Straits enclosure ecological survey

Nomenclature of the species recorded in the Straits quadrats follows Rose (1981) A Flora of Britain and Fitter and Fitter (1984) Collins guide to Grasses, Sedges, Rushes and Ferns of Britain and. Northern Europe. For ease of analysis, the plant list has been grouped into categories of woody shrubs and trees, perennial climbers, soft stem herbs (both annual and biennial), grasses and sedge, rush and fern groups. Rosa arvensis thus falls within the woody shrub group, whereas bramble, Rubus fruticosus comes within the climber group, along with woody nightshade, Solanum dulcamara. A general bulked category of mixed herbs was adopted where small herbs occurred only sporadically in one sample period and were individually of both small leaf area and biomass. This bulking procedure was also occasionally used for grasses where the leaf area of individual species was low, particularly during winter.

Open community

The first sampling period in May was less than 4 months after the completion of brashing, canopy thinning and clearance of understorey shrubs in one third of the enclosure. This process caused several side effects:

  • The creation of large amounts of fallen wood, occasionally in deep piles but more often remaining as poles scattered across the site
  • The creation of deep ruts through the use of heavy machinery, giving rise to areas of standing water and bare soil
  • Uneven cutting of the understorey, leaving behind large individual shrubs growing beside the remaining oak trees, which later posed problems in estimating average shrub regrowth.

Recent forest management thus created an uneven microhabitat for understorey growth, and consequent variability in the leaf area and biomass estimates from the limited number of quadrats that were assessed. Ten metre squares were located subjectively to minimise the inclusion of large individual shrubs.

Bare soil estimates were occasionally as high as 60%, but in wetter areas, rushes and clumps of grasses (mostly Deschampsia) quickly recovered to provide 15-20% cover in the middle of the year, and seedlings of birch, ash and oak were encountered in several quadrats by October.

This Open community is characterised at ground level by the vigorous growth of the grasses, Deschampsia flexuosa and wood millet (Millium diffusum) in dryer areas with wood sedge (Carex sylvatica) and rush (Juncus effusus) in damper places. Small patchy carpets of stitchwort (Stellaria holostea), with Viola , herb Robert (Geraninium robertium) and wood avens are common, with short stems of bramble, honeysuckle and Rosa arvensis growing through.

Ivy spreads sideways on the bare soil under the other herbs with moss on older decaying wood.

Herbs such as enchanters nightshade, wild strawberry and geum grow between grass clumps, and on more bare areas, mullein and wood spurge have also been recorded.

Mast community

This community was cleared of understorey shrubs in 1996 and is now characterised by re-growth of hazel (Corylus avellana), hawthorn (Crataegus monogyna), willow (Salix capraea) and blackthorn (Prunus spinosa) from the old stumps. Hazel is dominant with stem heights of 2 m plus, but nowhere in this community is this understorey canopy closed.

There are some locations where a distinct association of shrubs dominate at a small scale, such as on the floodplain of the brook, where blackthorn and new ash saplings 1-1.5 m high are common. In the drier parts, dominated by hazel, guelder rose (Viburnham opulus), honeysuckle and Rosa arvensis are present as woody associates, together with frequent bramble with stems up to 1.5-2.0 m in length.

This open shrub understorey allows a variety of herbs to flourish, such as Viola, Fragaria, Geranium, Stellaria, Mercurielis perennis, Stachys sylvatica and Urtica dioica between the clumps of the dominant grasses species, including Millium effusum, Deschampsia and Holcus lanatus (soft grass).

Tussocks of wood sedge and rush occur throughout, and are found associated with carpets of Lysimachia nemorum (Yellow Pimpernel) and the climber, woody nightshade (Solanum dulcamara) in the wetter areas. Well established clumps of fern, both Dryopteris felix-mas and D. dilatata are also common throughout this community.

Level II community

Understorey shrub re-growth in this treatment was five to six years old in 1999, having been cleared initially for the establishment of the Intensive Forest Health Monitoring site (Level II).

The fallen, decaying wood has become colonised by mosses, notably Rhytidiadelphus, and in some quadrats up to 25% of the ground area was entirely moss covered with trailing ivy. Hazel shrubs are commonly up to 2.5-3.0 metres in height in this community, with 10 plus live stems arising from each stool, although they do not form a closed secondary canopy. Hawthorn occurs in well-developed bushes 1.5 m high, but is less frequent than hazel, and a number of clumps of holly (1.5-2.0 m) and well-branched beech saplings (1.0-1.5 m high) are also present. Single stem ash seedlings occur throughout over a range of heights up to one metre. The growth of climbers in this community is good with honeysuckle both on the ground and up to the top of the shrub layer, particularly on hazel and hawthorn, and vigorous arching Rubus stems to 1.5 - 2.0 metres in length. Rosa arvensis is confined to the more open light patches.

This community has the greatest diversity of ground flora of the Straits woodland.

The abundance of grasses is lower than in the Mast quadrats, with Deschampsia less dominant, and wood millet and Holcus lanatus equally prevalent; tufts of the latter often show grazing damage, presumably by deer. Wood sedge is also widespread, but less wood rush is present compared with the other communities, except in ditchside locations. Both species of fern grow throughout.

Herb species which have not been recorded elsewhere include primrose, yellow archangel (Galeobdolon sp.), hemp nettle (Veronica hederifolia), betony (Betonica officinalis ) and cuckoo pint (Arum maculatum). Carpets of cleavers, stitchwort and enchanters nightshade replace the grasses, particularly in deeper shade and close to the base of trees. Herb Robert and trailing Veronica are also frequent as ground cover between the shrubs, and in areas with more ash and mature beech in the overstorey. Veronica seems to substitute for Stachys sylvatica here compared to the Mast community, where it damper soils are prefered.

Coupled with the decreased incidence of Juncus recorded in this community, there is some ecological indication of overall dryer ground conditions across the Level II plot.

Tall shrub community

The last period of management in this area appears to have been following the storms of 1990, and the largest hazel stems cut from the understorey show a maximum of 10/11 distinct growth rings. The average size stem (2.2 cm diam) has 7-8 rings and these stems are still over 4.0 m high and interlock to form a closed canopy over much of the area under. Although the hazel stools are still producing new side shoots, there is a considerable proportion of standing deadstem wood in the lower parts of the canopy, which had attained some three or four years growth  before becoming moribund.

The heavy shade cast by this under-canopy leads to the suppression of all other shrubs and herbs, and makes this community the most species poor in the Straits Enclosure. Some hawthorn persists here, but the proportion measured is less than one-third of the standing biomass recorded in the Mast and Open communities. Hawthorn is rarely more than 1.5 m high and other shrubs such as willow and blackthorn are completely absent. Beech saplings occur sporadically (3-4 years old), but in contrast to the other communities, ash seedlings are rare. The dominant climber is honeysuckle, which is also found as ground cover, whereas ivy is restricted to the oak trunks, and even the growth of bramble is suppressed and limited to etiolated short stems.

Ground cover is greatly reduced, with less moss than recorded in the Level II community, and large areas of bare soil, sometimes up to 90% of the total sample area. A few small clumps of grasses persist, mostly Deschampsia and wood millet, but even these become occasional to rare. There is similarly poor cover of wood sedge, and no occurrence of rush was recorded in any of the Tall shrub quadrats (though it can be found in major active ditch lines). Clumps of fern were recorded in almost every location, but it is noticeable that the smaller D. dilata becomes the dominant species over D. felix-mas. There are also rare, sporadic occurrences of herbs in the ground cover, notably wood spurge (Euphorbia amygdaloides), Viola, Fragaria, cleavers and Hypericum.

With the notable loss of Juncus from the species list, the lower incidence of mosses and the presence of butchers broom (Ruscus) in some other places within this community, there is good ecological evidence that ground conditions in this community are much dryer than other forest blocks in the Straits.