Biogeographers have long recognized that the spatial distribution of plant species, at a coarse resolution, mainly reflects each species' climatic requirements. However, few studies have carefully matched maps of a species distribution with maps of climatic variables to see where climate may fail to predict a species distribution, thus suggesting other ecological factors, such as limited seed dispersal or competition with other species.

In a new study published in the Journal of Biogeography, researchers analyzed the distribution of western hemlock, a common tree that occurs in the wet climates in two separate regions: along the Pacific Coast and in the Northern Rocky Mountains of North America. Daniel Gavin and Feng Sheng Hu, the two authors of the paper, showed that a single variable representing the amount of water evaporating off the land can explain the distribution of western hemlock quite well in the coastal region, but large areas of the Rocky Mountain region matched climatic patterns much less closely.

Spatial analyses also suggested that hemlock in the Rocky Mountains is better adapted to drier summer conditions than hemlock near the coast, and that hemlock has not completely expanded into its potential habitat in the Rocky Mountains. Gavin and Hu explain: "Hemlock became common in the Rocky Mountains only 2000-3500 years ago, compared with > 9000 years ago in the coastal region. The Rocky Mountain region is also a more competitive environment for hemlock, with frequent fires and disturbance-adapted species. The limited time for dispersal combined with intense competition have probably retarded the range expansion of this species in certain areas". The authors warn that similar mismatches between climate and species distributions may become more common with more rapid climate change.