Wind pollinated trees and hay fever

 As winter draws to a close I always look forward to the first hazel catkins, that signal a change in the seasons ......... but then, when spring finally does arrive, I remember that I suffer from hay fever. The early-flowering trees are wind-pollinated, producing vast clouds of sneeze-inducing pollen. 

Wind pollination is a chancy business and once the pollen is released it's rapidly diluted in the air, so the chances of an individual pollen grain landing on a female flower stigma, leading to the formation of a seed, decreases exponentially with the distance between then. So the only way to improve the odds is for trees to release great quantities of pollen

Male catkins of hazel.

Tiny, female stigmas of hazel

Male flowers of ash, about to open

Ash flowers, fully expanded

Silver birch male catkins, a major contributor to hay fever

Silver bird male catkins

Elm flowers. Before Dutch elm disease arrived, the bare crowns of mature elm trees were covered in a purple haze of flowers, a magnificent sight, now just a memory.

Most wind-pollinated trees produce their flowers on bare twigs, unencumbered by foliage that might hinder pollen release, but beech is an exception. It may be that beech bud burst is delayed because its foliage is very sensitive to late frosts. 

Evergreen coniferous trees like Scots pine tend to produce their male, pollen producing cones on the tips of their branches, well clear of the foliage, where pollen is easily carried away by the wind.

Tree silhouettes in winter

Every autumn, as the last leaf falls, I've had good intentions of recording a set of images of the characteristic silhouettes of different tree species in winter. They are all distinctive and beautiful in their various ways.

This winter I finally made a start but I hadn't bargained on how difficult it would be to find good isolated examples of trees silhouetted again a clear sky background, so progress has been slow. Anyway, here are a few, that I hope to add to before bud burst begins in a few weeks' time.  

Sycamore Acer pseudoplatanus. Only the tree on the left has had enough space to develop symmetrical growth; the two on the right have got in each other's way.

Horse chestnut Aesculus hippocastanum. Those almost horizontal heavy lower limbs are characteristic; in the rare absence of grazing animals they'll sweep almost down to the ground and are usually the first to be shed from old trees in gales.

Ash Fraxinus excelsior. The upward sweep of the rather thick twigs is characteristic.

Durmast oak Quercus petraea

Ivy - the all-year-round carbon dioxide fixer

Ivy's flowers provide a wonderful nectar and  pollen resource for insects in autumn (see some examples at the bottom of this post) and its berries are a vital food  resource for migrant birds in early spring, but it also has another important attribute with more general implications.

As an evergreen that stores carbon in its woody trunk, ivy is an all year-round carbon dioxide fixer, constantly removing the gas from the atmosphere. In that respect it contributes towards combatting the build-up of carbon dioxide in the atmosphere, which is the underlying cause of global climate change.

Deciduous trees like this ash have a short growing growing season in our seasonal climate. It comes into leaf in May and sheds its foliage in September, so for about half the year it fixes no carbon dioxide at all. But when it supports a luxuriant growth of ivy the two organisms together fix carbon dioxide 365 days a year.







































The combined efforts of an ash tree with ivy cladding are likely to be more effective at fixing atmospheric carbon dioxide than an ash tree alone.

The tangle of  thick, woody ivy stems on this sycamore are as effective a store of carbon as the tree's trunk.

Ivy can even turn a dead tree into a carbon dioxide fixing pillar of evergreen foliage, as it did with this dead oak at Egglestone in Teesdale a few years ago. Even better, in autumn this stump became a tower of flowers, humming with insects that came for the nectar and pollen. In spring it was covered in berries.

Another example, at Wolsingham in Weardale.

Sadly, there are still plenty of ivy-haters around who are convinced that ivy is a parasite (it isn't, it just uses trees for support). They'll usually also advance the argument that a top-heavy mass of ivy is likely to bring trees down in winter gales, but most trees that are felled in this way are moribund anyway and topple because they have already been weakened by root-rotting fungi, not by ivy. Logically, a good cladding of well-rooted ivy stems is more likely to anchor a shaky tree, delaying its demise. 

The fact that trees that have been killed by fungi often have a healthy covering of ivy suggests that it might well be resistant to some of the fungi that kill trees.

From all of this you might have gathered that I'm something of an ivy enthusiast - not least because of the insect fauna that its flowers attract in autumn. All of the pictures below were taken in a few minutes late last October on another ivy clad tree just across the road from the oak-clad stump pictured above. 

Ash flower buds

These flower buds of ash, which usually open about a month before the leaf buds, look like some strange kind of insect eggs or maybe caviar (not that I've ever eaten caviar). They are opening very late this year - these buds usually burst at the end of March or in early April.  Click here for more pictures of ash flowers in full bloom.

The Do-nothing Option

When we went walking (or should I say slithering) in the increasingly slushy snow today the snow surface was littered with these ash keys, torn off by gusty winds yesterday. How many will germinate and how many of those will be resistant to ash die-back is debatable, but since the trees produce these seeds in vast numbers it's likely that some will reach maturity. 

During our walk we passed through two quite different new woodlands.

This was the first - a Woodland Trust site where thousands of saplings of several species, including ash, have been planted in tree shelters over the past couple of years. The young trees have a good chance of establishment but it will be a very even-aged woodland when it matures. I can understand why woodlands are planted like this - it looks like an impressive amount of work that will turn grassland into woodland in the shortest possible time - but the next field that we walked through, with a different owner, revealed a different strategy.

This footpath runs through what was, until eight years ago, a pasture. On the right is old woodland. No one has planted anything in the pasture - the wind and the birds have done all the tree planting. The result is already a woodland of five metre-tall birches, three metre tall ash saplings and metre tall oaks. Their seeds have been planted by the wind and jays - no charge, no grant funding required. There's also a fair sprinkling of alder, rowan and holly. 

The only management that this site is receiving is the cutting of a few glades where there are some fine patches of marsh orchids and some trimming to keep the footpaths clear. There's already a shrub layer developing of wild roses and bramble. 

I suppose it's at the stage that is disparagingly referred to as scrub, but in summer it already hosts several butterfly species, a decent range of flowers and plenty of warblers. The birches, which are presently dominant, are short lived trees and by the time the oaks become substantial trees they'll be moribund and will contribute to the dead wood layer on the woodland floor, which is such a valuable habitat for fungi, beetles and a host of other invertebrates.

I know which method of woodland establishment I prefer - let nature take its course. Of course, this isn't the most desirable option when you are a charity that has spent a lot of money donated by members to buy land, who want to see evidence that a woodland is being created PDQ. 

I should say that I'm a supporter of the Woodland Trust's objectives and particularly admire the way in which they encourage the public to participate in woodland creation and encourage everyone to visit the woodlands that they own.  But I do also believe that the phase of scrub that naturally regenerating woodlands go through creates an incredibly valuable transitional habitat, and that allowing woodlands to re-establish naturally creates uneven-aged, mixed species stands of trees that are likely to be healthier and more biodiverse woodlands in the long run. 

Sometimes, in some circumstances, the do-nothing option is best.

Ash

Today's Guardian Country Diary is about an organism that seems certain to benefit from the death of ash trees. More of that later, but first some ash appreciation (amazing, isn't it, how the imminent loss of something makes you appreciate it so much more?).

Ash twigs are surely the most distinctive of any native tree, with those thick grey twigs and charcoal-black buds. The tell-tale ring of bud scale scars that you can see just above the bottom of this twig show that it didn't make a lot of growth last year - there was just a couple of inches of growth before it stopped and developed a new terminal bud. Above those old bud scale scars you can see the scars of the shed-leaf stalks under each resting bud which, with the exception of the terminal bud, formed in a leaf axil when the twig was growing in summer.

Bark of mature ash trees exhibits this quite distinctive criss-cross pattern of fissures. Almost everything about ash is ash grey in winter unless ...

... the tree has been colonised by lichens, which add a splash of colour. A recent survey of epiphytic lichens on ash trees in Sweden recorded 174 different species growing on this species.

This is the downward view that a perching crow enjoys when it looks down from the crown of an ash - in this case one that grows under the old railway viaduct on the Romaldkirk to Cotherstone line (now a footpath) in Teesdale. Here the ashes grow right up to the edge of the parapet, so you get a rare opportunity to visualise the birds' perspective on ash trees.

When ashes grow in places where plenty of rain trickles down the trunk and branches mosses also colonise the twigs. I think this is wood bristle moss Orthotrichum affine which is particularly common on ash.

This is the view from the old railway viaduct mentioned above, where it crosses the river Balder in Teesdale. All those grey tree crowns are ashes. Ash woods often grow best in the rich, moist alluvial soils alongside rivers.

From the viaduct again - those knobbly grey twigs  are actually quite brittle and snap easily. There's a little moth called the ash bud moth Prays fraxinella whose caterpillar destroys the terminal buds of ash shoots, especially in young trees, which promotes the growth of outward-facing buds further down the twig and tends to produce very open-crowned trees (and often low-branching, multiple-stemmed trees if it attacks young saplings)

Ash keys will germinate when they are green but develop dormancy when they ripen - and then it takes a couple of winters to break the seed dormancy. They tend to stay on the tree until spring ....

.... where they're often decorated with hoar frost. Once they do germinate they grow very rapidly and young trees set seed within a decade of growth. My garden is downwind of a large ash and if I didn't keep pulling out all the ash seedlings I'd be surrounded by a forest in no time!

This is a well-proportioned ash in the prime of life. Silhouettes like this will probably disappear from most hedgerows once ash dieback, caused by Chalara fraxinea, runs riot. Many mature ashes display heavy, almost horizontal lower branches that easily snap under the weight of snow and in high winds, leaving jagged holes in the trunk that rot and provide nest holes for birds and roosts for bats.

Ash has the shortest growing season of any native tree, coming into leaf in mid-May and losing its leaves by October. This tree, high on a hillside in Teesdale, is showing signs of autumn on its south-facing side. Ash sheds its leaves in two stages - leaflets first, followed by the leaf stalks on the following day.

The open crown of ash allows plenty of light to penetrate so old trees become swathed in ivy, which vastly increases the value of the tree to wildlife (ivy nectar and pollen for insects in autumn, berries for birds in spring and shelter under those evergreen leaves all year-round). Old trees take a long time to die and ivy-covered specimens often have this stag's antler appearance, with just the ends of branches protruding beyond the coat of ivy foliage.

Ash is far from being the most important tree in terms of the diversity of invertebrate life that depends on it (oaks and birch are way ahead on this score), but these strange growths are the work of the ash gall mite Eriophyes fraxinivorus, which infests the flower buds and prevents seeds forming. Whole trees can be infested. 

This is what healthy ash flowers should look like. The flower buds burst in March, around six weeks before any foliage puts in an appearance. Ashes have complicated sex lives. Individual trees can be male, female or hermaphrodite (with only the latter two types bearing seeds) or they can be one or other sex with occasional hermaphrodite branches. I know of one tree that's all male on the sunny southern aspect and all hermaphrodite on its north side, so only half the tree carries seeds. It's said that trees, and even individual branches, can change sex during their lifetime although I wonder who had the patience to find this out.

I guess we'll lose some rookeries when ashes begin to decline. Rook nests sit comfortably in those open crowns.

When Dutch elm disease took hold elm roots regenerated suckers, so elm is almost is common today as it ever was - it just grows as a shrub rather than a tree, succumbing to its fungal pathogen before it reaches tree proportions. Ash is also capable of regeneration. This pollarded tree, vigorously sprouting new growth, is in a wood near Richmond in North Yorkshire and ......

..... this ash stool is also vigorously sprouting new growth from its roots in the same woods. Will trees attacked by Chalara fraxinea regrow in the same way? No one seems to know yet.

Ash isn't often planted as an ornamental tree, with the exception of this weeping form which is unusual but not particularly graceful. The original weeping ash tree is supposed to have been discovered in woodland by the vicar of Gamlingay in Cambridgeshire sometime around 1760 and it's likely that some of the oldest weeping ashes in gardens are grafted descendants of this original specimen, which has long-since died.

And finally, this is the organism that stands to benefit most when ashes die - King Alfred's cakes fungus Daldinia concentrica, which grows almost exclusively on ash. The late, great mycologist Terence Ingold demonstrated that this strange fungus has an internal biological clock, whereby it only shoots out its spores during the natural hours of darkness, even if you keep it in a light-proof container in continuous darkness. 

A Tree-Spotter's Guide to Flowers: 3

Sloe (blackthorn) blossom is produced before any leaves appear, often in such profusion that whole hedgerows can look as though they're covered in snow.

Wild cherry (gean) blossom - clusters of large flowers set against expanding foliage that has a bronze tint when it's young.

Male ash flowers. Ash trees flower long before their leaves expand - this is usually the last tree species to come into leaf, in May. There are three basic kinds of ash tree. All-male trees like this produce dense clusters of crimson pollen-producing anthers when their flower buds burst. All-male trees never produce ash 'keys' in autumn.

 Female trees produce clusters of flowers with bright red stigmas and styles - the style will later develop into the wing of the ash 'key'. Female trees produce heavy crops of  'keys' in autumn.






















Close-up of female ash flowers. There are also hermaphrodite trees, whose flowers look similar to these female flowers but have a pair of pollen-producing anthers attached on either side of those slender green stalks. They too produce heavy crops of 'keys' in autumn.

Just to complicate matters even further, some trees produce any two of the above three flower types on the same tree i.e. male + female; male + hermaphrodite; female + hermaphrodite.

For more information on trees click here