Leaves and buds of the Pedunculate Oak
There are two native species of Oak growing in Britain – the Pedunculate (or English) Oak (Quercus robur- meaning strong, of the timber), and the Sessile Oak (Q. petraea – of the rocky places). These two species may be easily distinguished respectively by the presence or absence of acorn stalks. Curiously, the leaves behave in the opposite manner, with the Pedunculate Oak usually having no petioles, while the Sessile Oak has stalked leaves. A better means of distinguishing these two is found at the base of the leaves, the English Oak being lobed, while that of the Sessile Oak is un-lobed. In the Bristol area, the Pedunculate Oak is by far the most common. English Oak heartwood is of high quality in strength and durability, and has been in great demand throughout history, especially for the construction of barrels, houses and ships. Oak is very long-lived, some of our veteran trees being over 1000 years old. Most of our old Oaks have been managed by pollarding, that is, the harvesting of the timber by removal of the upper branches. This process can prolong the life of the tree by up to 50%. Good examples of ancient Oak trees may be found at Ashton Court, and on the south boundary of Towerhouse Wood. Both the National Trust and the Woodland Trust use Oak leaves in their logos.
The Greek word for the Oak – dryas (δρυας) may be found frequently in the English language – in words like Druid and Dryad, and also in the scientific names of many of our plants – like Dryopteris, Dryas and chamaedrys.
Acorn of the Pedunculate Oak that have been distorted by the Knopper Gall Wasp
The rapid spread of the Turkey Oak since its introduction in 1735 threatens to displace our native Oak trees. This tree can be identified by the ‘whiskers’ around the terminal buds, and by the cuneate (wedge shaped, hence ‘cuneiform’) leaf bases. It is the alternate host for the asexual Knopper gall (Dutch – ‘knop’ is a bud) wasp Andricus quercuscalicis, introduced in 1961.
The Pedunculate Oak probably supports the largest number of insect species of any British tree. It is also host to a large variety of fungi. Oak mildew (Microsphaera alphitoides) introduced in 1908 can cause the death of young trees by attacking the leaves, so reducing photosynthesis. Of the larger wood rotting Basidiomycetes, Fistulina hepatica (Beefsteak fungus, a brown rot) and Polyporus squamosus (Dryad’s Saddle, a white rot) both occur in Towerhouse Wood. (‘Brown rot’ fungi remove cellulose, leaving lignin, while ‘white rot’ fungi remove both lignin and cellulose.)
The Jay is probably mainly responsible for the dispersal of the acorns, since it has the habit of burying them in caches, failing to recover most. Acorns have had many uses. They were a source of food (pannage) for pigs that are able to tolerate the high concentrations of tannin. In both World Wars, an ersatz coffee was produced from the roasted acorns.
The tannins in Oak bark have been used for many centuries for the production of leather. More recently, other sources of imported tannins have been found, but there remains one company in Cornwall that continues this tradition, using native Oak.
Recent research has shown that the Oak tree, in common with Willows and Poplars, produces significant amounts of volatile organic compounds (VOCs), notably isoprene (CH2=C(CH3)CH=CH2), which interacts with oxygen by complex processes involving methylvinylketone and methacrolein to produce ozone (O3) and peroxyacetyl nitrate (MeCOO2NO2). Ozone is a pollutant in the lower atmosphere, toxic to plants and animals. It also contributes to the greenhouse effect, although in the upper atmosphere it is essential to prevent penetration of harmful UV light. The Ash tree by contrast with Oak produces only small amounts of VOCs, and may even tend to remove them from the atmosphere.