Nutrient availability and plant community composition of N-limited grasslands - A greenhouse experiment into the driving factors behind species composition
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Species richness is very important for the perseverance of Dutch grasslands. As eutrophication is one of the major threats to grasslands worldwide, more research is needed into the validity of accepted mechanisms and their ability to explain changes in species composition. The primary goal of this experiment was to give insights into the reaction of N-limited plant communities on eutrophication. This was done by measuring competition and productivity traits of three different functional groups, namely N-specialists, P-specialists and opportunists. Secondly was tried to find supporting evidence that these functional groups are useful in explaining plant community composition dynamics. To provide these results, a nine week during greenhouse experiment was set up using 1125 plants in these three specialisms. N-specialists used were Rumex acetosa, Crepis capillaris, Trisetum flavescens and Alopecurus pratensis. Succisa pratensis, Centaurea jacea, Briza media and N. stricta were used as P-specialists and finally Prunella vulgaris, Knautia arvensis, Carex oederi and Agrostis capillaris compiled the list of opportunists. All species were germinated and put into equal communities, which were divided over low and high nutrient levels under strong N-limitation (N:P < 5). Measurements show no differences in species richness and abundance levels for the different specialisms came back around 33%. Evenness also showed no sign of uneven distribution among species or specialism. Results for the productivity trait show that N-specialists produce more above ground biomass than P-specialists and opportunists. This was also observed for the light competitions traits (canopy height & stretched leaf length). CSR-strategy was compared with the specialism groups but gave no conclusiveness as to validity of either classification. In general this research shows that eutrophication leads to higher expression in productivity and light competition traits. The relative difference between specialism was more clearly visible under higher nutrient levels. Where N-specialists perform better than P-specialists and opportunists, the differences between the latter two is minimal. However, it is not unlikely that some experimental shortcomings have caused the obscure results regarding P-specialists and opportunists. More evidence for the validity of these functional groups is needed and can be provided by research into these specialisms and their reaction on changing nutrient stoichiometry.