BrownhillsBob's #365daysofbiking

On a bike, riding somewhere. Every day, rain or shine.

Posts tagged ‘oak’

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February 4th – I’ve always been puzzled why it might be that some deciduous trees don’t shed their dead leaves in autumn; the summer growth dies and goes brown, but doesn’t drop.

Someone asked the same question on social media over the weekend, so I thought I’d look into it.

The characteristic is called marcescence, and is exhibited mainly by oak, beech and hornbeam in the UK. It’s not clear what the evolutionary purpose of this curious feature is; it could be to shelter leaf buds from browsing animals like deer, and indeed, some oaks are only marcescent on lower boughs. Another theory says that the leaves attached to the beaches have their goodness absorbed back into the tree over winter, which is more efficient than them dropping and relying on conversion from leaf litter.

So I’m not really much wiser, but at least it has a name – and this marcescant oak was showing it’s dead leaves well beside the cycleway in Telford as I passed this morning, making me smile.

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August 23rd – I don’t know what it is in the season, but the acorns this year are prolific and absolutely huge. With the dry summer I’d have expected the opposite, but they are absolutely huge. 

The fecundity of the crop is, however, being affected by the large amount of knopper galls, that from from acorns attacked by the knopper wasp. Also a peculiar seasonal phenomenon, they are very,. very red this year, whereas it’s usually just tinges of colour. 

Wonder why?

August 13th – At Hortonwood in Telford, I found a small wayside oak sapling, which was growing round, large acorns. they were generally healthy and surprisingly large, with only a handful affected by knapper galls – and where they were, the actual effect of the acorn remained small.

The ants were clearly interested in the acorns, but I have no idea why, and I’m wondering if the low gall-count and manner of development is maybe signifying and evolved defence to these parasites.

Certainly is fascinating. Going to be a good crop of acorns this year I think, and a bumper hedgerow harvest generally from what I can see so far.

July 25th – These oak knopper galls actually took me by surprise a couple of days ago, but the photos I took weren’t great, so I revisited the saplings they’re growing on today.

I spotted them on a tree near Victoria Park in Darlaston and I don’t think there’s a single normally formed acorn on that tree at all, and yet several adjacent ones have no knoppers at all.

These mutations of normal acorns are of course caused by a tiny wasp that lays eggs in the acorn buds earlier in the year. A secretion the eggs are coated in causes the acorn bud to mutate and grow in a distorted form, forming a gall, with the wasp egg at the centre. 

The larva hatches, and eats it’s way out of the gall, which provides it enough nourishment to grow to maturity.

Insect galls like this don’t hurt the tree, but of course, do affect it’s fecundity.

Isn’t nature amazing?

May 8th – A first for the year: pottering back from an errand in Aldridge the first marble oak wasp galls are appearing on the oaks along the canal towpaths.

That’s a sign the season is already moving on…

September 13th – Also falling from trees now and altogether less of a hazard are the knopper galls, the genetically mutated acorn-cum-insect-cocoons that are bastardised from the normal oak fruit by the knopper wasp.

These seemingly dead, spent galls will most likely have larva inside them and they will overwinter in the fallen galls before boring their way out in spring – although those dropping in vulnerable positions like these on the footpath will be lost under feet, cycle tyres and to the wind and elements.

It’s not until you think about it you realise what a high rate of attrition there is with such things – just how many larva are lost and how this must affect the fecundity of the knopper wasp as a species.

Remarkable how they survive at all. 

September 11th – I had promised no more wasp galls. Sorry, just one more I missed. 

I’ve been looking at this type of gall for ages and not realised what they are – a small, coffee-bean sized growth, caused by genetic mutation provoked by an injected tiny wasp’s egg. These small, rough galls are tiny compared to the more familiar marble oak galls which are smoother  and rounder.

They function in the same way though, as a growth pod and foot source for the wasp larva that hatches within, and when ready, the wasp will eat it’s way out to freedom.

This poor tree at Darlaston had knapper galls, marble galls, common galls and cola nut galls. And plenty of acorns!

August 14th – Sorry, after this I promise no morale oak wasp galls!

This is an artichoke oak wasp gall, created the same way as all the others, this wasp selects acorn buds, which are corrupted into these neat little artichoke shaped growths to house it’s larvae.

These examples spotted on Clayhanger Common.

That’s it now, I think we’ve collected the set…

August 13th – More oak wasp galls, which I’ve gone all out to find this year for no other reason than they fascinate me.

On a small sapling by the canalside track at Hopwas, hundreds of thousands of almost annular, ring-like growths on the leaves, looking maybe like fungus or some odd egg. These are the delightfully named common spangle gall for the flat ones, and silk button galls for the rounder, more sharply defined ones.

These are all created by the same mechanism – a small wasp injects an egg into the leaf, and a chemical coating the egg disrupts the plant DNA to grow the gall, which leaves a light patch on the upper surface of the leaf where nutrients have been leeched away by the larva growing underneath.

I’m not sure why galls like this captivate me so much but they are absolutely fascinating.

August 10th – Oak galls continue to fascinate, and on this tree in Victoria Park, Darlaston, there’s quite a display of knopper galls, the first I’ve seen this year.

Like other oak parasites, the knopper wasp lays eggs in it’s host, secreted in a chemical that corrupts the cellular DNA of the host plant matter causing the gall too grow. In this case, the target is the acorn itself, and on this tree, one can see some acorns blighted by two such galls.

As with others, the egg hatches and ithe wasp larva eats the gall and grows safe in it’s corrupted acorn, before boring it’s way out when mature.

Also on this tree, the more conventional wasp gall – the common ‘oak apple’ of folklore, a spherical gall grown the same way.

These galls don’t harm the host, but do reduce the functional acorn crop. I’d love to know just why the oak is targeted so particularly with the and not so much other trees…