Woodland Watch – Week 15 – 26/06/2013

Summer seems to finally be here and although it is neither as warm or as predictable as we would like, at least it is not as cold as it has been! This week my walk in the woods was overcast, but warm and I hope you will join me to have a look at what is going on this week.

The grass in the meadow approaching the wood is getting taller and taller but closer to ground level you can see lots of little purple clover flowers.

Clover growing in the hay meadow

Clover growing in the hay meadow

This red clover (Trifolium pratense) is a native, herbaceous perennial plant which can grow 20-80cm tall. The genus is so named because of the leaf structure consisting of three leaflets, known as trifoliate leaves. Clover is an interesting plant, widely grown as a fodder crop and highly valued for its nitrogen fixing capabilities which increase soil fertility.

Nitrogen fixation is the process by which nitrogen in the atmosphere is converted into ammonia. In the atmosphere nitrogen exists as a diatomic molecule (the atoms go around in pairs, like oxygen). In this inert state it does not easily react with other chemicals to form new compounds. Nitrogen fixing nodules in the clover plant frees up the nitrogen atoms from their chemical diatomic form (N2) to be used in other ways.

Before the establishment of science, when peopled looked to religion and magic for medicinal cures, red clover was already playing a part in people’s lives. To early Christians, it was a sign of the Trinity while during the Middle Ages, it was regarded as a charm against evil. A mutation resulting in the well-known four-leaf clover is considered good luck when found, even to this day.

Heading into the woodland, the first difference that was immediately apparent was how dark the wood was compared to previous weeks. The canopy has now almost completely closed over with a hazel lower canopy and an upper canopy made up of a range of other broad leaf trees including oak and horse chestnut.

The woodland was noticeably darker now the canopy has closed over

The woodland was noticeably darker now the canopy has closed over

One of the lower level tree species that we haven’t yet touched on but which I have noticed on my walks is the holly (Ilex aquifolium). An evergreen shrub which most people can easily recognise it can grow 10-25m tall with sharp spiney leaves which last around 5 years. What many people don’t realise, and to the relief of tree surgeons such as ourselves, is that leaves on the upper branches of mature trees do not have spines as they are well out of the reach of grazing animals and do not require this particular defense mechanism. 

Lower leaves of a mature holly tree

Lower leaves of a mature holly tree

Holly is dioescious which means there are male and female plants, unlike the hazel which we saw developing the male catkins and female flowers on the same trees. The sex of the plants cannot be determined until they begin flowering, usually between 4 and 12 years of age. Between October and November, the flowers on the female plants will grow into red fruit which is eaten by birds and rodents.

Interestingly in the Irish/Gaelic ogham alphabet holly is called Tinne, a word believed to have originally meant ‘fire’. From this was derived the word ‘tinder’, referring to dry, inflammable matter used for kindling fire from a spark. This association between holly and fire has been known since ancient times when charcoal made from holly wood was used by armourers to forge swords and axe heads.

The holly will remain fairly constant through the changing of the seasons, but it is a highly valuable woodland species.

Sticking with the theme of trees, another species to note is the English elm. Once a classic English tree like the English Oak, the elm all but disappeared from our countryside after Dutch Elm Disease reached Britain in 1927.

Now, these small elm saplings around the edge of the wood, although relatively common at this stage of their lives, are unlikely to reach maturity.

English Elm saplings

English Elm saplings

The current DED epidemic is caused by the fungus Ophiostoma novo-ulmi which is dispersed by various elm bark beetles within the Scolytus genus. The fungus blocks water conduction, resulting in wilting and eventual death of the foliage.

The first signs of the disease are often yellowing/browning of foliage tips as the affected branches are gradually starved of water.  When the leaves fall, the remaining twigs often turn down to form ‘shepherds crooks’ which can be valuable for disease detection in winter.

The diagram below shows how the disease progresses.

Dutch Elm Disease

Dutch Elm Disease

Although the English landscape has certainly lost the majority of its mature English elms, the Ulmus procera species has not disappeared completely from the landscape. This is because the elm bark beetles require bark to be of a certain thickness for breeding to be successful.

When an English Elm is killed by the disease, some roots remain alive and new elms regenerate from these. This means that for every mature elm killed by DED, many more are able to replace it. We may not have many large elms, but there are millions of young elms growing all over the English countryside, such as these young saplings in our wood.

That’s it for this week. Join us again next week for another walk in the woods.

http://www.tree-creeper.com

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