Thursday, December 20, 2012
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.