When the island was first settled, it was extensively forested, with around 30% of the land covered in trees. In the late 12th century, Ari the Wise described it in the Íslendingabók as "forested from mountain to sea shore". Permanent human settlement greatly disturbed the isolated ecosystem of thin, volcanic soils and limited species diversity. The forests were heavily exploited over the centuries for firewood and timber. Deforestation, climatic deterioration during the Little Ice Age, and overgrazing by sheep imported by settlers caused a loss of critical topsoil due to erosion. Today, many farms have been abandoned. Three-quarters of Iceland's  is affected by soil erosion;  is affected to a degree serious enough to make the land useless. Only a few small birch stands now exist in isolated reserves. The planting of new forests has increased the number of trees, but the result does not compare to the original forests. Some of the planted forests include introduced species. The tallest tree in Iceland is a sitka spruce planted in 1949 in Kirkjubæjarklaustur; it was measured at  in 2013.