THE CASEBOOK OF DR. D. R. SLATER - No. 11 - "Oddities"

No. 11 - Oddities

Trees are a fantastic topic to teach about: there are so many aspects to their planting, establishment, growth, ecology, sex lives, ecosystem services, decay, management, assessment, and removal. It is quite easy to keep learning new things about trees each day of your adult life, should you have that ambition to more fully understand these glorious woody plants that adorn our planet.?

As a teacher, I like to include some of the odd things that trees or fungi do as part of my lessons. This is probably because bizarre facts are the easiest ones to learn: for instance, that scientists have found that the fungus ‘split gill’ (Schizophyllum commune Fries) exhibits over 20,000 different sexes.?By having so many genders, it makes it easier for one fungal propagule to find another compatible propagule when they are rather randomly distributed in the wind. Put another way, this fungus has much better odds of meeting a suitable partner than you would have if you hung around your local nightclub. Clever!?

In this article, I share a few odd things that trees do, as captured by my ‘Trees Over Time’ photographic project.?

Figure 1: Changes in the bark of a semi-mature aspen (Populus tremula) over 14 years.

WRINKLES?

Figure 1 shows changes in the bark of an aspen (Populus tremula L.) over fourteen years. I must admit that this image does make me feel quite old.?With age, of course, comes wrinkles… and we can see that has happened to this tree, just as much as I can see it in myself!?

It’s common to think of the outer bark of a tree as dead material: and that’s because it is! However, the outer bark undergoes a lot of changes over time that makes it more interesting than a simple ‘slab’ of dead tissue.?The most obvious changes are to the bark’s texture and hue as it ages.?A big driver for this is that the trunk or branch of the tree is expanding underneath the bark through the process of secondary thickening, so that continuing expansion inevitably causes the outer layer to stretch and crack, with some bark types breaking into smaller sections, known as ‘bark plates’.?It is a bit like a loaf of bread being baked in the oven: the internal expansion swells and alters the outer crust.?

Those are not the only changes that one can find in a tree’s bark.?Bark is adorned with ‘lenticels’ – pores that allow gaseous exchange to the inner living tissues of the tree – and the lenticels often change in size and appearance as the outer bark ages. In addition, although it is dead material, the outer bark of some tree and shrub species changes noticeably in colour through the year – species such as dogwood (Cornus) and willow (Salix) are used as cut ornamental stems for that reason, with cultivars whose outer bark on younger shoots go strong yellow, orange and red colours in the winter. It turns out that even the outermost dead layer of a tree is more interesting and odder than one might first think.?

Figure 2: Changes in a one-sided mature oak (Quercus x rosacea) over 10 years.

THE HAIRY HUNCHBACK

The main structure of a tree can tell you a lot about the environment in which it grew into maturity.?In a woodland near me, there are several oaks growing in a dense section of the wood, surrounded by yews, hollies, and sycamores.?However, these oak trees have long, low branches – identifying that they were once open-grown specimens, and those other trees developed around the oaks later.?

Similarly, you can often tell a mature tree that has been growing alongside another large tree for much of its life, due to a lack of branch development in the direction where the other tree was growing.?When that other co-dominant or dominant neighbouring tree is removed, the remaining tree can look spectacularly one-sided and gawky. This effect is often worse in scale-needled conifers, where removing one conifer reveals a large area of dead twigs in the neighbouring one that the owner wants to keep.?

Figure 2 shows an oak tree (Quercus x rosacea Bechst.) in a public park that has been subdominant to a beech tree for many decades.?The bigger beech tree became decayed and then had to be removed – resulting in the one-sidedness of this oak becoming much more obvious.?However, it’s making the most of the new space that has been opened to its growth. Note how much of the new growth is acting to re-balance the crown: it is not as simple as a tree ‘grows towards the light’ – they are far more sophisticated in how they develop their branch architecture.?

This was a particularly pleasing time-lapse image to capture, and I hope to revisit this oak in a decade to come and see how its form has changed further.?Note how much the ivy has developed up the tree’s trunk over the last ten years too: for the sake of the tree’s further development, I would be regularly ‘coppicing’ this ivy, so it didn’t come to disfigure or adversely affect this “old hunchback” of an oak. After all, this oak has been waiting long enough for its own ‘time in the sun’.?

Figure 3: The creation and occlusion of increment borer wounds in a silver birch (Betula pendula) over 10 years.

A PLUG FOR TREES

Working at an educational institution with several woodlands and more than a thousand individual trees at the students’ disposal, we do occasionally do odd or cruel things to trees for the sake of science.?

One group of MSc students under my charge were given the task of investigating the effectiveness of the ‘fractometer’, a wood testing tool that works by virtue of an internal spring attached to a lever that bends wood samples to assess the sample’s strength and flexibility. In one of the College’s woodlands, I allowed multiple wood core samples to be taken from four birch (Betula pendula Roth) and alder (Alnus glutinosa Gaertn.) that would eventually be thinned out – so it didn’t really matter that they would have several bore holes in their trunks.?

Despite good intentions, I didn’t get around to thinning out all four of the affected trees – and one birch and one alder persisted with these increment borer holes in them for ten years.?Both trees had continued to grow in height and diameter – and all the bore holes were still discernible as darker marks on the outer bark (Figure 3).?

Figure 4: Dissection of the common alder (Alnus glutinosa), bored 10 years before, to find a 'plug' of sapwood blocking each and every increment borehole made into its stem.

Out of interest, I got around to thinning out the alder tree and split the section that had been bored multiple times: a little ‘plug’ of callus growth was growing down every borehole (Figure 4) – there was almost no decay present - but the boring had caused a lot of dysfunction to the sapwood in the tree, which is seen by how much of the section had become discoloured. This ‘plugging’ is an important mechanism for trees to overcome minor injuries and defend against the potential damage that could be caused by woodboring insects.?

Figure 5: Touching its toes and then back up again - a rowan (Sorbus aucuparia) recovers partially from bending over in just ten months.

ROWAN TOUCHES ITS TOES?

Some species of tree have sex at a very young age: as early as “year two” for a fast-growing birch tree!?However, this can cause some trees problems, because of the subsequent fruit-bearing that then occurs.?This rowan (Sorbus aucuparia L.) became bent over in this planting scheme just one year after planting because of the heavy fruit it developed high up on its whippy young stem.?

Now, trees have means of correcting their stems and branches through a complex set of mechanisms.?To right a bending stem, trees can grow a specialised type of wood – known in science as a ‘reaction wood’ – that acts to push up (compression wood) or pull up (tension wood) a stem or branch into a more advantageous position. In simple terms, trees have ‘posture control’ – which is informed by phototropism (orientating in relation to the light) and gravitropism (orientating in relation to gravity).?

However, trees are not perfect organisms that ‘optimise’ themselves all the time: rather – they are like you, me, and all other living organisms: they are a complex compromise.?Over just one growing season, this rowan has managed to raise its bent stem considerably upward, but it is far from being straight as yet.?I hope to keep time-lapsing this tree to see how its form develops in the longer-term.?It seems likely to me that a small side branch will take over as the ‘leader’ for this tree, with what was the main stem becoming a curved side branch. One can learn a lot more about trees by observing them carefully than one can from just studying pure theory (e.g. phototropism and gravitropism) as trees don’t follow any of these theories to the letter.?

Figure 6: A trunk anomaly in a mature oak (Quercus robur) which essentially hasn't changed in the seven years of this time-lapse.

?A BELLYFUL OF WIRE?

Odd external growths on trees are quite common: for nearly all the parks I visited in Greater Manchester (about seventy parks and cemeteries), I was able to find at least one large gall, burr or unexplained protrusion – often there were many to be found.?

The concept of a ‘body language’ for trees was developed from too-much of a mechanical/engineering perspective.?This is a shame, as a tree is not solely a mechanical object – it is a living entity, and it does not necessarily produce all its wood to provide mechanical support to its structure.?That can be seen in the case of all these burrs, galls and so on, that stick out from the main shape of the tree’s trunk and limbs. In the case of a gall, the external growth is due to an infection; but there are many other reasons for odd external growths from the trunk of a tree.?

For the oak (Quercus robur L.) pictured in Figure 6, what would be the correct interpretation of the obvious bulges that can be seen? Some arborists trained in the 1990s in the UK might be tempted to say that these were signs of previous ‘fibre buckling’, that the tree’s trunk had partially failed in the past and this has caused these odd swellings.?

But, if you think about it, what type of loading or bending could cause such an even amount of bulging all the way around the tree’s trunk – and at two different heights??It’s a highly implausible bending scenario: rather, there’s a much simpler reason for these bulges, which haven’t increased in size over this seven-year time-lapse.?This tree is situated right at the edge of an old estate – and it has undoubtedly been used as a fence post, with strands of wire nailed to it – and then being occluded into its stem. This is a very common phenomenon where I live – inducing odd growths to the trunks of many hedgerow and farm trees.?

A key lesson learnt here is that bulges in trees can have a wide range of causes – and often they are not explained by biomechanics.?

Figure 7: Anastomosis of two stems in a semi-mature horse chestnut (Aesculus hippocastanum) over 11 years.

LET’S GET TOGETHER

This time-lapse of a horse chestnut (Aesculus hippocastanum L.) picked up the oddest change in a tree that I have yet found.?Look closely and you’ll see what I mean…

Two competing leaders of this tree had met each other – forming a natural brace.?Over this eleven year period, the two stems have not been able to move away from each other, because of the natural bracing – and so, instead, they have become fused together to become one single stem: albeit with some bark included in that join.?This behaviour is very common in yew trees (Taxus baccata L.).?Perhaps, if you have ever milled trees into boards and planks, you may have come across some odd and long sections of included bark in the stem of a tree or two – and perhaps not realised that this has occurred because two or more stems have fused together as part of the tree’s earlier development.

It is finding changes like this one that keeps me still immensely interested in trees - even after studying them for more than thirty years. Trees do some amazing things, about which we know too little.?

?*** THIS ARTICLE FIRST APPEARED IN PRO-ARB MAGAZINE, JULY 2020 ***

Dr. Duncan Slater is a senior lecturer in arboriculture at Myerscough College, Lancashire.?He is a current candidate for an MSc in Environmental Management, furthering his education in forestry, philosophy, arboriculture and plant biomechanics to now include ecology.