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Germination and growth of three Cirsio_Molinietum species in a Holcus lanatus sward at different nitrogen levels

Vink, S. (2003) Germination and growth of three Cirsio_Molinietum species in a Holcus lanatus sward at different nitrogen levels. Master's Thesis / Essay, Biology.

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Since halfway the last century heathlands and species-rich grasslands have been becoming increasingly rare. The reason for this decrease is mainly due to the transformation of nature areas into intensive farming areas, especially by the use of artificial fertilisers. In recent years there have been attempts to restore the former low production vegetation types. This is often done by topsoil removal, resulting in a decrease of soil nutrients and in a situation with a limited productivity. There are a number of areas where monitoring of the vegetation has been taking place after topsoil removal. At the start of monitoring it was determined that the abiotic conditions in most of the sites were favourable for the regeneration. However, after nine years, the establishment of target species of these low production communities has hardly occurred. Two possible reasons for a lack of re-establishment of certain species can be that either there is no seed source and/or that the conditions for germination/establishment is not favourable. In order to gain more knowledge about whether the presence of an existing vegetation (as is the case in many of the nature development areas now) and if increasing levels of nitrogen (as could be the case in the future) can affect the germination and establishment of target species plants, two experiments were carried out. In these experiments seeds and seedlings of target species were placed in an existing Holcus lanatus vegetation at different densities and at different nitrogen levels. The planting of H. lanatus in different densities did not lead to a significant difference in H. lanatus dry weight or light transmitted in either the growth or the germination experiment. The different nitrogen additions did, however, lead to significantly different H. lanatus dry weightsand light conditions. In the first experiment the growth of seedlings of three target species of the Junco-Molinion alliance; Cirsium(dissectum, Danthonia decumbens and Juncus conglomeratus was determined by using the relative growth rate and the final dry weight. The cumulative RGR curves for the control seedlings showed a large amount of growth during the experiment, with an influence of nitrogen. The seedlings grown in the presence of H. lanatus showed very little to no growth. This was the same for all the different nitrogen treatments. The final dry weights of the seedlings showed the same pattern as for the RGR, C. dissectum and J. conglomeratus control seedlings were significantly influenced by nitrogen, for D. decumbens this influence was not significant. There was no correlation between seedling dry weight and the percentage light transmitted. The seedlings in competition showed an enormous repression of growth by the vegetation, the percentage dry weight in competition/ dry weight without competition was the following: C. dissectum 14%, D. decumbens 16% and J. conglomeratus 4%. In the second experiment the germination of C. dissectum and D. decumbens in an established H. lanatus vegetation of varying density and at different nitrogen additions were examined. Both C. dissectum and D. decumbens showed normal germination curves. There were no significant differences in germination percentages for any of the treatments for both C. dissectum and D. decumbens. At the end of this experiment the seedling dry weights were also measured. They showed a similar result to the first experiment: a large difference between the control plants and the competition plants with a significant effect of nitrogen on the control plants but not on the competition plants. The fact that this occurred even at very low H. lanatus densities suggests that a mechanism such as allelopathy may also be involved. From these results it can be concluded that although germination of the examined species was independent of an existing vegetation, seedling growth was very heavily influenced by the H. lanatus vegetation. It can therefore be expected that if there is an existing (dense) vegetation in a nature restoration area the chance that a target species will be able to establish will be very small. It would probably be better to attempt to get target species established before the more common species become established in large numbers by sowing seeds or by planting seedlings of the target species.

Item Type: Thesis (Master's Thesis / Essay)
Degree programme: Biology
Thesis type: Master's Thesis / Essay
Language: English
Date Deposited: 15 Feb 2018 07:31
Last Modified: 15 Feb 2018 07:31

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