A STUDY OF THE LICHENS ON SOME YEWS IN EASTERN DEVON
B. Benfield. June 2006
This small study was the result of a meeting organised by East
Devon District Council in 2006 and led by Tim Hills of the national
Ancient Yew Group to highlight the importance of the ancient yew
tree at Plymtree. The research was prompted by a forester who assumed
that no lichens grew on the trunks of yews because all parts of
the wood and bark are toxic. It came as a surprised to him, therefore,
to hear that some, even nationally scarce lichens, can thrive on
the bark, decorticate timber and iron hard spikes of dead branches.
Admittedly these lichens are not obvious, often no more than green-grey
crusts and although none are specific to yews some are more often
found on them than on any other trees.
Yews in 43 churchyards were investigated in eastern Devon to collect
information about the lichen flora on the trunks in an attempt to
construct a theory regarding any connection between the girth and
age of the trees and the lichen flora and whether the toxicity of
the bark may affect the selection of lichens growing there.
Most large specimen yews are situated in churchyards or rectories.
Two types can be found, Taxus baccata, the Common or European Yew
and Taxus baccata fastigiata, the Irish Yew. The latter was discovered
as a different form of T. baccata in Ireland in 1780 so no ancient
specimens are known to exist. It has a characteristic, comparatively
low, bushy growth which makes it unsuitable for the development
of most lichens and has therefore been omitted from this study.
A list of ancient yews found in Devon and their sizes provided
by the national Ancient Yew Group was used as a base and any churchyards
near those sites were visited and the trees measured regardless
of size so that a comparison of the lichen flora on large and younger
trees could be made. Using tree size to try to age a yew is fraught
with problems after the tree reaches more than 4m due to the slowing
of girth increments and variation in growth habit. Mitchel (Trees
of Britain and N Europe 1974) pointed out the problems of dating
yew due to its tendency to rot in the centre but retain a healthy
perimeter or create new trunks and roots within the hollow. The
centres of the ancient yews at Payhembury and Hemyock have rotted
away leaving 2-4 spaced trunks indicating their once massive size.
This makes core sampling and ring dating impossible or inaccurate.
The trunk of English Yew rarely grows straight up from the base
and often large low branches splay out making measurements difficult
and dependent on the whims of the recorder. The straight growth
of yew trunks at Axminster, Calverleigh and Plymtree are exceptions.
Mitchel suggested as a rough guide young trees add 2-3 cm girth
per annum, old trees 0.5cm and very old 0.1cm. The yews visited
in Devon with known planting dates between 1636 and 1790 can be
calculated to have had average increments in the range of 1.2 and
1.6 cm per annum. A large percentage of the yews in the 43 yards
visited measure between 250 and 350cm which suggests a surge of
planting during the 18th century.
The splaying of large branches and fluted trunk formation leads
to deep crevices with slightly better lit areas forming different
available niches. The lichen flora on the trunks is very limited
partly due to the dry, flaking nature of the bark and intense evergreen
shade cast by the canopy. In fact, it is rare for any part of a
yew trunk to be well lit, unless lower branches have been trimmed,
but a few lichens are adapted to inhabit deeply shaded bark.
By the 1970s it was realised that some lichens only grow in places
which have been left undisturbed for centuries and a list was compiled
by Francis Rose (1976 updated to the New Index of Ecological Continuity
or NIEC in 1992) to be used to evaluate woods or parklands in terms
of conservation value. The habitat in churchyards will probably
have remained relatively unaltered for decades if not centuries
and of the 18 species found on the yews visited 4 are on the NIEC
list. (Cresponia premnea, Dimerella lutea, Opegrapha corticola and
O. prosodea.) The nationally scarce (NS) lichens found most frequently
were Lecidea doliiformis, Opegrapha prosodea and O. zerica.
Only two yews under 350cm girth were host to NIEC or NS lichens.
They are at Woodbury, with a known planting date of 1775, and at
Calverleigh where two trees, one at 388cm and its neighbour 327cm
were not far apart. The larger may have acted as a reservoir from
which lichens could colonise the smaller tree or the two yews may
have been planted at the same time despite the difference in girth.
Although almost all yews with NS lichens are over 350cm girth
some large specimens have no lichens at all or none of interest.
One imagines the ambience of a churchyard yew to have remained the
same for centuries but this is not always the case. A clue to relatively
recent changes in the surroundings of a trunk can be found at Withycombe
Raleigh where a bench under the yew is completely hidden by epicormic
branches and surrounding scrubby vegetation. Hawkchurch is an example
of a tree which is 510cm girth but appears to have no lichen cover.
Ivy had been removed very recently so the shade on the trunk of
this tree may have been too great for the presence of any species.
In folk lore ivy has a mixed reputation and may therefore have been
systematically removed from church yards in the past. However, at
the time of this study very few of the trees visited were devoid
of ivy and many had up to 75% cover. In Devon ivy growth has increased
in the last decade due to warmer winters so a lichen flora could
have developed before the extra shade created on the trunks became
established.
The Widworthy yew is more puzzling. With a girth of 410cm it should
have been a host to at least some NIEC or NS lichens. Arthonia spadicea,
growing in the bark runnels there, was not commonly found on any
of the yews investigated and is generally considered a species of
humid secondary woods. Possibly the growth of a laurel hedge has
altered the surrounding microclimate of the churchyard over the
last 100 years. Arthonia spadicea was also found on a large yew
at Stoodleigh where the humidity of the trunk may have increased
since grass clippings have been piled round the base or a young
holly growing from the bole of the tree has reduced air circulation.
The veteran yews at Stoodleigh (640cm) and Bampton (884cm girth)
appear to be thriving with stone collars encasing the trunks to
1.5m. The trees at both sites are host to nationally scarce lichens.
The planting date is not recorded but a note in the church porch
at Stoodleigh states that in 1717 a seat surrounded the tree trunk
and that Edward 1st (13th century) decreed that yews should be planted
as wind breaks.
All parts of the yew except the red flesh round the seed contain
alkaloids which are toxic but the young twigs and needles contain
the highest concentrations. It is not known to what extent this
level of toxicity drops with age or if it inhibits colonisation
on the trunks by any lichen species. Only the common species Lepraria
incana, Opegrapha vulgata and O. varia were found on trees with
a girth less than about 300cm and no lichens were recorded on the
trunks of yews under 250cm except where the lower trunks are well
lit and affected by nutrient enriched splash. This seems to indicate
that a lack of light, moisture and available nutrients restricts
common lichens but when the trunks mature a niche develops for more
unusual and rarer specialists.
Many yews have no lichens on their outer branches though some
have a considerable cover which suggests that the toxicity of the
twigs is no deterrent to lichen colonisation. Physcia tenella is
the most frequent and often the only lichen on outer branches often
growing with thick algal crusts but occasionally accompanied by
Anisomeridium polypori, Arthonia radiata, Hyperphyscia adglutinata,
Melanelia laciniatula, M. subaurifera, Parmotrema perlata, Phaeophyscia
orbicularis, Punctelia subrudecta, Ramalina farinacea, Xanthoria
polycarpa and X. parietina. This assemblage can be found where the
churchyard is adjacent to farmland and indicates that the branches
are subject to some nutrient enrichment. In more sheltered yards
the once common Hypogymnia physodes is occasionally found on well
lit twigs.
The rarity of Enterographa crassa, which dominates many tree species
in shaded woods throughout Devon is puzzling especially as it was
found with Porina chlarotica on exceedingly shaded small trunks
of Irish yew. It is possible this may be due to the higher humidity
of the comparatively young bark within these tight knit clumps.
Despite only 18 lichens (table 1) being found on the trunks of
yews in 43 churchyards in the eastern part of Devon, 4 of those
are on the list of ecological continuity, 4 are nationally scarce,
and two have the category of international responsibility (IR).
The number of trees on which the scarce lichen Opegrapha prosodea
was found suggests that yew bark is its favoured habitat. Of the
eighteen species, twelve contain Trentepohlia as the algal component.
Although not in East Devon a special note must be made for the
huge spreading yew at Stoke Gabriel beside the Dart Estuary, which
is the finest seen so far and is a mini ecosystem in its own right.
The main branches have been allowed to fall outwards and now touch
the ground propping the whole tree as with ‘flying buttresses’.
So apart from the habitats on the fluted trunk, the branches have
rain wetted or dry, shaded and better lit surfaces in a sheltered,
south facing humid site near the edge of the Estuary. It is host
to an impressive list of lichens including 2 Red Data Book species,
Cryptolechia carneolutea and Physcia tribacioides. This one tree
has a lichen conservation evaluation score of 600 which is greater
than many woods.
Lichens picking out the wet/dry niches on the trunk of the Stoke
Gabriel yew.
The white stripe is Opegrapha vermicellifera and the green Opegrapha
corticola. Opegrapha prosodea is mainly in the crevices formed by
the flutes or on iron hard spikes which have lost their bark. Cresponia
premnea is growing on the dry bark and is only found on ancient
trees.
© Copyright B.Benfield
The importance of all ancient trees in a European context is becoming
widely appreciated and the small but significant lichen flora of
ancient yews adds another dimension to their value and that of the
churchyards where they are found
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