Yew tree pollination

 February is the month when the tiny male cones of yew trees begin to shed their yellow pollen. These were photographed in Teesdale a couple of days ago. 

Yew trees are dioecious - either male of female - and the male pollen is carried on the wind to single female ovules hidden amongst the foliage on another tree. These are tiny and can be hard to spot, but here is one, ready to receive pollen.

After fertilisation the outer wall of the ovule swells to become a soft, fleshy, aril that surrounds the single hard seed inside. It turns bright red in autumn, to attract the attention of birds. 

Every part of a yew tree, except the aril, is poisonous to humans but thrushes, fieldfares and redwings quickly devour them. Nuthatches extract the hard seed, wedge it into crevices in yew trunks and hammer it open to extract the kernel. There's more about them in this Guardian Country Diary.

Yew

This is the time of year when yew 'berries' begin to become conspicuous. Yew is one of only three British native species belonging to that division of the plant kingdom known as the gymnosperms - seed-producing plants that don't have flowers and don't enclose their seeds in an ovary to form a fruit. The other two are juniper (whose seed has a succulent outer coating - seed picture at the bottom of this post) and Scots pine (which carries its seeds in woody cones - see picture below). 

In this picture you can seed the fully formed yew seed on the right, half covered by a pale green cup. This is the tissue that quickly expands to form the scarlet succulent aril, that you can see half-formed on the seed on the left. This squashy cup is attractive to birds that disperse the seeds, effectively judging from the numerous seedlings that appear in places where birds perch, such as crevices in old stone walls after the seed passes unharmed through the bird's gut. The seed itself is deadly poisonous to mammals but I've watched nuthatches wedge it in a bark crevice, hammer it open and eat it.

Here's a fully formed aril, which almost completely encloses the seed, like a scarlet doughnut.

Juniper 'berries'

Scots pine cone

The Last Day of Winter in Teesdale

Spent the last day of winter walking beside the river Tees through woodland downstream of Egglestone Abbey in Teesdale. The pace of spring is picking up!

Sunlight streaming through hart's tongue fern fronds, revealing the herringbone pattern of sori underneath.

Ivy berries ripening fast - a fine crop, food for many birds.

Wood anemones coming into flower.

Male cones of yew releasing pollen

Golden saxifrage in full bloom.

Money Tree

A few years ago, when we were following the Ingleton Waterfalls footpath, we came across the famous money tree, where passers-by have hammered coins into the trunk. Earlier this month, when we were following the footpath from Clapham towards Ingleborough we saw this one, where coins had been hammered into a tree stump and into the branches of an old living yew tree.

The coins in the stump (above) all radiate out from the centre, which says something about the anatomy of tree trunks. Radiating bands of living cells called medullary rays run outwards from the centre towards the bark, at right angles the vertical rows of thick woody cells that form the annual rings in the trunk, and when the tree dies the thin-walled living cells become lines of weakness and often form radiating splits. It's much easier to force coins into these than to hammer them into the tough woody woody tissue of the annual rings in between, so the coin hammers have unwittingly revealed the pattern of living cells that existed inside the trunk when it was alive.

These are coins forced into the think bark of the branch of the living yew. 


I've seen suggestions that hammering coins into trees like this is supposed to bring good luck (not for the tree, though!). I wonder when this started, and how many more trees like this there are around the country? Does anybody have any information?

Yew Midge Gall

This year some of the yew trees in Teesdale are heavily infested with this 'artichoke' gall, caused by a tiny midge called Taxomyia taxi. This insect has a strange two-speed life cycle. Eggs are laid on the yew shoot tip foliage in late spring, inducing the formation of a swollen terminal bud where they spend the winter. Some emerge as adults in the following year but others, like his one, have slower development and spend a second winter inside an enlarged and more conspicuous gall before emerging as midges two years after the eggs were laid.

Margaret Redfern's recent New Naturalist book, Plant Galls, is a mine of information on the strange world of these plant-insect interactions.

An Autumn Walk along the River Tees

The long, narrow gorge spanned by Egglestone's Abbey Bridge bridge provides one the most picturesque viewing points on the River Tees in autumn

This is the view from the bridge looking upstream - Egglestone Abbey is just  above the trees in the middle distance (double-click) and is ...

...... visible from the bridge now that the leaves are falling.

The view downstream - there are footpaths on both sides of the river and you can follow it down to its confluence with the River Greta, at the Meeting of the Waters.

The river squeezes through narrow gaps and tumbles over boulders ...

.... and you can hear it through the trees all the way along the path, even when it isn't in spate.

The high humidity in the gorge makes this a fine habitat for mosses and ferns like this polypody growing as an epiphyte on a tree branch over the river.

Yellowing horse chestnut leaves provide a sunbathing spot for flies whose days are numbered, now that frosts are on their way.

Nectar-rich ivy flowers provide a last-minute refuelling station for drone flies.

Shades of yellow - hazel, oak and beech autumn colours.

Some of the large beeches have been attacked by honey fungus - always fatal, but it can take decades to kill the tree.

A nuthatch, dangling down to pick beech nuts out of beech mast - it seems to be a 'mast year', with a very heavy crop.

Ripe holly berries are a reminder that there are only 50 shopping days until Christmas.

Downstream there are some wild cherries with a fiery display of autumn colours..

... and a fine crop of ripe yew berries.

A passing shower leaves a rainbow, which is wasted on those two sheep that are watching - they only have dichromatic vision and can't distinguish red from green.

Looking back upstream - on what Kenneth Grahame in the Wind in the Willows called a 'golden afternoon'...

Yew

These strange objects are the pollen-producing structures of a yew tree. There are separate male and female trees and at this time of year these little pollen-producing cones, known botanically as strobili, are very conspicuous on the male trees, releasing pollen that's carried on the wind to the females. Yew belongs in the division of the plant kingdom known as the gymnosperms that includes the cone-bearing conifers, but unusually it doesn't produce its seeds within a woody cone. Instead each seed is carried in a succulent red cup, which you can see by clicking here.

A Tree-Spotter's Guide to Fruits and Seeds: Part 2

Yew Taxus baccata, one of only three native conifers in Britain (the others are Scots pine and juniper) is a conifer that doesn't produce cones. The vast majority of its relatives carry seeds on the surface of whorls of woody bracts that form a cone, but yew produces single seeds surrounded by a red fleshy aril. In the picture above you can see an aril that has yet to swell and envelope its seed, just above and to the right of the seed in the centre of the image. Thrushes are particularly fond of the succulent aril. All parts of the tree are very poisonous apart from this conspicuous coat but the thick hard wall of the poisonous seed (which can take two years to germinate) means that it can pass through the bird's gut with no ill effects. There are separate male and female yew trees, so any that are not bearing seeds at this time of year are probably male. There is a rare mutant of yew (which I have never seen) known as var. lutea that bears yellow arils. In Sussex you can find one of the finest of all yew forests, at Kingley Vale National Nature Reserve, filled with venerable trees whose dense shade prevents anything from growing beneath them -well worth a visit if you are ever in that part of the country. Some churchyard yews in various parts of Britain are reputed to be over 2000 years old.

Usually privet Ligustrum vulgare grows as a shrub and is often clipped into a neat hedge, but it it's allowed to grow unchecked it will grow into a small tree, flowering prolifically, attracting butterfly and bee pollinators and producing these rather attractive indigo-hued berries. It's a member of the olive family (Oleacea), as is ...

... the common ash Fraxinus excelsior. Ash trees can be either male, female or hermaphrodite and it's only the latter two forms that bear these familiar ash 'keys', in large bunches. Ash seeds are dormant and can take up to 18 months to germinate. They begin to fall in late autumn and require a winter chill before any will germinate - and even then only a small percentage of the crop sprouts in spring. The rest are immature and the embryos inside continue to develop throught the summer, on the tree or on the ground, and will not germinate until the following spring.

Some potential ash seeds never mature, if the flowers are attacked by a microscopically-small gall-mite called Eriophyes fraxinivorus (also known as Aceria fraxinivorus). Trees like this one alongside the River Tyne at Wylam can become completely infested and produce few seeds.

For more posts on tree ID click here