Effects of climate variability on tree growth of Picea glauca across latitudinal and forest productivity gradients in southwest Alaska

Background/Question/Methods

Increased productivity at the northern and western margins of the North American boreal forest appears to be offsetting declines in growth and productivity in the interior.  How this variation in tree growth plays out geographically is poorly understood, particularly in the transition zone between the colder, drier forests of the interior, and the warmer, wetter maritime systems of western Alaska.  In this study, we investigated the effects of temperature and precipitation on tree growth of Picea glauca across latitudinal (ca. 59 to 61^o N) and forest productivity gradients in southwest Alaska.  We hypothesized that in lower-elevation and higher productivity (closed canopy) forests at the northern, interior end of the gradient, temperature is at or exceeding a critical threshold for growth.  In the highest elevation (treeline) and lower productivity (open woodland) forests at the southern, maritime end of the gradient, we expected that rising temperatures would continue to favor tree growth.  We evaluated growth trends for cores collected at 30 sites and developed standard ring-width chronologies from crossdated tree cores for each site (1302 cores total).  Ring-width chronologies were averaged by forest type (forest, woodland, treeline) and latitudinal zone (northern, central, southern) for estimation of growth-climate relationships across the gradient(s).

Results/Conclusions

We found substantial increases in radial growth since the 1970s across forest types and latitudinal zones, with the greatest increase in growth at treeline sites at the northern end of the gradient.  Temperature-related increases in growth at the southern sites have occurred only since 2000.  With the exception of treeline sites, however, the relationship between temperature and growth, and the rate of tree growth, has decreased since the late 1980s.  Growth in lower elevation, closed-canopy forests show a weaker response to temperature (correlation of >0.50 to 0.26), or a complete shift from positive to negative correlation with summer temperature (correlation of >0.50 to -0.21), since ca. 1986.  We also found a shift from positive to negative growth relationships with winter-spring temperatures (January-March) in all but the southern sites in this time frame, particularly for trees that established prior to 1910.  These results suggest that persistently warm summer temperatures (i.e., 11-12.6^oC average June temperature) and continued warming (e.g., an increase of 1-2^oC since 1986) may be inducing stress as trees experience temperatures beyond an optimum level for growth.  This may be especially true for older trees (pre-1910 establishment) at lower elevations in the study area.