Master’s Thesis Abstract by Michael Case (2004)

Information about the sensitivity to climate of Douglas-fir (Pseudotsuga menziesii) and lodgepole pine (Pinus contorta) is valuable because it will allow forest managers to maximize growth and better understand how carbon sequestration may change over time. Increased understanding of tree growth responses to climatic variability will also enable us to better model and predict future ecosystem responses to climatic change.

This study examined the effects of climatic variability on the growth of Douglas-fir and lodgepole pine along an altitudinal gradient in the North Cascades National Park, at annual and decadal time scales during the 20th century. I used factor analysis to identify two common growth patterns, and correlation analysis to help determine which chronologies associated best with each factor chronology. After correlating the factor chronologies with monthly, seasonal, and annual climate variables, I identified climate-growth relationships based on elevation. Mid-elevation chronologies were negatively correlated with growing season maximum temperature and positively correlated with growing season precipitation. In contrast, high-elevation chronologies were positively correlated with annual temperatures.

Douglas-fir and lodgepole pine growth is limited by both temperature and precipitation in the North Cascades. At low to mid-elevations, growth is limited by site water balance, which controls soil moisture during the summer dry period. Temperature becomes more limiting at higher elevations, where growth is largely affected by annual temperatures and low-frequency climatic variability (e.g., PDO). Higher temperatures melt the snowpack earlier and warm soil temperatures more quickly, thereby lengthening the period of time during which growth can occur.

Projected increases in summer temperatures will likely cause greater soil moisture stress in many forested ecosystems. The potential of extended summer drought periods over decades may significantly alter spatial patterns of productivity, thus impacting carbon storage. It is likely that the abundance and productivity of both Douglas-fir and lodgepole pine will decrease on sites with shallow, excessively drained soils, south and west facing aspects, and steep slopes.