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