Individual study: Management of farmland field ditches in peat areas to enhance plant diversity, the Netherlands
Twisk W., Noordervliet M.A.W. & ter Keurs W.J. (2003) The nature value of the ditch vegetation in peat areas in relation to farm management. Aquatic Ecology, 37, 191-209
In Holland, farmers could contribute to enhancing plant species diversity of water-filled ditches on agricultural land by undertaking more sympathetic management. However, knowledge as to what measures farmers can realistically undertake and methods of best practice are poorly known. Therefore, a study was set up in peat areas of the Netherlands to assess how dredging, ditch cleaning, and nutrient supply in the adjacent fields, affects ditch vegetation and thus make management recommendations for enhancing floristic value.
Study areas: The study was carried out between 1994 and 1996 in Utrecht, Noord Holland and Zuid Holland Provinces. A total of 240 ditches on 84 dairy farms were selected. The fields (improved grassland) were 30-60 m wide, separated by ditches 2-10 m wide, and 10-200 cm deep with water depths of 10-90 cm.
The following were recorded:
Physical conditions - soil type (mesotrophic or eutrophic peat), ditch width, ditch depth, ditch water table, hydrological isolation and Cl- concentration.
Ditch management -
Cleaning machine (ditch-scoop, mowing-basket, mowing-drum);
Cleaning period (July-September, September-October);
Dredging machine (pull-shovel, punched pull-shovel, suction pipe);
Dredging period (April-August, September-October, November-March);
Years since dredging (ñ1 year, ± 2 years, ± 3 years, >4 years);
Field management - i.e. field nitrogen (N) and phosperous (P) inputs.
Vegetation monitoring: Vegetation was sampled (giving a frequency score to each species present and calculating a 'nature-value' index) along a 150 m ditch section in 25 evenly distributed sections 2 m long x 1.5 m wide, from mid-May to the end of June (i.e., prior to farmers cleaning the ditches).
Increasing the water depth (by raising the ditch water table) is probably the most cost-effective short-term measure to enhance floristic value in the ditches; the 'nature-value index' of submersed and floating species, and the number of emergent species, increased with water depth. Analysis also indicated that reducing the N supply would also yield beneficial results (effect of P supply was not clear) as the number of species declined as N supply increased. Further floristic improvements could be achieved by choosing certain machines or specific management periods.
A significant effect of ditch cleaning was recorded for emergents: using a mowing-drum resulted in lower species numbers (19) and a lower nature-value index (43.3) than for the mowing-basket or ditch-scoop (21.0 and 46.4). Cleaning in September or October also resulted in a lower index for the emergents (44.3), compared with cleaning between July and mid-September (46.4).
The type of dredging machine had an effect on the nature-value index of the submersed and floating vegetation, index values were: pull-shovel (46.1), punched pull-shovel (44.6) and suction pipe (43.9). For emergents, there was a trend for the highest nature-value index (45.1) in recently dredged ditches (1-3 years ago) in September or October. Dredging by suction pipe between November and March resulted in the lowest nature-value index (36.4); all other combinations had intermediate values (42.4).
Effects of ditch management must take into account impacts on fauna, as well as flora.
Note: If using or referring to this published study, please read and quote the original paper, this can be viewed at: http://www.springerlink.com/content/g33p164436064150/fulltext.pdf