Glacier National Park in northwestern Montana represents the easternmost extent of western red cedar (Thuja plicata) distribution in North America. If climatic changes are to affect this species, it is likely that populations at the range-margin will be the first to show signs of stress. Past climate influence on growth may give insight to future effects of climate change on the species. Using tree rings, we identify the influence of past climatic variability on the annual growth of western red cedar, as well as the influence of topographic and edaphic settings. Plots were sampled (n = 35) in the Avalanche Creek Drainage of Glacier National Park. Pearson correlation coefficients were calculated to indicate the magnitude and direction of influence of climate on tree growth (increased or decreased growth relative to the mean). Comparisons of plot characteristics included slope, aspect, elevation, and proximity to water.
Results/Conclusions
At the population level, cedar growth was negatively affected by warmer temperatures throughout the mid-summer months (R = 0.270), but increasing levels of precipitation during the same time period had the opposite influence (R = 0.193). Warmer temperatures during the early growing season were positively correlated with increased ring widths, with precipitation being less important to increased growth, and even negatively correlated with growth if precipitation was too high during this time. Tree growth on northeast aspects showed a positive correlation with higher minimum growing season temperatures (mean R = 0.326), while southwest aspects showed a similar correlation only during July and August (mean R = 0.312). The primary difference between high and low elevation plots was a negative correlation (R = 0.227) between growth and maximum temperatures during May in low elevation plots where temperatures are generally warmer. Despite western red cedar being a relatively mesic species, precipitation was not an important predictor of tree growth throughout the year. Trees on moderate slopes were negatively affected by high May maximum temperatures (R = 0.234). Even these preliminary analyses suggest that an understanding of how climate and environmental settings affect the growth of cedar is important, and will allow land managers to better understand the potential effects of future climate change on this species at its range-margin. Further, they suggest that without the opposing influence of increased precipitation, increases in temperature may stress trees in topographic settings where trees are not currently stressed.