Overall, there was only a weak tendency for declining growth rates in mature red pines at Itasca (Fig. 2.5). Average radial growth rates held near 1 mm / year for most of this century. A depression in growth rates from 1989-91 corresponds to a drought. There was a slight tendency for older trees to have lower growth rates, but there were no effects of age on physiological stability as measured by interannual variation in growth rate, nor were old trees more likely to be in growth decline than young trees (Fig. 2.6). Furthermore, older trees suffered less impacts from drought and had higher resin flow than younger trees. These data indicate continuing growth and high defense in even the oldest red pines at Itasca. There was no suggestion of physiological senescence in even the oldest trees in our sample (up to 220 years old). In the absence of mortality from windstorms, bark beetles, fires, lightning, icestorms, and other disturbances, it seems realistic to expect that many mature red pines in Itasca could live for another century or more. This is very positive news for park management. One consequence is that it becomes appropriate to minimize any manageable risks of tree mortality. Given the long potential lifespan of these trees, even very small changes in mortality risk can have dramatic impacts on the lifetime of the forest (see Figs. 5.7 and 6.9 in Chapters 5 and 6).

We further tested the effects of tree age on anti-herbivore defenses by comparing resin flow in mature red pines at Itasca (100 - 220 years old) with young red pines (30 - 80 years old). Mature red pines had dramatically higher resin flow (about two-fold) than younger red pines growing on similar sites in the same area (Figs. 2.11-2.12). These results reinforce the conclusions that mature red pines at Itasca are not senescing and are not poorly defended against bark beetles.