Native grasslands are crucial reservoirs of biodiversity in the San Juan Islands archipelago of Washington State, but their extent has been drastically reduced since the arrival of European settlers. The San Juan Islands are part of the San Juan-Gulf Islands archipelago located in the central Salish Sea in a region characterized by a Mediterranean climate with very dry summers. Grasslands and neighboring Gary oak woodlands were historically more widespread on the islands, in part due to Indigenous land management with fire. Today, native grasslands are recognized as important reservoirs of biological diversity in the region. They are home to many rare and endemic species, some of which have limited ability to disperse due to the insular nature of this region. Unfortunately, native grasslands have become increasingly rare and fragmented. They remain threated due to a combination of land cover change, invasive species, and fire exclusion. While the reduced extent of grasslands in this region is widely recognized among conservation experts and practitioners, the long-term trends in the extent and patchiness of this ecosystem are not fully known. We are therefore using a combination of historical GIS, image interpretation, and classification of aerial imagery to create a spatially explicit dataset of grassland extent on the San Juan Islands at multiple time periods starting during the early years of European colonization. By digitizing hand-drawn historical land cover maps and historical aerial imagery, and using supervised classification to extract land cover classes from recent aerial imagery, we have been to create high-resolution land cover datasets for time periods centered on the 1890s, 1930s, and 2020s. Our preliminary results reveal a rapid decline in grassland extent following European colonization, with 10-30% of grassland area disappearing on most of the islands in the first 40 years of our record. The spatial data, analyses, and visualizations we create will improve understanding of historical ecological trends and help develop a historical baseline for conservation initiatives, which in turn will be useful in the development of conservation and restoration practices.
