Common alder & English oak - trees - September 2017
From left to right: Common alder, English oak and Common alder. Note that the Common alder on the right has been pollarded so the top / crown is not the usual conical shape. These trees are next to the bus station in Exeter city centre. ===Information about English oak=== English oak is probably the best known and loved of British native trees. It is the most common tree species in the UK, especially in southern and central British deciduous woods. The acorns are not produced until the tree is at least 40 years old. Peak acorn fecundity usually occurs around 80 – 120 years. English oak is a large deciduous tree up to 20-40m tall. In England, the English oak has assumed the status of a national emblem. As common oaks mature they form a broad and spreading crown with sturdy branches beneath. Their open canopy enables light to penetrate through to the woodland floor, allowing bluebells and primroses to grow below. Their smooth and silvery brown bark becomes rugged and deeply fissured with age. Oak tree growth is particularly rapid in youth but gradually slows at around 120 years. Oaks even shorten with age in order to extend their lifespan. The leaves are around 10cm long with 4-5 deep lobes with smooth edges. Leaf-burst occurs mid-May and the leaves have almost no stem and grow in bunches. The flowers are long yellow hanging catkins which distribute pollen into the air. Its fruit, commonly known as acorns, are 2–2.5cm long, borne on lengthy stalks and held tightly by cupules (the cup-shaped base of the acorn). As it ripens, the green acorn takes on a more autumnal, browner colour, loosens from the cupule and falls to the canopy below. Most acorns will never get the chance to germinate, they are rich food source, eaten by many wild creatures including jays, mice and squirrels. Acorns need to germinate and root quickly to prevent drying out or becoming victims of the harvest. Following successful germination, a new sapling will appear the following spring. ===Information about Common alder=== Alder is native to Britain and is also found throughout Europe as far as Siberia. Common name: alder, common alder, black alder, European alder It is conical in shape and mature trees can reach a height of around 20m and live to around 60 years. The bark is dark and fissured and is often covered in lichen. Twigs have a light brown spotted stem which turns red towards the top. Young twigs are sticky to touch. The purple or grey leaf buds form on long stems and the 3–9cm long dark green leaves are racquet-shaped and leathery, with serrated edges. The leaf tip is never pointed and is often indented. The flowers are on catkins which appear between February and April. Alder is monoecious, which means that both male and female flowers are found on the same tree. Male catkins are yellow and pendulous, measuring 2–6cm. Female catkins are green and oval-shaped, and are grouped in numbers of three to eight on each stalk. Once pollinated by wind, the female catkins gradually become woody and appear as tiny, cone-like fruits in winter. They open up to release seeds, which are dispersed by wind and water. The small brown cones, which are the female catkins, stay on the tree all year round. Its natural habitat is moist ground near rivers, ponds and lakes and it thrives in damp, cool areas such as marshes, wet woodland and streams where its roots help to prevent soil erosion. It can also grow in drier locations and sometimes occurs in mixed woodland and on forest edges. It grows well from seed and will quickly colonise bare ground. Because of its association with the nitrogen-fixing bacterium Frankia alni, it can grow in nutrient-poor soils where few other trees thrive. The wet conditions found in alder woodland are ideal for a number of mosses, lichens and fungi, along with the small pearl-bordered fritillary and chequered skipper butterflies, and some species of crane fly. Alder roots make the perfect nest sites for otters. Alder coppices well and the wood makes excellent charcoal and gunpowder. The roots have nitrogen-fixing nodules which make it an excellent soil conditioner. The trees are therefore used to improve soil fertility on former industrial wasteland and brownfield sites. They are also used in flood mitigation.
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