Disturbance Effects on Plant and Mycorrhizal Communities in Sandy Grasslands

Abstract: Many ecosystems have historically been disturbed, either naturally or as part of a cultural management. Due to loss of species as a result of too little disturbance, disturbance must be considered as a potential tool for conservation and restoration in various ecosystems. In this thesis work, disturbance effects on a grassland system were studied. The studied grassland had traditionally been severely disturbed, both by grazing animals and wind erosion, and through agricultural practices. By experimentally disturbing the soil in the dry sandy grassland, disturbance-induced effects on plant and arbuscular mycorrhizal communities in sandy grasslands was studied. Ploughing and harrowing had no effects on soil pH and only small effects on available phosphorous was found. A positive effect on plant species richness and abundance of red-listed annuals in dry calcareous grasslands was found. The plant species richness shifted towards one comprised mainly of early successional species, but during the four years of this study, the succession went towards the pre-disturbance state rather than the restoration target defined for the area. It was found that a functional trait approach to evaluate disturbance effects was not sufficient to fully explain the reasons for the disturbed plots to diverge from the target areas. However, the functional approach showed that disturbance increased the incidence of nitrogen favoured species in the system. By using Sanger sequencing it was shown that ploughing had a negative effect on the abundance and species richness of arbuscular mycorrhiza two years after disturbance. However, in a second experiment, at a different site and using pyrosequencing, small-scale disturbances was not shown to affect either species richness or composition of arbuscular mycorrhizal fungal communities 4.5 months after disturbances. Carbon flows through arbuscular mycorrhizal fungi was negatively affected by disturbance, mainly due to a decreased abundance of arbuscular mycorrhizal fungi and potential host plants. In summary, disturbance was shown to have both positive and negative effects on the studied organisms in dry sandy grasslands. Ploughing and harrowing were not good options for restoring this specific dry calcareous grassland type although some of the wanted annual targets increased in incidence. Mycorrhizal abundance could potentially be decreased following disturbance and with it the potential soil carbon sequestration. Using disturbance for restoration purposes involves complex responses with potentially disparate responses in different organisms within the same community.