Master’s Thesis Abstract by Melisa L. Holman (2004)

Forests of the Pacific Northwest are highly productive and may act as substantial carbon sinks as global atmospheric concentrations of carbon dioxide continue to rise. Few studies analyze how tree growth sensitivity to climatic variability varies over multiple spatial scales-including responses of non-dominant trees and trees that are not located at the extremes of their growth ranges. We quantified tree growth for all size classes and species in 76 forest plots (0.05 ha) spanning the diversity of forest types and growing conditions in the Hoh River watershed on the wet, west side of the Olympic Mountains, and the Dungeness River watershed, within the eastern rainshadow. We compared annual basal area increment (BAI) during two different time periods (1947-1976 and 1977-2000), representing approximately the two most recent phases of the Pacific Decadal Oscillation, a dominant system of interdecadal climatic variability in the Pacific Northwest. We performed analyses at different spatial scales (stand, forest type, watershed, subregion) to determine the scale of the dominant growth influence during these periods of contrasting climate. Growth correlations at small spatial scales suggest that trees are responding to very local growth conditions. However, significant positive growth correlations between geographically adjacent forest types (R=0.440-0.852) and between watersheds (R=0.430) indicate that there is a common overarching growth-limiting factor (or set of co-occurring factors) that similarly affects the growth of many trees over large areas. Because the magnitude and direction of future climatic variability and change remain uncertain at the regional scale, we also estimated the sensitivity of forest types to annual variability in growth-limiting factors. The Sitka spruce forest type in the Hoh watershed is the most sensitive to environmental change with the highest mean sensitivity (0.345), the highest potential for annual growth change (mean BAI=0.0047 m2), and the highest growth variability (coefficient of variation = 0.498, range in variability = 4.40 m2). In addition, this forest type is most likely to exhibit extreme positive growth responses (4.2% of years have BAI values two standard deviations above the mean).

In summary, while there are many growth-limiting factors acting at the local scale, widespread positive growth correlations at large spatial scales suggest that forests of the Olympic Mountains are responding to an overarching climate-growth signal. The Hoh Sitka spruce forest type and other highly productive Pacific Northwest forest types are relatively sensitive to changes in growth-limiting factors and will likely play an important role in storing carbon in a warmer future climate. Managers can employ the processes used in this study to evaluate the sensitivity and carbon storage potential of forests in other subregions within the Pacific Northwest (and elsewhere), thereby increasing our understanding of the potential effects of climatic change on regional productivity and carbon dynamics.