POST-MEDIEVAL AND RECENT CHANGES IN BRITISH VEGETATION: THE CULMINATION OF HUMAN INFLUENCE.

Abstract

Although measurable climatic fluctuation in temperature and wetness has occurred between A.D. 1700 and 1984, lack of earlier records and the complicating effects of human influence militate against detection of vegetational response in Britain. Vegetational change during this period is thus regarded as almost entirely the result of anthropogenic factors. Agriculture, now affecting c. 80% of Britain's surface, is overwhelmingly the most important such factor. Original vegetation had been mostly destroyed or heavily modified by a.d. 1700, notably through removal of the former forest cover and draining of lowland wetlands. The last 300 years have seen continuing development of farming and other land uses, to the point where very little natural vegetation now remains, and the main extent of semi-natural types is located in the uplands. Many distinctive semi-natural vegetation types actually created by man over several centuries of low input-low output management have decreased greatly since A.D. 1700, and rate of loss has even accelerated since 1940-1950. Only 5% of permanent lowland grasslands now remain agriculturally unimproved neutral grassland and only an estimated 20% of the extent of pre-1940 calcareous grassland is now assignable to that category floristically. Lowland acidic heath has declined by 78% in area since A.D. 1830, and by 40% of the 1950 level. Lowland rich fens are reduced to scattered remnants, the largest expanse (the East Anglian Fenland) having decreased from 3380 km2 in ad 1630 to 10 km2 in 1984. Forest cover was reduced to 5·4% by 1924, but has increased to 9·4%, largely through planting of alien conifers in upland districts. Since 1930 an overall 46 % of ancient semi-natural broadleaved woodland has been lost, largely by conversion to conifer plantation but also through grubbing out. An estimated 300000 ha of ancient semi-natural woodland remains, representing 14% of existing woodland and 1·3% of Britain's land surface. The total extent of hedges in England and Wales was reduced by an estimated 140000 miles (28%) between 1946-1947 and 1974. Urban-industrial growth, including roads, railways, water-use, mineral extraction and energy generation, has directly obliterated large areas of vegetation and caused chemical pollution. Waste ground associated with these activities often develops semi-natural plant communities through spontaneous succession, and may become floristically rich. Pollution affects especially fresh water through both toxic residues and eutrophication from nutrient enrichment (including run-off of agricultural fertilizers). Atmospheric pollution causes acid deposition especially damaging locally to the lichen flora, and induces biological changes in lakes and rivers in districts of acidic rocks. Recreational pressures have caused increasing disturbance to vegetation locally, and less directly through working of Sphagnum bogs for horticultural moss litter and surface mining of limestone pavements for rockery stone. Collecting of plants as a hobby has also brought some species, notably rare ferns, orchids and'alpines'to dangerously low population levels, or even local extinction. The British flora has been profoundly modified by human influence. Only 19 vascular species are known to have become extinct, but many have declined greatly during the last 300 years and, for species in some habitats, especially semi-natural grasslands, rate of decrease has accelerated since 1940. Of 317 nationally rare species 117 (37%) have declined by at least 33 % since 1930. Human agency has also introduced to Britain at least 239 plant species which now grow wild, and extended the distribution of at least 363 others. Some are still increasing but others, notably weeds of arable land, are decreasing again. Conservation measures include setting up nature reserves to contain adequate representation of all semi-natural and natural vegetation types, and of populations of all native plant species. The aim is to give protected area status to c. 10% of Britain's surface through appropriate control and management designed to maintain or enhance the nature conservation interest. At present c. 1% is managed as nature reserve of one or other category and another 6% has been designated as Sites of Special Scientific Interest. Wild plants are now legally protected against unauthorized picking and uprooting, and 61 rare species are protected by special measures. Wider environmental conservation measures to reduce damaging human impact on vegetation and flora include inputs to and by local authorities and their planning system, persuasion of landowners and resource managers, and pollution control; supported by an educational and publicity programme, and by relevant research and information.