Scottish Northern Isles Plant Conservation Project
Birch fragments in peat at Tresta
Much of the present land surface of Shetland is covered with blanket peat with plant communities dominated by heather, grasses and sedges. The flora is restricted, with only some 400 native vascular species, when compared with the vegetation covers which existed earlier in the Holocene and in previous warm intervals. This impoverishment reflects the long-term influence of man and especially grazing by sheep.
Spence identified the impact of sheep grazing by surveying the remnant vegetation of loch islets and cliff ledges. He found that the entire native shrub and tree flora of Shetland was largely confined to these locations and comprised Betula pubescens, Corylus avellana, Juniperus communis, Salix and Sorbus aucuparia. Wood remains in peat occur on Mainland, Foula, Whalsay, Unst and Yell and include roots, branches, leaves and seeds peat of birch, juniper, willow and alder and Corylus avellana (Jóhansen, 1985). The remains demonstrate that trees were formerly much more widespread on Shetland until around 8000 BP when the wetness of the Atlantic period led to waterlogging and peat development. Pollen records provide evidence for substantial disruption of vegetation at some time after 5000 BP, varying in its timing and rapidity between sites but generally involving a loss of woodland.
At Murraster, a lake basin near Walls, the pollen record shows an increase in the pollen of birch at the start of the Holocene. Until about 8900 14C BP the vegetation remained open, with some shrub and perhaps local woodland development. During the period 8900-8400 BP pollen of trees and shrubs reaches 50% of the total. Around 4650 BP pollen of Plantago lanceolata appears whilst willow and tall herbs disappear from the vegetation record. Jóhansen (1985) attributes this change to the arrival of Neolithic people and their grazing animals.
More recent investigations at Dallican Water and Gunnister, north Mainland support this general pattern of change. An early postglacial phase with woodland development is succeeded by blanket peat. After 3000 BP, tree pollen drops abruptly and it appears that the main period of landscape change associated with permanent human occupation occurred then.
The gradual nature of climate change and its low amplitude due of the moderating influence of the Atlantic Ocean means that global climate change is unlikely to be responsible for abrupt changes in pollen records and vegetation history. Icelandic volcanic eruptions might have produced short-lived climate deteriorations in Shetland, but there is as yet no evidence that they did so. Autogenic influences, through gradual soil leaching in a wet, cool climate probably influenced vegetation change during Holocene, forcing a shift from woodland patches towards blanket peat. The dominant factor forcing change on Shetland vegetation seems to have been human influence since ~7500 BP, and certainly from 4600 BP onwards. The recent dates from the shell midden at West Voe, Sumburgh, indicate that man had arrived on Shetland before about 6200 BP.