Remotely sensed estimation of shrub encroachment : a case study on the Great Alvar, Öland, Sweden

The increase of shrub extent in arid and semi-arid environments is reducing the species richness all over the globe. This process of shrub encroachment was found to result from changes in anthropogenic land use, especially from too high or too low grazing intensity. The Great Alvar on the island Öland, Sweden, is a unique grassland ecosystem with a high species richness. Changes in land use patter lead to shrub spreading, resulting in a reduction of plant and bird species. The aim of this study is to investigate if the current grazing intensity is effective in halting or reducing the shrub expansion. The past and current shrub extent of a low and a medium grazed area was identified by digitizing the shrub area based on aerial photographs from 2002, 2008 and 2017. Additionally, the growth rate of shrubs was estimated as well as a future prediction of shrub extent. The shrub coverage of the study plot in 2017 was found to be 30% in the low grazed area and 2.5 % in the medium grazed area. The annual average increase of shrub extent on the low and medium grazed area were 1.2% and 0.2%, respectively. The future prediction, based on a non-spatial model, forecasts a continuous increase in shrubs. The study results indicate an ongoing increase in shrubs extent, which could lead to a continuous reduction in species richness and in future ecosystem instability on the Great Alvar.Shrub Encroachment, which implies the increase in density, extent and cover of shrub species, is one of the driving processes for grassland changes on Earth. The wide spread increase in shrubs has a negative effect on the species richness and can lead to a complete ecosystem turn over. The process of shrub encroachment is a result from changes in anthropogenic land use, especially from too high or too low grazing intensity. A change from an intermediate grazing intensity to a low grazing intensity has led to increase in shrub cover for example on the Swedish island Öland. Öland is home to the Great Alvar which is a grassland ecosystem that can rarely be found elsewhere. The main characteristics of an Alvar are the shallow soils and the high lime content of the soils. Only certain plants can survive in this environment which is why there are plants on Öland that can not be found in the rest of Sweden. The dry and windy climate of Öland is an additional cause for the special vegetation. Shrub encroachment on the Great Alvar was noticed already 70 years ago. Öland’s unique grassland species have been found to decrease since then which is why the aim of this study is to investigate if the current grazing intensity on the Great Alvar is effective in reducing shrub encroachment. This was done by analyzing aerial photographs. The shrub extent was assessed for the years of 2002, 2008 and 2017 and compared afterwards. In addition, the growth rate per year and a future prediction of shrub extent was estimated. The results show a continuous increase of shrub extent in the 15-year period. Therefore, it has been shown that the current grazing intensity does not reduce shrub encroachment. Possible further decrease in biodiversity can be expected as the future prediction also shows further expansion of shrubs. Shrub control measures such as manual clearings, periodic fires and an intermediate grazing intensity need to be implemented to reverse the process of shrub encroachment. The understanding of the consequences of shrub encroachment on grasslands needs improvement. Finally, as the effect of climate change on shrub encroachment is unknown, further research is necessary to predict the future ecosystem changes more precisely