Effects of vegetation management and raising the water table on nutrient dynamics and vegetation change in a wet grassland.
The results of a restoration experiment carried out on a permanent grassland on peaty, heavy clay in the Netherlands are described. The experiment started in 1985, 7 years after fertilizer application had ceased, and was designed to provide insight into ecologically significant processes accompanying restoration. An analysis was made of the effect of management regime and of raising the water table on nutrient availability, dry matter production, tissue nutrient concentration, dynamics of species numbers and plant species replacement. Three management practices were compared: cutting and removal (RR), cutting and mulching (MM), sod removal in 1985, and thereafter cutting and removal of the hay (RS). Data are presented on changes during a 5-year period. No trend was discernible in soil pH, total C, or N and P in the RR treatment; extractable P and K decreased sharply in the field with the raised groundwater level. Nine years after fertilizer application ceased, dry matter production had fallen from 10-11 to 6-7 t ha-1 year-1. In the subsequent 5 years of the experiment it declined to 5-6 t ha-1 year-1 when all cut biomass was removed, and to about 4 t ha-1 year-1 after sod removal. Mulching caused an increase to 11 t ha- year-1. No effect was seen of the raised water level. The dry matter yield of the first June cut in the RR treatment decreased. The tissue K concentration also decreased, but no increase of the tissue P concentration was detected. It was concluded that the availability of K and to some extent of P was more important than N availability in explaining the decrease in dry matter production. The tissue nutrient concentrations were not influenced by the water table. Sod removal to a depth of 5 cm resulted in the lowest productivity and the lowest tissue concentrations of P, while tissue concentrations of N and K were not affected. Raising the water level resulted in a more rapid establishment of species indicative of wet conditions, some of which invaded from nearby ditches. The trends of dominant species are described with a set of response models. The species were ranked from disappearing to colonizing species. The relationship between rank order of replacement and indicator values of species was investigated. Raising the water table resulted in species indicative of wet conditions becoming dominant, independently of vegetation management. The removal of nutrients resulted in the appearance of species with a lower maximal height, indicative of lower P and K availability.