RESPONSE OF TREE GROWTH TO CLIMATIC VARIATION IN THE MIXED CONIFER AND DECIDUOUS FORESTS OF THE UPPER GREAT-LAKES REGION

Tree-ring data were used to define regional tree-growth anomalies (i.e., recurring spatial patterns of growth that differ froin long-term averages) for 11 species growing in a network of sites spanning the deciduous and mixed hardwood-conifer forest boundary in the upper Great Lakes region. Tree-ring samples were collected at 11 sites that are classified as mesic to dry-mesic based on species composition. At each stand at least 20 trees were sampled of each species dominant in the canopy, resulting in one to five species collections per stand and 31 chronologies in total. Principal component analysis was used to define the common variance among the chronologies. Three components explain 57.4% of the variation among the chronologies, indicating that common patterns of tree growth exist within the multispecies network. Component loadings indicate that (i) species to species variation is more important than site to site variation and (ii) species can be segregated into distinct groups based on their common patterns of growth through time. Correlations between the three-component-score time series and climatic data indicate that growth anomaly patterns are weakly, but significantly, correlated with growing-season temperature and precipitation variables. Extreme climatic events (i.e., greater than 1 SD above or below the mean) are very important in generating differential growth rates among the species sampled.