Relations of climate and radial increment of western hemlock in an old-growth Douglas-fir forest in southern Washington

The objective of this study was to examine the association of western hemlock (Tsuga heterophylla) growth to climate across structural levels in an old-growth forest. The canopy vertical gradient was sectioned into six 10-m height classes with western hemlock growth examined in the top five levels. All trees in a 4.0 ha plot (n = 645), greater than 15cm diameter at breast height (DBH) were cored for the recent 20-year growth record. Growth chronologies representing each height class were correlated to 10 climatic variables. The 40-50m height class exhibited the highest correlations to climate and the 10-20m height class exhibited the lowest. When moving averages of 3 and 5 years were applied to the growth index and climatic variables, the correlations of height class one (10-20m) became the strongest suggesting that growth of canopy dominants is determined by short term climatic conditions while understory growth was influenced by longer term climatic conditions. Understory growth at the 5-year temporal scale was strongly (-0.6 > R < -0.8) and negatively correlated to temperature while its correlation to precipitation was strongly positive. The annual depth of snow at the 5-year time scale was the only climatic variable to affect growth similarly for all height classes, indicating consecutive years of deep snow had a negative impact upon western hemlock productivity. Understanding effects of canopy structure upon the relationships between climate and tree growth can improve interpretation of past climatic conditions and stand dynamics from long tree cores. The climatic signal for stand chronologies that experienced suppression or competition could be adjusted for typical changes that occur with tree ascension to canopy dominance. Current uses of universal climate-growth relationships in forest ecosystems (e.g., gap family models), especially in structurally diverse old-growth stands, provide a questionable picture of forest growth and dynamicsRudnicki and Chen "Relations of climate and radial increment of western hemlock in an old-growth Douglas-fir forest in southern Washington." Northwest Science. 2000; 74(1): 57-6