Growth Variations in Old-Growth Douglas-fir Forests of the Puget Sound Area: Final Report

Brubaker, Linda; Ford, E.; Earle, Christopher; Vega-Gonzalez, Silvia; Ribic, Christine

The research reported herein focuses on the question, "Are recent changes in tree growth at low- to mid-elevation forests in western Washington greater than can be attributed to natural variability?" We examined temporal and spatial pattern in the radial growth of 400-600 year old Douglas-fir trees at nine old-growth stands in the Cascade and Olympic Mountains of Washington. The distribution of the sites spans a range of pollution levels in the Puget Sound region as inferred from knowledge of surface wind patterns relative to pollution source areas and from recent monitoring of atmospheric deposition (Edmonds and Basabe, personal communication). .... The first objective of our work was to describe temporal and spatial variations in tree growth for periods before potential pollution effects, as pollution effects must be judged against an understanding of natural growth variations. We chose the period 1700 to 1960 for characterizing natural (pre-pollution) growth variations. Though long-term air monitoring data are lacking, pollution in the Puget Sound regions was probably minimal prior to 1960. Because of the great stand ages, tree growth after 1700 was well past the stages of rapid juvenile growth and intense self-thinning, which strongly influence individual tree growth and complicate analyses of exogenous influences such as pollution. Over the period 1700-1960, short-term growth variations are most likely caused by weather fluctuations, and slow variations in tree growth are most likely caused by gap-replacement processes, disturbance and long-term climatic variations. Thus, the temporal and spatial characteristics of growth during 1700-1960 could possibly provide control conditions for assessing whether recent (post-1960) temporal and spatial growth variations are anomalous. However, the critical question in adopting such a strategy is the extent to which variation in the pre-1960 period can be characterized in a homogeneous way. In addition to causing clearly anomalous growth rates, pollution can disrupt relationships between growth and climate. In this case, growth during pollution periods may not be unusual with respect to the long-term mean, variance, etc., but would be unusual with respect to contemporaneous climatic variations. The second objective of our work, therefore, was to compare climate-growth relationships before and after 1960. Changes in these relationships between periods would suggest a pollution effect, even if growth variations since 1960 did not exceed the range of variation for earlier periods.