Joe Also report.pdf - Winston Churchill Memorial Trust

70 Pages·2015·8.7 MB·English

Report for the Winston Churchill Memorial Trust

Woodland and tree management in the wake of
Ash Dieback (Hymenoscyphus fraxineus):
Experience from Continental Europe

Denmark, France, Germany, Lithuania,
Luxembourg, Sweden and Switzerland.

JOE ALSOP - Churchill Fellow 2014
“To build may have to be the slow and laborious task of years. To
destroy can be the thoughtless act of a single day.”  - Winston Churchill

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Contents

Introduction 1
Project Aims 5
Ash dieback in continental Europe 6
Case study 1: Trial plot - Baden-Württemberg, Germany 17
Case study 2: Dalby Söderskog National Park, Sweden 19
Current Management: Practice & Consequence 22
Case study 3: Spreewald Biosphere Reserve, Germany 35
Case study 4: Elmias LIFE-Project, Island of Gotland, Sweden 39
Case study 5: Forêt domaniale d'Auberive, France 43
Conclusion 46
Recommendations 50
Acknowledgements 53
References 54
Itinerary 64

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Introduction
This report has been compiled on behalf of the Winston Churchill Memorial Trust. It
details the findings of a European trip in 2014 to study the serious tree disease, Ash
dieback. Meetings were held with a wide range of industry professionals including
forest pathologists, university research scientists, lecturers, species and habitat
specialists, foresters and site managers. The contents are a summary of what was
learnt and the understanding which was gained, and, where appropriate, this is
supported by relevant references.
The Winston Churchill Memorial  Trust  ‘funds British citizens, who are resident in the
UK, to travel overseas to study areas of topical and personal interest, to gain
knowledge and bring back best practice for the benefit of others, their profession and
community, in the UK’  (WCMT 2014).

The Significance of Ash______________________________________
The Ash tree (Fraxinus excelsior) has an extensive range throughout Europe, growing
as far East as Russia, throughout Ireland in the West, down to Greece in the South
and its Northern range stretches up into Norway (Euforgen 2010). The species is
important for its great historical, cultural, ecological, biodiversity, economical and
commercial value - possibly nowhere more so than in the UK.
The Ash has long been held in high regard. In old Scandinavian mythology the Ash
was known as the tree of life - ‘Yggdrasil’  (Mabey 1998) - and in the UK, its historical
significance  is  shown  in  that  there  are  more  places  with  ‘Ash’ in their name than any
other tree species,  except  ‘Thorn’  (Marren 2013). Ash occupies a fairly unique niche
within the limited suite of British native tree species, and, further still, in the even
smaller group of native trees which can attain canopy dominance. It coppices well,
can grow in a wide range of sites and, where conditions are optimal, it can grow fast,
producing a pure, off-white, valuable, straight-grained timber which can be worked
easily for a wide range of applications. It is has one of the toughest timbers,
absorbing shock well, making it well suited for tool handles and, where  quality  isn’t  
achieved, the timber still commands a good price for its excellent firewood. Ash
regenerates extremely well, has shade tolerant seedlings and, in addition, it is one of
the few hardwoods of timber value to be almost entirely unsusceptible to damage
from the Grey squirrel (Sciurus carolinensis) (Fraxigen 2005). Growth traits make the
Ash an intermediate between a permanent forest component and a pioneer species
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(Pautasso et al 2013) and, in certain conditions, it can excel at both. Ash trees are
usually dioecious, having separate male and female plants, although monoecious
specimens can occur (Harmer, Kerr & Thompson 2010).

In addition to its commercial significance, Ash is also of great biodiversity value,
providing a unique set of ecological traits: – whilst in leaf, the lightly shading canopy
provides conditions for a diverse ground flora, and once the leaves are shed, they
produce a shallow, rapidly degrading leaf litter which is nutrient rich and maintains a
high soil pH.  Of  the  955  species  which  use  Ash  (known  as  ‘Ash  associated  species’), 44
of these have been identified as only being found on Ash trees (Mitchell et al 2014a).
Ash is a significant component of the British countryside: it is the second most
frequent individual tree species (after Oak) and is the most common hedgerow tree
(Maskell et al 2013). Ash’s  importance  within  the  landscape and for biodiversity has
already been further enhanced by the loss of the majority of Elm trees (Ulmus sp),
due to Dutch Elm Disease (DED). Elm can host a large number of Ash associated
species (Mitchell et al 2014b) and, like Ash, it has a high bark pH. It exhibits comparable
growth traits and performance and grows well in a similar range of site conditions. In
addition, where the bark pH of some tree species has declined due to acidification
over the last century, it may now have rendered them as unsuitable hosts for some
lichen species, which are now more reliant on Ash (Bates & Farmer 1992 & Scheidegger
2014).

In the UK, Ash woodlands are more prominent than they are in mainland Europe
(Peterken 2013) and according to the Forestry Commission (2014), Britain has around
130,000 hectares, making  up  to  5.5%  of  the  nation’s  total woodland cover. With all
this in mind, it is entirely understandable that since 2012, when the presence of Ash
Dieback (Hymenoscyphus fraxineus) was first confirmed in the UK, (in a tree nursery
in Buckinghamshire and shortly after in the wider environment,) foresters,
conservationists, woodland managers and timber growers have all become
increasingly concerned as to what this disease will mean for the future of Ash in the
UK. This concern is further justified as there is no single alternative tree species,
native or otherwise, which can fulfil the unique role of Ash.

Ash Dieback_______________________________________________
Ash Dieback is a serious disease spreading through Europe and affecting the three
native Fraxinus species, most noticeably F. excelsior and F. angustifolia and to a lesser
extent F. ornus. The first symptoms of Ash Dieback were observed in Poland and
Lithuania in the early 1990s, but the cause was initially unknown  and  wasn’t correctly
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identified until over ten years later. Chalara fraxinea was first identified by Kowalski
(2006) as being the asexual form of the disease but it was initially believed to be
associated with a native saprophytic fungus, Hymenoscyphus albidus, which is a
decomposer of dead fallen Ash leaves. In 2011, Queloz et al correctly identified
Hymenoscyphus pseudoalbidus to be the sexual form of the disease, but, after the
rules relating to the nomenclature of certain fungal species changed in 2011, further
work by Baral et al (2014) declared the correct name for the fungus to be
Hymenoscyphus fraxineus. Interestingly, McKinney et al (2012b) suggests that H.
albidus may be the first extinction, at least locally, to be caused by the invasive
pathogen H. fraxineus.

Ash Dieback has been introduced into Europe from East Asia (Zhao et al 2012), most
likely from the importation of infected trees for planting (Drenkhan et al 2014). In its
native range where it has coevolved with native Fraxinus species, such as F.
mandshurica, H. fraxineus remains harmless. It is unclear whether H. fraxineus has

Joe Alsop, 2014
Figure 1: F. excelsior rachis infected with H. fraxineus - the black staining is symptomatic. Gotland
Sweden.
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become such a serious pathogen in  Europe  solely  because  coevolution  hasn’t  
occurred between host and pathogen, resulting in an extremely low levels of
resistance or whether the different climate and phonological sequences are also
factors.

The small cup-fungus fruiting bodies of H. fraxineus grow on the rachis of fallen Ash
leaves (Fig. 1) and release ascospores between June and October. These spores then
alight on living Ash leaves causing infection which then spreads down the rachis and
enters the twigs and stem causing dieback. When these newly infected leaves are
shed, they will produce fruiting bodies the following year, completing the disease
cycle.

In the UK, it can only be assumed that Ash dieback will have a similar impact on the
Ash tree population as it has in continental Europe. With this in mind, it is essential to
gain a good proportional understanding of what this impact will be, and whilst there
has been a lot written and reported on the subject, it  doesn’t  quite  compare  with  
first-hand observations and on-site discussions with tree and woodland specialists.

Joe Alsop, 2014
Figure 2: Ash dieback severely affecting a young stand of F. excelsior in Jura, Switzerland.
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Project Aims
As Europe has up to a 20-year head-start in managing trees and woodland in the
wake of Ash dieback, they have valuable experience of how the disease will spread,
the effect it has on trees and woodland and also any measures which can be
implemented to mitigate against its negative impacts. Whilst the main focus of the
project was to investigate the effect of Ash dieback in woodlands where conservation
is the primary management objective, many such woodlands also have commercial
interests so timber production was also considered.

The aims of the project were to:

1. Learn first-hand about the impacts of the disease on mature trees and
woodlands especially where Ash is a key species.

2. Gain knowledge of management options and techniques which can minimise
the negative effects of Ash dieback on woodland ecosystems and landscapes
in addition to economic and social factors.

3. Learn about how tolerant or resistant strains of Ash can be identified,
propagated and promoted to ensure their survival.

4. Learn how Ash dominated woodlands can be made more species-diverse to
enable them to be more resilient to pests and diseases.

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Ash dieback in continental Europe
Roy & Kirchner (2000) state, “Host  organisms  can  respond  to  the threat of disease
either through resistance defences (which inhibit infection) or through tolerance
strategies (which do not limit infection, but reduce or offset its fitness
consequences).” Throughout the range of F. excelsior populations in Europe currently
under infection pressure from H. fraxineus, none have been found to be completely
disease free (McKinney et al 2011, McKinney et al 2012b,  Pliūra  et al 2011, Pliūra  et al 2014 &
Stener 2012). Therefore it is likely that within these populations, rather than true
resistance, there are just differing degrees of tolerance. It is clear that genetics is the
primary factor which determines disease susceptibility (Stener 2012, McKinney et al 2011
& McKinney et al 2012a). In addition, it is important to understand that there are a range
of contributory factors which can also influence survival or mortality, but many of
these are yet to be wholly understood. Infection pressure from H. fraxineus acts like a
war of attrition, and in trees with low levels of tolerance, their energy reserves can
become rapidly depleted fighting the disease. Transversely, it appears that when
subjected to additional biotic or abiotic stress factors, even trees with higher levels of
tolerance can have their ability to resist infection reduced (Bakys, Vasaitis & Skovsgaard
2013). As many of these secondary factors are dynamic, the plight of a tree can
change over time resulting in trees succumbing which at first appeared to be
tolerant, and also trees which are at times greatly affected, later rallying.
Interestingly, many forest professionals commented on the fact that the condition of
F. excelsior could vary from year to year, and in both Sweden and Denmark, they
thought that generally trees appeared to be in better condition in 2014 compared to
previous years (Fig. 3). Similar observations of yearly variation are also reported by
Pliūra  et al (2014) and Stener (2012).
Regarding the proportion of the F. excelsior population which is considered to be
disease tolerant, 10% is often quoted, but one first needs to understand what is
actually being classified as tolerant. Assessment protocols can vary from country to
country and trees considered to have some degree of tolerance can range from
having no visible symptoms up to having 35% of the crown affected. Amongst others,
Enderle et al (2014), Metzler et al (2012), Kirisits & Freinschlag (2012), McKinney et al
2011, Stener (2012) all describe methodologies for scoring and classifying tree health
in relation to the levels of damage symptomatic of Ash dieback. As an example,
Enderle et al (2014) suggests classifying dieback and the associated defoliation as a
percentage of the crown affected, e.g. class 0 = no symptoms visible, class 1 = 1-10%,
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class 2 = 11-25%, class 3 = 26-60% and class 4 = 61-99%, and obviously with 100%, the
tree would be dead. Due to some trees which are severely affected by H. fraxineus
producing prolific epicormic growth (figs. 3 & 4), they could score well in this system.
Consequently, Enderle et al (2014) take this into account and also record the level at
which a tree is affected by the disease, by using the same classification system to
estimate the amount of epicormic growth present within the crown. It can be
assumed that the production of epicormic growth is an indicator of disease
susceptibility, whilst the extent of crown defoliation is an indicator of overall tree
vigour (Enderle et al 2014). So a tree with very few shoots and twigs dying back and
otherwise symptomless could be considered to have a good level of disease
tolerance, whilst a tree producing epicormic growth and has extensive dieback would
not be. Throughout the trip, many trees were observed which had clearly been
heavily affected by H. fraxineus in the past, with much of their outer crown having

Gunnar Isacsson, 2009 Gunnar Isacsson, 2014

Figure 3: A severely affected F. excelsior in Norra Sandby, Scania, Sweden. Whilst the tree looks
better in 2014,  it  can’t  be  considered  to  have  recovered, nor can it be considered as having a high
level  of  disease  tolerance,  due  to  the  secondary  ‘inner’  crown  which  has  developed  from  epicormic  
stress growth. What the pictures do show is the variation in tree health which can occur from year
to year.
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