Sub-ecoregions of the Pacific temperate rain forest ecoregion as defined by the WWF include the Northern Pacific coastal forests, Queen Charlotte Islands ecoregion, Vancouver Island ecoregion, British Columbia mainland coastal forests, Central Pacific coastal forests, Southern Cascades forests ecoregion, Klamath-Siskiyou coastal forests, and Northern California coastal forests ecoregions. They vary in their species composition, but are all predominantly coniferous, sometimes with an understory of broadleaved trees and shrubs. Most of the precipitation occurs in winter but summer fogs moisture is extracted by the trees and produces a fog drip keeping the forest moist. The Northern California coastal forests are home to the Coast Redwood (Sequoia sempervirens), the world's tallest tree. In the other ecoregions, Coast Douglas-fir (Pseudotsuga menziesii var. menziesii), Sitka Spruce (Picea sitchensis), Western Hemlock (Tsuga heterophylla) and Western redcedar (Thuja plicata) are the most important tree species. A common feature of Pacific temperate rain forests of North America is the Nurse log, a fallen tree which as it decays, provides ecological facilitation to seedlings. Trees such as the Coast Douglas-fir, Western Hemlock, Western Red Cedar, Pacific Yew, and Vine Maple are more closely related to coniferous and deciduous trees in the temperate forests of East Asia.