The Curse of the Black Spot

 

The curse of the black spot! Tar spot fungus Rhytisma acerinum, that disfigures sycamore leaves in summer, continues to develop on its dead, decaying foliage in the leaf litter. The black spots become increasingly wrinkly as winter progress, then hundreds of asci - microscopically small flasks filled with needle-shaped spores - develop along the ridges. When spring arrives the spores are released into the air, just in time for them to infect a new crop of sycamore leaves, where they germinate and invade through the breathing pores (stomata) in the leaf surface.

Tar spot fungus is less common on urban sycamores that on trees in rural areas and it used to be thought that this was because the fungus was susceptible to sulphur dioxide in polluted city air. A more likely explanation could be that the fungus is less common on city sycamores because their fallen autumn leaves tend to be removed by councils from public places, so the annual cycle of reinfection from dead infected leaves is broken.

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

Every leaf is a mite metropolis

Insects begin to attack tree leaves almost as soon as they emerge from buds in spring. Some eat foliage but others turn leaves into secure homes.......... 

......... like these little red galls on sycamore leaves produced by a microscopic mite called Eriophyes macrorhynchus aceribus.

These little eruptions on the surface of an alder leaf are caused by another eriophyid mite, Eriophyes laevis inangularis.

Eriophyid mites are not insects but are related to spiders. 

This is the underside of the leaf, with the little yellow, sausage-shaped mites crawling around the entrances to the chambers, which are lined with nutritive cells that provide sustenance for the mites.

Here they are at higher magnification .........

............ and at still higher magnification, when the elongated body with four legs at the head end is visible in the mite in the top, left-hand corner. Each chamber is home to a brood of mites and a tree with a severe infestation could be covered with hundreds of thousands of them. 

These are three of the mites, each being about one fifth of a millimetre long, with only four legs.

The outer cuticle of the animal has a distinct pattern that differs between species, although the easiest way to identify species is via the symptoms that they cause on the host plant.

Here is the head, legs and cuticle patterning at higher magnification.

Under the skin of a sycamore

Thursday's Guardian Country Diary is about the remarkable range of small invertebrates that live under the flaking bark of old sycamore trees.

When the bole of an old sycamore tree expands the old rigid bark cracks and begins to curl at the edges, while ....

.... a fresh, new layer of bark forms underneath. It might not look very attractive but these bark flakes, some as large as a slice of toast, take a long time to fall off and while they are still attached harbour a remarkable fauna of small invertebrates underneath. Here's a selection, most of which I've yet to ID.

Millipedes, a hatched moth pupa and some unidentified cocooons 

Lots more millipedes - these were just a few from under two bark scales - if this sample is representative there must have been hundreds sheltering under the bark of this tree

A minute scarlet mite

A rather beautiful little money spider, Gonatium rubens

A huddle of earwigs ....

... that raised their tail forceps in defence when they were suddenly exposed to the light. Male left, female on the right.

A spider that lives in a silken tent under the bark

Lots of slugs in areas where the bark is permanently damp, where rain water trickles down the tree trunk ..

.... together with snails ......

...... and woodlice

Another moth pupa, that looks as though it hatched successfully.


Sycamore sits low in the league table for tree foliage that supports insect biodiversity - a 1961 research paper on the subject found only 15 species, compared with the 284 hosted by oak (click here for details). But a quick look under the flaky bark of old sycamores casts them in a more favourable light, as a sheltered habitat for a host of invertebrates. 

The few examples shown here were just from a height that I could reach - there may well be a different array of species higher up the trunk and the hosted species most probably vary depending on the aspect (sunny & south facing or shaded & north-facing). Moisture must play a role too because there are well defined runnels where rainwater flows down the trunk from the branches and the bark there is always moist and often very wet. All in all, flaking sycamore bark is probably quite a complex habitat, with many interesting interactions between species that inhabit it.

A Tree-Spotter's Guide to Flowers: 1

Mature elm trees may be a rare sight now because of Dutch elm disease but shrubby hedgerow elms are still common. The twigs are covered with tufts of crimson flowers in March.

Tap an elm twig when it's in full flower and all those stamens release a cloud of pollen. Just imagine the amount of pollen that mature elms would have released in their prime.

Norway maple flowers open before any leaves expand...

... and each individual flower has conspicuous green petals.

Sycamore flowers are produced at more or less the same time as the leaves .....

... and unlike Norway maple the flowers have small petals - generally shorter than the clusters of stamens

The dangling sycamore inflorescences are composed of separate male and female flowers and pollination is by flies and bees (as in Norway maple). Some of the female flowers here have been pollinated and have begun to produce the vestiges of winged fruits - you can them about a third of the way down the inflorescence.

For more information about trees click here

The Insect that Turns Cars into Toffees

Take a close look at sycamore Acer pseudoplatanus buds at this time of year, just as they are beginning to swell, and you'll probably find some of these tiny insects huddled in groups around the edges of the bud scales. They're sycamore aphids Drepanosiphum platanoidis and they will have recently hatched from minute eggs that have survived the winter on the surface of the bark, just below the bud. It's often claimed that hard winters tend to reduce pest numbers, but that doesn't seem to be the case with this insect - buds on sycamores that I looked at today had a large pupulation of these aphids.

As soon as the tender new leaves begin to burst out of the bud scales the aphids will crawl onto them and begin to feed and breed - very rapidly. At this stage of their life cycle the aphids clone themselves, giving birth to live young that already have the next genetation developing inside them when they are born.

This is what the aphids look like at the moment, magnified about forty times. They cluster together with their antennae touching those of neigbouring insects, so any threats are transmitted through the colony very rapidly.

And this is what they do if they are threatened- all stick their tails up in the air. I'm guessing that they must be emitting something that might repel insect predators....

Once the sycamore leaf expands and begins to photosynthesise a winged generation of aphids develops. Later in the season these can migrate from tree to tree. Notice how most of them are lined up along the leaf veins. Their stylets, like hypodermic syringes, are inserted in the phloem inside the veins that transports the sugars, made by photosynthesis, to other parts of the plant. The sugary solution is rich in energy but very poor in other essential nutrients that the insects need for growth and reproduction, so they need to consume vast quantities of it, most of which is emitted from the tail end of the insect as sticky honeydew - and if you park your car under a sycamore on a hot summer's day when it's raining honeydew it will have the tactile qualities of a toffee when you return.

A Tree-Spotter's Guide to Fruits and Seeds: Part 4

A really good crop of beech Fagus sylvatica nuts is good news all round for wildlife, providing a food resource that lasts well into the winter. Bramblings are particularly fond of these seeds and in past 'mast years' I've sometimes seen large flocks of these finches in the beech woods at Stanhope Dene in Weardale, feasting on the fallen nuts.

We have three tree species in the genus Acer that commonly occur in the countryside but only this one - field maple Acer campestre - is native. It grows into a small hedgerow tree. The yellow autumn foliage is attractive and the winged fruits are unmistakeable. In field maple the wings of the fruits (which, botanically, are known as samaras) are held out almost horizontally, unlike .....

... those of the non-native sycamore Acer pseudoplatanus, whose wings are swept back at an angle of about 45 degrees. Sycamore is native to the mountains of central and southern Europe but has spread throughtout Britain since it was introduced in the fifteenth century.

 

The wings on Norway maple Acer platanoides fruits are held at an angle intermediate between field maple and sycamore, at about 30 degrees to the horizontal, and they are larger, broader-winged and altogether more attractive than either of the other species. They are also very efficient fliers - my garden is infested with seedlings that have arrived as an airborne invasion from a tree upwind of us.

 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Sycamore's autumn foliage of often marred by the black spots of tar-spot fungus and simply turns brown and withers in autumn, unlike the leaves of the closely-related Norway maple whose foliage turns a clear, vibrant yellow. This Norway maple tree, photographed at the weekend growing in a crevice in a  quarry wall at Stanhope in Weardale, is testament to the mobility of the winged fruits and the dazzling autumn display that the tree delivers.

Sea buckthorn Hippophae rhamnoides is more of a shrub than a tree, although I know of some specimens of a similar height and stature to large elders. At this time of year it produces a heavy crop of these dazzling orange berries that birds seem to ignore. Sometimes the fruits just decay on the branches and on warm spring days develop a rancid, vinegar-like smell. The underside of the leaves is coated in flat grey scales, that protect the plant against excessive water loss in the windy coastal habitats where it thrives, while its vicious thorns turn it into impenetrable scrub - good nesting territory for birds.

For more posts on tree ID click here

A Tree-spotter's Guide to Buds: part 1

A beech Fagus sylvatica bud. It always seems to me that buds are under-appreciated natural objects, not just because of their inherent beauty but because that explosion of greenery that we call spring is already pre-packaged inside, protected by bud scales but ready to unfold just as soon as a winter's chill breaks the bud's dormancy.

Charcoal-black ash Fraxinus excelsior buds on grey twigs are unmistakeable. Some buds will burst to reveal a mass of crimson stamens in early spring, others will burst much later to unfurl their foliage. Ash is always the last native tree to come into leaf in Britain.

Hazel Corylus avellana buds, with next year's catkins already formed and ready to elongate and shed pollen next February. Hazel twigs have bristly hairs on their surface. The leaves only unfurl after the catkins have shed their pollen, so as not to inhibit the flow of airborne pollen.

These are the distinctive winged fruits of hornbeam Carpinus betulinus, that cling to the twigs long after leaf fall. You can just see one of the small brown nuts attached to one of the bracts, bottom right.....

... and these are hornbeam buds, which are not quite so distinctive.

A sycamore Acer pseudoplatanus bud, which is particularly attractive in spring when it swells, elongates and in many trees becomes flushed with purple pigments.

And finally, a horse chestnut Aesculus hippocastanum 'sticky bud', with the horse shoe-shaped scar of one of last summer's leaf stalks. For generations of children (me included) who went to rural schools that had a 'nature table', the annual ritual of cutting these buds in early spring and watching them unfold in a jam-jar of water was an annual, memorable ritual that became an enduring totem of spring. For a close look at the marvel of microscopic packaging inside one of these buds, hop over to http://beyondthehumaneye.blogspot.com/2009/11/marvel-of-miniaturisation.html

For part 2 see http://cabinetofcuriosities-greenfingers.blogspot.com/2009/12/tree-spotters-guide-to-buds-part-2.html

For a Tree-Spotter's Guide to Fruits and Seeds, visit http://cabinetofcuriosities-greenfingers.blogspot.com/2010/10/tree-spotters-guide-to-fruits-and-seeds.html

For more posts on tree ID click here