The forest understory is a different matter. As a geologist passing through the understory of the forest I have developed a pretty good sense of the different understory plant communities. But it is more than rain fall totals that define where plants grow. The underlying geology can make a big difference as well. Silty or clayey soils hold a lot more water and that is reflected in the types of plants that will grow. I have gotten pretty good at predicting the types of soils I will dig into with my shovel by the type of plants I see.
May is the month when rhododendrons bloom in the Washington forest. They always provide a nice surprise particularly since they are common in some of the areas I work. Rhododendrons are not found everywhere. They prefer drier areas and a look at the leathery leaves on this evergreen plant indicates that they are drought tolerant. The rain shadow area of the Olympic Mountains creates a band where they are much more common, but well drained soils seem to help as well. The thickest area I have seen them is in the Quilcene area and on the Toandos Peninsula east of Quilcene. Port Townsend to the north has a festival celebrating the blooms. Quilcene is not dry in regards to total rainfall, but it has been my experience that the area gets a lot warmer than the rest of western Washington during the dry summer months. This is partially hinted at by another plant. The southwest tip of the Toandos Peninsula is called Oak Head. I have been there and Have never observed any oak trees. However, the steep shoreline slope does have a good crop of poison oak. I have also encountered thick poison oak thickets in the Brinnon area.
A companion plant frequently seen with rhododendrons is the evergreen huckleberry. Both plants combined can form nearly impassible thickets. But they both put on a great show in May, the Rhododendrons with their large white to pink blooms and the huckleberries with their new coppery leaves. The huckleberries produce very dark berries in the fall that I nibble on from October till January.