- Hager, Annie E.S.
University of Maine Graduate School
Predictions of the effects of climate change on forest ecosystems are often based on the predictions of computer models, which are in turn based on assumptions about forest ecosystem processes. One of the assumptions commonly employed by "gap" models is that the effect of temperature on tree growth can be represented as a parabolic curve relating heat sum during the growing season to radial growth of individual trees.
In order to test the validity of the gap models' assumption of temperature effects on tree growth,I collected tree-cores from several stands of pitch pine (pinus rigida) in New England and New York. I examined the relationship between radial growth of pitch pine and climate in two different ways: first, I used the standard dendroclimatological technique of the response function to correlate radial growth to monthly average temperature and monthly total precipitation over the period 1935 -1992 for four sites. Second, I compared average radial growth to growing season temperature sum (DEGD) at six sites.
Neither monthly average temperature and monthly total precipitation nor DEGD were strong, consistent predictors of radial growth. Radial growth of the Long Island stand showed significant positive responses to February temperature and June precipitation in the year of growth. None of the other three sites analyzed showed any significant response to monthly temperature or precipitation. Two sites from Massachusetts showed a positive correlation between DEGD and radial growth, two sites from Maine showed a negative correlation, and sites from Albany and Long Island, New York both showed no significant correlation.
While it is still unclear how climate influences yearly ring-width patterns in pitch pine, it appears from this study that the assumption made by gap models, that temperature has a predictable, parabolically shaped effect on annual radial growth, is inaccurate for this species.