Effects of mechanical ditch management on the vegetation of ditch banks in Dutch peat areas.
The effects of cleaning frequency, cleaning method and dredging on ditch bank vegetation were studied in peat areas in Zuid-Holland and the western part of Utrecht in 1983-86. Species-richness was highest with cleaning once every 2-3 years. Thirty-eight species, including many that are endangered in the peat district, did better with cleaning every 2-3 years or less frequently than with yearly cleaning. Only 5 common species (Holcus lanatus, Lolium perenne, Phalaris arundinacea, Polygonum hydropiper and Ranunculus sceleratus) had greater cover with yearly cleaning. The results suggested that the lower floristic richness with yearly cleaning compared to less frequent cleaning was 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. It is suggested that the somewhat lower species-richness of banks along ditches cleaned less frequently than once every 3 years reflects the effects of succession. Cleaning method appeared to have no effect on ditch bank floristic richness, probably due to variations in the operation of cleaning machinery. No floristic differences were found between banks examined 1-5 and >5 years after ditch dredging, presumably because the ditch mud was spread over the fields rather than the banks. It was concluded that ditches should be cleaned once every 2-3 years, provided this is compatible with proper water management, and that the lowest strip of the banks should be kept free from ditch sludge.