In this special guest blog piece, forester and pathologist Jim Pratt describes his growing obsession with, and love for, the Ash tree, along with his concerns about the causes and implications of Ash Dieback disease.
What started as a mild feeling of guilt ended up in a journey that took 6000
years to complete. Let me explain.
I am, by profession, a forester. I started an axeman, and cut my first tree in 1960, near Monmouth. It was an elm. Knotty, awkward, hard as iron and unforgiving. And my axe was blunt, and my skill negligible. It was a miracle I still had two feet.
Next to fall: an Ash. And what a difference. Clean, white and straight, it accepted the axe generously – as indeed it might. Because felling would lead to an extended life of centuries as coppice. A spiritual affinity, or bond, was formed, reinforced some years later making love under the dappled shade of another ash.
Forestry took me into research, and I became a pathologist, working, breathing, thinking and writing about Sitka spruce for 35 years.
Retirement was, however, skin deep, and in 2010, with a Danish colleague, I surveyed Scotland for signs of the far-eastern disease that was ravaging European ash: Hymenoscyphus fraxineus, or Ash Dieback. We found none. But, as dedicated professionals, we asked Government to cease importing Ash planting stock from diseased areas in Europe. They didn’t. Three years later, Ash Dieback arrived in Britain. It will cost £15 billion. I needed to take stock of my trees, and learn more about this wonderful native species if I was to make any progress bashing Government over the head for its lack of action.
In 2015, I became aware, for the first time, that two 20-year-old Ash trees either side of a gate looked different. That on the left was slightly smaller, with fissured bark while on the right it was slightly larger with smooth bark.
And when I looked at the buds on the branches, they, too, were different.
The buds from the tree on the left were knotty, gnarled with rough bark and many splits while those on the right were smooth, swept clean and graceful. It was winter, and it seemed prudent to keep an eye and watch what happened next.
It was April before I saw any change. The 28th in fact. My 73rd birthday. The buds at the end of the left hand tree seemed swollen, while the right were unaffected. A few days later, it was clear I was looking at something developing that I had never seen before. Flowers. I vaguely knew that Ash was diecious (that is, each tree carries either female or male flowers) but I had never looked at them before (1). Now, however, there were flowers at head-height. These flowers seemed so inconspicuous that a cursory glance seemed sufficient. It was not until the following day when I happened to walk past the trees with a x10 magnifier and looked at one in detail that I was blown away by their inconspicuous beauty. And this is the origin of my feeling of guilt. How could I ever have ignored such a stunning natural object? The urge to photograph these tiny structures, only a few mm long, and watch them develop over time turned out to be very expensive in specialized camera equipment, so I took no photographs that year, but by spring 2016 I was ready. I wanted to follow the growth of a single flower from both the female and the male tree: a difficult proposition in a site riven by strong winds. So, at a point where female and male shoots were touching, in winter I built a tower and made it partially windproof with reinforced polythene. I lashed down the branches and set up the equipment … and waited.
I won’t go into the problems of lighting, of shake, of movement, of comments by passers by and by dogs and (in one year) a complete absence of flowers of either sex. Nor of waiting for the sun (late afternoon to catch it waning). Nor of adjusting depth of focus or mastering focus-stacking. Suffice it to say that over the next three years I amassed several hundred photographs. And I became so obsessed that I had to get a friend to photograph the flowers during one fortnight when we travelled away, or earn a divorce.
What follows are from one year, 2016, when both trees obliged by producing flowers in the right place. In each of the following pairs of pictures, the female is on the left, the male on the right.
April 25 and 27 2016
The female flowers (left) emerged from swollen buds set either side of the terminal (leaf) bud and further up the shoot. In the early stage, as shown here, the bud appears covered in red fluff. Two weeks later, the fluff is better defined.
May 7 2016
The female flower (left) is technically an inflorescence (multiple flowers emerging from the same source). Each inflorescence contains maybe 50 separate ovules, each of which will make a samara (winged seed) if fertilized. The stigma (tip) and style developing from each ovary are approaching ripeness, but the absence of male pollen means that few have been fertilized (you can see that most stigmas are pink: once fertilized, their tips turn black.)
Look closely at the male flowers. They are just a bunch of enlarging sacs, here emerging from buds whose surfaces are splitting under the internal pressure. The sacs will contain pollen when they are ripe.
Fast-forward five days, to May 11 2016
The female stigmas (left) have ripened, and several have already intercepted windblown pollen and are dark at their tips. Others, especially those on the far left and near the camera, remain unfertilized. Each pollen grain produces a single pollen tube which grows down the style and into the ovary to form a fertilized ovule. This matures into a samara. White pollen tubes are present stuck to the stigmas.
The white pollen which has fertilized the stigmas is being released (right) from the male stamens from the anthers that are ripening within the male flowers which, a week earlier, were mostly closed. The pollen is wind-blown, so large quantities are produced and most fails to find a receptive stigma. These photographs are at roughly equivalent scale, and as we will see the male pollen sacs soon shrivel and disappear while the female stigmas are transformed into delicate, but very tough, seeds that rely on wind for their dispersal.
I particularly want, at this stage, to draw attention to the stems on which the female stigmas are perched. These are present in the previous photograph for the 7th May. Their correct name is pedicel. They are little studied and much ignored, as will become evident.
Three weeks later, considerable changes have occurred.
May 25 2016
A ripening ovule is present in the large backlit ovary on the female flower (left). This will mature into a seed, capable of reproduction and creating a new tree. But first it has to ripen, and then fall away from the mother tree sufficiently far to avoid deep shade. There are mechanisms that help that happen, some of which we will observe. Note, again, the pedicels that support each ovary, and which have been there since the start, and are just evident on April 25th.
These develop into very tough, flexible stalks each holding a samara. It is these, dried and withered, which are not shed and allow female trees to be identified for a couple of years. A microscopic examination of their structure would be rewarding … but that’s another story.
Meanwhile, the male flower (right) has given up all its pollen, has withered and is beginning to be reabsorbed or shed even as the leaf arising from the terminal bud is beginning to flush. This gradual swelling of the nascent leaves as the bud scales are forced apart reveals the fibrous glue that has helped keep the bud safe and complete throughout the winter. The expansion of the leaves over the next few days is quite magical and is mathematically precise, but is not a subject we will pursue further.
The period of flowering of females appears to be very much longer than that of the male and, I would guess, acts as a bigger drain on energy resources of the tree than that of the almost transient male flowers. This, if correct, is a matter of great significance to which we will return. It is with the female tree that, from now on, I am concerned. The period of maturation of the seed (or, more correctly, the samara) is relatively quick.
June 13 2016 and June 18 2016
Two weeks later, the females are recognizable as samaras. These are aerodynamically shaped with a twist, all in the same direction, to facilitate dispersal away from the mother tree when they are shed. I think the twist has another purpose: it allows bunches of samaras to bind together in strong winds and form an elliptical, egg-shaped aerofoil that can resist 50 mph gusts. Very necessary if the seeds are not to be shed before they are ripe. When the samaras are clustered close together, I suspect a venturi develops between samaras which binds them even closer. This, as far as I am aware, has not been proposed before. Evident on the ends of the samaras are the stigmas, now dessicated and dried, which were fertilized by the pollen on May 11.
These clusters of ripening samaras can be large, with over 100 seeds. You can now see where they came from in the young flowers. 2016 was a heavy seed (mast) year. Because the samaras hung on the tree until autumn and winter, it was possible to measure its fecundity.
October 10 2016, Female tree.
After leaf-fall, the density and distribution of ripe, desiccating and brown samaras throughout this crown are evident. This was after the first frost.
This tree had over 1000 clusters of samaras , probably containing 100,000 seeds , weighing in at over 10kg when green. This may not seem much in weight compared to the size of the tree, but seeds are expensive nutritionally, and may impact on the storage of starch in the roots. And this was not a load born by the male tree, whose flowers were small in bulk and short-lived in comparison. There is circumstantial evidence, a year later, that heavy seeding had influenced the following year’s growth in a subtle way.
20 Sept 2017 Female on left, male on right
In 2017, no flowers were borne on either female or male trees. This is perhaps not surprising for the female tree, but why did it happen in the male as well? Synchronicity is interesting, because with diecious trees the need for both sexes to fruit together is obvious for the survival of the species. I doubt it has been explained. Climate in the spring may be involved: in one year after this study, there were no signs on either tree of any flowers until May, after a long period of cold and frosty weather. However, early that month there was a spectacular change to warm weather for a few days early that month, and both trees produced a plethora of flowers shortly afterwards.
What is more significant is that during the dormant period 2016 – 2017 the female suffered significant upper-crown shoot dieback (see above), which is evident in the left photograph of the female tree, but is absent from the male on the right. This characteristic post-seeding dieback has been noted before, but it is has not been investigated as far as I am aware. important because it may easily be confused with Ash dieback. It is also possible that female shoots, because of their morphology and surface roughness, are just more susceptible to attacks from weak pathogens such as the fungal nectria spp. [species, plural].
What effect does heavy seeding (and crown dieback) have on growth in the following year? This is possible to measure retrospectively in trees since the terminal buds leave a tell-tale scar on the outer bark which allow annual growth to be measured.
Flowering had been particularly heavy on the female tree in 2015 and 2016. Axial extension in female shots appears to be less than male, as this picture of a female shoot (in the top half of the image) and male shoot (in the lower half) shows.
In this case one year’s increment in the male (2016) is greater than the length of 5 years growth on the female. I admit that you can’t tell much from one shoot, but this photograph shows how you can age a shoot and measure its yearly growth. To be precise, you slice the shoot in half along the pith, and look for the “nodal diaphragm”: a check in the pith which marks the point of the base of the terminal bud. Using this system, you could re-create the entire life of a tree no matter its age since the nodal diaphragm is not disturbed except by decay.
Another characteristic to examine is the yearly radial growth of the stem. A 5mm diameter core was extracted at chest height from the stem of both female (left) and male (right trees) in August 2019. Measuring the distance between the annual rings gives an estimate of the tree’s yearly growth.
These cores show significant growth reduction in 2017 in the female stem compared to the male. That is the year when crown dieback started, following two previous mast years. Here, an important feature to note on these cross- sectional cores is the relative width of the spring compared to the summer wood in both male and female tree. The width of spring wood (large, thin- walled vessels) is roughly similar in both trees, but there is less dense summer wood in the female. If this feature is related to heavy seed production, and if that occurs at regular intervals during the seed-bearing period, it has a profound effect on the strength and quality of the timber formed by the female tree. The higher the proportion of dense summer wood, the stronger and more flexible is the timber.
This is a tiny sample, from just two trees. How may it be relevant? Here, we need to throw our minds back thousands of years, and consider the uses to which Ash has been put during that period, A few examples will suffice to show what a utilitarian species this is.
Uses of Ash: Neolithic 3500BC
When man realised that fixing a haft to a stone made an effective axe (it quadruples the force that can be applied by a human arm) he (or she) needed a material for the handle that was tough, could absorb shock, was easy to carve and readily available. He chose ASH.
This, known as the Shulishader Axe, is one of a handful of neolithic stone axe-heads found in Europe still attached to a haft. During the next few thousand years, Ash was the chosen timber for hafts and tool handles right up to the present day.
One other example will suffice: the Roman army of the occupation of Britain.
In the first 80 years, using hundreds of thousands of tools with handles mainly made from ash, the Roman Army built:
2000km all-weather roads, 300 military forts, 6 major fortresses, 20 major towns, 115km border (Hadrian’s) wall.
The tools hardly differ from those we use today, as these examples from the Roman army fort “Trimontium” in the Scottish Borders show. Most are high- impact tools that require tough, non-splintering resilient timber handles or hafts that absorbs shocks: wherever timber survived it has been Ash.
Preparing the building materials using hammers and chisels was one thing; transporting them to the building site another. For example, Hadrian’s Wall consumed some 24 million facing stones, weighting around 4 million tonnes.
Evidence from elsewhere in the Roman world shows that horse or oxen-pulled carts were used. These need wheels. One example survived from Trimontium, and it is thought-provoking, because it shows a sophisticated technology, in common use even before the Roman occupation, which can only be attempted by one wheelwright in Britain today. Its relevance is that this technology demands a standard of timber that is hard to meet today.
The object is a wheel of a cart, one of a pair found in a pit filled with “rubbish” by the Roman forces when they abandoned the fort at Trimontium. This wheel (see below) is now in the National Museum of Scotland in Edinburgh, who kindly gave me permission to photograph it.
Iron-shod, around 1m in diameter, the ten spokes are of willow and the hub of Elm, turned on a lathe. The felloe, usually made from 6 or more interlocking lengths of timber cut on the curve, is in fact made of a single 3m length of Ash, steam-bent into a perfect circle and shrunk onto the spokes using a red-hot iron rim. A pair of such wheels, made by Robert Hurford of Taunton, Somerset, is shown in the right inset.
The state of preservation allows an appreciation of both the quality of the ash and the reason for its failure (inset, left). It looks as if an inherent weakness in the felloe formed by a branch initial coincided with the placement of one of the spokes, and it was at this point that the wheel failed maybe because it hit a rock. The 3m length was split from a clear length of Ash with at least 19 annual rings of regular width.
The evidence presented above would make me think that this timber was specially grown, almost certainly by coppicing which was a technique attested in the neolithic some 6000 years ago. Ash coppices well, and can live for many coppicing cycles (of around 30 years) such that coppice stools in Suffolk can be reliably dated as at least 800 years old. That would suggest the technique of coppicing ash is very much older.
Straight, fast-grown Ash which has the highest density (derived from abundant summer wood and regular growth) can be obtained from coppice such as this.
Having coppice stools that have been tended and valued for that period makes me wonder to what extent selection for coppicing of ash of this quality (suitable for bending, for absorbing shock , and for easy felling and working) was obtained mainly from male trees, which are not subject to growth interruptions following heavy seeding. However, we do not know if and whether such selection has, unknowingly, been carried out for thousands of years in Europe and Britain.
We shall never know. And finally: so what?
The facts are stark. European Ash may become extinct, both from Ash dieback and the follow-up depredations of the Emerald Ash Borer, an exotic insect which has gained access into Eastern Europe and North America, in the latter continent killing billions of Ash trees.
The observations described here, derived from a simple obsession with the shape of an Ash ovule, may be completely wrong. But our negligence in failing to control the inter- continental spread of tree diseases means that it may not be possible to find out whether this superbly utilitarian tree has been managed with the care and wisdom described here, or not. But I would make one final point which, in my mind at least, seals the value that I believe was placed on Ash for thousands of years.
This is the earliest representation of a human figure found in Britain. It was carved with stone tools around 6000 years ago, from Somerset. It is clearly hermaphrodite. It was made from Ash.
- This gender difference is thought to have originated when ancestral ash flowers were pollinated by insects, and was a way of ensuring self-fertilisation was kept to a minimum. The actual system determining gender is complex, such that the gender of an entire tree, or of a branch in a tree, can change from one year to the next. Furthermore within individual inflorescences of either gender, fruiting structures of the opposite gender can be seen on the male flower pictured above (May 11 2016).
Jim Pratt, West Linton, Scottish Borders, 27th June 2022.
Jim Pratt was born in Paddington in 1942. Having failed all his A levels, he became a forester and a forest research pathologist working throughout Britain, Europe and N America. He published widely on a common root disease of conifers and retired to the Scottish Borders in 2002.Since 2012, both his wife and he have survived serious cancer, and that drives him to take nothing for granted, no matter how small or insignificant or commonplace. His obsession with Ash flowers exemplifies that attitude, strengthened by guilt that the species is going extinct on his watch.