Centers of regionally endemic vascular plants in northwestern North America: a quantified approach

Certain areas of northwestern North America have been identified as plant endemism hotspots previously, but without repeatable methods or comparable results. The prevalent methods among endemism studies give misleading results from partial, often subjectively sampled taxa or by considering only jurisdictional endemics. A quantified approach was needed to assess and compare centers of endemism in northwestern North America to help guide landscape-scale conservation effort. Whole flora, per-taxon weighted endemicity methods were used to assign sum endemicity scores to 1 × 1 degree latitude–longitude grid cells, adjusting for cell area. Extralimital distribution data were considered for endemicity scores. Taxa with an endemicity score over 10 were rejected. Jurisdictional endemics are listed, but because many of these are distributed too widely to be included in the endemicity dataset, they are discussed only to highlight stewardship responsibility. The list of regionally endemic taxa falling within the criteria totalled 1224. Nearly all grid cells having sum endemicity values over 5 occur south of the southern ice-age glacial limits. The highest values occurred in grid cells corresponding to southern regions: the Siskiyou Mountains, the Wenatchee Mountains, the Columbia River Gorge, and the greater Blue Mountain system. However, the entire northern North American Cordillera form a globally significant center of endemism. The taxonomic structure of the regionally endemic flora is examined, and potential causes of the concentrations of regional endemics are explored.