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Today, 70% of the site is covered by woodland, primarily oak and elm, mostly with a 30 to 50 foot (10 to 15 metre) canopy. Some areas have a canopy lower than this and clearly represent heath that has been over run more recently. All the tall elm trees died in 1976 and 1977, opening up temporary glades, but these were rapidly filled with myriads of elm saplings at densities greater than 1 per square metre in places. The tallest of these saplings had grown to 23 feet (7 metres) around 1992, but a further attack of the disease in the mid- to late 1990s removed these. At present, elm sapling are continuing to thrive in many places, so it looks as if regular cycles of growth and retreat will continue in the future. In most of the high canopy wooded area there is little undergrowth beneath except at the edges.
Tall woodland with few plants on the forest floor is one of the poorest habitat types for birds, insects and mammals. The canopy provides nesting for some birds, and some day-flying moths and butterflies such as the tiny Adela Reaumurella and the Purple Hairstreak (Quercusia quercus) swarm among the tree tops. At Marshall's Heath the woodland canopy's most important function is as a home for the White-letter Hairstreak, a threatened species of butterfly that is declining over much of southern England, which spends most of its life at the top of tall elm trees. Any dead wood is also important as a home for the woodpeckers (see section on birds), and dozens of scarce insects. Another major benefit of the woodland is that it provides shelter for the heathland. When even quite stiff breezes sweep the adjoining farmland, the heathland can be virtually calm at ground level due to the protection of the surrounding trees. This means that butterflies and other insects are active for longer, and thus have more chance of survival. The trees also form a thick barrage against crop sprays from farms to the east and west.
Lawn and Play area
13% is taken up by the lawn and play area, where the grass is regularly cut throughout the summer months. Despite the sterility of wildlife, these areas have potential, since whenever cutting falls behind, a most beautiful crop of diverse wild flowers appears, 16 of which are not found elsewhere on the Heath. Grass that is cut every week or two can support very little insect life, and provides no cover or breeding places for birds or mammals. However, many species lie dormant and the situation would be greatly changed if mowing was reduced to two or three times per year, since the myriad wild flowers would make excellent nectar sources for insects, and there would be sufficient time for some butterfly species to breed.
True remaining heathland took up only 8% of the site in 1992, perhaps less now. It is nearly all found at the southern end, though a tiny isolated patch occurred well to the north of the main area in a clearing just to the west of the road, but this has now been over run with brambles. The wild heathland is the main ecological and conservational value of the site, and a variety of different grasses and wild flowers occur here. The large anthills for which the site is remarkable are confined to the western side of the road only, and take up only 4% of the site. Over the last decade, only the grass on this main area of anthills has been cut annually; much of the rest of the heathland has been cut intermittently or not at all. This means that the grasses on the anthill area can grow to about 5 foot tall (1.3 metres) at the height of summer in normal years, though in the mid-1990s the hot weather and comparatively low rainfall meant that it only grew to about 15 inches (40 cm) in most places.
This is the rarest and most important wildlife habitat to be found on the reserve. It contains one of the best colonies of Yellow Hill Ants (Lasius flavius) to be found in Hertfordshire, and it is here that most of the exceptionally rich collection of butterflies, moths and other insects live and breed.
Approximately 9% of the reserve is now over run by scrub, consisting of bushes, young trees, brambles and other creepers that have sprung up in areas where the grass has not been regularly cut. This will become woodland if left to itself. A small amount of scrub has been intermittently cut in the last few years.
Scrub can be an important site for mammals and birdlife as it provides both cover and secure nesting places for many species of birds. Shrubs are equally important for some species of butterfly, provided they do not get too dense and overgrown. The fact that the Green Hairstreak butterfly has not been seen on the Heath since 1987 may well be due to the fact that the shrubs that lined the western edge of the road in the 1980s became too dense, tall and overgrown at about that time.
The problem with scrub is that it is always a temporary habitat that rapidly reverts to woodland. To maintain the area, bushes have to be cut down every few years. This practice will hopefully provide a home for the Green Hairstreak again, as well as adding extra variety of habitat that would also benefit other wildlife.
This is also scrub, but there is a distinction between scrub which would not revert to heathland if cleared, and heathland that has only recently been lost to scrub and can be reclaimed. The areas that can be reclaimed are being progressivly shaded by trees and bushes, which also drop a carpet of leaves which smother the specialized heath plants and increase soil fertility which will also eventually kill them. However these plants can still be found beneath the carpet of leaves, but they will be killed if the scrub is not cleared. About 4% of the site could be reclaimed as heath by scrub clearance and felling small trees, provided that this is carried out soon.
It is suggested that a good balance of wildlife habitat at Marshalls Heath would be 40% heathland, 40% woodland and 20% scrub. The restoration and re-creation of heathland would present the most difficult, but not impossible challenge, as there have been recent advances in heath restoration which have had increasing success at other sites in Great Britain.
Updated 23 Feb 2003