faster initial growth than the first planted generation. Mean annual production in trials has reached a level of about 10 tons DM per hectare and year already after a few years.

15 FOTO: LARS RYTTER/SKOGFORSK

NUTRIENTS Studies about the nutrient balance in fast-growing poplar and hybrid aspen plantations are important for management to keep up high production rates as well as for environmental issues (leaching, fertilisation and water quality). So far, the main studies about soil nutrients have been carried out in Sweden and Estonia. The results cover only the first part of the rotation cycle and generally conclude that there are no major losses of soil macronutrients. Hybrid aspen tolerates lower soil acidity in comparison to poplars. Nutrient turnover is very intensive during the prime growth period in fast-growing plantations. The knowledge about micronutrients and heavy metals in the soil is poorly studied. Nutrients uptake by trees is so far mainly studied in the first half of the rotation cycle because of the sampling complexity of older trees. Recently, studies on nutrients uptake of second generation hybrid aspen plantations were published in Finland, but generally nutrients balance and uptake of different management scenarios in second generation plantations is unclear. Only few studies exist about fertilisation in hybrid aspen and poplar plantations in the region.

CARBON Even though forest ecosystems are highly important for the terrestrial carbon (C) balance, the studies about C balance in hybrid aspen and poplar plantations are still very rare in the Baltic Sea region (Sweden and Estonia). An ecosystem level study was carried out in Estonia on hybrid aspen plantations where the changes of C pools were studied for above- and below-ground level: the results showed the importance of former land-use type on the topsoil C pools; an intensive C sequestration was observed in the above-ground pool. Already in the middle of the rotation period, hybrid aspen plantation are C sequestrating ecosystems. There is lack of studies about C balance in second generation hybrid aspen stands. More precise estimations of C balance are needed in fast-growing poplar and hybrid aspen plantations during their whole rotation cycle.

16 BIOTIC AND ABIOTIC RISKS Attention should be given to the biological risks in poplar and aspen cultivation. There are several biotic and abiotic risks for poplar and hybrid aspen plantations and there may be some differences between countries. These risks are also a major reason for forest owners being reluctant to establish poplar or hybrid aspen plantations.

The most serious pathogens on hybrid aspen are stem and branch canker (Entoleuca mammatum, Leucostoma niveum) and leaf rust and bacteria canker (Melampsora larici-populina, Xanthomonas populi) on poplars. Yield and vitality can be drastically reduced, and the trees can eventually be killed. There is a large variation among clones, which is used in resistance breeding. The selected clones used in Sweden and Iceland, where rust on poplars have been a serious problem, have among others been selected for resistance to the pathogens mentioned. Leaf and shoot blight of aspen (Venturia macularis) is a pathogen causing deformities in terminal shoots on plants. The effects can be severe, but hybrid aspen seem to be less infected compared to ordinary aspen.

Moose, deer, hare, voles and/or sheep are a major threat for hybrid aspen and poplar plantations in most of the Nordic and Baltic countries. Fencing is one alternative for protection against herbivores, but quite expensive. Another alternative during the first years is using repellents. The best way to avoid vole damage is proper vegetation control during the two first years of establishment. It is known from for instance studies of birch that there is a clonal variation in palatability, which may be used for breeding less attractive plant material also for Populus species.

Damages related to the climate is of very high concern, especially at our northern latitudes and especially for introduced species such as poplars, since they are not fully adapted to our climate conditions. The identification of varieties with a relevant timing of budburst and growth cessation is crucial for a high growth performance in the boreal regions and will probably be more important in respect of the climate change.

One specific problem is frost cracks that drastically can reduce stem quality also for older trees. These cracks occur as a result of high temperature variations during early spring. The sun heats the bark of the trees during the daytime, while at night still freezing tem- peratures occur, resulting in tensions in the wood, just above the snow cover. The cracks occur at the south side of the stems and their occurrence seem to have a variation among clones.

BIODIVERSITY Poplar and hybrid aspen plantations offer temporal habitats for various species and improve the spatial variability of the landscape. Poplar and hybrid aspen plantations offer temporal habitats for various species and improve the spatial variability of the landscape. Biodiversity aspect of Populus spp. plantations has been studied mainly in Estonia. The results show that hybrid aspen on agricultural land has a positive effect on vascular plants and composition of mosses and the number of species increase with the development of the stand. Hybrid aspen plantations may already in a very early development phase host a high number of lichens. Comparison of hybrid aspen with native silver birch plantations shows no major differences in vascular plants composition. However, longer studies about biodiversity are needed where other groups of species are included (insects, mammals, birds, fungal communities etc.)

17 WOOD PROPERTIES Concerning wood properties, there has been a shift in research. During the early years of this century the main focus was on fiber and other characteristics of importance for the pulp and paper industry, especially in Finland. However, this declined around 2005. More recent studies, especially from Finland, deal with the genetic variation in wood properties of hybrid aspen for bioenergy, including nutrient contents. Recent studies from Estonia deal with chemical compounds of hybrid aspen wood.

ECONOMICS There has been very little research on the economics of hybrid aspen and poplar cultivation in the Nordic and Baltic countries. This may be partly due to the unclear markets for the wood, as was also pointed out in the results from the questionnaire. Also, additional costs such as fencing, have a large effect on the profitability of a plantation. Some practical economical outcomes of poplar and hybrid aspen cultivation has been published.

PHYTOREMEDIATION There are only a few publications from the Baltic Sea Region on the use of hybrid aspen or poplars for phytoremediation. Two of them are from a Finnish study, comparing different tree species, including hybrid aspen, for cleaning up oil-polluted soils. The other two are from Estonia where the suitability of hybrid aspen was tested for recovering reclaimed oil shale quarry.

CLIMATE CHANGE Fast-growing tree plantations may play an important role in climate change mitigation through intensive CO2 uptake. The pressure to reduce the use of fossil fuel-based energy will increase the demand for woody biomass, and with that the importance of fast- growing Populus spp. will also increase. On the other hand, tree breeding should enhance adaptation to a changing climate: 1) elongated growing season will allow growing tree genotypes from southern regions that have already adapted to a longer vegetation period and 2) the frequency of drought stress will probably increase during mid-summer, which might reduce the production of highly transpiring tree species (poplars, aspens). Tree breeding strategies aiming at finding genotypes with high water-use efficiency are needed.

18 Projects in progress The resources put into improvement of yield by for instance tree breeding and silviculture, for poplar species have varied and are still varying among the different countries in the Baltic region. This is indicated by the literature review and by the list below where the current (2018) research projects in hybrid aspen and poplars in the Baltic region are listed.

All the current projects are carried out at a national level, with a limited amount of material and data. Several aspects in current projects are not addressed in all countries but in one specific country, such as production and management in Sweden, wood quality in Finland and environmental aspects in Estonia. The reason for this can for instance be differences in priority of various research fields, access to appropriate research material as well as competent researchers. Cooperation among countries will in one way or another involve all member states in all activities resulting in an overall better knowledge of fast-growing hybrid aspen and poplars.

ESTONIA IUT21-4 "The carbon dynamics in Estonian forests affected by sustainable management. Subsection: Carbon dynamics in stands of fast growing tree species, short rotation forestry (SRF) (2014−2019)", Estonian University of Life Sciences, Institute of Forestry and Rural Engineering.

P170053MIMK "Effective and sustainable forest management“ (2017−2018)", Estonian University of Life Sciences, Institute of Forestry and Rural Engineering.

8-2/T15064MIMK "Smart biodiversity conservation in Estonian natural and managed forests (2015−2018)", Estonian University of Life Sciences, Estonian University of Life Sciences, Institute of Forestry and Rural Engineering.

8T160023MIMK "The management and environmental impact of the second generation hybrid aspen plantations (2016−2018)", Estonian University of Life Sciences, Institute of Forestry and Rural Engineering.

FINLAND AspenWill “More, faster, higher quality: potential of short-rotation aspen and willow biomass for novel products in bioeconomy”; Project manager: Tuula Jyske; Organisation: Luke; Duration: 2018-2020.

“Intensive production of wood biomass”; Project manager: Jyrki Hytönen; Organisation: Luke; Duration: 2012-2019.

Hybrid aspen is, as a minor tree species, part of the Finnish national tree breeding programme. This programme is run by Luke; Manager Matti Haapanen.

Hybrid aspen is also part of the forest production studies at Luke, looking at the effects different management practices; Manager Jari Hynynen.

19 SWEDEN Environmental effects of biofuel production in fast-growing plantations on abandoned arable land (Funding source: Swedish Energy Agency; Project manager: Lars Rytter; Organization: Skogforsk; Project no P30659-4; Duration: 2018–2020).

Sustainable cultivation of root sucker generated hybrid aspen – Growth and soil impact (Funding source: Swedish Energy Agency; Project manager: Lars Rytter; Organization: Skogforsk; Project no P41896-2; Duration: 2018–2021).

Nutrient recycling circular economy model for large cities - water treatment sludge to biomass for bioenergy (Funding source: Swedish Energy Agency; Project manager: Almir Karacic; Organization: Swedish University of Agricultural Sciences, SLU; Project no P45082-1; Duration: 2018–2021).

Improved seedling types and regeneration methods for establishment of poplars on Swedish forest soils (Funding source: Swedish Energy Agency; Project manager: Urban Nilsson; Organization: Swedish University of Agricultural Sciences, SLU; Project no P41931-1; Duration: 2016–2019).

Synthetic biology in crops – synthetic genetic circuits for extended biosynthesis of lignocellulose (Funding source: Swedish Energy Agency; Project manager: Ines Ezcurra; Organization: KTH; Project no P36662-1; Duration: 2013–2019).

Realising biomass supply – farmers’ perspective on straw and short-rotation forestry (Funding source: Swedish Energy Agency; Project manager: Kristina Blennow; Organiza- tion: Swedish University of Agricultural Sciences, SLU; Project no P45808-1; Duration: 2018–2022).

Productivity of hybrid aspen in southern Sweden (Funding source: The Swedish Forest Society Foundation; Project manager: Nils Fahlvik; Organization: Skogforsk; Duration: 2018–2019).

Forest production systems based on tailored trees (Funding source: Swedish Foundation for Strategic Research; Project manager: Ewa Mellerowicz; Organization: Swedish University of Agricultural Sciences, SLU; Project no RBP14-0011; Duration: 2015–2020).

20 Major conclusions One important conclusion from the questionnaire is that the presumptions for initiating joint projects about Populus species (hybrid aspen and poplars) among the countries in the Baltic Sea area and Iceland are good. Even if the activities of hybrid aspen and poplars are at a low level in some countries, there seems to be a positive attitude to start up joint research projects. This is in line with the objectives of the MEDROPLAN project.

The fundings for poplar research is a problem in all countries, which emphasize a cooperation between countries having similar growth conditions. Breeding is one essential part in such a cooperation. At present, each of the countries has its own breeding program and there are, at the same time, only a few hybrid aspen and poplar clones for use at these northern latitudes. Faced to an expected increase of the cultivated area of these species, it is necessary to test and select more clones to reduce the risk of economic and environmental setbacks. Climatic adaptation is a main selection trait and finding well performing clones for future use in an unknown climate is favoured by the establishment of tests at many sites. This could be achieved by a cooperation among the countries by planting the same test material in several locations in each country and after some years, make a joint evaluation. Furthermore, breeding is expensive and a cooperation among countries will make it more cost efficient and may result in faster introduction of new, superior clones for use in each country.

Silvicultural research with emphasis on establishment, management regimes, damage by wild game and the overall costs are other topics that seem to have high priority in most countries. A collaboration will improve and spread knowledge of establishment, management, production, environmental benefits and thereby the profitability of Populus species cultivations.

From the political and socio-economic regulations survey, it is clear that subsidies, legislation and environmental regulations vary in time and among countries. For sure they will have a large impact in establishing hybrid aspen and poplar plantations in the future. One way to decrease negative effects of such regulations is by increasing information to politicians and decision makers. The potential of using fast growing broad-leaved species on abandoned agricultural land should be obvious as one complementary way to become carbon-neutral in year 2050 without any environmental drawbacks.

The results from a review of recent publications relating to hybrid aspen and poplar cultivations in the Nordic and Baltic countries showed quite many studies dealing with topics like management, yield, wood properties and breeding. Economy, risk assessments and phytoremediation were little handled and should be focused more the coming years.

21 Appendix 1. Questionnaire – cultivation of hybrid aspen and poplar

No Question Hybrid aspen Poplars 1 What is the level of interest for cultivation of hybrid aspen or 0, 1, 2, 3, 4, 5 0, 1, 2, 3, 4, 5 poplars in your country?

2 What land type is/will be of concern? 2a Forest land 0, 1, 2, 3, 4, 5 0, 1, 2, 3, 4, 5 2b Agricultural land 0, 1, 2, 3, 4, 5 0, 1, 2, 3, 4, 5

3 Main objective of cultivation: 3a Biomass for energy production (short rotation) 0, 1, 2, 3, 4, 5 0, 1, 2, 3, 4, 5 3b Fibre production for the pulp and paper industry 0, 1, 2, 3, 4, 5 0, 1, 2, 3, 4, 5 3c Wood production for the mechanical wood industry (logs etc.) 0, 1, 2, 3, 4, 5 0, 1, 2, 3, 4, 5 3d Other (please specify) 0, 1, 2, 3, 4, 5 0, 1, 2, 3, 4, 5

4 What are the main challenges/problems? 4a Lack of available land 0, 1, 2, 3, 4, 5 0, 1, 2, 3, 4, 5 4b High costs at establishment 0, 1, 2, 3, 4, 5 0, 1, 2, 3, 4, 5 4c Poor knowledge about optimal establishment methods 0, 1, 2, 3, 4, 5 0, 1, 2, 3, 4, 5 4d Poor knowledge about silvicutural management (thinning, harvesting..) 0, 1, 2, 3, 4, 5 0, 1, 2, 3, 4, 5 4e Poor availability of genetically improved plant material 0, 1, 2, 3, 4, 5 0, 1, 2, 3, 4, 5 4f Poor general knowledge about the performance of the plant material 0, 1, 2, 3, 4, 5 0, 1, 2, 3, 4, 5 4g Poor knowledge about the production for different sites 0, 1, 2, 3, 4, 5 0, 1, 2, 3, 4, 5 (soil types, site fertilities) 4h Environmental regulations/public perception 0, 1, 2, 3, 4, 5 0, 1, 2, 3, 4, 5 4i Uncertain future market 0, 1, 2, 3, 4, 5 0, 1, 2, 3, 4, 5 4j Poor economical outcome 0, 1, 2, 3, 4, 5 0, 1, 2, 3, 4, 5 4k Problems with moose, deer... 0, 1, 2, 3, 4, 5 0, 1, 2, 3, 4, 5 4l Problems with pathogenes, insects 0, 1, 2, 3, 4, 5 0, 1, 2, 3, 4, 5 4m Greater overall risks compared to other species 0, 1, 2, 3, 4, 5 0, 1, 2, 3, 4, 5 4n Lack of subsidies (establishment, fencing, tending) 0, 1, 2, 3, 4, 5 0, 1, 2, 3, 4, 5 4o Other (please specify)

5-8 Please indicate if research and development in any of the following themes (item 5-8) would be relevant for hybrid aspen and poplar: 5 Breeding If so, classify the level of importance for each of these research groups 5a Growth and survival 0, 1, 2, 3, 4, 5 0, 1, 2, 3, 4, 5 5b Vitality (climatic adaptation) 0, 1, 2, 3, 4, 5 0, 1, 2, 3, 4, 5 5c Resistance to pathogens and insects 0, 1, 2, 3, 4, 5 0, 1, 2, 3, 4, 5 5d Stem and wood quality (straightness, branchiness, basic density,...) 0, 1, 2, 3, 4, 5 0, 1, 2, 3, 4, 5 5e Other (please specify)

22 Appendix 1. Continuation

No Question Hybrid aspen Poplars 6 Silviculture/management If so, Classify the level of importance for each of these research groups 6a Establishment by planting 0, 1, 2, 3, 4, 5 0, 1, 2, 3, 4, 5 6b Establishment by root or stump suckers (2'nd generation) 0, 1, 2, 3, 4, 5 0, 1, 2, 3, 4, 5 6c Weed control 0, 1, 2, 3, 4, 5 0, 1, 2, 3, 4, 5 6d Performance on sites of different fertility 0, 1, 2, 3, 4, 5 0, 1, 2, 3, 4, 5 6e Fertilisation and nutrient removal after harvest 0, 1, 2, 3, 4, 5 0, 1, 2, 3, 4, 5 6f Optimal management regimes (thinnings, rotation time...) 0, 1, 2, 3, 4, 5 0, 1, 2, 3, 4, 5 6g Economy 0, 1, 2, 3, 4, 5 0, 1, 2, 3, 4, 5 6h Other (please specify)

7 Wood quality from different end users point of view If so, classify the level of importance for each of these research groups 7a Stem straightness 0, 1, 2, 3, 4, 5 0, 1, 2, 3, 4, 5 7b Branchiness (number, angle and thickness of branches) 0, 1, 2, 3, 4, 5 0, 1, 2, 3, 4, 5 7c Wood properties (basic density, fibre angle …) 0, 1, 2, 3, 4, 5 0, 1, 2, 3, 4, 5 7d Chemical properties 0, 1, 2, 3, 4, 5 0, 1, 2, 3, 4, 5 7e Other (please specify)

8 Are there other factors of importance such as 8a Social values 0, 1, 2, 3, 4, 5 0, 1, 2, 3, 4, 5 8b Nature conservation issues 0, 1, 2, 3, 4, 5 0, 1, 2, 3, 4, 5 8c Effects on soil and water 0, 1, 2, 3, 4, 5 0, 1, 2, 3, 4, 5 8d Other (please specify)

9 What are the reasons for limited research activities for these species presently in your country? 9a Limited resources (money and staff) 0, 1, 2, 3, 4, 5 0, 1, 2, 3, 4, 5 9b Difficult to get fundings for this research 0, 1, 2, 3, 4, 5 0, 1, 2, 3, 4, 5 9c Other countries have research which can be utilized 0, 1, 2, 3, 4, 5 0, 1, 2, 3, 4, 5 9d Insufficient support from the forestry sector and/or policy makers for 0, 1, 2, 3, 4, 5 0, 1, 2, 3, 4, 5 research activities 9e Regulations 0, 1, 2, 3, 4, 5 0, 1, 2, 3, 4, 5 9f Other (please specify)

10 Would you or your organisation be interested in joining applications for funding for international projects on wood biomass production with hybrid aspen and poplars in Northern Europe?

11 Other comments

No of trials Others

P. trich. x P. delto. P.

P. trich. x P. max. P.

Poplars

yes / no yes P. delto- P. ides 0, 1, 2, 3, 4, 5 0, 1, 2, 3, 4, 5 0, 1, 2, 3, 4, 5 0, 1, 2, 3, 4, 5

P. nigra

P. maxi P. mowiczii

P. trich P. ocarpa

No. of trials

P. trem.x P. trem.x P. tremuloides

Aspen

yes / no yes P. tremu- P. loides 0, 1, 2, 3, 4, 5 0, 1, 2, 3, 4, 5 0, 1, 2, 3, 4, 5 0, 1, 2, 3, 4, 5 P. tremula P.

breeding activities for hybrid aspen and poplar hybrid activities for breeding – Question going on activites breeding any presently there Are hybrid aspen or other for organization within your poplars? been activites breeding If no (0), have previously? performed activities up new in starting interest any Is there hybrid- well performing selection of aiming at aspen/poplars? breeding up joint in starting interest any Is there within the countries with other together activities Region? Baltic do you (approx.) genotypes relevant many How breeding be used in future can that have testing)? crossing or (for activities in field trials as tested of these are many How trials? many clones and in how in field trials as tested of these are many How (seedlings) and in how progenies open-pollinated trials many in field trials as full- tested of these are many How crossings) controlled (seedlings from sib progenies trials many and in how been done between full-sib crossings Have clones selected genotypically been used and clones have parent many how If so, been established field trials have many how

2a 2b 3a 3b 3c 3d 4a 4b Nr 1a 1b Appendix 2. Questionnaire

No of trials Others

P. trich. x P. delto. P.

P. trich. x P. max. P.

Poplars

yes / no yes yes / no yes P. delto- P. ides 0, 1, 2, >2 0, 1, 2, 3, 4, 5 0, 1, 2, 3, 4, 5 0, 1, 2, 3, 4, 5 0, 1, 2, 3, 4, 5 0, 1, 2, 3, 4, 5 0, 1, 2, 3, 4, 5 0, 1, 2, 3, 4, 5 0, 1, 2, 3, 4, 5 0, 1, 2, 3, 4, 5

P. nigra

P. maxi P. mowiczii

P. trich P. ocarpa

No. of trials

P. trem.x P. trem.x P. tremuloides

Aspen

yes / no yes yes / no yes P. tremu- P. loides 0, 1, 2, >2 0, 1, 2, 3, 4, 5 0, 1, 2, 3, 4, 5 0, 1, 2, 3, 4, 5 0, 1, 2, 3, 4, 5 0, 1, 2, 3, 4, 5 0, 1, 2, 3, 4, 5 0, 1, 2, 3, 4, 5 0, 1, 2, 3, 4, 5 0, 1, 2, 3, 4, 5 P. tremula P.

Are commercial clones available for use in your use in your for clones available commercial Are country selected been genotypically have many how If so, country your in tests field from based on results based selected been genotypically have many How countries in other tests field from on results for regions or less defined more any there Are and if country your of aspen/poplar in utilisation many how so, such as is based on traits Selection/breeding survival growth, adaptation climatic …) branchiness quality (straightness, stem …) quality (density, wood resistance pathogen insect resistance Other (heritability, parameters of genetic The knowledge between correlations genetic G x E interactions, is traits) different for where exist trials demonstration Does relevant clones different for and vitality growth instance demonstrated species are and different Question be relevant clones could of the parent many How within the Baltic countries in testing field joint for sea region be relevant clones could of the progeny many How within the Baltic countries in testing field joint for sea region other to send material to agree you Would of for production countries in other organisations material. test

6a 6b 6c 7 8 8a 8b 8c 8d 8e 8f 8g 9 10 Nr 5a 5b 5c Appendix 2. Continuation

25 Appendix 3. Questionnaire – regulations of hybrid aspen and poplar

No Question Hybrid aspen Poplars Comments 1a Are there any regulations for cultivations on forest land? yes /no yes /no What is regulated: 1b Is it ok to use a single clone yes /no yes /no 1c Any limits in the cultivation area of one clone yes /no yes /no 1d Is it ok to use a mixture of clones yes /no yes /no 1e Min number of clones in clonal mixtures 1f Min number of plants per clone in clonal mixtures 1g Max area of clonal mixtures to be used on an estate 1h Are there different regulations for material of different yes /no yes /no test categories (1. Source-identified (known origin), 2. Selected (in stands), 3.Qualified (individual phenotypic selection), 4.Tested (individual genotypc selection) 1i If so, describe in what way

2a Are there any regulations for cultivations on yes /no yes /no agricultural land? What is regulated: 2b Is it ok to use a single clone yes /no yes /no 2c Any limits in the cultivation area of one clone yes /no yes /no 2d Is it ok to use a mixture of clones yes /no yes /no 2e Min number of clones in clonal mixtures 2f Min number of plants per clone in clonal mixtures 2g Max area of clonal mixtures to be used on an estate 2h Are there different regulations for material of different yes /no yes /no test categories (1. Source-identified (known origin), 2. Selected (in stands), 3.Qualified (individual phenotypic selection), 4.Tested (individual genotypc selection) 2i If so, describe in what way

3 Other comments

Appendix 4. Questionnaire – subsidies of hybrid aspen and poplar

No Question Hybrid aspen Poplars 1a Are there any cultivation subsidies? yes /no yes /no 1b If so, describe this shortly

x x x x x x x Subsidies

x x x x x x x Regulations Questions

x x x x x x x x x x Breeding

x x x x x x x x x x x x x x x x x x x x x x x x General

Type of Type organisation Research Company Company Major estate Major estate Major estate Research Company Other Research Research Research Nursery Research Company Company Company Company Major estate Nursery Nursery Research Research Research Research Other Organisation University of Copenhagen University Greengold OÜ Metsad Södra Center Forest Private Foundation forestry private municipality, Kohila Tornator Sciences of Life Univeristy Estonian Osuuskunta Metsäliitto Authortity Safety Finnish Food Evira, Finland Institute Resources Natural LUKE, Research Icelandic Forest “Silava” Institute Research Forest State Latvian Euromediana Forestry of LAMMC Institute Hushållningssällskapet AB Syd Skogsutveckling AB Technologies SweTree STT, AB Sydved AB Utvecklings Skåneskogens AB Sundins Skogsplantor Skogsplantor Södra Linneaus university Skogforsk Alnarp SLU, Uppsala SLU, Jorsbruksv. Skogsstyrelsen, Information about the participants and the distribution on their involvement on their involvement and the distribution about the participants Information in the different parts of the questionnaire parts in the different Country Denmark Estonia Estonia Estonia Estonia Estonia Estonia Finland Finland Finland Iceland Latvia Lithuania Lithuania Sweden Sweden Sweden Sweden Sweden Sweden Sweden Sweden Sweden Sweden Sweden Sweden

Name Nielsen Ulrik Bräuner Holmlund Aleksandra Vahter Tarmo Heiki Hepner Heiki Hepner Martin Tishler and Tullus Hardi Lutter Reimo Hakkarainen Matti Leinonen Kari Egbert Beuker Halldór Sverrisson Zeps Martin Silininkas Mindaugas Pliura Alfas Persson Mats Henrik Nilsson Anneli Adler Thomas Höijer Lomholt and Anders Björn Humble Hedegaard Fridell Pierre Johan Jonsson Säll Harald and L-G Stener Rytter Lars Henrik Böhlenius Almir Karacic and Hazell Per Markensten Tobias Appendix 5.

27 Appendix 6. aspen and poplar. hybrid of cultivation general regarding the questionnaire from Results (Tot) overall means for the scores attitude. High positive the more very much (or similar), i.e. the higher score 0 = no or nothing … 5 where each question used for 5 was 0 to from A scale are bolded. respectively, hybrid aspen and poplar for organisations and country means within overall from the deviations bolded and underlined high positive each species are for

28 Appendix 6. Continuation

29 Appendix 7. Results from the questionnaire regarding breeding of hybrid aspen and poplar.

A scale from 0 to 5 was used for each question where 0 = no or nothing … 5 = very much (or similar), i.e. the higher score the more positive attitude.

No Question Country Organisation Scores Hybrid aspen Poplar 1a Are there presently any breeding Denmark Univ_Cop 0 0 activites going on within your Estonia EULS 0 0 organization for hybrid aspen or Finland Luke 3 0 other poplars? Iceland IFR 0 0 Latvia SILAVA 1 0 Lithuania Euromed 0 0 Lithuania LAMMC 4 4 Sweden Skogf 3 3 Sweden SLU, Upps 0 4 Sweden STT 0 3 Mean 1,1 1,4 1b If no (0), have breeding activites Denmark Univ_Cop 0 0 been performed previously? Estonia EULS 0 0 Finland Luke 0 1 Iceland IFR 0 5 Latvia SILAVA 5 0 Lithuania Euromed 3 0 Lithuania LAMMC 0 0 Sweden Skogf 0 0 Sweden SLU, Upps 0 0 Sweden STT 0 0 Mean 0,8 0,6 2a Is there any interest in starting up Denmark Univ_Cop 3 4 new activities aiming at selection Estonia EULS 4 3 of well performing hybridaspen/ Finland Luke 4 3 poplars? Iceland IFR 1 5 Latvia SILAVA 2 1 Lithuania Euromed 1 5 Lithuania LAMMC 4 5 Sweden Skogf 5 5 Sweden SLU, Upps 5 5 Sweden STT 0 5 Mean 2,9 4,1 2b Is there any interest in starting up Denmark Univ_Cop 3 4 joint breeding activities together Estonia EULS 5 5 with other countries within the Baltic Finland Luke 5 3 Region? Iceland IFR Latvia SILAVA 5 4 Lithuania Euromed 0 5 Lithuania LAMMC 4 5 Sweden Skogf 5 5 Sweden SLU, Upps 5 5 Sweden STT 0 5 Mean 3,6 4,6

30 Appendix 7. Continuation

No Question Country Organisation Scores Hybrid aspen Poplar 3 Selection/breeding is based on traits such as 3a growth, survival Denmark Univ_Cop 4 Estonia EULS 5 5 Finland Luke 5 Iceland IFR 5 Latvia SILAVA 5 5 Lithuania Euromed Lithuania LAMMC 5 5 Sweden Skogf 5 5 Sweden SLU, Upps 5 Sweden STT 5 Mean 5,0 4,9 3b Climatic adaptation Denmark Univ_Cop 4 Estonia EULS 4 2 Finland Luke 4 Iceland IFR 5 Latvia SILAVA 5 5 Lithuania Euromed Lithuania LAMMC 5 5 Sweden Skogf 5 5 Sweden SLU, Upps 5 Sweden STT 5 Mean 4,6 4,5 3c Stem quality Denmark Univ_Cop 2 (straightness, branchiness …) Estonia EULS 2 2 Finland Luke 2 Iceland IFR 4 Latvia SILAVA 4 Lithuania Euromed Lithuania LAMMC 4 5 Sweden Skogf 3 3 Sweden SLU, Upps 4 Sweden STT 5 Mean 3,0 3,6 3d Wood quality (density, …) Denmark Univ_Cop 2 Estonia EULS 2 2 Finland Luke 1 Iceland IFR 0 Latvia SILAVA 3 Lithuania Euromed Lithuania LAMMC 2 2 Sweden Skogf 0 0 Sweden SLU, Upps 3 Sweden STT 0 Mean 1,6 1,3

31 Appendix 7. Continuation

No Question Country Organisation Scores Hybrid aspen Poplar 3e Pathogen resistance Denmark Univ_Cop 4 Estonia EULS 1 1 Finland Luke 2 Iceland IFR 4 Latvia SILAVA 3 3 Lithuania Euromed Lithuania LAMMC 2 2 Sweden Skogf 5 5 Sweden SLU, Upps 4 Sweden STT 0 Mean 2,6 2,9 3f Insect resistance Denmark Univ_Cop 4 Estonia EULS 1 1 Finland Luke 2 Iceland IFR 1 Latvia SILAVA 3 3 Lithuania Euromed Lithuania LAMMC 0 0 Sweden Skogf 0 0 Sweden SLU, Upps 0 Sweden STT 0 Mean 1,2 1,1

32 Appendix 8. References in the litterature review A review was made of recent (year 2000 or later) publications (mainly peer reviewed, also incl. doctoral dissertations), published in English, relating to hybrid aspen and poplar cultivations in the Nordic and Baltic countries. In total 118 publications (Appendix 8) were identified.

1. Bernhardsson, C. & Ingvarsson, P.K. 2012. Geographical structure and adaptive population differentiation in herbivore defence genes in European aspen (Populus tremula L., Salicaceae). Molecular Ecology 21: 2197–2207.

2. Beuker, E. 2000. Aspen breeding in Finland, new challenges. Baltic Forestry 6: 81–84.

3. Böhlenius, H. & Övergaard, R. 2014. Effects of direct application of fertilizers and hydrogel on the establishment of poplar cuttings. Forests 5: 2967–2979.

4. Böhlenius, H. & Övergaard, R. 2015. Growth response of hybrid poplars to different types and levels of vegetation control. Scandinavian Journal of Forest Research 30: 516–525.

5. Böhlenius, H. & Övergaard, R. 2015. Exploration of optimal agricultural practices and seedling types for establishing poplar plantations. Forests 6: 2785–2798.

6. Böhlenius, H. & Övergaard, R. 2016. Impact of seedling type on early growth of poplar plantations on forest and agricultural land. Scandinavian Journal of Forest Research 31: 733–741.

7. Böhlenius, H., Övergaard, R. & Asp, H. 2015. Growth response of hybrid aspen (Populus × wettsteinii) and Populus trichocarpa to different pH levels and nutrient availabilities. Canadian Journal of Forest Research 46: 1367–1374.

8. Christersson, L. 2002. Cultivation of American poplars and aspen in Sweden – Potentials and some results. In: IEA, Bioenergy: Task 17 Short-Rotation Crops for Energy Purpose (eds. Christersson, L. & Kuiper, L.). Swed. Univ. Agric. Sci., Dept. Short Rotation Forestry, Rep. 70, Uppsala, pp. 36-39.

9. Christersson, L. 2006. Biomass production of intensively grown poplars in the southernmost part of Sweden: Observations of characters, traits and growth potential. Biomass and Bioenergy 30: 497–508.

10. Christersson, L. 2008. Poplar plantations for paper in the south of Sweden. Biomass and Bioenergy 32: 997-1000.

11. Christersson, L. 2010. Wood production potential in poplar plantations in Sweden. Biomass and Bioenergy 34: 1289-1299.

12. Christersson, L. 2011. The potential for production of biomass for biofuel by the cultivation of hybrid poplar and hybrid aspen in the south of Sweden. In: Economic Effects of Biofuel Production (eds. Aurélio, M. & Bernardes, S.), Intech, pp. 163-180.).

13. Dimitriou, I., Mola-Yudego, B. 2017. Nitrogen fertilization of poplar plantations on agricultural land: effects on diameter increments and leaching. Scandinavian Journal of Forest Research, 32:8, 700-707.

33 14. Haapala, T., Pakkanen, A. & Pulkkinen, P. 2004. Variation in survival and growth of cuttings in two clonal propagation methods for hybrid aspen (Populus tremula × P. tremuloides). Forest Ecology and Management 193: 345–354.

15. Hamberg, L., Malmivaara-Lämsä, M., Löfström, I., Vartiamäki, H., Valkonen, S. & Hantula, J. 2011. Sprouting of Populus tremula L. in spruce regeneration areas following alternative treatments. European Journal of Forest Research 130: 99–106.

16. Heräjärvi, H. 2009. Effect of drying technology on aspen wood properties. Silva Fennica 43: 433-445.

17. Heräjärvi, H. & Junkkonen, R. 2006. Wood density and growth rate of European and hybrid aspen in southern Finland. Baltic Forestry 12: 2-8.

18. Hjelm, B. & Johansson, T. 2012. Volume equations for poplars growing on farmland in Sweden. Scandinavian Journal of Forest Research 27: 561–566.

19. Hjelm, K., Mc Carthy, R. & Rytter, L. 2018. Establishment strategies for poplars, including mulch and plant types, on agricultural land in Sweden. New Forests https://doi.org/10.1007/s11056-018-9652-6, 19 p.

20. Hjelm, B., Mola-Yudego, B., Dimitriou, I. & Johansson, T. 2015. Diameter-height models for fast-growing poplar plantations on agricultural land in Sweden. Bioenergy Research 8: 1759–1768.

21. Hjelm, K. & Rytter, L. 2016. The influence of soil conditions, with focus on soil acidity, on the establishment of poplar (Populus spp.). New Forests 47: 731–750.

22. Hytönen J. 2018. Biomass, nutrient content and energy yield of short-rotation hybrid aspen (P. tremula x P. tremuloides) coppice. Forest Ecology and Manage- ment 413: 21-31.

23. Hytönen J., Beuker E., Viherä-Aarnio A. 2018. Clonal variation in basic density, moisture content and heating value of wood, bark and branches in hybrid aspen. Silva Fennica vol. 52 no. 2 article id 9938. https://doi.org/10.14214/sf.9938.

24. Häikiö, E., Freiwald, V., Silfver, T., Beuker, E., Holopainen, T. & Oksanen, E. 2007. Impacts of elevated ozone and nitrogen on growth and photosynthesis of European aspen (Populus tremula) and hybrid aspen (P. tremula × Populus tremuloides) clones. Can. J. For. Res. 37: 2326-2336.

25. Häikiö, E., Freiwald, V., Julkunen-Tiitto, R., Beuker, E., Holopainen, T. & Oksanen, E. 2008. Differences in leaf characteristics between ozone-sensitive and ozonetolerant hybrid aspen (Populus tremula x Populus tremuloides) clones. Tree Physiology 29, 53–66.

26. Häikiö, E., Makkonen, M., Julkunen-Tiitto, R., Sitte, J., Freiwald, V., Silfver, T., Pandey, V., Beuker, E., Holopainen, T. & Oksanen, E. 2009. Performance and secondary chemistry of two hybrid aspen (P. tremula L. × Populus tremuloides Michx.) clones in long-term elevated ozone exposure. Journal of Chemical Ecology 35: 664–678.

34 27. Jansons, Ā., Zeps, M., Rieksts-Riekstiņš, Matisons, R. & Krišāns, O. 2014. Height increment of hybrid aspen Populus tremuloides × P. tremula as a function of weather conditions in south-western part of Latvia. Silva Fennica 48(5): id 1124, 13 p.

28. Jansons, Ā., Zurkova, S., Lazdiņa, D. & Zeps, M. 2014. Productivity of poplar hybrid (Populus balsamifera × P. laurifolia) in Latvia. Agronomy Research 12: 469–478.

29. Johansson, T. 2002. Increment and biomass in 26- to 91-year-old European aspen and some practical implications. Biomass & Bioenergy 23: 245-255.

30. Johansson, T. 2011. Site index curves for poplar growing on former farmland in Sweden. Scandinavian Journal of Forest Research 26: 161–170.

31. Johansson, T. 2013. A site dependent top height growth model for hybrid aspen. Journal of Forestry Research 24: 691–698.

32. Johansson, T. 2013. Biomass production of hybrid aspen growing on former farm land in Sweden. Journal of Forestry Research 24: 237–246.

33. Johansson, T. & Hjelm, B. 2012. Stump and root biomass of poplar stands. Forests 3: 166–178. (B179).

34. Johansson, T. & Hjelm, B. 2012. The sprouting capacity of 8–21-year-old poplars and some practical implications. Forests 3: 528–545.

35. Johansson, T. & Hjelm, B. 2013. Frequency of false heartwood of stems of poplar growing on farmland in Sweden. Forests 4: 28–42.

36. Johansson, T. & Karačić, A. 2011. Increment and biomass in hybrid poplar and some practical implications. Biomass and Bioenergy 35: 1925–1934.

37. Karačić, A. 2005. Production and ecological aspects of short rotation poplars in Sweden. Acta Universitatis Agriculturae Sueciae, Faculty of Natural Resources and Agricultural Sciences, Doctorial Thesis No. 2005:13 (Diss.), 42 pp., Uppsala.

38. Karačić, A., Verwijst, T. & Weih, M. 2003. Above-ground woody biomass production of short-rotation Populus plantations on agricultural land in Sweden. Scandinavian Journal of Forest Research 18: 427–437.

39. Karačić, A. & Weih, M. 2006. Variation in growth and resource utilisation among eight poplar clones grown under different irrigation and fertilisation regimes in Sweden. Biomass & Bioenergy 30: 115-124.

40. Karlsson, K. & Holm, S. 2002. Industrial aspen plantations in Finland. In: Aspen in Papermaking (eds., Pulkkinen, P.; Tigerstedt, P.M.A. & Viirros, R.), University of Helsinki, Dept. Applied Biology, Publications 5, pp. 5-8.

41. Kasanen, R., Kurkela, T. & Hantula, J. 2002. Neofabraea populi in Scandinavia. Finnish Forest Research Institute, Research Papers 829, pp. 146-151.

42. Kärki, T. & Vainikainen, V. 2004. Determining the quality of aspen (Populus tremula) logs for mechanical wood processing in Finland. Forest Products Journal 54: 64-71.

35 43. Luoranen, J., Lappi, J., Zhang, G. & Smolander, H. 2006. Field performance of hybrid aspen clones planted in summer. Silva Fenn. 40: 257-269.

44. Luoranen, J., Zhang, G. & Smolander, H. 2005. Production of even-sized hybrid aspen plants from root cuttings: transplanting, height grading and planting dates. Baltic Forestry 11: 20-28.

45. Lutter, R., Tullus, A., Kanal, A., Tullus, T., Vares, A. & Tullus, H. 2016. The impact of short-rotation hybrid aspen (Populus tremula L. × P. tremuloides Michx.) plantations on nutritional status of former arable soils. Forest Ecology and Management 362: 184–193.

46. Lutter, R., Tullus, A., Kanal, A., Tullus, T., Vares, A. & Tullus, H. 2016. The impact of former land-use type to above- and below-ground C and N pools in short- rotation hybrid aspen (Populus tremula L. × P. tremuloides Michx.) plantations in hemiboreal conditions. Forest Ecology and Management 378: 79–90.

47. Lutter, R., Tullus, A., Kanal, A., Tullus, T., & Tullus, H. 2017. Above-ground growth and temporal plant-soil relations in midterm hybrid aspen (Populus tremula L. × P. tremuloides Michx.) plantations on former arable lands in hemiboreal Estonia. Scandinavian Journal of Forest Research http://dx.doi.org/10.1080/02827581.2017.1278784.

48. Lutter, R., Tullus, A., Tullus, T., & Tullus, H. 2016.Spring and autumn phenology of hybrid aspen (Populus tremula L. × P. tremuloides Michx.) genotypes of different geographic origin in hemiboreal Estonia. New Zealand Journal of Forestry Science 40:20, http://dx.doi.org/10.1186/s40490-016-0078-7.

49. Lutter, R., Tullus, T., Tullus, A., Kanal, A., Tullus, H. 2017. Forest ecosystem recovery in 15-year-old hybrid aspen (Populus tremula L. × P. tremuloides Michx.) plantations on a reclaimed oil shale quarry. Oil Shale, 34, 368–389.

50. Mandre, M., Tullus, A., Klõšeiko, J., Lukjanova, A. & Tullus, H. 2011. Variation of carbohydrates and lignin in hybrid aspen (Populus tremula × P. tremuloides) on alkaline soil. Cellulose Chemistry and Technology 45: 299–311.

51. Mc Carthy, R. 2016. Establishment and early management of Populus species in southern Sweden. Dissertation, Swedish University of Agricultural Sciences, Southern Swedish Research Centre, Doctoral Thesis No. 2016:52, Alnarp, 69 p.

52. Mc Carthy, R., Ekö, P.M. & Rytter, L. 2014. Reliability of stump sprouting as a regeneration method for poplars: clonal behaviour in survival, sprout straightness and growth. Silva Fennica 48(3), id 1126, 9 p.

53. Mc Carthy, R., Löf, M. & Gardiner, E. 2017. Early root development of poplars (Populus spp.) in relation to moist and saturated soil conditions. Scandinavian Journal of Forest Research https://doi.org/10.1080/02827581.2017.1338751.

54. Mc Carthy, R. & Rytter, L. 2015. Productivity and thinning effects in hybrid aspen root sucker stands. Forest Ecology and Management 354: 215–223.

55. Mc Carthy, R., Rytter, L. & Hjelm, K. 2017. Effects of soil preparation methods and plant types on the establishment of poplars on forest land. Annals of Forest Science 74: 47, 12 p.

36 56. Mukherjee, S., Heinonen, M., Dequvire, M., Sipilä, T., Pulkkinen, P. & Yrjälä, K. 2013. Secondary succession of bacterial communities and co-occurrence of phylo- types in oil-polluted Populus rhizosphere. Soil Biology & Biochemistry 58 (2013) 188e197.

57. Mukherjee, S., Sipilä, T., Pulkkinen, P. & Yrjälä, K. 2015. Secondary successional trajectories of structural and catabolic bacterial communities in oil-polluted soil planted with hybrid poplar. Molecular Ecology 24, 628–642.

58. Nikula, S., Manninen, S. & Pulkkinen, P. 2011. Growth and frost hardening of European aspen and backcross hybrid aspen as influenced by water and nitrogen. Annals of Forest Science 68:737–745.

59. Pulkkkinen, P. (2002) The effect of wood properties on the possibilities of propa- gating hybrid aspen vegetatively. In: Aspen in Papermaking (eds., Pulkkinen, P.; Tigerstedt, P.M.A. & Viirros, R.), University of Helsinki, Dept. Applied Biology, Publications 5, pp. 34-39.

60. Pulkkinen, P., Vaario, L.-M., Koivuranta, L. & Stenvall, N. 2013. Elevated tempera ture effects on germination and early growth of European aspen (Populus tremula), hybrid aspen (P. tremula 3 P. tremuloides) and their F2-hybrids. Eur J Forest Res 132:791–800.

61. Randlane, T., Tullus, T., Saag, A., Lutter, R., Tullus, A., Helm, A., Tullus, H., Pärtel, M. 2017. Diversity of lichens and bryophytes in hybrid aspen plantations in Estonia depends on landscape structure. Canadian Journal of Forest Research, 47, 1202−1214.

62. Ranua, J. 2002. Industrial use of aspen fibres. In: Aspen in Papermaking (eds., Pulkkinen, P.; Tigerstedt, P.M.A. & Viirros, R.), University of Helsinki, Dept. Applied Biology, Publications 5, pp. 1-4.

63. Rautio, M.; Kangas, T.; Auterinen, T.; Álen, R. & Pulkkinen, P. 2002 Wood quality components of hybrid aspen for paper making. In: Aspen in Papermaking (eds., Pulkkinen, P.; Tigerstedt, P.M.A. & Viirros, R.), University of Helsinki, Dept. Applied Biology, Publications 5, pp. 19-26.

64. Rytter, L. 2002. Nutrient content in stems of hybrid aspen as affected by tree age and tree size, and nutrient removal with harvest. Biomass and Bioenergy 23: 13-25.

65. Rytter, L. 2006. A management regime for hybrid aspen stands combining conven- tional forestry techniques with early biomass harvests to exploit their rapid early growth. Forest Ecology and Management 236: 422–426.

66. Rytter, L. 2013. Growth dynamics of hardwood stands during the precommercial thinning phase – Recovery of retained stems after competition release. Forest Ecology and Management 302: 264–272.

67. Rytter, L. & Jacobson, S. 2018. Clonal variation in root suckering ability of hybrid aspen (Populus tremula L. × P. tremuloides Michx.). Scandinavian Journal of Forest Research 33: 523-528.

68. Rytter, L. & Rytter, R.-M. 2017. Productivity and sustainability of hybrid aspen (Populus tremula L. × P. tremuloides Michx.) root sucker stands with varying management strategies. Forest Ecology and Management 373: 56–65.

37 69. Rytter, L. & Stener, L. 2003. Clonal variation in nutrient content in woody biomass of hybrid aspen (Populus tremula L. × P. tremuloides Michx.). Silva Fennica 37: 313-324.

70. Rytter, L. & Stener, L. 2005. Productivity and thinning effects in hybrid aspen (Populus tremula L. × P. tremuloides Michx.) stands in southern Sweden. Forestry 78: 285–295.

71. Rytter, L. & Stener, L-G. 2014. Growth and thinning effects during a rotation period of hybrid aspen in southern Sweden. Scandinavian Journal of Forest Research 29: 747–756.

72. Rytter, L. & Jansson, G. 2009. Influence of pruning on wood characters in hybrid aspen. Silva Fennica 43: 689–698.

73. Rytter, L., Johansson, K., Karlsson, B. & Stener, L.-G. 2013. Tree species, genetics and regeneration for bioenergy feedstock in northern Europe. In: Forest Bio- Energy Production (Kellomäki, S., Kilpeläinen, A. & Alam, A., eds.), pp. 7–37. Springer Science+Business Media, New York.

74. Rytter, R.-M. 2012. The potential of willow and poplar plantations as carbon sinks in Sweden. Biomass and Bioenergy 36: 86–95.

75. Rytter, R.-M. 2016. Afforestation of former agricultural land with Salicaceae species – Initial effects on soil organic carbon, mineral nutrients, C:N and pH. Forest Ecology and Management 363: 21–30.

76. Rytter, R.-M. & Rytter, L. 2018. Effects on soil characteristics by different management regimes with root sucker generated hybrid aspen (Populus tremula L. × P. tremuloides Michx.) on abandoned agricultural land. iForest: 619–627.

77. Šēnhofa, S., Zeps, M., Matisons, R., Smilga, J., Lazdiņa, D. & Jansons, Ā. 2016. Effect of climatic factors on tree-ring width of Populus hybrids in Latvia. Silva Fennica 50(1), id 1442, 12 p.

78. Šēnhofa, S., Zeps, M., Gailis, A., Kāpostiņš, R. & Jansons, Ā. 2016. Development of stem cracks in young hybrid aspen plantations. Forestry Studies 65: 16–23.

79. Smilga, J., Zeps, M., Sisenis, L., Kalnins, J., Adamovics, A. & Donis, J. 2015. Profitability of hybrid aspen breeding in Latvia. Agronomy Research 13: 430–435.

80. Soo, T., Tullus, A., Tullus, H. & Roosaluste, E. 2009. Floristic diversity responses in young hybrid aspen plantations to land-use history and site preparation treatments. Forest Ecology and Management 257: 858–867.

81. Soo, T., Tullus, A., Tullus, H., Roosaluste, E., Vares, A. 2009. Change from agriculture to forestry: floristic diversity in young fast-growing deciduous plantations on former agricultural land in Estonia. Annales Botanici Fennici, 46 (4), 353–364.

82. Stener, L.-G. 2002. Hybrid aspen improvement in Sweden during the period 1939-2000. In: Aspen in Papermaking (eds., Pulkkinen, P.; Tigerstedt, P.M.A. & Viirros, R.), University of Helsinki, Dept. Applied Biology, Publications 5, pp. 9-13.

83. Stener, L.-G. & Karlsson, B. 2004. Improvement of Populus tremula × P. tremuloides by phenotypic selection and clonal testing. For. Genet. 11: 13-27.

38 84. Stener, L.-G. & Westin, J. 2017. Early growth and phenology of hybrid aspen and poplar in clonal field tests in Scandinavia. Silva Fennica 51(2), id 5656, 22 p.

85. Stenvall, N. 2006. Multplication of hybrid aspen (Populus tremula L. × P. tremuloides Michx.) from cuttings. Dissertationes Forestales 33, Univ. Helsinki, Faculty of Agriculture anf Forstry, Helsinki, 33 pp.

86. Stenvall, N., Haapala, T., Aarlahti, S. & Pulkkinen, P. 2006. The effect of soil temperature and light on sprouting and rooting of root cuttings of hybrid aspen clones. Can. J. For. Res. 35: 2671-2678.

87. Stenvall, N., Haapala, T. & Pulkkinen, P. 2006. The role of a root cutting's diameter and location on the regeneration ability of hybrid aspen. For. Ecol. Manage. 237: 150–155.

88. Stenvall, N., Piisilä, M. & Pulkkinen, P. 2009. Seasonal fluctuation of root carbohydrates in hybrid aspen clones and its relationship to the sprouting efficiency of root cuttings. Can. J. For. Res. 39: 1531-1537.

89. Suvanto, L.I. & Latva-Karjanmaa, T.B. 2005. Clone identification and clonal structure of the European aspen (Populus tremula). Molecular Ecology 14: 2851-2860.

90. Suvanto, L. & Pulkkinen, P. 2004. Gene flow between European (Populus tremula) and hybrid aspen (P. tremula × P. tremuloides. In: Forest Genetics and Tree Breeding in the Age of Genomics: Progress and Future (eds. Li, B. & McKeand, S.), IUFRO Joint Conference of Division 2, Conference Proceedinhs, November 1-5, 2004, Charleston, South Carolina, USA, pp. 171-173.

91. Thelin, G. 2009. Bio energy production in birch and hybrid aspen after addition of residue based fertilizers – establishment of fertilization trials. Värmeforsk, Rapport 1094, Stockholm, 21 p.

92. Thelin, G. 2012. Ash based fertilization in young stands of birch and hybrid aspen. Värmeforsk, Rapport 1213, Stockholm, 22 p.

93. Tullus, A. 2010. Tree growth and the factors affecting it in young hybrid aspen plantations. Thesis, Estonian University of Life Sciences, Tartu, 67 pp.

94. Tullus, A., Lukason, O., Vares, A., Padari, A., Lutter, R., Tullus, T., Karoles, K. & Tullus, H. 2012. Economics of hybrid aspen (Populus tremula L. × P. tremuloides Michx.) and silver birch (Betula pendula Roth.) plantations on abandoned agricultural lands in Estonia. Baltic Forestry 18: 288–298.

95. Tullus, A., Kanal, A., Soo, T. & Tullus, H. 2010. The impact of available water content in previous agricultural soils on tree growth and nutritional status in young hybrid aspen plantations in Estonia. Plant and Soil 333: 129–145.

96. Tullus, A., Mandre, M., Soo, T. & Tullus, H. 2010. Relationships between cellulose, lignin and nutrients in the stemwood of hybrid aspen in Estonian plantations. Cellulose Chemistry and Technology 44: 101–109.

97. Tullus, A., Rytter L., Tullus, T., Weih, M. & Tullus, H. 2014. Short-rotation forestry with hybrid aspen (Populus tremula L. x Populus tremuloides Michx.) in Northern Europe. Scandinavian Journal of Forest Research, 27:1, 10-29, https://doi.org/10.1080/02827581.2011.628949.

39 98. Tullus A., Sellin A., Kupper P., Lutter R., Pärn L., Jasińska A.K., Alber M., Kukk M., Tullus T., Tullus H., Lõhmus K., Sõber A. 2014. Increasing air humidity – a climate trend predicted for northern latitudes – alters the chemical composition of stemwood in silver birch and hybrid aspen. Silva Fennica vol. 48 no. 4 article id 1107. 16 p.

99. Tullus, A., Tullus, H., Vares, A. & Kanal, A. 2007. Early growth of hybrid aspen (Populus x wettsteinii Hämet-Ahti) plantations on former agricultural lands in Estonia. For. Ecol. Manage. 245: 118-129. (2336, B223).

100. Tullus, A., Sellin, A., Kupper, P., Lutter, R., Pärn, L., Jasińska, A.K., Alber, M., Kukk, M., Tullus, T., Tullus, H., Lõhmus, K. & Sõber, A. 2014. Increasing air humidity – a climate trend predicted for northern latitudes – alters the chemical composition of stemwood in silver birch and hybrid aspen. Silva Fennica 48(4), article id 1107, 16 p.

101. Tullus, A., Soo, T., Tullus, H., Vares, A., Kanal, A. & Roosaluste, E. 2008. Early growth and floristic diversity of hybrid aspen (Populus × wettsteinii Hämet-Ahti) plantations on a reclaimed opencast oil shale quarry in north-east Estonia. Oil Shale 25: 57-74.

102. Tullus, A., Tullus, H., Soo, T. & Pärn, L. 2009. Above- ground biomass characteristics of young hybrid aspen (Populus tremula L. × P. tremuloides Michx) plantations on former agricultural land in Estonia. Biomass and Bioenergy 33: 1617-1625.

103. Tullus, A., Tullus, H., Vares, A. & Kanal, A. 2007. Early growth of hybrid aspen (Populus x wettsteinii Hämet-Ahti) plantations on former agricultural lands in Estonia. For. Ecol. Manage. 245: 118-129. (2336, B223).

104. Tullus, H., Tullus, A. & Rytter, L. 2013. Short-rotation forestry for supplying biomass for energy production. In: Forest BioEnergy Production (Kellomäki, S., Kilpeläinen, A. & Alam, A., eds.), pp. 39–56. Springer Science+Business Media, New York.

105. Tullus, T. 2013. Understorey vegetation and factors affecting it in young deciduous forest plantations on former agricultural land. Thesis, Estonian University of Life Sciences, Institute of Forestry and Rural Engineering, Tartu, 66 p.

106. Tullus, T., Tullus, A., Roosaluste, E., Lutter, R. & Tullus, H. 2015. Vascular plant and bryophyte flora in midterm hybrid aspen plantations on abandoned agricultural land. Canadian Journal of Forest Research 45: 1183–1191.

107. Tullus, T., Tullus, A., Roosaluste, E., Kaasik, A., Lutter, R., Tullus, H. 2013. Understorey vegetation in young naturally regenerated and planted birch (Betula spp.) stands on abandoned agricultural land. New Forests, 44, 591–611.

108. Tullus, T., Tullus, A., Roosaluste, E., Tullus, H. 2012. Bryophyte vegetation in young deciduous forest plantations. Baltic Forestry, 18 (2), 205–213.

109. Tuñón, T.B. 2009. Methods of poplar propagation and establishment on forest and agricultural land in Sweden. SLU, Dept. Energy and Technology, Examensarbete 2009:10, Uppsala, 38 p.

40 110. Taeroe, A., Nord-Larsen. T., Stupak, I.. & Raulund-Rasmussen, K. 2015.Allometric biomass, biomass expansion factor and wood density models for the OP42 hybrid poplar in southern Scandinavia. Bioenergy Research 8: 1332–1343.

111. Yu, Q. 2001. Can physiological and anatomical characters be used for selecting high yielding hybrid aspen clones? Silva Fennica 35: 137–146.

112. Yu, Q. & Pulkkinen, P. 2003 Genotype-environment interaction and stability in growth of aspen hybrid clones. For. Ecol. Manage. 173, 25-35.

113. Yu, Q., Pulkkinen, P., Rautio, M., Haapanen, M., Alén, R., Stener, L.G., Beuker, E. & Tigerstedt, P.M.A. 2001. Genetic control of wood physicochemical properties, growth, and phenology in hybrid aspen clones. Can. J. For. Res. 31, 1348-1356.

114. Yu, Q.; Pulkkkinen, P. & Tigerstedt, P.M.A. 2002 The relationships between growth and wood properties in hybrid aspen clones. In: Aspen in Papermaking (eds., Pulkkinen, P.; Tigerstedt, P.M.A. & Viirros, R.), University of Helsinki, Dept. Applied Biology, Publications 5, pp. 27-33.

115. Yu, Q., Tigerstedt, P.M.A. & Haapanen, M. 2001. Growth and phenology of hybrid aspen clones (Populus tremula L. x Populus tremuloides Michx.). Silva Fennica 35: 15-25.

116. Välimäki, S. & Heliövaara, K. 2007. Hybrid aspen is not preferred by the large poplar borer (Saperda carcharias). Arthropod-Plant Interactions 1: 205–211.

117. Zeps, M. 2017. Potential of hybrid aspen (Populus tremuloides Michx. x Populus tremula L.) production in Latvia. Doctoral thesis Latvia University of Agriculture/ Latvia State Forest Research Institute “Silava” (summary). 33-51.

118. Zeps, M., Gailis, A., Smilga, J., Miezite, O., Sisenis, L. & Zariņa, I. 2016. Hybrid aspen clone wood mechanical properties. Agronomy Research 14: 1147–1152.