Effects of mechanical ditch management on the vegetation of ditch banks in dutch peat areas

Summary

Many grasslands in the now heavily agricultural peat polders of the western Netherlands were formerly rich in plant species. Due to the intensification of dairy farming, including increased use of fertilizers, species-rich plant communities are now largely confined to the banks of the ditches bordering the cattle pastures.

It had been suggested that ditch banks could be excluded from fertilizing and manuring and protected from intensive trampling by cattle to maintain or restore their floristic value. Changes in the methods of ditch management by farmers might increase the species diversity of banks. Management at the time of study was prescribed by regional Water Boards to maintain water discharge. All plants in or near the ditches must be removed frequently to prevent ditches from filling with sediments. As a result, the banks are buried with plant material and mud removed from the ditches. To meet specified conditions conditions, many ditches are dredged once every 5- 10 years. Farmers are free to choose the method of cleaning.

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.

Study sites: Study sites were located in the peat areas of Zuid-Holland and the western part of Utrecht. Historically, ditches were dug to drain the peat for agricultural use resulting in a polder landscape with long, narrow fields separated by ditches, most 3-6 m wide with 0.3-0.6 m of water.

The effects of ditch management were studied during 1983-86. When examining low cleaning frequencies, only banks along ditches that had been cleaned at least three times by the same method where selected. Forty seven banks were observed over three years (1984-86) after cleaning to examine changes in the vegetation with time.

Vegetation sampling: Ditch-bank vegetation was sampled prior to ditch cleaning (end of June-mid September) in the same month each year. Ditch vegetation was not recorded. On each of the 320 banks examined, a 50-m length relevé was made. Each strip sampled ran 70 cm up the bank from the water's edge, as this zone contained almost all bank species. Three indices of floristic richness were assigned:

i) number of species.

ii) number of quality-indicating species.

These included all except very common species characteristic of very intensive
agriculture (e.g. common chickweed Stellaria media) and species that only occasionally occur in grasslands and on ditch banks, but frequently in other habitats (e.g. purple dead-nettle Lamium purpureum).

iii) a 'nature-value index' was determined from estimates of species' regional (Zuid-Holland), national and world rarity, and their rate of decline. The index was calculated by summing the nature-values of the species, taking into account their plot cover values.

Ditch management: Information about ditch management and grassland use was obtained from about 150 farmers and through field observations. Four cleaning frequency classes were distinguished: yearly; once every 2 years; once every 3 years; and > 3 years.

All the current methods of cleaning ditches in Dutch peat areas were involved:

i) Cleaning by hand – water plants, some submerged organic debris and soil from the ditch bank are pulled onto the banks with a hook.

ii) Cleaning by mowing-basket - a machine carries a basket with a cutter-bar in front which mows the water plants and renews the edge of the ditch bank. Part of the cut material and ditch bank edge falls back into the ditch through the bars of the basket; the rest is deposited on the banks.

iii) Cleaning by ditch-scoop - the bank edge is cut loose and water plants, some submerged debris and soil from the bank edge is dumped onto the banks. Usually, the gaps in the scoop are smaller than those of the mowing-basket and therefore more material can be dropped onto the banks.

iv) Cleaning by specialized cleaning equipment, mainly by bottom auger - this loosens water plants, ditch bottom and ditch edge; material is removed, pulverized and resulting sludge squirted over the banks.

v) Cleaning by two different methods alternately.


Physical factors and field management: Bank soil samples (from the surface 0-l0 cm within 70 cm of the water) and peat mud in ditches were collected once in the autumn, a few months after the ditches were cleaned. Samples were analysed for: P and K content; pH; total N; and total C.

Characteristics of factors that may influence bank vegetation (soil type, slope aspect, gradient, ditch water table, adjacent land use, and fertilization of adjacent fields) were recorded.

Almost all farmers in the study area cleaned their ditches between September and November thus the effect of management season was not examined.

Plant species-richness was highest on banks with ditch cleaning once every 2-3 years, with 38 species, including many endangered regionally, doing 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 floristic differences between cleaning once every 2-3 years and cleaning less frequently.

The lower floristic richness with yearly cleaning appeared related to a higher nutrient-rich sludge supply, smothering and physical damage to the plants. Soil acidification due to sludge deposition was unimportant.

The lower species-richness along ditches cleaned less frequently than once every 3 years was attributed to successional effects.

The cleaning method appeared to have no effect on bank floristic richness, probably due to variations in the way the cleaning machinery is used.

Most ditches were dredged once every 5-10 years but no botanical differences were found between banks examined 1-5 and >5 years after ditch dredging,

Conclusions In terms of conserving or enhancing botanical interest, 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.


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