Ageing the Yew- no core,
no curve?
Fergus Kinmonth reviews current thinking on this controversial
topic.
This article first appeared in The International Dendrology Society
Yearbook 2005. It is also published in 2006 yearbook of Der
Eibenfreund (Friends of the Yew).
The distribution zone of Taxus baccata extends from Finno-Scandia
to Asia Minor and the Caucasus and southward into north Africa,
but mainland Britain contains the greatest concentration of truly
ancient trees. The range of this species is defined by climatic
extremes in central and western Europe; it is ideally suited to
the mild oceanic climate of the UK where limiting growth factors
are rainfall and growth season thermo period. In keeping with its
climatic preferences, it is often a component of montane forest
to the south of its zone and of lower altitudes to the north.
About 45 specimens of exceptional girth have been gazetted by the
Conservation Foundation in London, some of which have achieved
girths of up to 10.75m. Tentative age assessments have been made
using current guidelines, arriving at a possible age for these
trees of 2000 years. Some 8000 cuttings have been propagated from
these “methuselahs” and planted in the churchyards
of England and Wales.
Such gargantuan specimens as have survived these modern times
are set apart from the forest herd, treated with reverence and
accorded mythic status. |
Propped yew at Crom Castle, Fermanagh. 1902.
Very few old yews of any size are to be found in Ireland. Revisionist
extirpation? |
These are refugees from beyond the age of reason. A spirit of positive
ageism is engendered by the lobby of wishful thinkers and romantics
who have made it their totem tree.
We cannot test for immortality whilst the trees are still standing,
since common decency prevents us from counting their annual rings.
Radioisotope dating is imprecise for the time span involved and
is defeated by core decay in older trees.
Conventional ring count techniques are in any case complicated
by the yew’s enigmatic propensity for anomalous growth. The
typically fluted trunk results from discontinuous growth in different
sections of the tree, leading to eccentric lobes which make it
hard to locate the core.
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Open-grown yew. Chalk downland, south aspect. Leatherhead, UK.
1.7m gbh, 48cm diameter. 240 ± 20 rings © Fergus
Kinmonth
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Dendrochronological analysis relies on core sampling by increment borer. Ideally,
sample deviation is minimised by transects from four cardinal points in the stem.
After several centuries, older specimens appear to attain an optimum
crown volume whereafter growth waxes and wanes across the tree,
increasing sporadically after the collapse of a major component
or ceasing altogether for long periods. Until this time, growth
rate seems to be quite rapid and a simple formula relates girth
to age.
Allan Mitchell’s rule of thumb remains a better guide than
most, and is consistent for trees below a girth of 4.5m. It
also reads well in imperial units.
This is summarised as 1ft in 30 years, give or take 5′ in
trees exceeding 22′.
This formula accounts well for a girth of 30ft in at least 750
years.
John White’s system includes adjustments to account for
distinct changes in the tree’s growth pattern, characterised
as formative, mature and senescent phases.
After a period of rapid early growth the mature state is reached
between 30 and 60 years of age according to site conditions, such
as: woodland interior, exposed site, churchyard or parkland. A
steady state of 3mm ring width marks the mature phase.
Recent dendrochronological analysis of a group of 15 trees at Norbury
Park in Surrey, found girths ranging from 1.92 to 7.92m. According
to John White’s methodology, the oldest of these could be
expected to exceed 2000 years of age, since a test fragment demonstrated
extremely low growth rate in the “senescent” phase.
The assessed minimum ages on this basis were 2536 years (7.27m)
2806 years (7.65m) and 2917 years (7.92m).
Despite its durability, Taxus is susceptible to the heart-rotting
fungi Ganoderma lucidum below a height of 1m and Laetiporus
sulphureus above.
As a result, the core is often decayed in trees exceeding 4.5m
girth, confounding attempts to establish a reference curve beyond
about 400 years.
Consistent results have however been obtained by extrapolating ‘partial’ core
samples using data from younger trees on the same site.
A reference series has been constructed using data from 12 yews
planted around 1700 at Hampton Court Palace, with mean annual growth
increment of 1.08mm.
Indicator years cross-matched significantly with figures for rainfall
and extent of summer growth season recorded at RBG Kew. The unusually
high growth rate of these trees supports theories regarding the “heat
island” effect of urban environment and underlines the variability
of growth.
On the other hand, current findings among “wild” northerly
yews show an increased ring density compared with southerly “churchyard” trees.
A special study of clifftop specimens indicates that the upper
age limit is governed only by the rate of rock mass wasting. These
trees are likely to be older still than their complacent churchyard
counterparts.
Recent studies have also shown a correlation between ring width
series in yew and those of oak and lime.
Annual rings of equal width will progressively increase in area
as the stem expands over the first 60 years or so. Until optimum
crown size is attained, the annual increment of new wood remains
constant at about 1.6mm.
John White refers to CAI (optimal cross sectional area increase),
a stage reached in middle age. After this growth stage, growth
rings of CAI are maintained at a constant rate (in other words,
the rings become progressively narrower).
Unfortunately for our growth formula, yew seems able to return
to “formative” growth rate at almost any stage of
its life.
Fortunately for the dendrochronologist, there are indications that
despite climate change, core growth in some species of old trees
has not varied for centuries.
Furthermore, core samples demonstrate a dramatic decrease in growth
rates of the largest trees, implying an age greatly exceeding conservative
estimates.
We rely on anecdotal and archaeological data to establish planting
dates and supplement this knowledge with intermittent girth measurements.
Comparison can often be made where larger trees have been recorded
in photographs and engravings.
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Yew at Ankerwycke, Middx. c.1840.
Was it associated with the Magna Carta? |
Favoured by optimum growing conditions in the Thames floodplain.
No individual trees were ever recorded in the Domesday Book, but
a curious trend might tell us more about the situation of our our
oldest trees. The current distribution of veteran yews from Kent
through the southern counties and along the Welsh borders follows
the demarcation line of the Danegeld, and might imply an ethnic cleansing
by our viking invaders. The ancient yew is certainly conspicuous
in its absence from east anglia. If this prejudice were to be confirmed
it would give us another historic baseline.
Stratographic archaeology does provide additional clues in cases
where trees form part of a configuration or overlie old earthworks,
but even these can be misleading.
A celebrated example is at Tandridge in Surrey, where a tree measuring
35’ in girth is established close to the foundations of a twelfth
century church. An apocryphal story arose regarding the discovery
of Saxon vaulting constructed to span an existing root. Had this
been so it would certainly have confirmed suppositions of great age. |
The Ankerwycke yew, 2001. Girth 30′11′′.
Was this a 12th century tree? © Caroline Forbes
1. Yew tree growing from the rubble of Waverley Abbey in Surrey:
21′gbh.
Does it pre-date the Abbey (1128) or the dissolution of the monastries
(1540)? © Fergus Kinmonth
|
Root material contains clearly legible growth rings and can frequently survive
and regenerate following destruction of the tree itself . Unfortunately this
evidence has wizened on close inspection.
Clues can be gathered as to the maximum possible age of trees established
on earthworks or burial mounds, in association with churchyards or
as marker trees near routes of pilgrimage or holy shrines. It does
appear that some association exists with the activities of early
christian missionaries, such as the 6th century St Afan at Discoed
in Powys. This may be a case of sanctity by association, but does
not necessarily imply that such trees were particularly valued or
preserved by earlier pagan cults. That they may have been marker
trees would make sense in view of their conspicuous character. They
certainly seem to have served as meeting places for the hundredal
courts or moots, which declined in the 12th century. Such groups
of trees as those at Knowlton in Dorset could possibly have arisen
since the last use of the site for saxon burials in the 7th century,
but their girths indicate that they could be older still. Churchyard
trees could be conveniently dated according to the transition from
circular llan or rath to rectangular yard, if only
the timing of this change had been sufficiently clear-cut. Examples
such as the ring of 13 veteran yews at Llanelly in Brecon (20′gbh)
and those at Llanfihangell nant Melan (22′gbh) were certainly
established prior to the 13th century.
The National Tree Register lists measurements between 1952 and
1992 and incorporating earlier records. These have been further
extended by TROBI.
Records from trees of known planting date include:
Dryburgh Abbey, Selkirk. A tree planted by the monks in 1136 measured
11′4′′ in 1896 and 12′2′′ in
1984. This represents 10′′ girth increase in 90 years
and 136′′ in 758 years, arriving at 12′ in 858
years (John Lowe, 1896).
Some 17th century trees have achieved 1′′ per 2-3 years
for the first 300 years:
Hughenden, Bucks (1613) measured 10′ (1980).
Waltham St Lawrence (1655) measured 12′11′′ (1966).
Sutton, Surrey (1660) measured 12′3′′ (1966).
The growth rate of older trees seems to slow, averaging 1′′ girth
over 4-10 years.
Paul Tabbush at Alice Holt has devised a shortcut to yew age estimation
based upon extensive sampling of churchyard yews. This maintains
the assumption that girth is a function of age, on the simple premise
that a plant’s cells must divide for it to survive.
This simplifies the mathematics of John White’s formula:
Age = girth² ÷ 300cm.
The dating of yew remains a fuzzy science despite the arsenal
of special effects brought to bear by archaeologists and dendrochronologists.
Whatever the facts, no upper age limit has been established for
this species- there is clearly a case for the ageist lobby as the
debate continues.
References:
Chetan, A & Brueton, D. 1994. The Sacred Yew.
Harte, J. 1996. How old is that old yew? At the Edge.
#4.
Larson, D.W. et al. 1999. Ancient stunted trees on cliffs.
Nature, 398.121-122.
Mitchell A.F. 1972. Conifers in the British Isles. (HMSO.
Forestry Commission)
Mitchell A.F & White, J.E.J. Forestry Commission National
Tree Register (unpublished)
Mitchell, A.F. 1996. Trees of Britain.
Moir A.K. 2004. Tree ring analysis at Druids Grove. Unpublished
study for the Conservation Foundation.
Moir A.K. 1999 Dendrochronological potential of modern yew…with
reference to Hampton Court Palace. New Phytol. 144.
479-488.
Tabbush, P. 1997. Estimating the age of churchyard yews.
Proc.
White, J.E.J. 1998. Estimating the age of large and veteran
trees in Britain. Info. note 250.
© Fergus Kinmonth
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