Posts Tagged ‘fungi’

Ivy – Hell Bent or Heaven Sent

August 24, 2011

Written as a blog in response to a letter by Lt Col Paul French (Rtd.) in the Daily Telegraph of Monday 28th March 2011.

Most of us who love trees for their timber and role in the landscape, regard Ivy as a foe.  The wild variety in the UK is named Hedera helix, though from its growth pattern it would be more aptly named Hedera rete i.e. Ivy net.  However, there are three views of  ivy: that of the tree; that of the birds, bats, insects and fungi that live in and on it; and that of humans and the browsing animals.  In past centuries: before the reign of plastic, huge human populations and high wages; all trees, farmland, woodland and forestry, were cared for by the humans who worked the land.  Ivy was left to grow only on those trees that were already misshapen, or of a species whose timber was not considered to be useful.  Even then Ivy was controlled by deer and cattle that were permitted to seek the shelter of the woods in winter, where they kept down the undergrowth.  Their grazing kept the glades open so that Wild Garlic and Bluebells could get enough sunshine to grow and flower ahead of the herbs and thin grasses in the spring.

Viewed from the tree’s perspective it is definitely a devil.
The creeping stems of Ivy are coated on one side with tiny suction rootlets that enable it to climb up the trunk and cling to the bark of  the tree.  Generally it does not attack until a young sapling has become too inflexible to fling off the unwanted guest as it bends in
the wind.  Then, when after several years the trunk has lost its whippy characteristic, and is suitably firm and steady, the Ivy colonizes.  Unless it is broken off by humans or browsers, ivy grows steadily upwards branching as it goes, until it reaches a level at
which the branches are still whippy enough to shake off the tendrils for the time being.

Like all evergreen species, although it does not shed its leaves seasonally, they gradually age and drop off.  They then cause even more trouble: by lodging in the angles of the boughs, and with other leaves they rot down leaving a black leaf mould that in turn plays host to epiphytes – mosses, nettles and ferns.  The bark beneath then starts to rot, thus weakening the branch, and allowing the rot to penetrate the heart wood of the tree.  The living Ivy leaves also cause problems by sheltering the bark from the rain, and the ‘rootlets’ absorb any water running down the surface of the bark.  This leaves the bark dry, brittle, and cracked so that it is easily penetrated by insects and fungal spores.  These often carry diseases, both bacterial and viral, which are, in turn, transported into the sap that runs in the living layers just beneath the bark.  In this way pathogens can be carried up into the leaves of the canopy or down into the root system.

Worse still Ivy keeps its leaves throughout the winter, thus adding weight to the branches and extra surface area to catch the winds.

The ivy here has climbed nearly to the top of the birch tree and the weight of its leaves is slowly pulling the tree over.

Slow felling of a Birch by Ivy

  This at a time of year when our deciduous trees have shut down and shed their leaves, thus protecting themselves not only from the cold, but also from damage by winter storms.  In a young sapling the situation is not so serious, as it is often both strong and flexible enough to carry the extra leaf area without cracking; especially so since the ivy is young too and equally flexible, but the story is very different for the older tree.  Veteran trees often have ivy stems of over 6cms diameter half buried in their bark; and these vast old creepers add substantially to the weight to be carried by the branches, made even more fragile by the lack of moving sap during the winter months.

The result is very visible damage.  Some unlucky trees are completely thrown and partially up-rooted; some escape; but most veteran trees suffer the breaking of large boughs and even smaller branches in the crown of the tree.  These breaks leave scars that not only spoil the visual shape of the tree, but may also unbalance the tree, making it more susceptible to wind damage in the future; not to mention the increased risk of disease entering the broken timber via rain and spores.  All this damage hastens the slow demise of the tree.  However . . . .

Viewed from the denizens of the tree the story is very different; for everything, that the tree could be expected to regard as negative, is positive to them.  In fact the more the tree is damaged, the better the better it becomes for them.  Though as it disintegrates, they will all have to move on, starting with the birds and insects and ending with the decomposers growing on the rotting wood.  This is the way that it has always been.

In regions where there are no convenient conifers, ivy-laden trees provide some of the only safe shelter for small birds throughout the winter months; besides providing an every ready larder of spiders, flies, beetles, millipedes and mites throughout the year.  Bats too, can be found sheltering under the ivy during the summer; though it is possible that they are really looking for suitable warm sheltered holes to use for winter hibernation.  As for the rest: the birds’ living larder and the fungal spores; the ivy provides warmth, shelter and nice little cracks in the bark with good access to sap – an ideal growth medium for the fungi and any pathogenic bacteria and viruses spread by the insects.  Finally . . . .

Viewed from the requirements of humans and browsers the story becomes split.

On the ‘Up side’, the latter seem to regard Ivy as we would a tasty herb.  However, it is probable that their actions are related to self medication, as Ivy is a known anthelmintic that has been used as such in the past.  Animals may not be able to read books, yet many seem to either know or learn that certain ‘herbs’ are beneficial when eaten in small quantities.  So long as the plant is bitter they don’t exceed the safe dose; it is only when the plant is sweet, either alive or when dead e.g. Ragwort, that they over eat and poison themselves.

Humans too have used Ivy leaves as a medicine for thousands of years; though now-a-days its use is limited to external applications because of unpleasant side effects and its ability to destroy red blood cells if imbibed in excess.   However, externally the leaves still provide a useful herbal poultice for leg ulcers, enlarged glands, painful joints and other pains that can be reached from the outside!

This oak is completely encased in ivy, only the dead branches remain sticking out above the luxuriant growth.

Ivy engulfs its prey - a veteran oak dies

On the ‘Down side’ humans have always grown trees, not only because of their beauty, but also because of their many uses: for food and the energy to cook it; for medicines; for timber with which to make houses, tools, and all the plethora of things that are necessary for a good quality of life; and for ashes to make lye for soap making.  So overall, anything that affects the health of trees qualifies as a devil rather than an angel from the point of view of humans.

But that is not the full story and Ivy too has its uses.  Perhaps the solution is: to remove ivy* from all straight trunked trees and those that are suitable for structural or ornamental timber; then remove it from 25 – 50 percent of the remaining trees.  25% of trees with ivy is quite sufficient in our heavily treed areas e.g. Dorset and Devon; but in areas where trees are more scarce e.g. Northumberland then 50% of the remaining trees with ivy might be acceptable.  Let common sense prevail, and preferably prevent  our non-governmental public bodies and allied groups from removing all our conifers on the grounds that they are not native, or that their English names fail to suggest that they should live south of the border e.g. Scots Pine (Pinus sylvestris) which is prevalent right across Europe from North to South.  To my certain knowledge trees are totally unaware of geographical boundaries – unless they coincide with environmental attributes such as climate, aspect, rainfall and altitude!  However, the conifers do provide very good protection from inclement weather for wildlife and particularly for small birds, many of whom crowded into our group of Lawson Cypresses at dusk on a winters evening, and provided a chirrupy sort of concert for any who cared to stand under and listen!

*Ivy removal is simple.  Pull the small stems off young trees using fingers, before they become established.  For older, thick stems of ivy use a saw, billhook or chopper, to cut and remove a 5cm/2 inch segment from the base of the trunk.  Try to avoid damaging the bark where possible; but a little damage to a small piece of bark is preferable to much the much greater damage caused by the ivy.