Individual study: The impact of mechanical harvesting regimes on the aquatic and shore vegetation of ditches in agricultural areas of the Netherlands
Best E.P.H. (1994) The impact of mechanical harvesting regimes on the aquatic and shore vegetation in water courses of agricultural areas of the Netherlands. Vegetatio (now Plant Ecology), 112, 57-71
The flora in the Netherlands has become rapidly impoverished over the 20 years, reasons for this including drainage, eutrophication, acidification and toxification, largely due to agricultural intensification. Many species that have disappeared are associated with wet habitats. The largely cultivated landscape of the Netherlands is frequently dissected by ditches, and formerly ditch management practice was aimed at maintaining their function i.e. securing the transport of water from areas with a surplus, involving the periodic removal of most vegetation by mechanical methods. Now however, it is recognized that these ditches may serve as refugia for species which have disappeared elsewhere. As such, management, as well as maintaining ditch function, takes into account maintenance of water and ecological quality
This study aimed to quantify the impact of different mechanical cutting regimes on the species composition and richness of the aquatic and shore vegetation of ditches relative to other factors (site, soil and water quality). The cutting regimes differed in frequency and timing but were carried out with a mowing basket.
Study areas: The ditches studied in the western Netherlands were in agricultural areas representative of low-lying, eutrophic areas of the country, three on a sand substrate: Hazerswoude Dorp, Callantsoog and Hazerswoude Rijndijk, and two on peat: Reeuwijk and Tempelpolder.
Cutting regimes and experimental design: The effects of four mechanical cutting regimes (all using a mowing basket) were assessed. The criteria for the choices of these were that they are currently in use and satisfy legal requirements to maintain ditch function. The following were applied over three years (1989-1991):
i) once/year (May);
ii) twice/year (May, July);
iii) 3 times/year (May, July, September);
iv) late November (i.e. later than usual August-September cut).
The cut plant material was deposited on the ditch sides, as commonly practiced in the Netherlands. A stretch of 100 m was selected form study, each stretch divided into five, 20 m long blocks in a south-north orientation. Cutting regimes were randomized within the blocks.
Vegetation: Plant species were recorded annually (mid-July) in five plots of 5 x 4-6 m (aquatic vegetation) and 5 x 0.30 m (shore vegetation). Each year relevés were made of the aquatic vegetation and the shore vegetation (both sides of the ditch) and percentage cover per species estimated. Filamentous algae were considered as one group. Mosses were excluded.
Sampling and soil and water analysis: Representative soil samples were taken at the beginning of the experiment: from the ditch bottom, the shore, and each side of the ditch. Surface water samples were taken monthly at each site (1989-1991) and analysed for pH and HCO3. Water temperature and transparency (Secchi disk) were recorded in situ. The soil samples were analysed for granular composition; organic matter content, CaCO3, total nitrogen and total phosphorus.
In total 136 plant species (52 semi-aquatic and aquatic; 84 terrestrial) were recorded. The cutting regime affected the species composition of the aquatic and shore vegetation communities to only a very small extent; the effect was significant only for the first year on aquatic vegetation and for the first and second years on shore vegetation. Other factors, e.g. the between-site and the within-site variation, and several soil and water quality parameters, were far more important. Cutting in November had the largest effect, in that it caused the greatest extremes in species cover (i.e. from high cover prior to cutting, to lower cover subsequent to cutting). Only 22 (16%) of the species was significantly influenced by the cutting regime.
Cutting repeated within a year opened up the vegetation allowing colonisation by new species, improving light conditions for seedlings already present; but it exhausted carbohydrate reserves of solitary species. Cutting once/year in November caused retardation of spring growth of shore vegetation due to the mulching effect of the not yet decomposed plant material deposited on the ditch banks in late autumn. The highest species richness was attained as follows: for aquatic vegetation: on sand, by cutting 3 times/year (in May, July and September); and on peat, by cutting once/year (in November); and for shore vegetation: on sand, once/year (in May) and twice/year (in May and July), and on peat once/year (in May or November).
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