1.2.3 Cultural

The scientific interest of Portsdown is largely due to centuries of interaction of people with the environment. Agriculture, chalk extraction and military construction have all left their mark.

1.2.3.1 Archaeology/past land use

As with many hill sites on Southern England's chalk, Portsdown has been a site of human occupation from prehistoric times. Neolithic, Bronze Age, Iron Age and Saxon burial sites have been found within the S.S.S.I. (Corney, 1967) and elsewhere on the hill, (Rudkin, 1989). The hill forms an important defensive barrier for Portsmouth and so has strategic military importance. During Roman, Saxon and Norman times inhabitants must have taken account of this fact and so it is likely that Portsdown has been under constant human influence for many centuries.

In addition to military exercises and disturbances during the world wars, the hill has been used for leisure purposes. Picnicking and tobogganing are well recorded and large fairs were held on part of the site until the early 20th century.
Although there is some evidence of historic occupation on Portsdown much archaeological evidence was lost during the building of five brick hill forts during the 1860's. The Victorian forts, which dominate the skyline today were obsolete soon after being built. The disturbance that occurred when building the forts and constructing wartime defences has left, in places, a varied soil profile and surface layer.

The most important land use was that of grazing. Tithe maps of 1839 describe the hill as pasture with arable land to the north and south. Paintings and various accounts, e.g., Cobbett, (1830) describe sheep grazing on open grassland. The effect of grazing is discussed in section 1.2.4.

1.2.3.2 Past Conservation management

Conservation management of Portsdown Hill S.S.S.I began in 1991 with the formation of the Portsdown Hill Countryside Management Project, which became the Portsdown Hill Countryside Service in 1997. In the first few years, a small amount of scrub clearance was carried out.

The previous management plan was written in 1994 and identified the following long term objectives:

  1. To maintain existing habitats and associated fauna and flora.
  2. To improve, maintain and increase (to 70%) the area of species-rich calcareous grassland.
  3. Safeguard all notable species.
  4. To provide for public access and educational use of the site and to enhance public awareness and appreciation of downland habitat, except where it compromises objectives 1, 2 and 3.

The most significant result of the plan was the application and implementation of a programme of scrub clearance and grazing supported by a 10 year Countryside Stewardship application. This is due to finish in 2005. An earlier, defaulted, stewardship scheme application, for compartment 11 was reinstated to complete the necessary work there. The project review, section 4.1, outlines the previous management work.

Fencing and grazing has been extended over half the site and limited scrub clearance has been carried out in many areas. The scrub clearance has been aimed at connecting grassland across the hill and preventing scrub encroachment onto particularly species-rich grassland.

Where winter grazing with cattle has occurred the grassland is more diverse especially where there is an active rabbit population. Where there are no rabbits, scrub seedlings are becoming established. All scrub clearance has been followed by vigorous regrowth. Mowing and herbicide spot treatment has kept approximately half this regrowth in check.

1.2.4 Environmental relationships which may have implications for management

1.2.4.1 General considerations of calcareous grassland

Chalk grassland is Britain's most diverse habitat. Many plant species can occur in close proximity to form a short flower-rich grassland. This is recognised as community CG2 in the National Vegetation Classification, (Rodwell, 1992).

A variety of plants in turn support an even greater variety of insects, especially those that require pollen and nectar. The sparse vegetation allows the sun to warm the soil that in turn encourages warmth-demanding invertebrates.

The floristic diversity of calcareous grassland is due to restricted plant-plant competition, (Grime, 1990). The conditions found in chalk downland favour low-growing perennials and other species that can tolerate the chronic environmental stress associated with chalk grassland. Therefore, many species of small stature can grow together.

Environmental stress on plants is due to the interaction of centuries of grazing on an inherently infertile soil. Grazing animals remove nutrients from the pasture and incorporate them in their bodies. With the absence of nutrient replacement, e.g. fertiliser application, downland developed a characteristically impoverished soil. In addition, the site's geology has influenced the vegetation because soils derived from chalk have high calcium levels and a high pH, but tend to be low in nitrogen, phosphorous and potassium. The combination of thin, infertile, basic soil with a poor water holding capacity, plus continual moderate defoliation by grazing leads to the suppression of dominant competitive plants.

The ecology of Portsdown Hill, like all chalk grassland sites, is therefore strongly modified by centuries of extensive grazing and to some extent intermittent cultivation which followed the clearance of primeval forest by Neolithic farmers about 5-6 thousand years ago. Other human activities have led to the variations in soil depth through the formation of trackways and excavations. Patches of thin and deep soil give rise to distinct abrupt variations in vegetation. This is a common feature of chalk grassland, (Ratcliffe, 1977). The vegetation types are correlated with different soil depths. The deeper the topsoil the more vigorous the plant growth.

Without grazing, organic material builds up from uneaten vegetation. Organic matter retains water and nutrients and therefore encourages more vigorous plant growth and therefore the dominance of a few competitive species, especially those which are intolerant of grazing. If grazing is not re-established scrub will invade and dominate until is replaced by woodland. Chalk grassland is considered a plagio-climax in which natural succession is held in check by human intervention.

There is a direct relationship between grazing pressure and vegetation type. Only since the 1960s has the taller vegetation been allowed to develop when grazing by livestock and rabbits stopped. Today's vegetation cover of predominantly scrub and tall species-poor grassland is historically unusual. For much of the last thousand years short species-rich downland dominated.

Chalk downland is a highly threatened, ecologically diverse habitat. However, there are also some positive ecological attributes to scrub and tall grassland communities. Both scrub and coarse grassland provide suitable habitats for many species that would not flourish on open downland. They also improve the wider habitat for some species by providing shelter in adverse weather conditions. Each habitat type is described below with the ecological considerations of restoring chalk grassland.

1.2.4.2 Coarse grassland

Most of the coarse grassland on the site can be described as NVC community CG3 (Rodwell, 1992). The dominant grass, Bromopsis erectus (upright brome) produces a bulky, resistant leaf litter and grows outwards from tussocks. As the expanding tussocks join, the grass dominates the sward, (Austin, 1968a).

Bromopsis dominated grassland, (CG3) is seen as a successional stage of Festuca-Avenula species-rich grassland, (CG2). CG2 will develop into CG3 in the absence of grazing. It is generally agreed that this is reversible.

The abundance of scrub on Portsdown suggests that bushes invade the coarse grassland, however this may be slowed by a large population of Microtus agrestis (Field Vole) that eat the bark of developing scrub, (Smith 1980). The thatch of dead litter in Bromopsis erectus (upright brome) grassland provides ideal habitat for the voles. The leaf litter may also inhibit scrub colonisation.

Tall grassland communities have a lower botanical diversity than the short downland communities they replace. However, they do add to the overall habitat range. For instance they provide larval food for species like Melanargia galathea (marbled white butterfly) and over-wintering sites for many invertebrates. Tall grassland also provides shelter and food for small mammals, which in turn supports a range of predators. If a full complement of downland wildlife is to be retained, the site should be managed to retain some tall grass in order to add structural diversity to the site.

Management techniques for increasing the diversity and reducing the vigour of coarse grassland on chalk include: burning, mowing and grazing.

Burning

Burning is not appropriate on Portsdown Hill, as it is unsightly and too near housing. Deliberate grass fires are common on the site therefore, it is unwise to encourage the practice. Several ecological problems are associated with burning, they centre on the detrimental effects on the insect fauna and the invasion of burnt ground by ruderal plants.

Fire will remove the build up of leaf litter but the Bromopsis will re-grow very quickly from the fire resistant tussocks. Unless grazing/mowing occurs the sward will soon be once again dominated by Bromopsis.

Mowing

Mowing can be an effective means of removing the choking layer of dead leaf litter and reducing the vigour of dominant species, especially Bromopsis erectus. The results of mowing depend on the timing and intensity of the operation. Cutting large areas on a particular day will have a disproportionate effect on those species that happen to be at a vulnerable stage of their life cycle when the grass is cut. Mowing can create a uniform sward that limits the diversity of insect and plant communities, therefore the timing of cutting has to be carefully regulated, so that a constancy of habitat is maintained.

The aim is to have the maximum effect on Bromopsis erectus. Wells (1971) recommends mowing the first growth of the year in an effort to deplete the plant's energy reserves. Most research recommends that conservation mowing is carried out in autumn after seed is set, Crofts and Jefferson, (1999).

As a short-term measure mowing is a useful management technique that can retain the short-turf species and prevent invading scrub from becoming established. To be effective in producing soil impoverishment and a variable sward height, mowing has to be carried out in a controlled pattern. The benefits of mowing can be further increased by harrowing to break up the dense Bromopsis erectus (upright brome) sward. Breaking up the sward is necessary to produce gaps for the seeds of downland plants to germinate.

In order to increase the diversity of the grassland it is essential that the cut grass is removed to prevent shading and soil eutrophication - in effect, a form of haymaking is necessary. There are practical problems with this on much of Portsdown Hill, as it is too steep to use mowing machinery. Only reasonably flat areas could be managed by tractor-mowing, the rest would rely on pedestrian mowers and hand-raking. Even small mowers would not work on many parts of the hill as the ground is not sufficiently level due to tracks, anthills and depressions.

Grazing

From an ecological standpoint, grazing is the most effective management tool. It was continuous grazing that produced downland and maintained it for hundreds of years. The response of grassland to grazing centres on what is eaten (defoliation), where the livestock walk (trampling) and what they leave behind (manuring). These factors vary in their effect according to the timing, intensity and duration of grazing and the type of grazing animal.

Grazing - defoliation

The herbage that livestock choose to eat affects botanical composition. Some plants flourish in grazed grassland because they are tolerant of, or resistant to herbivory. Tolerant species such as plantains (Plantago) have growing points close to the ground while other species simply grow quickly and produce many seeds. Plants that are resistant to grazing are often unpalatable e.g. Thymus (thyme), poisonous e.g. Senecio jacobaea (ragwort) or are spiny e.g. Cirsium (thistles). Plants that are actively selected by grazers and are unable to grow back quickly will eventually be driven from the site.

The nature of the grazing has important implications for the invertebrate fauna. The correct grazing pressure can retain short species-rich turf whilst maintaining areas of tall grass habitat. An uneven sward, with a variety of microhabitats is suitable for a wider range of species. Livestock remove vegetation gradually and so do not have the catastrophic effect on invertebrates, as mowing. Small mammals and reptiles also benefit from a range of vegetation densities.

The timing of any grazing will also affect the grassland's response. Many plant species have flowering periods restricted to only part of the growing season. If grazing animals remove all the flowers then the recruitment of new seedlings will be adversely affected. In plants with long-lived seeds and/or perennial species the loss of a year's seed will not affect the population, however, it may affect any invertebrates that are dependent on the seeds or flowers. Rhinanthus minor (Yellow rattle) is an example of an annual plant with short-lived seed that will be reduced by early summer grazing. The grazing preferences of different stock are another important factor.

Grazing - trampling

Livestock, especially cattle, break up accumulated dead vegetation and create bare ground as they move about the site. A low level of such ground disturbance is beneficial as it stimulates the regeneration of plants from the seed bank. Patches of soil exposed to the sun's warmth are important during the underground larval stage of many invertebrates. Where excessive trampling by heavy livestock occurs susceptible plants may be lost.

Plants are more sensitive to disturbance when they are actively growing. Therefore, the timing of grazing has important considerations. Heavy grazing at the beginning of the growing season will adversely affect one suite of species whilst leaving other (late growing) species little affected.

There are small areas of species-rich grassland scattered across the site. Where animals are obliged to walk over these areas due to the funnelling effect of scrub the risk of damage by trampling is increased. Bushes have been cleared to open these bottlenecks and alternative routes through cleared scrub made available.

Grazing - manuring

Chalk grassland is a nutrient-poor habitat. Livestock deposit urine and dung and therefore have the potential to raise the nutrient status and thus cause a shift in the vegetation towards coarse competitive species. Provided that supplementary feeding does not occur and livestock only eat vegetation from the site all they are doing is concentrating some of the nutrients that are already present. When they leave the site they are, in effect, removing nutrients and perpetuating conditions that give rise to downland by reducing soil fertility.

Whilst grazing animals lower the nutrients within a system they can increase the rate at which they are recycled. Nutrients locked up in dead or old vegetation are available for new growth. Where grazing has been established cattle have cleared much of the ivy from the scrub and deposited a proportion of the nutrients on other parts of the site.

The physical removal of dung may speed up the export of nutrients although it has to be remembered that there are many invertebrates and fungi that benefit from dung, as do the predators that in turn feed on them. It is important that livestock are not wormed using a prophylactic bolus that administers an Avermectin based compound. This drug persists in the dung and prevents invertebrates from colonising.

With the right grazing pressure all habitats can be retained and the need for other active management greatly reduced.

Stocking rates and timing of grazing

From the experience of the first 5 years it takes approximately 8 weeks of winter grazing at the stocking rate of 1 cow per ha to remove the annual growth of grass. The time taken to do this is less than it was at the outset of grazing because the dominant grass is now less vigorous and produces less biomass. Also much of the fodder value associated with the scrub (ivy) has been eaten and it is unable to regenerate to pre-grazing levels.

A guide to the carrying capacity of calcareous grassland is 0.25 LU/ha/yr, see Crofts and Jefferson (1999). A LU (livestock unit) is 550 kg of animal and is a means of comparing livestock of differing age and species. e.g. 4 adult ewes at 60 kg are equal to a 1 year old beef animal at 240 kg - both equal approximately 0.5 LU, Crofts and Jefferson, (1999).

The cattle that have grazed Portsdown Hill have been approximately equivalent to 1 LU, giving a stocking level of 0.16 LU/ha/yr. Agriculturally improved grassland rates are several times higher (2.0 LU/ha/yr) than this which suggests that the grassland on Portsdown is unproductive. However, it has to be remembered that much of the area is covered in scrub and there is little grass regrowth during the winter when the site is grazed.

On dry south-facing slopes like Portsdown Hill, winter grazing with cattle has produced the desired effect of suppressing the Bromopsis erectus (upright brome), and opened up the sward and allowed other species to grow. Grazing has been restricted to a relatively short time during a time of year when most plants and animals are dormant. Although there is some evidence of poaching on paths there seems to be little trampling damage on the best areas of grassland. Existing areas of fine downland turf retain their characteristic species e.g. Thesium humifusum (bastard toadflax).

From a grassland management perspective winter grazing at 1 cow per ha is increasing botanical diversity and suppressing Bromopsis. Scrub is not controlled by cattle grazing, neither are scrub seedlings that have established themselves in the grassland. In order to control scrub a more elaborate grazing regime is necessary for example grazing during the growing season and the use of scrub-eating livestock such as goats.

Grazing - choice of livestock

Each livestock species has unique grazing characteristics that will in turn have a distinct effect on the vegetation. The essential characteristics for sheep, cattle, goats and ponies are given below. Differences in breed and age also affect grazing behaviour.

Sheep: Sheep are the preferred animal at many sites, (Bacon, 1990, 1993). Their nibbling mouth action is recognised as producing the best (i.e. finer) downland turf. They are capable of grazing on steep slopes and cause less soil erosion than larger animals. They are not as susceptible to the toxic effects of ragwort as other livestock and some breeds will eat a certain amount of scrub.

They are selective feeders that tend to take flower spikes but leave grass stems, tussocky grass and dead vegetation. Unpalatable species tend to be avoided. A management consideration of sheep is the threat of dog worrying and likelihood of becoming entangled in brambles.

Cattle: Cattle are good at removing coarse grass and feed non-selectively by wrapping their tongue around the vegetation and biting it off. Feeding in this way they have produced a short sward on Portsdown. They are less susceptible than sheep to dog worrying and other problems caused by the close proximity of an urban area. Cattle will push deep into scrub looking for ivy and therefore make subsequent scrub clearance much easier. If droppings are to be removed, they make this procedure much easier by producing cowpats. They can easily cause excessive trampling, especially in wet weather.

Goats: Goats graze, strip bark and browse. They will eat a variety of scrub and herbaceous vegetation often concentrating on one particular type of vegetation for a while before turning their attention to something different. From trials with domestic goats on the site and in Fort Widley as well as with semi-feral goats in Fort Southwick it is clear they are efficient at controlling scrub of any type and grazing off rank grass. There is plenty of scope for their use on the site. Like sheep, they are vulnerable to dog worrying.

Goats heft, that is they tend not to stray from a chosen location. On Portsdown they moved barely 200 m from where they first introduced to the site in 3 months. This means they graze and browse a small area thoroughly.

Horses and ponies Horses can graze very close to the ground due to their forward pointing incisors and so potentially have a role in managing chalk grassland. On a nearby field a good mixture of downland species is maintained by low level horse grazing. However, they are not usually recognised as suitable grazing animals. A commonly seen situation is that of poached overgrazed horse paddocks that are prone to Senecio jacobaea (Ragwort) invasion. They are known to produce localised concentrations of nutrients by dunging in selected areas. This would be a problem if they were to choose a patch of species-rich grassland. Shod horses would soon damage sensitive grassland so are not suitable.

Low level horse grazing may be an appropriate option provided adequate control is maintained, (Gibson, 1996).

Grazing - wild grazers There are roe deer (Capreolus capreolus) on the site, but too few to have a noticeable effect on the vegetation. Rabbit grazing is significant at the western end of the site, i.e. compartments 11, 1 and 2. Few rabbits are found east of compartment 3. The rabbit population has increased considerably in the last five years to the extent that it has reduced the need for grazing. The potential for rabbits to influence the need for grazing management is considerable and rabbit numbers should be monitored.

Grazing - summary of ecological considerations The effects of grazing depend on when the site is grazed, the stocking rate, and the animals used. This is due to the interaction of the effects of grazing with the life strategies of the different plant species. In principle, as long as the grazing is not prolonged, intensive and simultaneously applied to the entire site, it will be beneficial. Ongoing defoliation and nutrient removal by grazing animals is the best way promoting a grassland habitat. The choice of livestock and grazing regime should reflect the aim of gradually reducing coarse vegetation without damaging the remaining patches of species-rich grassland.

1.2.4.3 Scrub

In the last fifty years the area of scrub on Portsdown has increased from 5% to over 60%. Bushes and brambles are replacing chalk grassland and most grassland will be gone within a few years if no management were to occur. All grassland areas have small, but established scrub plants and the rate at which it is spreading is increasing. Except where scrub control has been carried out the remaining patches of grassland are becoming isolated.

Once scrub is established it spreads effectively as it increases the fertility of adjacent soil through leaf and seed fall. Scrub species are deep-rooted and so can draw moisture from deeper in the soil profile than grass and herbs. This means that plant growth and organic accumulation continues, even when drought restricts the growth of herbaceous plants.

Although a threat to calcareous grassland, scrub is also a valuable habitat for many insects and birds. Many species are associated with the scrub edge habitat and downland species benefit from the shelter provided by bushes. Scrub adds value to a site's conservation value. However, the habitat associated with scrub, changes as it grows. If scrub is to be retained on a site there is a case for managing it on a coppice cycle, (Oates, 1990).

Advantages and effects of coppicing scrub:

Scrub regrowth forms a distinct, if temporary, habitat it is own right and benefits invertebrates such as Bush Crickets. The flowering ruderal plants that occupy recently cleared ground provide nectar and pollen for a variety of insects.

Whilst a programme of long-term scrub coppice may have some advantages it poses the problem of disposal of arisings. They can not be left on site as they will lead to a localised build up of nutrients and so encourage coarse vegetation.

Converting scrub back to grassland Effective scrub clearance is difficult. Most scrub species coppice vigorously when cut and so it has to be uprooted, poisoned or repeatedly defoliated in order to kill it - sometimes all three as it often regenerates from severed roots. If it can be removed, it leaves behind an area of enriched soil (laden with scrub seeds) that does not favour re-colonisation by downland species.

The most appropriate method of clearance depends on the structure of the scrub. Individual bushes or discrete scrub blocks surrounded by intact grassland are best felled and removed intact and the stumps treated or extracted. Large expanses of springy thickets are best shredded in situ with a tractor-mounted flail.

Disposal of arisings creates as much work as cutting down the scrub. They can not be left on site so they have to be burnt on an area that was previously dense scrub, and the ashes removed. Otherwise, the material has to be removed for composting. Finances and slope permitting a collection machine can be used, otherwise they have to be raked up.

Inevitably, much of the scrub is a mixture of established shrubs surrounded by a younger halo of privet, clematis and bramble. Therefore, a staged clearance is more appropriate. The mass of tangled, pliable scrub can be flailed out of the way revealing the more substantial bushes that can be treated separately. The steepness of the slope and the proximity of extraction point and/or species-rich grassland influence the choice of technique.

The scrub is concentrated on the lower slopes suggesting there is a positive relationship between soil depth and scrub growth, McIntosh, (1997). The removal of scrub and reversion to chalk grassland is likely to be more successful if the scrub control reflects the distribution of the least overgrown areas. Where scrub is to be retained the densest scrub should be left or coppiced as this will be the most difficult to return to grassland.

Where scrub has been cleared to ground level the site has been smothered in vigorous bramble-dominated regrowth after a growing season. There will also be opportunistic ruderal species, e.g. groundsel and thistles. There will also be a small number of downland plants which germinate from the seed bank, e.g. violets. In order to achieve grassland on these areas regular mowing with clearance and/or grazing is necessary to stunt the scrub species and favour grassland plants. This can take years or even decades.

Mechanical excavation of scrub roots and the enriched soil has the advantage of reducing the follow up work necessary to regenerate grassland. It is straightforward to reseed the stripped ground with seed collected from the same site and so retain genetic variation associated with the site. It poses the problem of extraction and disposal of material. The scale of the operation also requires vehicular access. It is important that transportation routes are aligned to avoid areas of intact species-rich grassland.

Goat grazing can quickly reduce the vigour of scrub, and given time even kill it. In a relatively short while it can make it easier to clear scrub. For comments on goat grazing see the discussion on livestock in 1.2.4.2.

Whilst it is clear scrub cover should be reduced, the previous comments also suggest that some scrub should be retained, but if it is left unmanaged many of its positive aspects will be lost. The conservation benefits of scrub can be maintained in a much reduced area. The greatest biological value of the site is found in chalk grassland.

1.2.4.4 Woodland

Woodland has developed on the deeper soil at the base of the slope. Large trees occupy a small area of the site, adding both visual and ecological interest. It would be difficult and undesirable to attempt to return woodland to grassland and so these areas have been designated as woodland in the plan.

Where seed-bearing Acer pseudoplantaus (Sycamore) and Quercus ilex (Holm oak) occur they are spreading to both grassland and scrub and are capable of dominating any stands of trees. Both trees have few associated invertebrates and their presence has serious implications on the site's biodiversity. Unless Sycamore and Holm oak are controlled before they are old enough to set seed, they will invade all other habitats.

Native tree species with many associated insects e.g. Fraxinus excelsior (ash) and Ulmus sp. (elm) have established themselves and where woodland is to be retained species such as these can be left whilst Sycamore and Holm oak are felled. Felled timber that is left will support deadwood dependant invertebrates and fungi. If there is little risk of trees falling on walkers consideration can be given to producing standing dead timber by ring barking trees that are to be removed.

Woodland management under the high voltage powerlines should aim to avoid the need to cut back trees and scrub during the growing/nesting season. A form of coppicing would be more appropriate as it would diversify the woodland and prevent the need for poorly timed emergency tree-cutting operations.

1.2.4.5 Species considerations

It is notoriously difficult to identify all the environmental requirements of a species. Using Lysandra coridon (Chalkhill Blue Butterfly) as an illustrative example, it shows how involved some life cycles are. Hippocrepis comosa (horseshoe vetch) is necessary as a larval food-plant, certain ants belonging to the genus Lasius and Myrmica protect the larvae, a range of flowers are needed to provide nectar sources for the adults. Sheltered roosting sites in tall vegetation greatly assist its survival. Other controlling factors are predators and pathogens that attack it throughout its life cycle. In addition, the weather has a dramatic effect on the breeding success. A very bad year can wipe out a weak colony. Once insects have failed to breed on a site they have to re-colonise from elsewhere. Unlike plants, insects cannot survive adverse conditions as seed.

Management cannot fine-tune the environment to suit a few desirable species. It has to provide a habitat in which the desired appropriate species can live in association with everything else in an ecosystem. The ecosystem that developed on Portsdown Hill did so under a form of low-intensity livestock based agriculture and the best way of retaining it is to reintroduce and maintain a similar form of management.

For the purposes of illustration, listed below are the important environmental factors associated with several of the key species found on Portsdown Hill:

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