The relationship between old age and growth in trees is a fundamental question of tree life-history biology and may be important for the management of old forests. Although age-related growth decline has been reported at the stand-level in many forests, these studies seldom address the question of how individual trees grow over their lifetimes. The observation that old trees in a stand grow more slowly than younger trees often carries an unstated assumption that these growth differences are strictly age-driven. However, an alternative hypothesis is that intrinsically slow-growing trees are more likely to reach old age than fast-growing trees. In preliminary results using tree rings to measure long-term basal area growth in ponderosa pine trees we found that old, slow-growing trees also grew relatively slowly when they were young. These results challenge the notion that “old trees grow slow” by suggesting instead that “slow trees grow old”. A relationship between slow growth and longevity may be driven by underlying physiological mechanisms, such as hydraulic limitation, that imply size- rather than strictly age-related growth limits in trees. If size-limitation is important, measures of tree vigor such as basal area growth rate and growth efficiency (diameter growth per leaf or sapwood area) should decline more strongly with size than with age. We extracted and analyzed increment cores from young and very old ponderosa pine trees at two remote sites in the Selway-Bitterroot region of Idaho in order to determine 1) whether the oldest trees exhibit lifetime basal area growth rates below the average sustainable growth rate observed at that site, 2) whether basal area growth declines are related to relatively high average growth rates prior to decline, and 3) whether basal area growth and growth efficiency decline more strongly with size than with age.
Results/Conclusions
Preliminary results from one of the sites showed no consistent trend of age- or height related decline in basal area growth rate. However, growth efficiency declined more strongly with height than with age. These preliminary results suggest that individual old trees can maintain slow basal area growth and stable growth efficiency for centuries after “age-related growth decline” at the stand level occurs. Further tests and analyses will be used to investigate the relationship between slow-growth and longevity in these trees.
