THE VEGETATION OF THE ISLE OF STROMA

J. K. Butler

The island of Stroma is in the Pentland Firth 3km. from the mainland at Huna. One might therefore expect that its vegetation is strongly influenced by the proximity of the mainland and by the surrounding sea, and this turns out to be largely correct. The island is a remnant of Old Red Sandstone of the Mey Beds series, and the nature of this rock has a marked influence. The east and central part has a fairly thin cover of clay till (up to 2m thick) fed by calcareous minerals from the rock to form a fertile soil. On the west side there are presumably less minerals released from the rock and the till has become sour and peaty, particularly in the southwest corner. The wind from the west drives considerable quantities of salt spray over the land, while this seems to happen to a lesser degree on the east coast.

This fertile island has been densely inhabited in the past; the peak population seems to have been about 350 in 1880, this being largely self-sufficient and subjecting the land to intensive grazing and cultivation to the detriment of the original diverse vegetation.

The vegetation was previously described by Arthur Bennett (Annals of Scottish Natural History (1900), 108) who used material and notes gathered by Miss A. M. Geldart of Norwich when she visited the island for about five weeks during June and July 1899. She stayed with the Rev. W. Dundas and his wife, and obviously occupied a good part of her time studying the botany, if we are to judge by her thorough account of it. The population at that time was 340, and some of the changes in the plant population reflect the dramatic reduction of the human population in recent years. Bennett gives some general remarks about the island in his introduction, including the following anecdote:

"In an old topographical work on Scotland it is said that a dispute arose as to whether Stroma belonged to the Orkneys or to Caithness, and that it was decided in the following quaint manner. Some venomous animals (of what kind we are not told) lived in Stroma. Of these a certain number were shipped at the same time to colonise Orkney and Caithness. Those that were brought to Caithness took kindly to the soil, as to a congenial habitat; while those that we sent to Orkney, from the unfavourable effects of the climate, sickened and died. By this singular method Stroma was adjudged to belong to Caithness."

The Rev. Dundas was of the opinion, from his own observations, that the vegetation of Stroma is three weeks later than on the mainland opposite, and that there was considerably more sunshine on Stroma than on the mainland.

Recent studies of the Stroma plants have been made by the author in 1972 (1 day), 1973 (1 day), 1974 (2½ days), and 1978 (1 day); accompanied by Mr. A. Currie in 1974, and by Miss McCallum-Webster in 1978; the following lists are a compendium of their findings. List 1 in the Appendix gives the plants observed; those also seen by Miss Geldart have an asterisk attached. List 2 gives the plants observed by Miss Geldart which were not recorded recently.

List 1 contains 178 species, and we may conjecture that 17 more from list 2 have merely been overlooked (these are marked # on List 2) giving a total flora of 195 species. We may compare this with 746 species recorded in the whole of Caithness and 261 species recorded for the mainland directly opposite - i.e. the recording square ND37. It is clear that some common species are not recorded on the island - understandable ones are trees and riverside species, but others are missing presumably because they did not establish themselves before the sea rose and isolated the island or because they have been eradicated by human population pressures more recently. Some examples are Gorse, Common Ragwort, Eared Willow, Polypody, Common Butterwort, Bog Myrtle, Water Avens, and Hard Fern. We can say more specifically that some species have been eradicated recently because arable farming has virtually ceased, and because even if it continued the modern seed cleaning practices have stopped the spread of some weeds which were common in the fields in Miss Geldart's time. From List 2 Mugwort, Corn Marigold, Sunspurge, Cut-leaved Cranesbill, Bugloss, Pineappleweed, Knotgrass, Field Madder, Lesser Trefoil, Scentless Mayweed and Common Vetch are all of this type.

There is a good calcareous marsh in the middle of the island, which in Miss Geldart's day would hive been dammed to provide the water supply and to drive the Mains watermill. Now it has partly drained to form marsh and muddy pools whose plant population is similar to that frequently encountered in Orkney but not so commonly in Caithness.

The western border has, even on the tops of the high cliffs, extensive saltmarsh vegetation with much large areas of Glaux maritima, Triglochin maritima, Juncus gerardi and Plantago coronopus sward. It also extends along the north coast where it is mixed with a richer Carex flacca sward containing Primula scotica and a poorer expanse of Littorella uniflora dominant. There is also a patch just north of the manse which has the same type of Carex flacca sward containing a large and healthy population of Primula scotica. This latter site is alleged to have appeared quite suddenly, but each account of this event is different, and it has clearly been there since 1860, the type of ground and the accompanying species being quite typical of a Primula scotica locality.

Much of the inland area is moderately grazed grassland while the clifftop parts are quite varied. In the north-west they are bare and exposed, yet in the south-cast lush enough to support Bracken and a single Dog-rose bush. There is also a prominent Phragmites community in the south-east on a flushed wet slope falling away to the sea. There are smaller more level flushed areas in the west with only a short herb community.

There are no unusual species present in the plant population, although the suggestion has been made that Monk's Rhubarb (Arctium lappa) grows there. There is a small population of Winter Heliotrope (Petasites fragrans) clinging to the clifftop to the east of the main harbour, and this is presumably the source of the report.

APPENDIX

LIST 1 - Plants Recorded from Stroma in 1972-1978

(* = those also seen by Miss Geldart in 1899)

Achillea millefolium *

Aconitum anglicum

Agrostis canina

Agrostis stolonifera

Agrostis tenuis *

Aira praecox *

Alopecurus geniculatus *

Alopecurus pratensis *

Angelica sylvestris *

Antennaria dioica *

Anthoxanthum odoratum *

Anthriscus sylvestris *

Anthyllis vulneraria *

Apium inundatum *

Armeria maritima *

Arrhenatherum elatius *

Aster tripolium *

Atriplex glabriuscula *

Avena sativa

Bellis parennis *

Callitriche stagnalis *

Calluna vulgaris *

Caltha palustris *

Capsella bursa-pastoris *

Cardamine pratensis *

Carex binervis *

Carez demissa

Carex dioca

Carex flacca *

Carex nigra *

Carex ovalis

Carex panicea *

Carex pilulifera

Carex pulicaris *

Carex serotina

Cerastium atrovirens

Cerastium holosteoides *

Cirsium arvense *

Cirsium palustre

Cirsium vulgare

Cochlearia officinalis *

Coeloglossum viride *

Crocosmia x crocosmiflora

Cynosorus cristatus *

Dactylis glomerata *

Dactylorhiza purpurella

Dactylorhiza maculata ssp cricetorum *

Deschampsia cespitosa *

Deschamipsia flexuosa

Eleocharis palustris *

Eleocharis quinqueflora *

Empetrum nigrum *

Epilobium palustre

Equisetum arvense

Equisetum palustre *

Erica cinerea *

Erica tetralix *

Eriophorum angustifolium *

Euphrasia agg *

Festuca ovina *

Festuca pratensis

Festuca rubra *

Filipendula ulmaria *

Fumaria officinalis *

Fumiaria muralis ssp boraei

Galeopsis tetrahit *

Galium aparine *

Galium saxatile *

Galium palustre *

Galium verum *

Glaux maritima *

Glyceria fluitans *

Gnaphalium uliginosum

Heracleum sphondylium *

Hieracium pilosella agg.

Holcus lanatus

Hydrocotyl vulgaris *

Hypericum pulchrum *

Hypochoeris radicata *

Iris pseudacorus *

Juncus articulatus

Juncus bulbosus agg. *

Juncus conglomeratus *

Juncus offusus

Juncus gerardii *

Juncus squarrosus *

Lamium moluccellifolium

Lamium purpureum *

Lathyrus montanus

Lathyrus pratensis *

Lemna minor

Leontodon autumnalis *

Ligusticum scoticum *

Linum catharticum *

Littorella uniflora *

Lolium perenne *

Lotus corniculatus *

Luzula campestris *

Luzula multiflora

Lychnis flos-cuculi *

Menyanthes trifoliata

Molinia caerulea *

Montia fontana *

Myosotis arvencis

Myosotis caespitosa *

Myosotis discolor *

Myosotis secunda

Narcissus poeticus

Narclus stricta *

Narthecium ossifragum *

Pedicularis palustris

Pedicularis sylvatica *

Petasites hybridus

Phalaris arundinacea *

Plantago coronopus *

Plantago lanceolata

Plantago major *

Plantago maritima *

Poa annua

Poa subcaerulea

Poa tribialis

Polygala serpyllifolia *

Polygala vulgaris *

Potarmogeton natans *

Potarmogeton polygonifolius *

Potentilla anserina *

Potentilla erecta *

Potentilla palustris

Primula scotica *

Primula vulgaris *

Prunella vulgaris *

Pteridium aquilinum *

Puccinellia capillaris

Ranunculus acris *

Ranunculus flammula *

Ranunculus hederaceus *

Ranunculus rapens *

Ranunculus trichomanes

Rheum rhaponticum

Rosa canina agg. *

Rubus saxatilis *

Rumex acetosa *

Rumex acetosella *

Rumex crispus

Rumex obtusifolius

Sagina maritima *

Sagina procumbens *

Salix repens *

Scilla verna *

Selaginella selaginoides *

Senecio aquaticus *

Senecio vulgaris *

Sieglingia decumbens *

Silene dioica *

Sinapsis arvensis *

Solidago virgaurea

Sonchus arvensis

Sonchus asper *

Sonchus oleraceus *

Spergula arvensis *

Spergularia marina

Stellaria media *

Succisa pratensis *

Taraxacum agg. *

Trifolium pratense *

Trifolium, repens *

Triglochin maritima *

Triglochin palustris

Tripleurospermum maritinum *

Tussilago farfara *

Urtica dioica *

Urtica urens

Veronica serpyllifolia

Vicia cracca *

Vicia sepium *

Viola palustris

Viola riviniana*

Viola tricolor *

Viola canina

LIST 2 - Plants seen on Stroma by Miss Geldart but not recorded recently.

(Note: It is possible that those marked # have merely been overlooked)

Achillea ptarmica #

Agropyron repens #

Achusa arvensis

Arctium lappa

Artemisia vulgaris

Asplenium marinum #

Botrychium lunaria #

Carex echinata #

Carex flava agg. (Probably C. demissa or C. serotina)

Chrysanthemum segetum

Eleocharis uniglumis #

Euphorbia helioscopia

Geranium dissectum

Helictotrichon pubescens

Honkenya peploides #

Isolepis setacea #

Juncus bufonius #

Marticaria matricarioides #

Mertensia maritima

Myriophyllum alterniflorum

Parnassia palustris #

Pinguicula vulgaris #

Poa pratensis

Polygonum aviculara

Potentilla anserina

Radiola linoides #

Rhinanthus minor agg. #

Sambucus nigra (planted)

Schoenus nigricans #

Sedum rosea (planted)

Sherardia arvensis

Spergula media

Stallaria alsine #

Thymus drucei #

Trichophorum cespitosum #

Trifolium dubium

Tripleurospemum inodorum

Tritucum repens

Veronica agrestis #

Vicia sativa