Growth response of bigcone Douglas fir (Pseudotsuga macrocarpa) to long-term ozone exposure in southern California.

Long-term radial growth of bigcone Douglas fir (Pseudotsuga macrocarpa) was studied throughout its range in the San Bernardino Mountains of southern California, where ambient ozone has been high for approximately the past 40 years. A gradient of both ozone concentration and precipitation exists from west (high) to east (low). Growth rates of bigcone Douglas fir are considerably lower since 1950 throughout the San Bernardino Mountains, with the largest growth reductions in the western part of the range where ozone exposure is highest. Needle retention is also somewhat lower at high ozone sites. Lower annual precipitation since 1950 may have some impact on long-term growth reductions, and short-term growth reductions induced by drought are an important component of long-term growth reductions at sites with high ozone exposure. An ozone-climate stress complex may be responsible for recent reductions in the growth of bigcone Douglas fir.