Long-Term Records of Temperature and Precipitation in the Pacific Northwest Derived from Tree Rings

Annual growth records from climatically sensitive trees growing in Washington, Oregon, and northern California are used to reconstruct annual temperature and precipitation variation in the Pacific Northwest over the last several hundred years. Response surfaces indicate that growth of mountain hemlock (Tsuga mertensiana) and subalpine larch (Larix lyallii) at timberline in the Cascade Range of Washington is complexly related to variation in summer temperature and spring snow depth. Interactions between these climatic variables in governing tree growth therefore make it difficult to separately reconstruct either of these seasonal climatic variables using standard methods. Mean annual temperature values, however, combine information on both summer temperature and spring snow depth. Mean annual temperature values were therefore reconstructed at Longmire, Washington (46°47′N, 121°44′W; 842 m) using a regression model with larch and hemlock tree-ring chronologies as predictors. The reconstruction shows mean annual temperatures between 1590 and 1900 to be approximately 1°C lower than those of the 20th century. Only during a short period from 1650 to 1690 did temperatures approach 20th century values. Long-term regional precipitation variation within the Pacific Northwest is reflected in reconstructions of three mean annual precipitation indices representing the “Western Lowlands” (western Washington and northwestern Oregon), “Columbia Basin” (eastern Washington and northeastern Oregon) and “Southern Valleys” (southeastern Oregon and northern California). Tree-ring chronologies from drought sensitive Douglas-fir (Pseudotsuga menziesii) and ponderosa pine (Pinus ponderosa) are used to reconstruct each precipitation series back to 1675. The precipitation reconstructions do not reveal long-term changes in mean conditions but show episodes of wet and dry conditions that differed in timing between the regions. During the first half of the 19th century, precipitation equaled or exceeded the long term average in the Western Lowlands and Columbia Basin but was below average in the Southern Valleys. During the second half of the 19th century, the Southern Valleys experienced above average precipitation while precipitation was below average in the Columbia Basin. Single year drought events show great spatial homogeneity implying that severe dry years are caused by circulation features of sufficient size to affect the entire Pacific Northwest.