Vegetation structure and distribution, while influenced by many factors (e.g. topography, organisms, topography, and soil parent material), is largely determined by global climate patterns. Climate controls such as precipitation, solar radiation, wind, temperature and humidity directly and indirectly affect plant physiology.
A characterization scheme of climate controls was devised by Henrick Walter. Between the years of 1957 and 1966, Walter and Lieth compiled a world atlas of climatic diagrams. These charts depict both the seasonality of thermal regime and of moisture availability. Walter diagrams therefore show many key climatic features.
Each diagram depicts the monthly averages for both temperature and precipitation for a year, with the months delineated by tic marks along the horizontal axis. For the northern hemisphere, January is the month on the far left; therefore, the middle of the diagram depicts the astronomic summer. A dry season is indicated with the precipitation curve dipping below the temperature curve; conversely, when the temperature curve is superseded by the precipitation curve, this marks a wet season. Climate diagrams from regions with similar climatic variations resemble each other.
An ecological province - or subdivision of a region - has a distinctive combination of geological features and ecological sites. In an ecological province, general regional differences in vegetation complexes among ecological sites are related to basic differences in underlying geology, geomorphology, and climate on a relatively broad scale that encompasses a number of ecological sites.
Within Oregon, the ecological provinces stratify the natural variation that exists across watershed basins and political boundaries such as counties, national forests, and Bureau of Land Management districts.