. Végétation de l'Oregon et de Washington. Botanique botanique de l'écologie, de l'Oregon Washington (État) de l'écologie. limites. À des limites altitudinales en lave- tonne, les forêts de Pinus ponderosa soit dans une prairie ou une steppe d'Artemisia. À certains endroits dans le nord de l'Oregon et de Washington, le lavage- du sud il y a une ceinture d'écotone entre Quercus garryana forêt de Pinus ponderosa et des steppes. La discussion de l'Pinus ponderosa Zone qui suit considère seraï Pinus pon- derosa forêts ainsi que les forêts dans lesquelles l'Pinus est l'orgasme. Le Pinus ponderosa Zone, au sens strict, comprend que ce dernier ; je

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. Vegetation of Oregon and Washington. Botany Oregon Ecology; Botany Washington (State) Ecology. limits. At lower elevational limits in Washing- ton, Pinus ponderosa forests grade into either grassland or Artemisia steppe. At some loca- tions in northern Oregon and southern Wash- ington, there is an ecotonal belt of Quercus garryana between Pinus ponderosa forest and steppe. The discussion of the Pinus ponderosa Zone which follows considers serai Pinus pon- derosa forests as well as forests in which the Pinus is climax. The Pinus ponderosa Zone, in the strict sense, includes only the latter; in this narrower definition, the Pinus ponderosa Zone correlates with the Ponderosa Pine- Bunchgrass Zone of Krajina (1965) and Bray- shaw (1965). It is important to realize that in many locations there is no belt of climax Pinus ponderosa forests between steppe and areas of Pseudotsuga menziesii (Brayshaw 1965; Johnson 1959) or Abies grandis (Hall 1967) climax. The Pinus ponderosa Zone, more broadly defined, con-elates roughly with Merriam's Arid Transition Zone (Barrett 1962; Bailey 1936) and includes representa- tion of Kuchler's (1964) Ponderosa Shrub and Western Ponderosa Forests. Environmental Features The climate of the Pinus ponderosa Zone is characterized by a short growing season and minimal summer precipitation (table 20). Average annual precipitation ranges from about 355 to 760 millimeters, much of it fall- ing as winter snow. Diurnal summer tempera- tures fluctuate widely, with hot days and cold nights. In many areas, frost may occur any night of the year. The months of July, Au- gust, and September are very dry, with rain- fall averaging less than 25 millimeters. Much of this summer rain is ineffective, as it usually comes during brief, high-intensity convection storms. Winter temperatures are generally low; as a result, snow often accumulates to considerable depths. Since Pinus ponderosa occupies drier sites than any other forest type (except Juniperus occidental

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