The expansion of common reed (Phragmites australis (Cav.) Trin. ex. Steud.) in the anastomosing river valley after cessation of agriculture use (Narew River Valley, NE Poland)

Results of the survey into expansive growth of the common reed (Phragmites australis (Cav.) Trin. ex Steud.) carried out in the upper Narew River valley (NE Poland) within the whole area of Narew National Park (6810 ha) (NNP) are presented in the paper. The Narew River is an anastomosing river in the part subjected to the analysis and an open swampy bog ecosystem has formed within its valley. Sedge communities and to a lesser degree meadow communities are typical plant communities in the study area. Till the end of the 1970's, the area was agriculturally used. Since the beginning of the 1980's when grassland mowing was discontinued, expansion of reed communities has been recorded. The analysis of reed expansion has been based upon data on its range originating from interpretation of aerial NIR photos made in 1987 and 1997. The field survey and analysis of the air photos have shown that the reed expansion rate differs depending on the location in the study area. Assessment showed that in 1987 the total area of the common reed stands equalled 1214.75 ha (18% of the NNP area), and in 1997 - 2300.6 ha (34%). Due to considerable homogeneity of habitat features within the valley and habitat requirements of reed, a basic hypothesis has been put forward that the rate of reed expansion in terms of the increase of reed community area is related to bankline density and length. The material obtained and statistical analysis have confirmed that assumption. Besides, it was mentioned that cessation of mowing sedge grasslands along with a change in character of spring surface flooding also contributed to reed expansion.