percent of this precipitation occurs between October and March; July and August mark the driest months with means below , while November, December, and January represent the wettest with averages greater than . Prevailing winds originate from the south during winter and from the northwest during the summer season, with the exception of prevailing winds at the mouth of the Columbia River Gorge, where winds predominantly move to the east. The southern winds have the highest velocities of the three, only rarely occurring with potentially destructive force. Ecology The Portland area has a moderate climate, and precipitation is not typically very heavy, allowing for vegetation, which can hamper fieldwork in the area. Many forests that covered the area were partly cleared for agriculture, timber, or cemetery applications in the early 20th century. These cleared and burned land plots sustain rich stands of secondary forest, featuring gorse, huckleberry, nettles, poison oak, salal, and blackberry. Myriad species of fern, as well as rapid-growth deciduous trees like alder and vine maple are also frequent. Forests support stands of Douglas fir, western hemlock, western redcedar, Pacific dogwood, bigleaf maple, Oregon ash, red alder, cascara buckthorn, Pacific madrone, and Oregon white oak; within swamps and moist areas in creeks, the shrub Devil's club can be observed. Other trees that sometimes dominate forest areas include black cottonwood and red alder. Forest communities have many additional shrubs including Indian plum, western hazel, and snowberry. Ground layer plants include the herbaceous sword fern and stinging nettle. In contemporary times, clearing of forests for housing development have left about half of the Boring Lava region still forested. As a result, water quality has decreased due to higher sedimentation and turbidity, and flooding has worsened over time. Streams within the area are of either first or second order, with moderate to low flows and average gradients between 10 and 12 percent. Cool and clear, many sustain macroinvertebrates, and a smaller number support amphibians and fish. The riparian zones in the Lava Field area host diverse species, and they are influenced by uplands that serve as migration connections for birds, mammals, reptiles, and some amphibians. The United States Fish and Wildlife Service provided a list of potentially threatened or endangered species in the Boring Lava area, labeling them "sensitive" species. Among plant species, they determined the following species to be sensitive: white top aster, golden Indian paintbrush, tall bugbane, pale larkspur,