Thursday, August 7, 2008

PS 60-86: Age class, longevity, and growth rate relationships: Evidence for increasing growth in both young and old trees

Sarah E. Johnson and Marc D. Abrams. Pennsylvania State University


Tree growth rates and patterns are a function of many factors, and are a dynamic resource that allows us to study past changes in forests, as well as predict the future impacts of a changing environment.  Growth rates have been correlated both with species and individual longevity in the past, but this has not been documented over a wide range of species of varying life history traits.  We hypothesize that individual trees are able to approach the maximum species longevity by growing very slowly throughout most or all of their life.  Within that framework, species longevity, and thus growth rate, will vary by shade tolerance and site quality.  This study utilizes data from the International Tree Ring Data Bank website and tree cores collected in the field to explore the relationship between growth rate (basal area increment; BAI) and age class (from young to old) for eight contrasting tree species in the eastern U.S.; bigtooth aspen (Populus grandidentata), black oak (Quercus velutina), red oak (Q. rubra), chestnut oak (Q. montana), white oak (Q. alba), pitch pine (Pinus rigida), hemlock (Tsuga canadensis), and blackgum (Nyssa sylvatica).


Our results support the hypothesis that the oldest trees within each species have very slow growth, and that growth rate and longevity vary inversely with shade tolerance and site inferiority.  In addition, we found that younger trees (<60 years of age) within each species are consistently growing faster than the older trees, but that both young and old trees exhibit increasing BAI throughout their lives (i.e., an expected sigmoidal growth plateau was not observed.  A separate analysis involving only trees within a species growing on the same site revealed similar trends of decreasing growth with increasing age class, increasing growth with age within each age class, and younger trees growing faster than older trees at the same age.  The pattern of increasing growth throughout the life of trees suggests that global change phenomenon, including land-use history, elevated CO2, global warming, and altered precipitation chemistry (increased nitrogen input) has increased the growth rate of trees during the last half-century or more in the eastern U.S.