Introduction

In the United Kingdom (UK) there are two main aims of the Government’s Renewable Energy Strategy (DECC 2012); to reduce emissions of carbon dioxide and to improve energy security. This is to be achieved through producing 15% of the energy in the UK through renewable means by 2020, which represents an increase of seven times the 2009 contribution within a decade (DECC 2012). The lead scenario generated within the Strategy suggests that 30% of electricity and 12% of heat could be provided through use of renewable sources of energy (DECC 2012). By 2014, 7% of energy was derived from renewable sources, of which 72% was obtained from bioenergy (DECC 2015). Of this approximately 40% was from wood or other plant biomass (DECC 2015). The transition from fossil fuels to renewable sources of energy has been supported by the Renewable Heat Incentive (RHI) and the Feed-In Tariff (FIT). The RHI is focussed on providing payments to encourage the production of heat from renewable energy sources (OFGEM 2019a), while the FIT is aimed at increasing renewable electricity generation (OFGEM 2019b). The FIT scheme is closing to most applications in April 2019 but the RHI continues.

As a source of renewable energy, biomass has certain attractions; it can produce energy at times of peak demand, it involves low fossil fuel inputs for production and tried and tested technology is available for its efficient conversion. The UK Bioenergy Strategy (DECC 2012) predicted that while biomass imports will form the bulk of supply, domestic production can provide an important, stable and reliable resource. If demand for biomass were to be met domestically, the area under energy crops would need to increase dramatically. The UK Biomass Strategy (DEFRA 2007) anticipated that 350,000 ha of perennial energy crops would be required by 2020, which contrasts with the 2009 area of 15,500 ha of Short Rotation Coppice (SRC) and Miscanthus (SAC 2009). Woody biomass is an attractive option compared with agricultural biomass crops as it requires the input of relatively low levels of fertiliser, pesticides and herbicides and can also be established on marginal land, thereby not competing with food production (Hastings et al. 2014).

Two approaches to bio-energy production using tree species have been adopted: short rotation coppice (SRC) and short rotation forestry (SRF) (Read et al. 2009). These two approaches are compared in Table 1. Since the 1990s the development of dedicated woody energy crops in the UK has focused on using SRC of clones of willow (Salix spp.) or poplar (Populus spp.). However, a comparative analysis of both systems identified SRF using fast growing tree species as being the most cost-effective and time-efficient approach to reducing greenhouse gases (Matthews and Broadmeadow 2009). However, there are very few examples of SRF in the UK, with the largest plantation an area of 24.2 ha of eucalypts established at Daneshill in Nottinghamshire, eastern England as an energy forest (WooddisseFootnote 1 pers comm). SRF contrasts with conventional production forestry in several ways. Conventional production forestry uses conifers, which are more productive than broadleaves in the UK over traditional rotations of 40–70 years. Normally SRF does not include thinning during the rotation, whereas after canopy closure production forestry stands are thinned on a cycle of 5–10 years. Due to the short rotation, measures are taken to accelerate establishment and maximise growth such as highly intensive weed control and fertiliser application (Purse and Leslie 2016b).

Table 1 Characteristics of SRC and SRF (modified from SAC 2009)
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The silvicultural attributes of an ideal SRF tree crop are described in Table 2. Ideally it should grow rapidly, with mean annual increment peaking early, have a low environmental impact and the wood should have ideal properties of a fuel, such as low moisture content and appropriate chemical composition. A review by Hardcastle (2006) identified species and genera with potential for SRF in the UK, these being ash (Fraxinus excelsior), alder (Alnus glutinosa), birch (Betula pendula), Eucalyptus gunnii, Eucalyptus nitens, Nothofagus spp, poplar (Populus spp.) and sycamore (Acer pseudoplatanus). Of these ash, alder, birch and some poplars are tree species native to the UK. Since the publication of Hardcastle’s (2006) report, the introduction of exotic pathogens has reduced the viability of some species, e.g., ash dieback (Hymenoscyphus fraxineus) on ash (Thomas 2016).

Table 2 Attributes of the ideal SRF tree crop
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In the UK there has been research to identify site suitability criteria for eucalypts through an understanding of the limitations to survival and growth (e.g., Evans 1980, 1986; Purse and Richardson 2001; Leslie et al. 2011, 2012). However, the scale of planting of eucalypts in the UK has been modest; in the 5 years between 2011 and 2016 nurseries in the UK have sold about 220,000 seedlings (Purse and Leslie 2016b), which represents less than 100 ha of establishment. As such, there remains uncertainty about the risks of large-scale planting of Eucalyptus SRF in the UK.

In this review paper, we compare eucalypts with other appropriate species or genera as a source of biomass for energy production then identify the limitations of eucalypts in the UK and potential ways in which they may be overcome. There was found to be limited literature on short rotation forestry in general in the UK. The review has therefore relied on compiling information from two sources. The first is literature on cold-tolerant eucalypts that has been collected by the authors over a period of 15 years and includes material no longer on-line. The information on other species, has relied on extracting relevant material from publications covering more general aspects of the silviculture, growth and wood properties of the tree species. A constraint to comparing growth of SRF species is that there were few data for trees grown under intensive short rotation management in the UK. Whilst yield models have been developed for commercial stands under longer rotations these only provide estimates of stand growth after canopy closure (from 10 to 25 years of age) depending on the maximum mean annual increment (Edwards and Christie 1981).

This review is timely as recent research studies (e.g., Harrison 2011; Leslie et al. 2014b; McEvoy 2016; Leslie et al. 2018) and reviews (e.g.. McKay 2011; Leslie et al. 2012; Purse and Leslie 2016a, b) have led to a better understanding of the potential of eucalypts but also their limitations. The findings are focused on the UK, but are relevant to other areas with cool oceanic temperate climates in western Europe.

Yield of eucalypts and other SRF species

The main attraction of eucalypts is their high productivity, being among the most productive trees in the UK. Volume growth rates have been estimated from small plot trials, to be as high as 30 m3 ha−1 y−1 for E. nitens (Purse and Richardson 2001). In Ireland a plantation of E. nitens felled at 16 years of age, with 740 stems ha−1, yielded a mean annual increment (MAI) of 26.1 m3 ha−1 y−1 (Hutchinson et al. 2011). Historic Forestry Commission data provides evidence of MAIs of 16 m3 ha−1 y−1 for E. gunnii over a 20 year rotation (Leslie et al. 2018). Purse and Richardson (2001) conclude that higher yields of 10–15 oven dry Mg ha−1 y−1 (approximately 20–30 m3 ha−1 y−1) over 8–10 year rotations are possible from plantations of E. gunnii. In Redmarley, Gloucestershire in the south of England (Fig. 1), E. gunnii was estimated to have grown at a MAI of 25 m3 ha−1 y−1 over a 11 or 12 year rotation (Purse and Richardson 2001). The coppice from this stand was assessed at 10 years of age and the standing volume of mainly E. gunnii with some E. dalrympleana combined was 317 m3 ha−1 or 31.7 m3 ha−1 y−1 with 4746 stems ha−1 (McKay 2010). Furthermore, when E. gunnii and E. nitens that had been damaged or killed by extreme cold in the winter of 2010–2011 was harvested at Daneshill in Nottinghamshire in the north of England (Fig. 1) at 5 years old an average of 85 Mg were extracted per hectare giving a green weight production of 17 Mg ha−1 y−1 (Wooddisse pers comm).

Fig. 1
figure 1

Locations of sites with shaded area representing the − 14 °C minimum temperature monthly lowest (January) isotherm for (1961-90) (Met Office no date). The locations are as follows: (1) Moray, (2) Kilmichael, (3) Glenbranter, (4) Wark, (5) Daneshill, (6) Thetford, (7) Redmarley, (8) Exeter, (9) New Forest, (10) Chiddingfold

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While eucalypts can be highly productive in the UK yields vary considerably across sites due to edaphic and climatic conditions, the quality of silvicultural practices and the genotype of the planting stock (Kerr and Evans 2011). This was highlighted by a study of four trials from the 1980s that demonstrated the difficulties in consistently achieving high levels of productivity (Kerr and Evans 2011). It was only at one trial at the New Forest (Fig. 1), in southern England where the potential of fast growth of eucalypts was realised; biomass at 7 years of age of E. gunnii and E. glaucescens was at least three times that of the other species planted, which were Alnus cordata, Nothofagus obliqua, Populus balsamifera and Pteryocarya x rehderiana. This trial was planted at two spacings and the results showed that increasing stocking significantly increased yield over a 7 year rotation. For E. gunnii a MAI of 2.7 m3 ha−1 y−1 was achieved at 2.8 m spacing and of 13.9 m3 ha−1 y−1 at 1.4 m spacing. For E. glaucescens the respective MAI was 8.1 m3 ha−1 y−1 and 17.4 m3 ha−1 y−1.

SRF species should exhibit a peak in mean annual increment (MAI) at a relatively young age. The potentially most productive genus other than Eucalyptus is Nothofagus and UK yield models predict maximum mean annual increments (MMAI) of 10 to 18 m3 ha−1 y−1 (Tuley 1980). The mid MMAI of this range for Nothofagus shows MAI peaking at 14.0 m3 ha−1 y−1 at 29 years of age (Tuley 1980). Poplars have been used in short rotation coppice due to their rapid early growth (Mitchell et al. 1993) and have been predicted to produce yields of 30.5 m3 ha−1 y−1 or 22 Mg ha−1 y−1 (at 35% moisture content) at age 15 years (Forest Research 1997). Stokes et al. (2017) has demonstrated the fast growth of hybrid aspen (Populus x wettsteinii) and commercial hybrid poplar clones in two Scottish trials at Moray and Kilmichael (Fig. 1). An assessment was made after 22 growing seasons at Moray and 21 growing seasons at Kilmichael. Clones of commercial poplar hybrids attained an average height of 19.98 m and 14.15 m respectively and a diameter at breast height (dbh) of 39.98 cm and 24.29 cm. Hybrid aspen exhibited more rapid growth with a height of 17.38 m and dbh of 30.99 cm at Kilmichael. Native aspen (Populus tremula) grew more slowly, achieving a height of 10.75 m and dbh of 14.97 cm at Moray and a height of 6.54 m and dbh of 8.86 cm at Kilmichael.

A review of the silviculture of alder across Europe showed that current annual increment (CAI) peaks at 20 years and MAI at between 30 and 50 years (Claessens et al. 2010). In Europe sycamore also exhibits an early peak in CAI and MAI and it is described as growing more rapidly than beech (Fagus sylvatica) up to an age of around 40 years and as being more productive than ash, on both poor and fertile sites (Hein et al. 2009). In most European countries birch is slower growing than sycamore or ash (Hynynen et al. 2009). The mean and range of MMAI and biomass productivity for these SRF species are presented in Table 3, while biomass productivity is based on SRF rotations, the MAI of species other than the eucalypts is based on conventional silviculture and rotations.

Table 3 Estimates of mean annual increment and biomass yield for potential SRF species (cf. Hardcastle 2006)
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Wood properties and important physiological and morphological characteristics

There are physiological, morphological and wood characteristics that are attractive in a SRF species. The first is the ability to coppice (Dickmann 2006; Hinchee et al. 2009; Guidi et al. 2013), as it avoids the costs of replanting and results in enhanced growth rates in the second and subsequent rotations (Pukkala and Pohjonen 1990). All tree species described in Hardcastle (2006) will coppice or sucker, except for E. nitens which has a very limited ability to coppice (Neilan and Thompson 2008). While birch coppices after cutting it only does so weakly and so this not a recommended means of regenerating stands (Cameron 1996). For eucalypts the ability to coppice also confers a degree of resilience to damage. During the winter of 2010–2011 stems and foliage of E. gunnii were killed at a planting at Daneshill, Nottinghamshire in the north of England, however many of the trees later produced coppice shoots and remain healthy to this date, whereas trees of E. nitens were killed. This supports the recommendation that in the UK, only species that coppice, such as E. glaucescens, E. gunnii and E. rodwayi (Sims et al. 1999) should be planted, rather than the few species like E. nitens that do not have this capability (Neilan and Thompson 2008).

For wood used for solid fuel, rather than conversion into liquid fuels, the main factors determining its suitability are moisture content, heating value, proportion of fixed carbon and volatiles, the ash content and the alkali metal content (McKendry 2002). Moisture content strongly affects the net heating value of wood when it is burned (Huhtinen 2006). There is considerable variation in moisture content between the tree species selected by Hardcastle (2006) (Table 3). Eucalypts have relatively high wood moisture content and experience in Ireland has shown that drying E. gunnii can be problematic. The wood only dried rapidly when the bark was removed and this itself was difficult using machinery because of its fibrous nature (Leslie 2013). The heating value of dry wood of different tree species varies relatively little with conifers having a higher value per unit of dry wood than broadleaves, due to higher lignin content and the presence of resins (McKendry 2002). However, wood density varies considerably between the tree species. Dense wood is an attractive trait for fuelwood (Senelwa and Sims 1999), as higher densities represent a higher mass of material and therefore energy per unit volume and allows a higher mass of wood to be transported per unit volume. Alder and poplar exhibit a low wood density, the eucalypts and Nothofagus moderate density and sycamore, ash and birch exhibit higher density wood (Table 3).

Many of the species have not been burned for energy on an industrial scale in contrast to short rotation coppice. Ash, is known to produce good domestic fuel wood (Savill 2013) while in Sweden, birch is widely used as a source of domestic heat (Hedberg et al. 2002). A study of the effects of torrefaction, a heating process that improved the quality of wood fuel in terms of combustion and gasification on wood from trees including birch, aspen and eucalypts showed that the two eucalypt species tested contained chlorine concentrations eight times that of aspen and six times that of birch (Keipi et al. 2014). Chlorine can be corrosive to boilers and pipework in power plants. High levels of chlorine have also been reported from eucalypts grown in Ireland (Teagasc 2015).

A regular, straight stem enables more efficient handling, storage and processing (Walker et al. 2013), although if the material is to be chipped on site when harvested this is less important. Potential SRF species known to exhibit good stem form include E. nitens (Neilan and Thompson 2008), Populus (Savill 2013) and silver birch (Hynynen et al. 2009), while the stem form of Nothofagus alpina is as good as poplar (Tuley 1980). The other SRF species show a wide variation in stem form between individuals. Ash is sensitive to frost damage and this can result in poor stem form through death of the leader and so frost prone sites should be avoided (Dobrowolska et al. 2011). Sycamore shows considerable variation in stem form (Hein et al. 2009). Young alder often exhibits a straight stem with a compact pyramidal crown, but stem form becomes more variable as the trees age (Savill 2013). Stem form of E. gunnii is variable and often poor (Purse 2010; Marriage 1977), but improved material used in France exhibits good stem form (AFOCEL 2007).

Abiotic and biotic limitations

To be a productive tree for biomass, it must be well adapted to the abiotic and biotic environment. The following sections examine the limitations imposed on eucalypts and the other tree species identified by Hardcastle (2006).

Abiotic limitations

Most of the tree species in Hardcastle’s (2006) list of potential SRF species are well adapted to climate of the UK and will grow well on a range of soil types. However, periods of low temperatures can be damaging to Eucalyptus spp. and Nothofagus spp. (Deans et al. 1992). Eucalypts are at the margins of their climatic limits in most of the UK: over the last decade there have been two winters (2009–2010 and 2010–2011), with extended periods of < − 10 °C overnight temperatures, causing widespread mortality to E. gunnii and E. nitens across a range of experimental sites planted in England (Harrison 2011) and in Scotland (McEvoy 2016). However, 2009–2010 was the coldest winter in 30 years and in some parts of England in 100 years (Prior and Kendon 2011) and 2010–2011 was only a little less severe (Met Office 2011). Furthermore, in a review of the impact of climate change on eucalypt plantations in general, Booth (2013) assessed their vulnerability as being moderate. He also noted that the short rotations often associated with eucalypts, compared with conventional forest rotations offered greater opportunities to change genotypes and silvicultural practices. Murray et al. (1986) noted the high risk of Nothofagus being damaged: however, if southern provenances are used and the hardiest individuals are selected, they are suitable for planting in most lowland parts of the UK.

Biotic limitations

The risk of damage to trees from biotic agents is predicted to increase (Logan et al. 2003, Sturrock et al. 2011) and is already having major impacts on forestry in the UK. European larch (Larix decidua) is no longer planted due to the impact of Phytophthora ramorum (Forestry Commission 2014), while planting of Corsican pine (Pinus nigra ssp. laricio) has ceased because of damage from Dothistroma septosporum (Brown and Webber 2008). Wainhouse et al. (2016) provide useful predictions of those pests and pathogens likely to be particularly damaging to trees in the UK in the future while another resource for appraising the risk from insect pests and pathogens is the online UK Plant Health Risk Register (UK Plant Health Risk Register no date). This does not provide a combined overall risk rating for a tree species. However, the risk from pests and pathogens to Hardcastle’s (2006) potential SRF tree species can be broadly described as follows.

There are tree species currently at high risk from damage, such as ash which is no longer being planted due to the predicted damage from ash dieback (Woodward and Boa 2013). Furthermore, an additional risk is that from Emerald ash borer (Agrilus planipennis), which is now present in Russia (Straw et al. 2013), with the combined impacts likely to be severe (Thomas 2016). Sycamore can suffer extreme damage by grey squirrels (Sciurus carolinensis) and planting this species for timber is uneconomic in areas where high squirrel populations are present (Savill 2013). For pathogens of sycamore, Webber et al. (2011) note that Phythophthora spp and Verticilium wilt can be damaging in nurseries or newly planted stock. Cryptostroma corticale also affects sycamore and remains dormant in the tree until it becomes stressed by prolonged dry conditions and the pathogen then causes an ailment known as sooty bark disease, which results in crown dieback and can cause death of the tree (Savill 2013).

Of the tree species there are those where potentially serious pests or pathogens are already established in Britain. This group includes alder and Nothofagus which are at threat of damage from Phytophthora alni (Gibbs et al. 1999) and Phytophthora pseudosyringae (Scanu et al. 2012) respectively. This group also includes poplars, plantations of which have been severely damaged by rusts (Melampsora spp) in the UK (Forestry Commission 2005). Damage by P. alni was first noted in Britain 1993 primarily infecting and killing the native alder, but also grey alder (A. incana) and Italian alder (Alnus cordata) (Gibbs, et al. 1999). Other recent work (Černý et al. 2012) demonstrates that cold temperatures will kill the pathogen and suggests that with predicted increases in winter temperatures due to climate change, persistence of this pathogen may increase damage.

Infection by P. pseudosyringae of Nothofagus was first noted in 2009 in a stand of N. obliqua in Cornwall in the south of England, where in four plots between 50 and 72% of trees had become infected. The susceptibility of Nothofagus to this disease prompted Scanu et al. (2012, p. 27) to comment ‘A consequence of this damaging new disease is that future use of N. obliqua and N. alpina in UK forestry as suitable species for climate change adaptation strategies could be limited’. This view is supported by a recent review of Nothofagus in Britain (Mason et al. 2018) which identified N. obliqua and N. alpina as having potential, provided P. pseudosyringae does not prove to be highly damaging. Poplars are susceptible to attack by rusts (Melampsora spp). Rusts cause premature leaf fall and can also disrupt hardening in some hosts and other damage can include a reduction in growth, shoot die back and when severe, tree death. Developing varieties of poplar resistant to rusts and to the highly damaging Xanthomonas populi that causes stem cankers is the main strategy to produce disease free stands. In the past, the Forestry Commission published a list of resistant varieties (Forestry Commission 2005) and a mix of resistant clones was planted to reduce risk further, however at present there are no fully rust resistant varieties available (Tabbush and Lonsdale 1999).

Birch is currently relatively free of major damaging biotic agents, although it is susceptible to attack by Armillaria (Webber et al. 2011). Furthermore, there have been problems of crown dieback reported in recent plantings of birch, in Scotland, due to three pathogens; Anisogramma virgultorum, Marssonina betulae and Discula betulina (Green 2005). However, it is the threat from a pest, currently absent from the UK, that gives greatest cause for concern. The bronze birch borer (Agrilus anxius), if introduced would have a devastating impact (Nielsen et al. 2011) as silver birch and downy birch (Betula pubescens) are highly susceptible. Within 8 years of planting in a trial in the USA, all individuals of these birch species had been killed by the borer (Nielsen et al. 2011). The probability of detection of bronze birch borer in wood chips was extremely low using the current protocols in Europe (Okland et al. 2012), although the European and Mediterranean Plant Protection Organisations (2011) risk assessment suggested current measures meant the likelihood of bronze birch borer arriving in Britain is relatively low.

Finally there are the eucalypts which currently are probably at the lowest risk of damage from pests and diseases in the UK as few native pests of eucalypts have been introduced to plantations outside Australia (Fanning and Barrs 2013). The eucalypts identified as being suited to SRF in Britain are not those most susceptible to Phytophthora spp or to foliar pathogens (Webber et al. 2011). There are no records of major pest outbreaks in the UK, however there have been outbreaks of pests in Ireland. In the late 1990s a psyllid, Ctenarytaina eucalypti was introduced to Ireland (Chauzat et al. 2002). Chemical control was not particularly effective and so a parasitic wasp, Psyllaephagus pilosus was introduced and this effectively controlled the psyllid (Chauzat et al. 2002). In 2007 a leaf beetle, Paropsisterna selmani caused severe defoliation in multi species plantings of eucalypts (Fanning and Barrs 2013, Horgan 2012). Fanning and Barrs (2013) describe the beetle as being a serious threat to eucalypts in Ireland, the UK and more widely in Europe as the adults are strong fliers, capable of surviving long periods without food and can tenaciously cling to various forestry residues.

Discussion

Eucalypts possess many of the silvicultural attributes that are attractive for SRF for biomass, namely rapid early growth, good stem form and methods for propagation and establishment that are well understood. This review of evidence supports the proposition that eucalypts could provide a productive source of biomass in the UK and in other countries with oceanic climates in Europe. In Ireland, well-adapted species of eucalypts have been identified (Neilan and Thompson 2008) and spacing and other trials are underway (Tobin et al. 2016). In France there has been a programme establishing pulp plantations of cold-tolerant eucalypts that has been established for 35 years in the mid-Pyrennes. About 2000 ha have been planted of E. gunnii and Eucalypts x gundal, its hybrid with Eucalyptus dalrympleana (FCBA 2018).

The climate of the UK however is generally colder than Australia and the parts of France where cold-tolerant eucalypts are planted. Over most of the UK absolute minimum temperatures dip to below − 14 °C over a 30 year period (Fig. 1), a temperature that will damage or kill most eucalypts, particularly young trees or when temperatures have dropped rapidly. It is mainly coastal or southern areas where absolute minimum temperatures remain higher (Met Office no date). Eucalypts are particularly vulnerable to cold damage because, unlike most temperate broadleaves they do not have a defined dormant period. Growth of the naked buds begins above a certain minimum temperature threshold, which contributes to their high productivity (Beadle et al. 1995). A study of E. nitens in Tasmania showed maximum winter growth rates were only marginally less than those in summer (Davidson et al. 1995). Hardening in cold-tolerant eucalypts usually commences below a temperature of between 2 °C (Paton 1983) and 4 °C (Davidson and Reid 1987) and there are differences between species in the rapidity of hardening and in the minimum temperature they will tolerate before damage. Work by Black (no date) in Ireland developed an index of hardiness for eucalypt species in Ireland based on lethal minimum temperature and rate of hardening and the index ranked species tested in the following order, from most hardy to least hardy; Eucalyptus rodwayi, E. glaucescens, Eucalyptus subcrenulata, Eucalyptus delegatensis, E. gunnii, E. coccifera and E. nitens. This is a useful approach, although it raises questions as to why E. gunnii, a highly cold-tolerant species ranked so low. Details of the provenance used was not provided and it may be that a less cold-hardy one was used.

The limited area of both Eucalyptus (Purse and Leslie 2016b) and Nothofagus (Mason et al. 2018) makes matching species to site imprecise but colder and more exposed sites must be avoided. To identify climatically suitable sites for planting Nothofagus and Eucalyptus, Ray and Sing (2006) created a map of England, Scotland and Wales that showed areas where minimum temperatures of − 14 °C or below occur more than once every 50 years (Fig. 1) as a very broad indication of sites that had a low risk of cold damage (cf. Murray et al. 1986). An indication of relative cold of a site is currently not incorporated into Forest Research’s Ecological Site Classification (ESC) (Pyatt et al. 2001) decision support system, a web-based programme for matching species to site.

An important means of reducing risk is ensuring suitable species and provenances of Eucalyptus are planted. Results from trials established in the 1980s (Evans 1986) or informal planting (Purse and Leslie 2016a) have yielded useful information on species and provenances that are well-adapted to parts of the UK. It is clear a wider range of eucalypts could be planted (Purse and Leslie 2016a) than E. gunnii and E. nitens as proposed in Hardcastle (2006) and incorporated into ESC. This has been recognised and the species most planted in the UK between 2011 and 2015 was Eucalyptus glaucescens (Purse and Leslie 2016b). Table 4 describes recommended provenances for five promising eucalypts and the areas of the UK where they are best suited. However, obtaining seed of superior provenances has proven difficult and it is likely that sub-optimal origins have been planted in recent decades in the UK.

Table 4 Recommended origins for selected eucalypts
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The risk of damage by cold can be reduced further by focusing on species that readily coppice as they are likely to recover from the main stem being killed by cold and thus replanting costs can be avoided. Furthermore, coppice rotations are more productive than first-rotations (Pukkala and Pohjonen 1990). A further approach to reduce damage is the use of intensive silviculture to accelerate establishment and growth as larger trees are more resistant to cold than smaller ones (Leslie et al. 2014a). Finally, to reduce the risk of complete failure, plantations should comprise stands reflecting the full range of ages, from those recently established to those of rotation age, as younger stands are more susceptible to damage from cold.

A further positive influence on the suitability of eucalypts to the climate in the UK and other temperate maritime environments is the likely effect of climate change on the frequency and duration of extreme cold events. Climate models predict a reduction in the frequency of cold events during winters in Western Europe, and in general a reduced severity (Peings et al. 2013). Gloning et al. (2013) predict warmer winter temperatures and no increase in the frequency of extreme winter temperature events for the UK. However, warmer springs may lead to greater frost damage; a study by Augspurger (2013) at a woodland in Illinois, USA described increased risks of frost damage to woody species over the last 100 or more years associated with an increased frequency of periods of warming in March followed by frosts in April. The species other than eucalypts listed by Hardcastle (2006) are pioneer species and so their phenology is influenced more by photoperiod and less by temperature (Basler and Körner 2012). It is likely that climate change will extend their growing season, but this will also increase the risk of frost damage (Basler and Körner 2012). A potential negative factor for eucalypts are increased levels of carbon dioxide. Heightened levels (twice ambient concentrations) of atmospheric carbon dioxide have been shown to increase susceptibility to frost damage through an increased ice nucleation temperature in cold-tolerant eucalypts, probably caused by bacteria on the leaves (Lutze et al. 1998).

The main constraints for using the wood from eucalypts as a source of biomass is the high moisture content and chemical composition of the wood. The high moisture content and requirement for some species for debarking for effective drying require additional inputs to produce a quality wood fuel. However, most of the plantations of woody energy crops in the UK have been SRC and wood from this source has as high a moisture content as eucalypt SRF. The high chlorine content of eucalypt wood creates acid gases and corrosion of boilers, however the concentration of chlorine can be considerably reduced (as much as 90%) by pre-heating the wood before burning through torrefaction (Kiepi et al. 2014).

If eucalypts were to be planted on a large scale in the UK, the environmental impacts would need to be quantified. The risk of invasiveness of E. gunnii and E. nitens is low. For both species, seed germination can be poor and the seedlings are susceptible to frost damage and, for E. gunnii, it is also palatable to deer. Furthermore, for both species the small seed size means that seedlings have few reserves and are vulnerable to competition from other plants; there are few UK sites where natural regeneration of E. gunnii has been observed and none where E. nitens has been recorded (Dickinson 2011a, b). In general, Booth (2013) highlights the low risk of invasiveness of eucalypts in frost prone areas of the world. Nothofagus is also not considered to be a threat and is included in the exotic tree species where planting is not regulated under the Wildlife and Natural Environment Act of 2011 (Forestry Commission Scotland 2015).

There are limited studies on the effects of eucalypts on flora and fauna in the UK. Surveys of fungi showed that most of the mycorrhizal fungi associated with eucalyptus had originated from Australia, with a limited number of native British species (Pennington et al. 2011), although another survey identified rare fungal species, including three species representing three new genera to the UK (Hobart 2012). A study of earthworms under SRF and in comparison with pasture (Rajapaksha et al. 2013) suggested SRF should focus on native species but also Eucalyptus spp, which also supported dense populations of earthworms. Under the Great Britain Non-native Species Risk assessments (Dickinson 2011a, b), reviews were conducted of the environmental risk associated with E. gunnii and E. nitens. The conclusion of the analysis for both species was that they were both in the upper level of the ‘low’ risk category for environmental impact.

In a review of predicted impacts of SRF on water quality and supply no problems were associated specifically with eucalypts, rather with intensively grown plantations (Nisbet et al. 2011). Furthermore, evidence shows that eucalypts are very efficient at using water to produce biomass (Dvorak 2012).

Conclusion

Eucalypts have many properties that make them attractive as a source of woody biomass in the UK. Where eucalypts have the greatest advantages is their potential growth rates that exceed those of the other species and are achieved over short rotations, although in practice high yields are often not achieved due to poor survival due to low temperatures. The risk of poor survival can however be mitigated by adopting four management approaches. The first is to focus on planting eucalypts that readily coppice as they are likely to recover from the main stem being killed by cold and thus replanting costs can be avoided. Furthermore, coppice rotations are more productive than first-rotations (Pukkala and Pohjonen 1990). A second means of reducing risk is the use of origins that have been proven to be best adapted to UK conditions. There is a growing body of information on the most suitable provenances for a limited range of eucalypts (Table 4). A third approach to reduce damage is the use of intensive silviculture to accelerate establishment and growth as larger trees are less damaged by cold than smaller ones (Leslie et al. 2014a). Finally, plantations should comprise stands reflecting the full range of ages, from those recently established to those of rotation age, as younger stands are more susceptible to damage from cold.

The high moisture content of green wood and the high chlorine content released during their combustion (Keipi et al. 2014) are constraints to the use of eucalypt wood as a fuel. However, the moisture content is not higher than some other species, such as poplars (Table 3) and for SRC willow and methods have been devised for drying this material (Whittaker et al. 2018). The problem of chlorine release on combustion can also be addressed through pretreatment by torrefaction (Keipi et al. 2014).