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Habitats in Perivale Wood Local Nature Reserve
The Reserve occupies a total of 27 acres (11 hectares), with a rich variety of habitats:
18 acres (7.3 hectares) of ancient woodland
5 acres (2 hectares) of grassland and pasture
2 acres (0.8 hectare) of damp scrub
2 acres (0.8 hectare) of (relatively) recently disturbed land, which has a very different vegetation from the ancient wood
Records of the Wood date back to the Middle Ages, and it is a remnant of the forest that once covered all of southern England, although it has long been managed by the traditional coppice-with-standards system. It is a traditional English oak woodland, with a rich variety of species; the solitary sycamore is the only tree that is not native.
The 20 or so elms, which were 200-250 years old, succumbed to Dutch elm disease in the early 1970s, and the stumps have now either fallen or been felled for safety. There was a further outbreak in 1983-6, and all of the 20-25 year old suckers that survived the earlier outbreak are now dead.
The Wood was known in earlier centuries as Braddish, Braddidge, or Broad Hedge Wood, reflecting the broad hedgerow along the eastern side of the wood, which marks the boundary between the parishes of Greenford and Perivale. Confusingly, Perivale Wood itself is actually in the parish of Greenford.
The importance of light
Plants are totally dependent on light as a source of energy for the process for photosynthesis, the production of sugars from carbon dioxide. This is why very dense woodland, and especially evergreen woods, have little ground flora, and why more species flourish at the edge of the wood, and in clearings.
The struggle for light is won by the tallest trees, since they can grow above the competition. It is interesting to compare the shapes of oak trees growing in close competition in the heart of the wood with the same species growing in open spaces. There is a very fine solitary oak in the pond-field, and it has a typical spreading canopy quite unlike that of the trees in the wood.
Trees at the edge of the wood receive more light than those in the wood itself, and therefore they have fuller, more luxuriant, canopies. This in turn means that they can synthesise more food, and therefore they can grow faster. Some of the oaks at the wood edge have gained about a centimetre in circumference each year since 1967, when measurements were first made.
The natural regeneration of a wood, and also sustainable management of woodland as a source of timber, depends on the availability of light. Although the Wood itself may be thousands of years old, hardly any of the individual trees are more than a couple of centuries old.
Where a tree has fallen (or been felled), there is a clearing where more light reaches the woodland floor. This means that seedling trees have a chance to grow, together with herbaceous plants that do not normally grow in the dense shade of the leaf canopy. Initially there will be a great many tree seedlings, and in time one or two will grow faster than the others, and come to dominate the area. The deeper shade on the woodland floor will mean that the herbaceous plants will once again disappear as the clearing is filled.
Storms of have done damage in Perivale Wood, although less severe than the damage in more exposed areas. A large number of branches have been broken off, and some of the trees have odd shapes as a result. The effect of this was to increase the amount of light reaching the woodland floor - possibly by as much as 2 - 3% on average. As a result, there are thousands of seedling trees and shrubs, some of the herbs that were rare have become quite common. There has also been an increase in the amount of ivy, cow parsley and hogweed - all plants that are rare in ancient woodlands and hedgerows. At the same time, the fungi that grow on dead wood have flourished.
One of the many gradual changes that is occurring in the Wood is the growth of holly seedlings. Thirty five years ago holly was a rarity, yet now there are hundreds of holly bushes, and some have reached a considerable size. A number of different factors may be involved, including perhaps a change in the bird population, with an increase in a species that carries the berries, a change in the local climate, or the disappearance of the rabbits, which has given more seedlings a chance to grow without being eaten. However, the most important factor is almost certainly the increased amount of light on the woodland floor, which has given the seedlings a chance to become firmly established.
Light on the woodland floor
In summer, the trees cast a deep shadow, and there is little light for the growth of plants on the woodland floor. In winter and early spring, the leafless branches cast very much less shadow, and plants that make all their growth early have a chance to flower and set seed before the leaf canopy is too dense.
Foremost among such plants is the bluebell; Perivale Wood is a typical English bluebell wood, with some 4-5 million flowers in spring. On a calm day in late April or May, the scent of the massed blooms is delightful. Bluebells are long-lived plants, lasting for 20 or more years, as long as the leaves are not trampled in spring, and each year the bulb grows larger; they only flower after several years, when the bulbs are large enough.
Mosses and liverworts can be found throughout the Wood. They obtain their nutrients from material in the water that runs over them, not from the substrate on which they are growing; although they are anchored to the soil or other substrate by rhizoids, these are not true roots. Like the ferns, mosses and liverworts are spore-forming plants, and the sporophyte-bearing lobes are fascinating and beautiful when seen under a hand lens.
The ferns and horsetails are more advanced plants. They are the most primitive of the vascular plants, with vessels that carry water and nutrients up from the roots, and down from the leaves. This enables them to grow considerably taller than algae, mosses, or liverworts; before flowering plants evolved these were the dominant species. The great forests of the carboniferous age, whose fossilised remains are coal, consisted of plants closely related to modern ferns, horsetails, and cycads. The common horsetail can be found abundantly at the northern end of the Paddock, while the male fern and other ferns are common throughout the Reserve - including the tiny adder’s tongue fern.
Management of the Wood
A wooden ship of the line, as used in the Battle of Trafalgar in 1805, required the timber from some 600 mature oaks. Historical accounts show that the last major felling of trees in Perivale Wood was before 1810, and the oaks reflect this - they are of uniform age and degree of maturity; from their girth they are about 180 years old. The Gilbert White Clearing was created in 1965, by felling five large oaks and coppicing several dozen hazels.
Overall, the leaf canopy in the Wood is so dense that there is little natural regeneration of trees. The Management Committee is working on a programme of planned felling over the next 50 years, to create a series of clearings in which seedlings will have a chance to become established, and rejuvenate the Wood. This would have been the traditional style of management - cropping one small area at a time, so as to ensure a succession of mature trees, and natural regeneration of the woodland.
About 19% of the total area of the Reserve (2 hectares = 5 acres) is grassland. Apart form the small area around the hut, it is pasture land, and is grazed in summer, as would have been the traditional grassland management of pasture. Left alone, scrub will rapidly invade grassland, and it would eventually revert to woodland. Grazing, or mowing for hay, prevents this invasion of scrub, and maintains the grassland, although there is still a need for some manual control of invading scrub.
The paddock was a corn field until about 1700, and thereafter was a hay meadow. It is still managed in a somewhat similar manner; the grass is left until late summer, and only then are the horses allowed in to graze it. This means that many of the typical flowers of hay meadow, such as knapweed and cuckoo flower (lady’s smock have a chance to flower and set seed. The flowers are attractive to a wide range of butterflies and moths, and it is not unusual to see half a dozen or more skippers, burnet moths or plumed moths around a single knapweed flower.
If you are patient and quiet, you may see some of the small mammals of the Reserve in the Paddock - shrews, field mice, voles, and harvest mice are all known to be there - but you are more likely to see the remains of a shrew that has been caught by a cat than to see the live animal.
Slow worms (legless lizards) are found in the Paddock and Little Elms Meadow under sheets of corrugated iron that are placed there to provide them with a suitable habitat.
Traditionally, beehives would have been put in orchards and meadows in summer, both to provide a source of nectar and pollen for the bees (and hence a source of honey for the bee keeper), and also to ensure pollination of the fruit trees in the orchard. We maintain this tradition, and two or three hives of honey bees are kept in the Paddock each summer. In addition to these domesticated bees, the Reserve has a good population of wild bees.
Little Elms Meadow is different from the Paddock; it has been a grazed meadow, with unimproved pasture, for as long as records are available, and may never have been cultivated. This traditional management continues, and 3 or 4 horses graze the Meadow each summer; in late summer they are also let into the Paddock.
The name Little Elms Meadow was always misleading. Originally the meadow was bordered by majestic elms at the woodland edge, so it was properly the little meadow of the elms, rather than the meadow of the little elms. It is even less meaningful now. The elms succumbed to Dutch Elm disease in the 1970s.
A new habitat was created in 1958, when a fence was put across Little Elms Meadow and grazing was confined to the eastern part. A large pond was dug, access was restricted, and the area was left to develop. However, a number of non-local, trees, such as lombardy poplar, osier, weeping willow, and Scots pine were planted. Ideas of conservation have advanced since then, and we would certainly not introduce such exotic species today.
The shallow pond in the south-western corner of the meadow was originally dug in 1970, and has been redug, enlarged and dredged several times since then. Part has been fenced off, to keep the horses at bay, and it supports a good variety of pond life). Kingfishers have been sighted here fairly commonly, and an occasional heron.
The hedgerow at the edge of the Wood provides a valuable habitat, different from both the woodland on one side and the open grassland area on the other. Hedges have long been used to mark boundaries, and the Eastern Hedge marks the boundary between the parishes of Greenford and Perivale. They also serve the important purpose of keeping animals in the fields and out of the woodland. Because the Meadow in the Reserve is grazed in summer, this is an important function of the hedges, and a great deal of management is involved in ensuring that the hedges remain stock-proof.
Hedgerow management is also important to ensure that the plants that make up the hedge do not encroach into the meadow. Left to itself, even if grazed, open grassland may revert to scrub, as the hedgerow encroaches on the open space. Examples of this can be seen in the Paddock and in the northern part of Little Elms Meadow, where there are broad areas of blackthorn scrub.
Blackthorn is so called because it is indeed a wickedly thorny plant, and it flowers in early spring, before the leaves have opened. The cold snap, when the blackthorn flowers is sometimes known as the “blackthorn winter”. An area of blackthorn in flower is a beautiful sight. In autumn the sloes, the fruits of the blackthorn ripen. They are small, dark purple and intensely sour and astringent; they are not edible, but have long been used to make the traditional liqueur, sloe gin. The wood is used to make the Irish shillelagh or blackthorn stick.
The traditional method of hedge management is laying. The vertical stem is partially cut at the base, then bent. This promotes vigorous growth from the stool, while the cuts allow the stems to be bent over to form horizontals for the hedge. As the newly laid hedge grows, so it forms a dense stock-proof boundary, and a habitat for birds and small mammals, and the many herbaceous flowers of the hedgerow.
Unlike a garden hedge, a woodland hedge does not consist of just one species. Although the main hedge plants in Perivale Wood are hawthorn and blackthorn, you will also find wild roses, bramble, spindle and other plants, as well small trees such as crab apple, which are left as standards when the hedge is laid. As a general rule, the more species that are present, the older the hedge. The 14 woody species in the Eastern Hedge indicate that it is about 700 years old, which is consistent with references in the thirteenth century to the broad Parish boundary hedge for which the Wood was originally named.
Between the hedge and the wood there is commonly a path to allow access for maintenance of the hedge. This, and the hedge itself, provide a sheltered habitat with good light for a wide variety of small flowering plants - these include many of those that we regard as the typical wild flowers of springtime. The sheltered zone between the hedge and the wood also provides an ideal habitat for a variety of butterflies, feeding on the nectar of the hedgerow plants and laying eggs on plants such as garlic mustard and nettles.
The northern boundary of Perivale Wood is a relatively recently disturbed area; it was used to dump the waste when the Grand Union Canal was dug, and then the area was a Victorian refuse dump for rubbish brought out from Paddington by barge. This area is some 3 metres higher than the rest of the Reserve (there are steps between Nature Trail points 9 - 10, and again along the West Canal Path), and has a very different vegetation.
The main trees in this area are elder, which is a good sign of recently disturbed land, and ash and elm, which grow relatively fast. Elder is a useful plant, as a source of food and wine, dye, wood for flute making, skin cleaner, insect repellent, pith for microscopists, and hard wood for making cogs and skewers. The tree is also valuable to birds, as a source of food in autumn, and as a nesting site in its many holes. The willow tit, which used to nest in the elders, is no longer recorded in Perivale Wood, and indeed is declining nationally. The ground cover in this area is also very different from the rest of the wood. There is a dense carpet of moss under the elders, and at a number of sites on the mound there are colonies of the delightful little pink-beige fungus Ramaria stricta
Along the Western Canal Path there are masses of nettles. Nettles need a rich soil, but as they die down each year, the nutrients are recycled back into the soil. In this way, a single episode of soil enrichment, such as organic matter in the Victorian waste, can lead to a permanent patch of nettles. Along the Eastern Canal Path, ground ivy is well established, as are brambles. Much of the dustheap has been colonised by Japanese knotweed, an extremely invasive garden escape. However, it seems to have reached its natural limit in Perivale Wood, and is not spreading to any significant extent.
Water is an essential part of the natural environment. If the water table in the soil sinks too low, many plants will be unable to survive - after all, rainfall is not predictable, and in a severe drought even deeply rooted trees are at risk, as has been only too apparent in recent summers. Other plants need very damp conditions to survive, and many can only live when they are wholly or partly immersed in water.
Artesian springs result from the collection of rain-water in the chalk uplands that surround the London basin; the North and South Downs. This chalk continues under the clay of the London basin, and is water laden. In places the water rises out of the ground as small springs, which eventually become the many rivers of the area. The combination of urban development, with its increasing demand for water, and a series of dry summers has meant that the water table is falling throughout southern England, and ponds and streams are in danger.
Perivale Wood has damp areas, especially around the base of the Mound, and some parts of Little Elms Meadow and the Bridle Path, where the water table is near the surface. Indeed, in a wet summer there may well be pools in the Meadow for much of the seasons, so that they develop a considerable collection of algae and aquatic insects. The same can happen with the small hollows in the Paddock outside the hut.
The Reserve also has streams which arise just outside the boundary fences, at least one of which actually rises underneath the Grand Union Canal, and then flows along the western edge of the Reserve. Much effort has been expended over recent years to ensure that this stream continues to flow through the land outside the Reserve. It feeds the Upper Woodland Pond and the wet area at the foot of the Mound, the wet land and pond of the Pond Field, and the shallow pond in Little Elms Meadow, before it runs into drains outside the south of the Reserve under the railway embankment.
Some of the life of a pond or stream is immediately obvious to a casual observer the larger plants and beds of filamentous algae, pond skaters on the surface and shoals of small fishes. Dragonflies (above) are a familiar sight flying over the surface of a pond in summer; the closely related damselflies are very similar, but their bodies are more slender. Other pond life is smaller, and dipping a jar or small net into a pond or stream will yield a rich collection of small animals and plants. A handful of the algae from the stream near the hut will reveal several hundred different species of snail plants and animals, ranging from microscopic single celled organisms, like Paramecium (the slipper animalcule) and Amoeba , small colonial organisms like Hydra , to larger species like the Daphnia (water fleas) and Cyclops, the one eyed water flea, rotifers, which move by means of a helicopter-like rotor, and up to relatively large animals like the fresh-water shrimps (up to about 50 mm long) and several types of snail.
Frogs and toads have become rare throughout Middlesex as a result of the gradual fall in the water table since the 1950s, the drainage of many ponds, and more recently, a viral disease that has caused considerable losses of frogs. Increased ultraviolet radiation, as a result of loss of the ozone layer, may also prove fatal to tadpoles in shallow ponds. Nevertheless, frogs spawn regularly in both the Upper Woodland Pond and the pond in Little Elms Meadow.
Copyright © 2015 The Selborne Society. This page updated January 11, 2015