Sitka spruce (Picea sitchensis) is a characteristic tree of the coastal "fog belt" of the Pacific Northwest. Arthur Kruckeberg, in _Natural History of the Puget Sound Country_, shows a nice map of the "Sitka spruce" zone's extent relative to the "western hemlock/Douglas-fir" zone in Washington State. Essentially, the Sitka spruce zone extends in a belt approximately 20 miles wide along the coast and the Strait of Juan de Fuca, but ends at the Olympic rain shadow at the San Juan archipelago; it does not extend south through the inland waterways of Puget Sound. The explanation given is that this species lacks the ability to regulate transpiration, and so requires the high humidity of the fog belt.
However, in my years of observations, I have found at least three populations of Sitka spruce in central and southern Puget Sound, far outside the regular "Sitka spruce zone." These populations are small in extent, with the species absent from the rest of the central and southern Puget Sound basin. I have been curious about what factors have allowed these populations to establish and persist outside the fog belt, but I struggle to formulate any workable hypotheses. Has anyone else worked with any similar phenomena? Jason Hernandez
