2. What are we trying to conserve?

The most important parkland habitats are those of considerable age and with a long continuity on the site. These include the mosaic of veteran trees and shrubs, and long-established grasslands. Wetlands are also important in some parks.

2.1 Grasslands
The most valuable park grasslands from a tree health and a nature conservation point of view have the following characteristics:

• a 'natural' species composition of wild plants reflecting the geology and soils, rather than a fertilised or agriculturally modified sward;
• several types of grassland, reflecting variation in soils and drainage or other features;
• a species-rich sward, generally gauged by the number of vascular plants - although recognising that some grasslands (e.g. acid grasslands) are naturally poorer in species than others;
• a varied structure, with variations in height and tussock development, and good flowering and seeding, at least in places. In some parks anthills are a valuable element of structure, as well as being important micro-habitats in their own right. Stands of bracken in parks can contribute important diversity.

©National Trust 2004
Dinefwr Deer Park has very species-rich grassland and some of the finest anthills found in any National Trust park. The anthills support acid grassland, while the sward between them is mesotrophic, giving an interesting diversity. The Park is grazed extensively by White Park cattle and is in very good condition.

All these features will affect the invertebrate fauna of the park, not just the species associated with the grasslands but also many of those associated with the veteran trees. The more natural and varied the grassland, the richer the fauna. Many insects pass part of their life in parkland trees, and utilise nectar and grassland plants, as well as other habitats, at other times.

The mix of grassland rich in insects, and scattered mature trees, is attractive to many birds, such as spotted flycatcher, starling and skylark. Anthills attract green woodpecker. Tall grassland is of value to small mammals that may attract birds of prey such as tawny owl and kestrel and provides important feeding habitat for bats.

All these features are significantly affected by livestock grazing and management.

2.2 Trees
Trees, and particularly mature and veteran trees, have long been valued as the most prominent feature of parks. In addition to the many cultural and aesthetic values associated with such trees, they have a very significant importance for biological reasons.

The value of veteran trees has recently been described in detail in 'Veteran Trees - a guide to good management' (Read 2000). Features of nature conservation value include a large girth, trunk cavities, fungal rotting of the heartwood, dead wood in the canopy, sap runs, crevices, fungi creating decay and forming unique associated fungal communities, fungal fruiting bodies and dead wood in various stages of decomposition on the ground nearby.

These features support a unique associated fauna and mycota (fungal communities) and many parks have nationally or internationally important invertebrate and fungal assemblages.

The bark of wood surfaces of trees, especially older trees, support important fungal, moss and lichen (epiphyte) communities. If sheltered habitats have persisted in the park, these have woodland lichen assemblages, along with other ancient tree specialists. Like vascular plants, some species will indicate undisturbed more natural habitats, others will indicate pollution and enrichment.

The holes and crevices in old trees are also important for a wide range of hole and crevice nesting birds including the woodpeckers but also species such as nuthatch, tree creeper, stock dove, tawny owl, starling, jackdaw and many of the tits. Hollow trunks, loose bark and rot holes also provide roost sites for bats with several woodland species being particularly associated with wood pasture and parkland.

The following aspects of the trees in a park have a bearing on the value of the site overall, not just for biodiversity but for landscape and aesthetic qualities too:

Tree cover
The density of trees will vary from park to park and it is difficult to recommend a 'right' tree cover. However, from a biodiversity point of view if trees or tree clumps are too widely spaced, species cannot move from tree to tree and the park loses its integrity. Conversely, if the trees are too dense, the ground habitats become too heavily shaded and other aspects of the parkland character are lost. English Nature and parkland specialists suggest that as a rule of thumb, to satisfy ecological needs an ideal tree cover should be of 20 - 30%.

If the trees are large and full-canopied, a tree cover of 20% equates to about 6 mature trees / ha.

A tree cover any less than 1 mature tree / ha does not give a park-like appearance and is probably unsatisfactory from a biological point of view.

Tree health
It is obviously important to have a population of healthy trees, so that fine stature, longevity and continuity on the site are achieved. Parkland management significantly influences tree health.

It is important to distinguish between the symptoms of natural ageing, and those associated with stress and poor health. As a full-canopied tree ages, it physiologically adjusts the area it has to supply with food and water by reducing the higher part of its crown; hence it will have dead branches at the top and become 'stag-headed'. This is a normal response of a healthy tree to ageing. By contrast, unhealthy trees will have dead branches throughout the canopy. They will often die rapidly, rather than very gradually.

Heartwood hollowing is also a natural feature of healthy trees. Non pathogen fungi relieve the central core of dead tissues without impacting on tree health. The trees themselves benefit from the release of the otherwise locked up nutrients.

Tree age structure
A varied age structure, with young, middle-aged and old trees present, is important to ensure the continuity of trees, particularly old trees. Many National Trust parks have an unfortunate lack of trees of about 70 - 100 years old, so a dearth of ancient trees is likely to become more critical in the future. This makes it even more important to conserve the existing trees in older categories indefinitely.

Young trees are well represented in many National Trust parks. Naturally regenerating trees are of most value from a nature conservation point of view, as species are more likely to show a natural distribution and reflect local variation in soil and environment. However, conditions for natural regeneration are often not provided in grazed parks, so most park trees are planted.

©National Trust/ 2004
Hatfield Forest has many elements of a rich parkland landscape from a nature conservation point of view: a variety of native trees, a good age-range, with fine old-growth trees and pollards, and semi-natural grasslands. Young trees regenerate naturally in the scrub clumps.

Tree species
A number of tree species dominate in parks, most often pedunculate oak, together with other natives such as ash, beech and hawthorn. Non-natives such as lime, sweet chestnut and horse chestnut are common. More exotic species such as red oak, Norway maple and cedars are also often planted and tend to be most abundant in more recently created parks.

Native and long-established species are of most value biologically. They support the greatest variety of mosses and lichens, and invertebrates, both foliage-feeding invertebrates and those associated with decaying wood. However, non-native broadleaves can also be of value, particularly for the fungi and invertebrates associated with decaying wood. The brown rot of oak, for example, is very similar to the brown rot of sweet chestnut. Beech, ash, lime and horse chestnut are also similar in the way their wood decays.

Coniferous species are not generally of nature conservation interest for either the foliage fauna or the wood decay communities.

The flowering shrubs such as blackthorn and hawthorn have abundant nectar early in the year, which is a valuable food resource for many of the invertebrates which breed in ancient trees.

2.3 Dead wood
Dead wood is very important for its associated fauna and as it decays it is essential to nutrient re-cycling and restoring soil nutrients. English Nature has developed favourable condition tables for dead wood habitat in the New Forest. These have been adapted to provide an assessment of dead wood within parks as follows:

STANDING
Good = 1 - 2 large dead trees or trees with large dead limbs (>50cm dia.) visible at any one time.
Average = 1 - 2 trees with medium sized dead limbs (<50>5cm dia.) visible at any one time.
Poor = Trees with dead limbs scarce.
Absent = Trees with dead limbs >15cm absent (evidence of dead limbs having been removed).

FALLEN
Good = 1 - 2 large fallen trunks (>50cm dia) visible at any one time in view.
Average = 1 - 2 large fallen limbs or only smaller material 5 - 50 cm in view.
Poor = Even smaller material scarce in view.
Absent = Nothing >15cm in view.

2.4 Fungi
Only a few parks have detailed information on fungi as yet, but there are undoubtedly important communities associated with park trees and with unimproved grasslands.

The mycorrhizal association between tree roots and fungal hyphae confers many advantages to the tree, including improved seedling establishment, improved growth, survival during key stress periods, mineral and water transport, resistance to fungal pathogens, enhanced reproductive output in low phosphorus soils and even enhanced tillering in some species. The effective foraging distance of a tree root system can be increased to many hundreds of square metres, with fungal connections.

2.5 Parkland soils
Long-established, undisturbed soils under permanent vegetation are often of particular interest and nature conservation value, and are likely to be well represented in parks. More recent soils may also have value, as for example soils preserved under medieval ridge and furrow, which seal and protect horizons reflecting medieval management practices.

Organic layers have the potential to be particularly rich in parks, given good delivery from both ground vegetation and tree leaves and twigs. Parkland soils in their natural state are therefore likely to drain well, be moisture-retentive, have some natural fertility and be well able to survive climatic stress.

2.6 Scrub
Scrub is a vital component of grazed park landscapes. The nectar from shrubs such as blackthorn, hawthorn and bramble, is important for many of the scarce parkland invertebrates that develop in veteran tree habitats. Several scrub species, including elder, are important hosts for epiphytic mosses and lichens, and large numbers of birds are associated with scrub.

As well as being important in its own right, scrub plays a key role in providing protection from grazing to developing tree saplings. Natural regeneration is possible in a park with scrub and bramble patches and similar protective habitat.

2.7 Wetlands
A good variety of wetland habitats is represented in National Trust parks, from large lakes to small ponds, springs, flushes, streams, rivers and canals. These are of most value when they have natural profiles, good water quality, adequate water supply and natural transitions to other habitats. Water quality will be best in catchments which are unintensively used.

Water bodies heavily stocked with fish or ornamental water fowl, or regularly dredged and disturbed, or those with hard constructed edges, are generally of limited or negligible interest.

2.8 Animal dung
Livestock dung is another important feature if parks. Dung is a rich habitat for invertebrates and fungi. Dung invertebrates in turn provide valuable food for a number of birds and mammals, especially bats. In particular, the larger bat species such as noctule, serotine and the horseshoe bats are known to be especially reliant on dung invertebrates.