3. Threats to parkland ecology - what can damage the features of value?

Livestock grazing is essential in conserving the array of important and valuable features outlined in the preceding section. However, unfortunately grazing and aspects of livestock management can cause a great deal of damage to parks.

Generally, extensive grazing with limited numbers of stock is the best way to conserve parks, whereas intensive grazing, with large numbers of animals and all the associated practices such as fertilising swards and supplementary feeding can destroy them. This section outlines the effects that various grazing and other agricultural management practices can have on parks.

3.1 Stock management

3.1.1 Stocking rates
As indicated above, whereas grazing is needed in parks, there is a great difference between grazing extensively with low numbers of animals, and grazing intensively with high numbers. High densities of livestock have many damaging effects on parklands, notably:

1. Trampling, compaction and eutrophication (enrichment) of soils around trees. These result in soil damage and change leading to inadequate water and nutrients for the tree and loss of fungal mychorrhizae. Loss of tree health and premature death result.
2. Bark stripping and rubbing by livestock. This can result in a decline in the health of the tree, or death in younger trees.
3. Heavy and prolonged grazing results in a dull and species-poor grassland sward with a uniform lawn-like structure.
4. Eutrophication of bark from deposited urine and dung leads to growth in algae and loss of ephiphytic lichen and bryophyte flora.
5. There is no likelihood of natural regeneration of trees - all trees have to be planted (and fenced).
6. Erosion of soil, and run-off of both sediment and nutrients, into lakes and other wetlands, causing rapid infill and pollution.

'Ideal' stocking rates were not recommended in this study, but 'high' densities could be anything greater than 1 Livestock Unit (= 1 dairy cow or equivalent) / ha . From an agricultural point of view this is extremely low, and this should be recognised.

Under-grazing can be a threat in parks, leading to coarse, species-poor grasslands and wetlands, and excessive bracken or scrub invasion. Undergrazing is not generally a threat in National Trust parks at present (being of more concern in wood pastures).

3.1.2 Stock types and breeds
The type and breed of stock have less effect than the sheer numbers. However, some problems can be linked with the type of stock, for example:

• commercial breeds may not maintain healthy condition on unimproved grassland swards. Pressures may arise for improving productivity, and supplying supplementary feed, which are incompatible with parkland conservation;
• heavy or continuous sheep grazing causes more damage to grasslands than heavy grazing by cattle. Sheep graze very low and tight, and few wild plants can tolerate this. Sheep also shelter within tree hollows, extending the area of damage;
• grazing and browsing by horses can cause particularly bad bark stripping;
• cattle tend to lie up in the shade of trees and cause the damage to soils etc. already outlined. It is possible that native cattle breeds would disperse themselves better than commercial breeds, but there is little information to support this at present;
• cattle may be more likely to have supplementary feed than sheep or ponies, with all the associated problems as outlined below.

If there is a threat from invasion of coarse, unpalatable grasses, or bracken, then certain types or breeds may be necessary to tackle it, often hardy breeds of cattle being best.

Overall, hardy and traditional breeds of cattle are the most suitable livestock for parkland grazing, with or without the addition of sheep at certain times of the year.

3.1.3 Grazing season and period
Grazing season and period do not generally constitute a threat - again numbers of animals are more significant than the period when they are grazing. However, as a general rule of thumb longer periods of lower intensity stocking are less likely to cause damage than short periods of heavy stocking. The 'long and low' regime tends to create a more diverse sward structure.

Winter grazing can have damaging effects on parklands if supplementary feeding is required - see below. Winter grazing can also be associated with trampling and poaching damage.

If there are threats from invasive species, the grazing period might be critical in control, e.g. grazing in June and July helps control bracken.

3.1.4 Supplementary feeding
Feeding livestock from ring feeders is a common practice in parks with over-wintering stock, and it may also take place through the summer. Feeding sites are often placed in sheltered conditions under parkland trees. This leads to a number of damaging impacts similar to those listed in connection with stocking rates above, although in this case more localised and intense. Soil erosion and enriched run-off are also associated with ring feeders. A similar concentration of animals can occur around water troughs and hence these need to be carefully located.

3.1.5 Use of veterinary products
Herbivore dung is an important habitat within all grazed ecosystems, including parks. Treatment of livestock with veterinary medicines to control parasites can have a significant impact on the dung-associated invertebrate fauna and hence mammal and bird species that feed on this. Avermectins are a group of such medicines that have become widely used in livestock since the 1980s. The most commonly used drug is a semi-synthetic derivative of avermectin known as ivermectin. Concern has been expressed that ivermectin and related veterinary products remain active in the dung of treated animals for a considerable time. This is known to have an adverse effect on dung flies and beetles, with larvae and emerging adults being particularly at risk.

Routine antibiotic use in animal husbandry is also causing concern at present due to the likely impact of these on soil micro-organisms, and in particular fungi with mycorrhizal associations with trees. More information about this relationship is needed.

©National Trust/ 2004
Stinging nettle rings under trees are characteristic of heavily stocked parks and are generally associated with poor condition for nature conservation and poor tree health.

back to top

3.2 Agricultural practices associated with grazing management
Various practices associated with livestock grazing have impacts on parks. It is often the case that the more intensive the stock management, the more damaging the associated practices.

3.2.1 Fertiliser use

Effects on grasslands
Application of fertiliser reduces the floristic richness and interest of grasslands. Most grassland plants thrive best under natural conditions of nutrient impoverishment. When this is altered by addition of fertiliser, a few robust and fast-growing species, respond and soon out-compete the majority of other plants. "Weedy" plants such as broad-leaved dock and creeping thistle will also thrive under conditions of enrichment. The aesthetic and biodiversity value of the grassland is very soon lost.

Slurries and manures have a similar effect in this respect to inorganic fertilisers - in fact slurry is particularly damaging. Other organic fertilisers should also be avoided.

Even low levels of fertilisers and manures, which might be deemed necessary to maintain agricultural productivity, will damage conservation interests.

Effects on soils, fungi and trees
Excessive or prolonged use of artificial fertiliser can cause damage to soil. Addition of nitrogen and phosphorus (especially the former) causes a short-term explosion in the number of certain microbes, which consume the organic matter in the soil. Once this is depleted, the soil loses inherent fertility, water retentive capacity and structures which aid free drainage. Waterborne pathogens such as Phytophthora can move very quickly through such damaged soils.

Parkland trees will be more prone to drought stress (or waterlogging) on soils which are fertilised.

The soil fungi will be detrimentally affected along with much of the rest of the soil biota. Although this subject is poorly understood at present, there is a direct positive relationship between fungi and tree health. Once the fungal mycorrhizal connections with tree roots are destroyed, the trees will be even more prone to stress from drought, pathogens and physical damage.

3.2.2 Silage and hay production; cutting, rolling, harrrowing and topping
Silage production is associated with heavy fertiliser or manure use, and thus the problems outlined above. Silage is often also associated with re-seeded swards. Overall it is not appropriate in parkland.

Hay production without fertiliser and using the 'natural' sward could conserve the flora of particular types of grassland, but the sudden loss of nectar source and the lack of varied year-round structure mean that this is not optimum management for invertebrates nor various other animal groups.

Both silage and hay require access by machinery with the associated problems outlined below. There are similar reservations with rolling, harrowing and topping.

Some parks are managed by cutting if grazing cannot be arranged. If limited to one or two cuts a year, with areas left on longer rotation, and most cuts being late in the season, this can be satisfactory in conserving grassland flora.

Cut material should be removed. If it is not, it acts as a smothering mulch and it can cause undesirable enrichment of the soil.

If topping is necessary to control weeds, it should be kept to the minimum necessary. If bracken control is deemed necessary, by cutting or rolling, then particular care should be taken not to allow the machinery to damage trees or saplings.

Rolling, harrowing and topping will all tend to produce a homogeneous sward which is not the type of structure valued in parklands. These operations are generally inappropriate for an extensively managed grazed grassland, quite apart from the damaging effects of machinery access.

3.2.3 Arable cultivation or cultivation for re-seeding
Although this report concentrates on livestock grazing, it is worth noting that cultivation in parks causes considerable damage, if not destruction. Ploughing around parkland trees has a significant impact upon tree health leading to die-back and premature death. Rarely if ever is dead fallen timber tolerated and left to rot within cultivated parkland.

Where trees shade the crop, it is common practice to limit the tree crown size as much as possible. Tree damage from farm machinery is most severe in arable parkland. Arable farming is also often in conflict with the establishment of new trees. Indeed it might generally be stated that healthy parkland and arable farming, or any form of cultivation, are incompatible.

Cultivation leads to soil erosion and loss to water courses, causing pollution and rapid infill of lakes.

3.2.4 Use of farm machinery
Threats to parkland from machinery include damage to trunks from collisions (occurring regularly in some parks), tree surgery and dead wood removal deemed necessary for access, and soil compaction around trees which directly damages the soil, the roots and their fungal associations, leading to poor tree health or death.

Machinery should not be allowed in the vicinity of trees. It should not be allowed to damage the roots. These extend a long way from the trunk, even for tree species with tap roots. Thus machinery should not be allowed under the 'dripline' of the canopy, or not within a distance 15 times the diameter of the trunk, whichever is the greater (ancient trees often have small canopies, hence the canopy rule does not always hold).

3.2.5 Use of herbicides
Herbicides are not a great threat unless agricultural management is intensive, and they are used in blanket applications, in which case they will damage the flora of grasslands, young trees, wetlands and other features.

3.2.6 Drainage
Again, this is not a great threat to parklands at the present time, although in the past elaborate drainage systems in parks must have altered the ecology considerably. Wetlands should be valued and left undrained. Drains are sometimes repaired with the aim of enhancing the water supply to a lake. However, delivery can then become seasonal and in fact less reliable. Repair of drains will generally have a damaging effect on water quality, if run-off is speeded up and water is not allowed to filter slowly through soil and vegetation. Sediment loads will be greater and lakes in parks will become silted up more rapidly.

Not necessarily linked with livestock grazing and so beyond the scope of this report are other threats to wetlands noted earlier, including heavy stocking with fish or waterfowl, and regular dredging of lakes. All these can cause serious and long-term deterioration in water quality.

back to top

3.3 Tree and woodland management
The threats to trees in parks result chiefly from the practices outlined in the preceding paragraphs. However, there are additional threats to trees resulting from management associated with the trees themselves, as follows:

Tree removal
Wide spreading tree crowns and fallen dead wood restrict access by stock and farm machinery, and shade grassland and crops. In parks where agricultural production is a main objective trees or branches and dead wood are often removed. Planting replacement trees is not given a high priority, and establishment of any which are planted can be poor. It is very noticeable that parks in which intensive grassland or arable production is practised have the lowest densities of parkland trees and those trees that are retained tend to be of least nature conservation value.

Dead wood removal
The dead wood habitat within a park is removed or damaged for a number of reasons, in addition to the agricultural ones mentioned above. These include:-

• Removal of fallen dead wood for use or sale as firewood.
• Variable interpretation and implementation of safety policy concerning dead trees and tree limbs.
• Removal of dead limbs and standing dead trees for visual and aesthetic reasons.

The net result is significant depletion of a valuable nature conservation resource and depletion of soil nutrients, the latter impacting on tree health.

Scrub clearance
Scrub has little chance to develop where stocking rates are high. However, removal for 'tidiness' purposes, although not noted in the parks in this study, has been known in other parks. This not only compromises a valuable habitat in its own right, but an effective tree nursery for natural regeneration.

Fencing veteran trees
To protect veteran parkland trees from stock or other damage, fences may be erected around individual or groups of veteran trees. Whereas this may have some benefits in protecting the trees from certain forms of damage there is a danger that in the absence of any grazing, trees will become encased in dense scrub growth. This can lead to shading of epiphytic lichen and bryophyte flora, and can have adverse effects on dead wood and veteran tree associated invertebrates. Fencing should be seen as a short term measure not a long-term solution to the underlying management problem.

3.4 Threats to wood pastures associated with parks
Historically, many parks were associated with adjacent areas of wood pasture. These shared characteristics with the parkland, but were more densely wooded and less heavily grazed - objectives could have included timber production, but not deer management, for example.

There has been a move over the last century or so to separate the denser wood pasture from the scattered tree covered areas of the park. This has resulted in the wood pastures becoming fenced and managed more intensively for timber production, with former wood pasture and veteran trees sometimes being engulfed within the timber plantation. In the park, management has been directed more towards landscape and ornamental considerations. This separation of management objectives tends to result in the loss of both the historic context and the nature conservation value within both the former wood pasture and the park.

back to top