Spring: How trees tell the arrival of new season

THE third month of the Christian calendar takes its name from Mars, the Roman god of war, and rightly so because March roars in like a lion to blow away the last cobwebs of winter. Romans called the month Martius, while the name ‘Hlyd Monath’, meaning ‘loud or stormy month’ was bequeathed by the Angles. They, along with other Germanic tribes (Saxons, Jutes and Frisians), invaded the British Isles in the 5th century. This evolving conglomeration of Germanic tribes crossed with native Britons, and known as Anglo-Saxons, called the month ‘Hreomonath’ or ‘Rhed-monath’. Rheda was the name of an Anglo-Saxon god which appears to have been borrowed from Rhea, the ‘mother goddess’ of Greek Mythology. 

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Whatever the origins of the name, March stirs slumbering living things across the ecological arena, with the re-foliation of trees giving shade and safety to forest floor plants and early breeding birds. Broadleaf deciduous trees have been on the move for weeks, with winter buds swelling slowly but surely to give subtle changes in colour.

March is the month when woodland, bathed in sunshine and viewed from afar, assumes a new, vibrant look due to discrete colour changes in billions of bud scales coalescing into a collective change from winter canopy colouration. 

‘MAKE-YOUR-MIND-UP TIME’ FOR SPRING

Catkins of great sallow (pussy willow) and common hazel (lamb’s tails) were once traditional features of March but are now seen much earlier. (Image: FJ)

Sure signs of the spring arrive on a special day in March. This is not a pre-determined day like March 1 (the first day of the calendar spring) or March 20/21/22 (the vernal equinox and first day of the astronomical spring, depending on the year), and definitely not on the first warm, sunny day, because spring is a cumulative effect of rising temperature over a period of time. Nor is it a visual experience but an olfactory one, because spring is the scent of volatiles released from the soil by rising temperatures. 

This is something recognised, if not completely understood, by poets and authors for a very long time. Ted Hughes, appointed poet Laureate in 1984 and holding the position until his death in 1998, likened spring to a woman recovering from an operation (winter).

The earth invalid, dropsied, bruised, wheeled
Out into the sun,
After the frightful operation.
She lies back, wounds undressed to the sun,
To be healed,
Sheltered from the sneapy chill creeping North wind,
Leans back, eyes closed, exhausted, smiling
Into the sun. Perhaps dozing a little.
While we sit, and smile, and wait, and know
She is not going to die.
From ‘March morning unlike others’ by Ted Hughes (1930–1998)

Climate change continues to confound the longer-term picture. Milestone March events from past years, decades and centuries, like the loosening and lengthening of yellow, pollen-dusted male catkins of common hazel (Corylus avellana) now happen in February and sometimes even earlier, while well-established April events like the re-foliation of white-flowering horse chestnut trees are now observed in March.

From time immemorial, March has been the spring-cleaning month. It was a month of blustery, blue-sky days but clear nights with frost still coating the ground and grass. 

William Wordsworth devoted a whole poem to this, written while resting on the bridge at the foot of Brother’s Water, a small lake in the Hartsop Valley, in the eastern region of the Lake District.

Like an army defeated
The snow hath retreated...
Small clouds are sailing,
Blue sky prevailing;
The rain is over and gone.
From ‘Written in March’ by William Wordsworth (1770–1850)

What a difference two centuries make. Apart from 2018, when the ‘Beast from the East’ spilled over into the first four days of March, I cannot remember the last time a measurable ground frost graced the grass during the month within the London area. 

That’s in stark contrast to a March night in 1962, when my senior scout troops took part in a night-time hike. In groups of four we were blindfolded and driven out at midnight into the wilds of Hertfordshire with torches, maps and compasses and with strong, well-dubbined leather boots on our feet. That’s because we had to make it back to base camp in south Hertfordshire by ‘Shanks’ Pony’ (on foot), some 20 km from the drop-off point. 

Large parts of Hertfordshire still retain their country character, but the county as a whole was much more rural then. I recall taking off the blindfold, walking down a country lane and finding the first clue to our location – a road sign saying ‘Codicote 1 mile’. Codicote was a tiny village in North Hertfordshire. It was cold, crisp and the frozen grass crunched beneath our feet as old English elm trees in the hedgerows loomed large against a clear, moonlit sky. That year (1962) experienced the coldest March of the 20th century and was the 10th coldest since 1797, when three stones at Stonehenge fell on 3 January due to heavy frost.

Dutch elm disease left its mark in more ways than one. The disease removed an age-old relationship between the rooks coming back year after year to nest in their elm tree habitats, a harbinger of spring lost forever. This was recalled by Anglo-Welsh poet Edward Thomas.

Over the land freckled with snow half-thawed
The speculating rooks at their nests cawed
And saw from elm-tops, delicate as flower of grass,
What we below could not see, Winter pass.
From ‘Thaw’ by Edward Thomas (1878–1917)

March is ‘make-your-mind-up time’ for spring with things prepared to go either way.

Spring has arrived early in recent years, with long spells of warm weather beginning in early March, and what normally happens in March and much of April gets compressed into a couple of fast-moving weeks. However, I can remember many instances further back when March came and went without a scent of spring and was essentially an extension of winter, with trees and wild flowers in suspended animation well into April.

The poet and author Thomas Hardy, born and bred in Dorset, highlighted the tardiness of spring when faced with a cold March.

This year with frost and rime
To venture one more time
On delicate leaves and buttons of white
From the selfsame bough as at last year’s prime,
And never to ruminate or remember
What happened to it in mid-December.
From ‘A backward spring’ by Thomas Hardy (1840–1928)

However, early warm weather and the consequences of short, speedy springs is not everyone’s cup of tea – and certainly not the Rev. Cannon Andrew Young LL.D, Scottish-born naturalist, poet and clergyman, who wrote:

Stay spring, for by this ruthless haste
You turn all good to waste;
Look, how the blackthorn now
Changes to trifling dust upon the bough...
So late begun, so early ended!
Lest I should be offended
Take warning, spring, and stay
Or I might never turn to look your way.
From ‘Stay, spring’ by Andrew Young (1885–1971)

TREES TAKE IT AWAY FOR MARCH

Late March – dandelions booming in set-aside rough pasture land. Oak trees still in winter condition but with buds fast colouring up. (Image: FJ)

First to re-foliate are native hedgerow hawthorn (Crataegus monogyna) and naturalised cherry plum (Prunus cerasifera), both botanically related though membership of the plant family Rosaceae. Hawthorn starts to re-leaf in March but doesn’t usually flower until early May, as the colloquial name ‘May blossom’ suggests. Cherry plum re-foliates and flowers at the same time during early March, this being the best way to distinguish the tree from the closely related native blackthorn (Prunus spinosa) which flowers before it re-foliates.

Other native trees like common hazel, common alder (Alnus glutinosa) and silver birch (Betula pendula) in the plant family Betulaceae, and great and grey sallow (Salix caprea and Salix cinerea) of the plant family Salicaceae, are already covered in catkins (male and female flowers) in March. These flowers are subsequently discarded during March, although the new spring leaves won’t appear until April. March provides a short window of opportunity to see spent male sallow flowers forming uniquely beautiful patterns across the surface of the many small ponds, around which grey sallow and great sallow commonly grow.

The non-descript flowers of native common ash (Fraxinus excelsior) are among the early spring risers, bursting out of the chiselled, jet-black winter buds in March, although common ash doesn’t re-foliate in earnest until May and is one of the last to do so. This is strange, seeing that common ash is a thoroughbred native, and one of the first trees to lose its leaves in autumn. 

Flowers of native common ash are drab, but those of a botanically-related woody shrub, adopted by many suburbanites as a harbinger of spring, are anything but. Indeed, the vibrant yellow flowers of Forsythia shrub species, most of which are native to eastern Asia, stand out like no other on the suburban spring landscape of the British Isles. That said, Forsythia species and hybrids which grow here are nothing compared with the vigorous and large-flowered types which I witnessed in the gardens of Adelaide as South Australia’s capital city came out of winter in August 2012. Native common ash and exotic Forsythia are both members of the olive family – Oleaceae.

An interesting tree to monitor in March is the white-flowering horse chestnut (Aesculus hippocastanum), originally exotic, latterly upgraded to domicile and perhaps nothing at all if bacterial bleeding canker caused by Pseudomonas aesculus advances as quickly over the next 20 years as it has over the past two decades. 

The beauty and notoriety of the white-flowering horse chestnut tree is due to the size and stickiness of its winter buds, caused by a coat of protective resin to make the tree a favourite with children in spring. But perhaps not as much as in autumn when the big, brash seeds are used in the traditional game of conkers. 

Horse chestnut buds and the scales which cover them are considerably bigger than those of all other British trees, so much so that you can almost see the buds burst in slow motion. The first stage of re-foliation is a rapid swelling of the winter buds and shedding of the sticky bud scales. 

I remember standing by a lake and hearing a continuous faint flutter which turned out to be horse chestnut leaf bud scales striking the surface of the water. This was made all the more memorable by the big buttercup-like flowers of the marsh marigold, commonly called ‘king cups’, growing in abundance at the water’s edge.  Over the following days the rudimentary compound palmate leaves and candelabra-like blossoms in the same winter bud began to unfurl like a young bird hatching from an egg, arching its fledgling wings along the way. The covering of down-like, glutinous material covering the newborn leaves and flowers added to the perception of an avian birth rather than the re-foliation of a tree. 

March is the month when you will usually see the first early flying native butterfly having overwintered in the imago (adult butterfly) stage. Peacock butterfly seen here soaking up the mid-March sun. (Image: FJ)

This memory of sticky bud scales falling onto the water cements my belief that climate warming is affecting the phenology of some tree species more than others. The event happened some 60 years ago and my notes from the time suggest it was in April, not March. That particular white-flowering horse chestnut tree has long gone, having swiftly succumbed to bacterial bleeding canker when the disease burst onto the  scene soon after the dawn of the new millennium. The marsh marigolds still grow around lake, but now bloom in March, not in April as before.

Other tree species appear oblivious to the measurably warmer and earlier springs of recent years. Over time I have assembled a collection of unrelated events as my own personal harbingers of spring. One is the day on which native wild cherry or gean (Prunus avium) comes into flower. That was traditionally 16 April (give or take a couple of days) in South Hertfordshire. Fast-forward 50 years in the same location and the timing has hardly changed.

March sees the seeds from trees start to germinate for the next generation. Interest in the botanical intricacies of seed germination and tree seedling establishment has been snuffed out by gene technology and the rest, but 100 years ago it was all the rage and at the forefront of forest botany. But that is a story for another time.