The Mountain Dogwood, a.k.a. Pacific Dogwood (Cornus nuttallii) is one of the great pleasures of Yosemite in the spring. They always seem to be just right for Mother's Day weekend. This was true in 2011 when we had one of the biggest snowpacks in history and it was true in 2012 after our warm, dry winter. Local lore also says that it always snows once after the dogwoods bloom.
The Mountain Dogwood has large white blossoms made up not of the flower petals, but of the bracts. When the flower first blooms, the bracts are fairly green, but they whiten over time, coming into their full glory over the space of a few days. Most commonly, the flower has six bracts, but can have anything from 4-7 according to Jepson.
The dogwood is variously classified as a tree or a shrub — the largest specimens barely make the definition of tree, being a certain diameter at a certain height (usually six inches at five feet) and coming out of the ground as a single trunk. Dogwoods like shade and thrive when they have a bit of cover from other trees or possibly a cliff or other source of shade cutting out as much as two-thirds of the light.