The Changing Status of Reedswamp in the Norfolk Broads

(1) A study was made of changes in the distribution and area of reedswamp in eighteen Broads by the use of aerial photographs, which covered the period 1945 to 1978. Additional information was obtained from the c. 1880, 1905 and 1926 Ordnance Survey maps. (2) Between 1880 and 1905 the colonization, by reedswamp, of open water exceeded losses, by succession, to fen. After 1905 the area of reedswamp decreased mainly by rapid succession to fen, and a slower rate of colonization. (3) From 1946 onwards there was a dramatic loss of reedswamp. The area decreased from 121.5 ha in 1946 to 49-2 in 1977. These losses were almost entirely through reversion of reedswamp to open water. (4) It is concluded that grazing by coypus could be primarily responsible for the major part of the reed decline, particularly from 1950 to 1963. Evidence is presented showing that if reedswamp, a food source preferred by coypus, formed not more than 66% of the coypu-diet this could account for all the changes observed. Further, if the coypus selected only buds and young shoots this figure might be as low as 6-6%. (5) Grazing by wildfowl, especially feral geese, could explain the present limitation of reedswamp in Bure and Ant Broads. (6) Experiments designed to test for direct effects of eutrophication on reedswamp were inconclusive. The growth of Phragmites australis was not inhibited by an atmosphere of nitrogen surrounding roots and stem bases, nor when potted in mud from sites where regression had occurred. (7) Eutrophication, however, could have had an indirect effect on reedswamp. It raised the rate of sedimentation, and so could have increased the susceptibility of reeds to grazing, because the buds are more accessible in soft mud and because, under anaerobic conditions, the plants might be more susceptible to damage when shoot grazing cuts off the oxygen supply to the roots.