Effects of Mechanical Ditch Management on the Vegetation of Ditch Banks in Dutch Peat Areas

(1) The effects of three aspects of mechanical ditch management (cleaning frequency, cleaning method and dredging) on the ditch bank vegetation in Dutch peat areas were studied. (2) The optimum relationship between species-richness and cleaning frequency was examined; species-richness was highest with cleaning once every 2-3 years. (3) Thirty-eight species, including many that are endangered in the peat district, did better with cleaning every 2-3 years than with yearly cleaning. Only five common species had greater cover with yearly cleaning. There were only small botanical differences between cleaning once every 2-3 years and cleaning less frequently. (4) The results suggest that the lower floristic richness with yearly cleaning compared to less frequent cleaning is related to a higher supply of nutrient-rich sludge, smothering of the vegetation and physical damage to the plants. Acidification of the soil from sludge deposition was not important. (5) The somewhat lower species-richness of banks along ditches cleaned less frequently than once every 3 years presumably reflects the effects of succession. (6) The cleaning method appeared to have no effect on floristic richness of the banks, probably due to variations in the way the cleaning machinery is used. (7) No botanical differences were found between banks examined 1-5 and >5 years after ditch dredging, presumably because the ditch mud is spread over the fields rather than the banks. (8) Cleaning ditches once every 2-3 years is recommended, provided this is compatible with proper water management. In all management practices, the lowest strip of the bank should be kept free from ditch sludge.