Trials of time
The Trials of Time
Despite their ability to live for over 1000 years, the life of an oak tree is fraught with trials and is a true test of
time. Normally I can confidently estimate the age of oaks just by assaying their size and crown (I guessed the
age of Cromwell's Oak correctly, before I learnt that it was dated at 600 years from a core drilled from the tree
to enable the counting of the annual growth rings). However, the tree above is a rare case that defies my best
estimates. The cragged hollowness of its bough is reminiscent of an oak of some 600 years of age. Often
oaks remain solid even at 400 years. However, this oak is short and not very large, it's like an ancient oak in
miniature. It could be a youngish oak of some 200 years that was hit early by lightning or set alight by people,
accounting for its hollow burnt out shell. On the other hand it could be over 400 years old and kept dwarf by
harsh growing conditions. It is certainly isolated, and isolated trees don't grow as tall as those in a dense
woodland canopy, because they have no competition for the light and height is a disadvantage when one has
to stand up to high winds all alone and without the shelter of nearby trees. Such trees often do grow thick,
however, as this helps them resist storms that throw or break thinner trees. On poor soils oaks grow slowly
and assume dwarf form, as at Wistman's Wood on Dartmoor, where old oaks huddle together on rocky
ground that was unfit for cultivation and so left uncleared for centuries, but the oaks are dwarfed twisted forms
that hang close to the ground, trapping in fog, mist and moisture. There is evidence that the oaks of
Wistman's Wood are still slowly growing taller, centuries after they should have reached full height and be
growing more outwards than upwards. The soil where this tree grew was arable land that generally supports
massive oaks, but maybe the drainage here is not as good as it should be, or perhaps the soil has been
badly leached of its nutrients? Trees certainly find it hard to recolonise land. Even centuries after tree
clearance, much of the grasslands and moorlands of Britain are still treeless, or support the old aspen or
dwarfed birch, but then is largely due to overgrazing by sheep and deer. Grazing aside though, when land is
cleared of its trees much of the goodness is removed in the trees themselves as they are taken away for
timber or firewood and the denuded land is left bare to winds and rains which slowly blast and leach away the
goodness. This hinders recolonisation by trees, especially if grazing or scrub clearance has kept the land
treeless for a very long time, altering the soil beyond that which trees prefer.
The hollow trunk might make you think that this tree is in trouble, but the truth is otherwise. A hollow trunk is
usually still strong, since most of the forces that winds exert on the trunk are concentrated in the outer layers -
the inner trunk has little roll to play in support once a tree surpasses a certain girth. A wide hollow trunk can
resist bending very well and most of the wind simply whistles through without pushing against the tree. In high
winds the young, solid but thinner trees are most at risk from windfall. In part this is due to resonance as the
tree bends in the wind, and gusts at just the right intervals can reinforce this swaying, until the tree fails.
Thicker trees sway less to start with. A hollow tree also has less weight of its own to support. Sometimes,
however, in old pollards that are left to grow wild, the crown exceeds the weight that the bough can bear and
old hollow trunks can be ripped apart. This happens because when a tree is pollarded it responds by putting
out many new shoots (if it is in an area prone to damage by herbivores, then more small shoots stand a better
chance as at least a few may escape ungrazed until they become sufficiently woody to make them inedible.
The trunk will also thicken, since the tree may have been damaged by winds and so it takes precautions
against a similar recurrence. However, when a pollard that was cut at regular intervals is left unattended for
decades, the many shoots all become big shoots and the weight may be too great for the old trunk to support.
Anyway, pollarding generally increases the lifespan of trees, so such trees are likely past their due time in any
case. People often go through great lengths to keep such imbalanced and ancient trees alive, by proping up
branches and fencing off the area to prevent the roots being tramples. I can understand this, but in the end if
we had more trees of middle age waiting to take their place then we would not need to worry so much about
the old veterans, since the old trees must fall, sooner or later, to make space for the new. The trouble is, that
decades of overgrazing often means that trees of middle to old age are often in short supply, and the ancient
veterans are out on a limb. Thus, losing them brings much sadness.

As for our tree here, of uncertain age, it is still very much alive (!) as the summer view below reveals:
Notice the newly constructed fence to protect the roots and bough of this tree. Trees obtain most of their
nourishment from the top few centimetres of soil, by way of feeder roots that may even project up into the leaf
litter. These roots are easily damaged by trampling or careless treatment of the soil. A young tree can withstand
having half of its feeder roots concreted over or tramples, as it compensates by rapidly growing new feeder roots
where the soil is still usable, but an old veteran might not be able to adapt rapidly enough and may die.Compare
the summer view with the winter views of the same tree:
To the casual observer the tree in winter may be dead or alive, but of course a closer inspection will reveal
the living but dormant buds on the winter tree that yet lives. However, even from a distance I would have
guessed this tree to be alive because its crown is well formed, after a few years as deadwood many of the
smaller branches will have fallen or rotten away. Typically the crown of oak trees does not die all at once, but
part of it may die, whilst new growth continues on other branches. This may give an old oak the characteristic
'stag's head' appearance with denuded antler-like branches reaching above a lush but diminished crown. The
Cromwell Oak certainly has a much diminished crown. Even so, such a tree may eek out its existence for
several moor centuries. Oak trees not only grow slowly, but they typically die slowly! Disease, drought and
destruction or trampling of surrounding soil can hasten this process, however, and repeated droughts
probably bring on the stag's head crown a few centuries earlier.

Leaf-fall is a protective measure. In winter large, flat broad leaves that served so well to catch sunlight in
summer become a liability. In high winds they add greatly to the weight that the tree must support, especially
as the wind drags upon them and the more so if they are laden with snow. When the soil is cold and frozen,
the tree experiences drought and large leaves would simply lose too much water. In any case, such leaves
are prone to freeze and might not work well in winter in any case. For this reason, most temperate
broad-leaved trees, like our oak, are deciduous, meaning they shed their leaves in autumn and grow new
ones in spring from buds that overwinter in a dormant state. Not all is wasted! The tree reabsorbs some
useful materials from the leaves first, whilst taking the opportunity to leave waste materials behind, thus
autumn leaves turn yellow, red or brown as the chlorophyll leaves them and waste remains. The fallen leaves
slowly rot as fungi, bacteria and insects, aided by earthworms and the weather, break down the leaves and
recycle their goodness into the soil for the tree to re-use. Some trees also use this system in another useful
way - they may fill the leaves with chemicals noxious to competitors, but harmless to themselves, preventing
other plants and other unwanted arrivals from growing beneath their canopies. Conifers, walnuts and beech
trees are particularly good at this chemical warfare. Walnuts shed the chemical juglone in their leaves, nut
shells and also secrete it from their roots. Juglone (a quinone) is very noxious to rival plants and unwanted
micro-organisms, but is harmless to the walnut and those microbes it needs. Oak trees have their defenses
too, but are remarkably enduring and support more species of life than any other British tree.

Fire is another danger that trees face. The oak above has obvious fire scars, which may be the natural
result of lightning, or the result of careless or malicious people. Either way, the tree survived. In games like
Dungeons and Dragons fire seems deadly to trees and ents (treemen) as players expect their alchemical
firebombs to reduce a great and formidable ent to ash within seconds. However, the truth is that trees are
quite resistant to fire, probably more so than animals. Small saplings may be destroyed, but larger trees are
certainly resistant and the fire mostly burns away only the dead\wood, including the heartwood which no
longer conducts sap. In our tree fire has scorched the inside of the hollow bough, but the living wood around
it is largely undamaged, or at least survived. The sap makes the outer wood wet and fire resistant, but dry
deadwood burns easily. It is much easier to start a fire with dry deadwood and dry tinder than with green
sap-filled logs! Buds might be burned away however, so fire certainly traumatises an oak tree even if it does
not destroy it. Some tree, notably conifers like the giant sequoia, are especially fire resistant as fires are a
frequent and widespread occurrence in many conifer forests. Not much chance of a widespread fire in a wet,
temperate deciduous woodland where the oaks grow, but lightning still frequently scorches trees.

Windfall is a big destroyer of trees, though many trees that lie fallen and mistaken for dead often surprise
everyone by resuming growth with perhaps a vertical branch becoming a new trunk, as one or more roots
may remain in place even when a tree falls. Apart from resonance, root breakage and top-heavy crowns,
trees often fail when the soil fails. If soil becomes waterlogged then the soil may fail mechanically and sheer
as the whole root disc and surrounding hemisphere of soil swivel and slide, toppling the tree. I once saw a
tree fall as it grew in a filled in pond and the soil added to fill in the pond simply detached from the underlying
soil bed and the whole tree, roots and all, simply pivoted and fell into a precarious leaning position, supported
only by the strength of those roots still anchored. The tree could have been set upright and it may have
consolidated its roots again, but for the sake of safety and convenience it was simply cut up and dug up. In
nature it may well have survived as a leaning tree as the soil may well have reconsolidated. Such trees will
naturally attempt to right themselves. A small sapling may pull itself up completely by pushing or pulling with
special reactive wood in the stem. Older trees are too heavy for this to work so well, but the ends may bend
upwards and new growth will in any case proceed toward the light and away from gravity, creating a tree with
a kink in it. Alternatively, a branch that is now upright may become the dominant shoot and develop into a
new trunk. Branches that strike the ground may grow roots and so become new trees in their own right. Some
trees are particularly good at this. Yews and plane trees have branches that tend to sweep toward the ground
and often take root in the natural cause of things (a process called self-layering). People often prune back
willow trees as these are very prone to splitting and falling apart, but then they are designed to! The crack
willow (
Salix fragilis) is especially good at this, but willows in general will shed twigs and branches into the
water by which they prefer to grow. These members may get swept downstream, only to take root further
along the bank and so grow into new trees.

Diseases may take their toll on trees. The case of the English Elm and Dutch Elm disease is a sad story. I
have never seen a mature English Elm tree (though I missed an English Elm tree that grows in the grounds of
the governor's offices in Sacramento California, which I noticed labelled on the map only after I departed - I
wonder if it is mature?). I have seen small wych elms growing in Cornwall, twisted into fantastic shapes by high
winds, but I have never seen a stately towering English Elm as depicted in old paintings. They were said to be
the hallmark of the British countryside, a role they shared with the oaks. Dutch elm disease simply wiped out
the English elms. However, many such trees once thought to be dead, have put out new shoots from their
root stocks, but the disease strikes when these sheets reach maturity and so the cycle repeats. The question
remains as to whether the elms can overcome this periodic setback - will they slowly grow in strength or waste
away? Perhaps the disease will become less virulent over time (it does not pay to kill one's hosts too quickly!)
or perhaps the trees will become more resistant and so someday return. However, part of the elm's problem
was climate, perhaps a changing climate. It really was hanging on in England and was unable to seed in
Britain's climate (as it does on the European continent) and reproduced asexually by cloning itself. This
meant that whole counties were occupied by perhaps one or a few individuals cloned into many trees.
Individuals of a clone have the same genetics, so if one is susceptible to disease the others most likely are
too. Perhaps, in the end, the story of the English Elm is simply part of the natural changes that every
ecosystem experiences over time. As great trees eventually fall and die, so whole races of trees, like human
empires, are destined also to fall, in the end.

The honey fungus is the most serious disease that kills trees and shrubs in Britain. This fungus forms
honey-coloured toadstools, often found growing on dead tree stumps belonging to their former victims. The
fungus puts out bootlace like feelers just beneath the soil surface or within the leaf litter, that creep about as
they grow, until they make contact with a new tree. They will then invade the roots and infiltrate the shoots as
they grow beneath the bark, feeding on the trees reserves and quite often killing it. They then put out more
bootlaces in search of new victims! (Note that some harmless fungi also spread by bootlaces and may 'infect'
tree roots upon invitation from the tree as the two form a mycorhizal partnership). Oak trees are apparently
quite resilient against honey fungus and may eventually outgrow the fungus.

Old age will eventually kill a tree if nothing else does! However, it is not immediately obvious why old age
kills trees at all. Plant tissues do not senesce in the same way as animal tissues, after all many plants can
grow indefinitely from cuttings or by natural asexual means (like self-layering or fragmentation or by suckers).
However, there are engineering reasons why an individual tree structure eventually fails. Wood becomes
non-conducting after a few years, typically as the vessels cavitate and block (especially in cold weather) and
so, in order to survive, a tree must constantly grow and increase in girth - adding new wood to the outside of
its trunk and existing branches and it must bud forth new branches to replace those it has lost. However, as a
tree gets thicker, it takes more and more new wood to cover its circumference each year. At first this is no
problem, since the crown and roots have increased in size too and is providing the tree with more
nourishment such that it actually can grow more. However, at some point the balance is increasingly tested
and eventually upset. An old tree needing lots of new wood each year might face several years of drought,
killing part of the crown or its roots might get damaged or disease might tax its resources and stretched as it
already is it may be unable to cover itself with new wood. If this state of affairs continues for several years
then parts of the trunk may contain no conducting wood and the parts of the crown and roots they supply will
whither and die. Now the tree has even fewer resources and is facing a slippery slope.

Sometimes regions of the trunk may thrive whilst others die and completely rot away - the trunk may break
apart into several smaller living fragments. For those trees whose wood is especially resistant to disease and
insect attack, such as the resin-rich woods of conifers like the yew, this might not be the end and each
fragment may become a mature tree. Indeed, in this way the yew tree in particular might occasionally be
capable of immortality. For most trees, however, declining vitality hastened by environmental stress or
disease eventually spells the end. Still, by that time, a healthy tree would have shed many spores (pollen
and/or seeds) and so its work is done and the cycle of Nature repeats.