Some Management Consideration for our Ancient Yews
by Russell Ball
On 4th November 2000 the Conservation Foundation hosted the Alan
Mitchell memorial lecture in a day devoted to the Yew Tree.
Russell Ball from the European office of the International Society
for Arboriculture (now Technical Director of Árbol EuroConsulting,
Madrid, Spain: www.arboleuroconsulting.com) spoke on the subject
of how to care for yew trees:
The UK is probably unique in Europe in having a priceless resource
of ancient yews (those with 1,000 years plus). We must then consider
their careful management and look towards caring for the ancient
yews of the future.
First some general points on trees (future ancient yews
included!):
Tree roots generally grow in the top 60cm of soil.
Tree roots generally grow out as a ‘shallow plate’
reaching the outer most branch tips and sometimes beyond.
Tree roots need oxygen, water and free space in which to grow.
Tap roots are rare.
Tree have stored ´food´ reserves in their trunks,
branches and woody roots. Thus a tree with fatal damage may survive
for 3-5 years living off these reserves until it dies: spiral
of decline.
Always follow the pruning recommendations in BS:3998*.
Tree & construction: follow the recommendations in BS:5837*.
Trees & utility trenching: follow the recommendations in NJUG
10*.
Trees may be protected by Tree Preservation Orders or be growing
in Conservation Areas: seek advice before any management works*
* Further details available from your
council tree/forestry officer.
Management of ancient yews:
The best policy is non-intervention. Prune to
the absolute minimum. These trees have survived for hundreds of
years without the need of a ‘hair-cut’. Let’s
remember that pruning wounds harm trees. Exceptions may be formative
pruning young trees or pollarding/coppicing. Obviously trees may
need pruning to abate their structural defects. Therefore, only
crown lifting a few branches to allow access or removal of some
deadwood over a path or other targets may be necessary (remember
the wildlife value of deadwood).
Why non-intervention? Ancient yews have an unfavourable
mass:energy. In other words, they have a large mass to support
(e.g. trunk & scaffold limbs) with only a relatively small
canopy (leaves that photosynthesis to make sugars that feed the
tree: autotrophic). Therefore removing portions of the canopy
(i.e. the infamous just-give-it-a-haircut) can shock (affect its
autotrophic – food making - capability) the tree and send
it into a slow but sure mortal, spiral of decline.
Taking advice. Arboriculture is a distinct art
and science to be separated from horticulture: a tree is not just
a tree. Unfortunately, myths and misguided best intentions are
common, so please take sound arboricultural advice: With much
respect, not the word/opinion of a gardener.
If they must be pruned. Crown reductions or
thinning should be done in staged pruning events (e.g. perhaps
one side first then the top and the other side over a 7-9 year
period) to give the tree time to recover between pruning events
(see mass:energy above). 30% should be the absolute maximum. Please
remember that many ancient yews may have Tree Preservation Orders
or be in Conservation Areas: consult your local Tree/Forestry
Officer (a good source of tree advice in any event) or the local
Planning Department. Other local sources of advice and information
may include the ISA´s Certified Arborists (www.isa-uki.org),
the AA Approved Consultants & Contractors (www.trees.org.uk)
or The Tree Council’s Tree Wardens.
Pollarding (started when the tree is young by
removing canopy) is sound a tree management practice that must
be done every 3,5, or 7 years thus creating a pollard head with
only small pruning wounds. Mature/ancient trees with a full canopy
cannot be pollarded: this is topping or lopping (see below).
Topping or Lopping. Simply never. This is the
language and ´craft´ of cowboy tree surgeons. It creates
large wounds that maim and kill trees.
Non invasive propping or cabling. Large scaffold
limbs may need this treatment if they are in danger of failing.
Such systems need regular inspection*. Please seek specialist
advice.
* I was once asked to look at an ancient
yew apparently suffering from drought stress. When I arrived on
site, I was met by a tree wrapped up with tens of metres of steel
cable. With the cable now embedded – after many years –
in the bark, the tree was literally being strangled to death (re:
impeded water flow in the vascular tissue). Careful removal of the
cable (leaving sections too deeply embedded in situ) saved the day!
Ivy. This can smother the canopy and kill trees
(see autotrophic capability above), so removing the basal ivy
stems (in 20cm lengths to stop them re-grafting) will kill off
the ivy above in the canopy.
Ancient yews and hazards. For trees to be hazardous a target
usually has to be present (e.g. a footpath or a bench). If an
ancient yew has structural defects – e.g. cavities or cracks.
Consider, firstly, can the target be moved? Secondly, can the
tree be fenced off with the appropriate signage? If no, then some
tree pruning may be necessary to abate the hazard.
Ancient yews: vigour.
As with all living organism this decreases over time. You can
‘help’ ancient yews by:
* Not by digging (or with herbicide) but
by carefully scraping off with a spade so as not to damage surface
tree roots.
Yews and compaction: The effect of compaction
(by foot & vehicular traffic) reduces oxygen, water availability
and (root) growing space in the soil. This can seriously harm
or kill trees. Fencing off trees or placing a raised board walk
for visitors can be a solution.
Ancient yews in church yards. They offer valuable
shade and a refuge in which to sit and ponder: not a spot to locate
fuel tanks, compost heaps or store bags of cement or other building
materials. The area beneath such trees must be regarded as sacrosanct.
Mulch only please.
So please let’s consider ancient yews as special
case. Or shall we be known as the generation that neglected and
disregarded a priceless resource.
Copyright © Russell Ball |
