Nature 2015 December

As an ook cometh of a litel spyr

In 1374 Geoffrey Chaucer was the first to record the occurrence in the English language of
this well-known every-day proverb: albeit one that has evolved over time. Today the English oak tree stands as one symbol for the stability and longstanding of the nation. However, it also sits at the centre of a whole ecosystem which contains microscopic, subterranean fungi; both metaphorically and in reality in the roots of the tree of life. At the other extreme, ithelps support ourselves, the self-appointed pinnacle of the tree of life.

There are several varieties of oak to be found in the British Isles. The most common is known as the English or pedunculate oak. This was the dominant tree of English woodland from a time when a warming climate after the last Ice Age 10,000 years ago reached an ambient temperature and there had been an accumulation of sufficient soil deposits. Together these would sustain and physically support this slow growing large tree. Oaks flourished and large specimens, maybe 800 years old or more, would have been commonplace in the landscape. Unlike the related species, the sessile oak, the English oak was favoured by medieval farmers for its ability to grow large and produce better quality acorns, on which pigs were let loose in the woodlands to feed during the autumn.

Massive ancient oaks remained a feature of the English landscape. This was despite the increasing influence of man, which led to wide-scale clearance and management of woodlands for agriculture and to make use of the oak’ s timber. However, all this changed in the 15th century when the dominance of these large oaks was suddenly and drastically reversed. The cause was the urgent demand for the largest available timber to support the rapid expansion of Henry VIII’s navy, due to concerns over an imminent invasion by the French. Almost at a stroke nearly all the largest and most ancient oaks were lost from southern England. Again, towards the end of the 18th century, there was a further cull of oaks as, once more, the British Navy needed timber to build ships to fight in the Napoleonic Wars. Ships of this period made an even greater demand of timber than in Henry’s day. HMS Victory alone needed over 6,000 trees for its last refit before the Battle of Trafalgar.

Though this had devastating consequences and today there are few specimens of oak over 500 years old, there are still many trees of medium size and age we can marvel at. A typical oak tree, one around 30-40 metres high, weighs over 10 tons. To maintain itself at this size, remain in a healthy condition and to add a further 230kg of wood each year requires a substantial manufacturing unit. A medium-sized tree produces over 700,000 leaves each year. These leaves provide the tree with a total photosynthesizing surface area of some 700 square metres, the equivalent of three tennis courts, which is needed for the production of nutrients, building materials and energy. The term ‘annually’ when referring to the production of leavesand timber is something of a misnomer, as even in southern England the length of the peak growing season is, at most, just 71 days. Over this period the tree needs to draw 58,000 litres of water up from the ground. Vital minerals are drawn up with this water. One chemical an oak cannot extract from the soil is phosphate, which is essential for producing proteins, including complex molecules such as DNA. Over millions of years of evolution the oak has developed what is called in the trade a symbiotic ectomycorrhizal relationship with fungi. In more everyday terms the fungi is able to dissolve the phosphate from mineral particles in the soil and transfer it to the root-system of the tree. In return the fungi can transfer sugars it is unable to produce by tapping directly into the tiniest of the tree’s roots.

The oak tree provides a habitat for over 1000 species of insect, spiders and other invertebrates. In turn, over 50 species of woodland and garden birds regularly rely on this tree to provide them with energy- rich food. Without this vigilant army of predators an oak tree would be stripped bare of most of its leaves, typically by caterpillars, before the leaves reach full-size and productivity. Birds are programmed to build nests and produce offspring to coincide with the period when the numbers of insects infesting the tree are at their height. As the nesting season comes to an end, birds move on from foraging largely for insects to relying heavily on seeds. Though the number of insects in the tree reduces as the season progresses, there would still be a sufficient number to debilitate the tree and cause it to weaken, and season-by-season this infestation would eventually take its toll. To counter this, the tree has developed a defence. As the leaves mature they start to produce phytotoxins which build up and are stored in the leaves. This makes the leaves unpalatable to caterpillars.

Like all members of the plant kingdom the oak contributes to our survival. Over a year it produces around 235,000 litres of oxygen. In other words, each person would depend on around three oak trees to supply them with sufficient oxygen to survive a year.

During the remaining period of the summer the efforts of the tree are dedicated to its production of the iconic fruits, acorns. Acorns are rich in carbohydrate, which both provides the seedling with a jumbo food supply when it germinates but also makes it an attractive food source to mammals and birds. Two in particular, which also provide the additional advantage to the tree of burying their acorns in the ground, are squirrels and jays. Both are intelligent animals and are capable of memorising where their stashes of acorns were hidden in the autumn when returning in late winter and spring. However, their memories are not perfect and some of the acorns escape retrieval. So from these very acorns mighty oaks may grow!

An oak in winter is still easy to find. Whilst there are no leaves on the branches they will be found scattered on the ground, so look out for one on your walks around the villages.

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