1.2 Environmental information
1.2.1 Physical
1.2.1.1 Climate
There are few weather readings available from the site. The nearest meteorological recording station is on Southsea Common, 7 km to the south. The highest monthly average temperature occurs in July and August (approximately 17.5OC). The lowest monthly average is in January and February, (approximately 5.0OC).
Portsdown Hill S.S.S.I. is a warm site in an area that enjoys warmer weather than much of the country. Winter temperatures remain higher than inland sites due the proximity of the sea and large urban area. Its south facing slopes are protected from cold northerly winds and therefore are noticeably warmer than the surrounding area during cold weather. Shelter from wind is easily found within the scrub whatever the wind direction.
Extremely high summer temperatures occur due to the southerly aspect and the shelter provided by scrub. The average yearly rainfall is 800 mm, which is 100 mm higher than the coast.
1.2.1.2 Geology
Portsdown Hill is the product of an anticline in Upper Cretaceous chalk (84 to 90 million years old). It is considered an outlier of the South Downs. The chalk forms a continuous stratum that outcrops to the north to form the South Downs and to the south where it forms chalk cliffs on the Isle of Wight. A borehole found the chalk to be 400 metres thick.
The highest point of Portsdown's ridge is 120 metres. Within the S.S.S.I. the height ranges from 50 to 110 metres. The average slope is approximately 1:4.
1.2.1.3 Soil
The soil classification of England and Wales (Avery, 1980) place the soils of Portsdown Hill within the Upton 1 series where they are described as a 'chalky grey rendzina' with some loessial silt. An average soil pH of 7.83 has been recorded. The soil becomes more clayey at the base of the escarpment where the depth reaches 30 cm due to Coombe deposits. Higher up the slope the soil is less than 3 cm in places. There is a considerable variation in soil depth over short distances i.e. less than a metre. These variations are caused by past disturbance such as the construction of defensive structures, trackways and turf stripping.
1.2.2 Biological
The warmer conditions encourage plants and invertebrates that are at the northern extent of their range and thus uncommon in the rest of Britain. The site's accessibility and close proximity to large centres of population has meant the hill has attracted many naturalists over the years. Many biological records exist. The process of putting the available information on to a computerised database is ongoing. See appendix 2 and 3.
1.2.2.1 Flora
The vascular plant species-list exceeds 200 and it is increasing all the time as new species are found, see appendix 2. Notable plants i.e. locally rare, nationally uncommon or butterfly food plants which occur in the S.S.S.I. include:
| Orobanche elatior | Knapweed Broomrape |
| Ophrys apifera | Bee Orchid* |
| Helianthemum nummularium | Rock-rose* |
| Hippocrepis comosa | Horseshoe Vetch* |
| Arabis hirsuta | Hairy Rock-cress |
| Linum bienne | Pale Flax |
| Anthyllis vulneraria | Kidney Vetch |
| Gentianella amarella | Autumn Gentian |
| Spiranthes spiralis | Autumn Lady's Tresses |
| Thesium humifusum | Bastard Toadflax* |
| Phyteuma orbiculare | Round-headed Rampion* |
Whilst many plants typical of downland are present many of these are found in very small numbers. The entire colonies of three species of orchid would fit within the area of an A4 sheet of paper.
Of the 27 plants recognised as key species, only Thesium is considered as nationally rare. i.e. in less than 100 10x10 km grid squares, (Britain is covered by approximately 3500 such squares). Plants, which have been recorded in the past but are no longer found include Coeloglossum viride (Frog Orchid), Campanula glomerata (Clustered Bellflower), Centauria calcitrapa (Star Thistle).
Portsdown Hill also is known to be of interest for its lower plants including the lichens Catapyrenium lachneum, Toninia coeruleonigricans and Leptogium schraderi and bryophytes including Ditricum flexicaule.
1.2.2.2 Fauna
1.2.2.2.1 Invertebrates
Much of Portsdown's scientific interest stems from its invertebrate community. The diversity of food plants and microclimate supports a considerable range of invertebrates. Appleton et.al. (1975) produced the most complete record of Lepidoptera, Hymenoptera and Orthoptera. The findings of more recent workers are less detailed, however, it appears that a diverse invertebrate community remains.
Lepidoptera
414 species recorded
There are healthy populations of Lysandra coridon (Chalkhill Blue) as well as many of the commoner butterflies. Aricia agestis (Brown Argus) and Argynnis aglaia (Dark Green Fritillary) have recently been seen after an absence of several years. Two moth species, Cynaeda dentalis and Hyopchalcia ahenella have their only known Hampshire locations on Portsdown Hill.
Other species e.g. Plebejus argus (Silver Studded Blue) and Lysandra bellargus (Adonis Blue) have disappeared from the site in recent decades. Two butterfly transects are carried out on the site; 30 species were recorded in 1999.
Hymenoptera
51 species recorded
There is a rich bee and wasp fauna associated with the hill. The sun warmed sparsely vegetated banks are ideal habitat for many species that have life-cycles with an underground larval stage. Notable species include Mutilla europaea and Mellitta tricincta.
Orthoptera
8 species recorded
Grasshoppers and crickets are well represented. There is a large population of Tettigonia viridissima (Great Green Bush Cricket) and Conocephalus discolor (Long-winged Conehead).
Diptera
21 species recorded
Notable species are Asilus crabroniformis Hornet Robber fly, Machizus rusticus, Eudorylas horridus, Cistogaster globosa.
Coleoptera
232 species recorded
Notable species are Odonteaus armiger
Arenea
110 species recorded
Notable species include Atypus affinis (Purse web spider) and a large colony of Argiope bruennichi, a large colourful spider with a limited distribution.
1.2.2.2.2 Vertebrates
Birds
Many birds can be seen on the hill as it provides a variety of habitats, food and shelter. The scrub is ideal habitat for warblers such as Phylloscopus collybita chiff-chaff and (Phylloscopus trochilus) willow warbler. Also breeding on the site are sylvia communis (Whitethroat), sylvia curruca (Lesser Whitethroat). Other scrub nesting birds include Saxicola torquata (stonechat) and Emberiza citrinella (yellowhammer). Alauda arvensis (skylark) and Anthus pratensis (meadow pipit), nest in the open grassy areas. The abundant hawthorn berries and ivy provide winter food. The hill's value is increased for birds as it is on a migratory route for many species.
A common bird census of the eastern part of the S.S.S.I. and adjacent farmland carried out in 1997 and 1998 found 20 species of breeding bird.
Reptiles
The site supports large populations of Anguis fragilis (slow worm) and Lacerta vivipiara (common lizard).
Mammals
Small mammal surveys reveal healthy populations of Apodemus sylvaticus (Wood mouse), Microtus agrestis (Field vole), and Sorex araneus (Common Shrew). A large population of Oryctolagus cuniculus (Rabbit) occurs in the western compartments 1,2,3 and 11. Small numbers of Capreolus capreolus (Roe Deer) live on the hill all year.
1.2.2.3 Communities
Any description of communities has to acknowledge the fact that they merge into each other and are difficult to separate. Any account looses accuracy as the community boundaries change. The vegetation is dynamic, scrub is spreading rapidly in most areas but is being cleared in others.
A habitat survey, carried out in 1992, PHCS file 120, described a wide range of habitats due to variations in grassland and scrub density. Most it was described was as upright brome-dominated grassland that was 'moderately rich'.
A NCC survey of the site in 1987 identified the following NVC communities: MG1 alongside roads; CG3d - to describe tall Bromopsis grassland and CG3a - to describe the short herb-rich grassland, see Rodwell, (1992). A Hampshire County Council Habitat assessment team carried out a site survey in 1985, PHCS file 120 and described 5 types of grassland: 'short rich'; 'short moderately rich'; 'short poor'; 'tall Upright Brome' and 'reseeded or disturbed'. Eleven areas of short-rich or moderately rich grassland were described and mapped. More cent survey work reveals species-rich grassland in all compartments that could be assigned to CG2, although these patches are often very small and fragmented.
All surveys describe or map extensive areas of scrub. This is mostly Crataegus - Hedra scrub, W21. Most of the scrub has few ground-layer plants associated with it.
Dense scrub covers over 60% of the site. Very little of the grassland is free from small scrub. Small scrub plants occur in most 2 m2 quadrats of apparently homogenous grassland. The scrub is predominantly Crataegus monogyna (Hawthorn) with other species including: Prunus spinosa (Blackthorn), Thelycrania sanguinea (Dogwood), Ligustrum vulgare (Privet), Ulex europeaus (Gorse), Clematis vitalba (Clematis), Rubus fruticosa (Bramble), and Fraxinus excelsior (Ash). The proportions of each vegetation type are shown in figure 1.
Below is a brief description of each compartment's vegetation and some of the uncommon or ecologically important species that occur, see appendix 1 for maps. Scrub clearance has occurred in all compartments, and this has promoted tall regrowth of scrambling invasive plants, ruderals and even the re-establishment of downland species e.g. Anthyllis vulneraria (Kidney Vetch). The state of the regrowth depends on the level of subsequent management.
Compartments 1 and 2
Predominantly dense tall scrub with some rabbit grazed "lawns" of remnant chalk grassland. The scrub's age varies. Much has recently invaded the grassland, however, there are large areas of established scrub that have been present for at least fifty years. Post-war aerial photographs clearly show scrub below Fort Southwick. The scrub in these areas contains species associated with established woodland, Daphne laureola (Spurge Laurel), Arum maculatum (Cuckoo Pint), Ruscus aculeatus (Butcher's Broom), Mercurialis perennis (Dog's Mercury) and Sanicula europaea (Sanicle). Rhamnus catharticus (Buckthorn) is common.
Compartment 3
The compartment above Paulsgrove Chalk pit differs from the others in that Ulex europeaus (Gorse) is much more common and locally dominant. Frequent fires have given opportunistic plants such as Echium vulgare (Viper's Bugloss), Reseda lutea (Mignonette) and various thistles a chance to establish. The grassland in this compartment varies from species-rich to strongly dominated by Bromopsis erectus. An old track way in the east of this compartment has short species-rich grassland on its banks.
Compartment 4
The largest expanse of scrub-free grassland occurs in this compartment. According to Francis Rose this area is known to have a good terrestrial lichen community, as well as some uncommon mosses. Various banks and pathways have been disturbed by motorcycle scramblers, so bare chalk is an obvious feature. To the east, the scrub becomes much thicker, but it retains patches of Hippocrepis comosa (Horseshoe Vetch) rich grassland. At the bottom of the slope Ash-Elm woodland is well established.
Compartment 5
The predominant cover is low open scrub. Patches of moderately diverse grassland remain between the bushes. Fires regularly occur here. The strip next to the road is tall Arrhenatherum elatis (False Oat grass)/ruderal grassland community with scrambling plants such as Rubus (bramble) and Clematis.
Compartment 6
Scrub dominates, but some gaps contain rich grassland. Some tall dense scrub with Arrhenatherum (False Oat grass) grassland is adjacent to Southwick Road.
Compartment 7
Tall grassland with many ruderal species form a mosaic with scrub and short grassland. The vegetation alongside Portsdown Hill Road is dominated by species-poor Arrhenatherum (False Oat grass) grassland.
Compartment 8
This compartment contains old earthworks and abandoned trackways with steep slopes. These areas have been used by motor cycle and mountain bike scramblers, thus have bare chalk with diminishing trampling associated with them. Areas of short species-rich grassland occur on the steeper slopes with much of the area dominated by Bromopsis erectus (upright brome). Scrub and scattered tree species are obvious and the margin next to Southwick Road is almost totally scrubbed over.
Compartment 9
At the bottom of the slope tall scrub with emergent woodland, e.g. Quercus ilex (Holm oak), Ulmus (Elm) and Acer pseudoplantaus (Sycamore), is dominant. Much of the rest of the site is dense scrub with tall moderately rich grassland. Grazing in recent years has much improved the diversity of the grassland. Several small areas of short rich grassland add to the diversity of habitats. Plants such as Thesium (Bastard Toadflax) occur in these areas. This area is most used for recreational purposes and has several paths crossing it.
Compartment 10
Dominated by tall grassland with small patches of scrub. The grassland is much improved by grazing and made more interesting with the presence of Helianthemum nummularium (rockrose). Scrub with emergent Sycamore trees is concentrated in the southeast corner.
Compartment 11 (Portchester Common)
Bromopsis erectus (upright brome) grassland with less scrub than other areas of the Hill. It has a moderately rich flora associated with it. The numerous paths and localised disturbances make the vegetation rather diverse. Scrub is dense in the southeast corner.