The biology and management of bark beetles in old growth pine forests of Itasca State Park
| Executive summary part VIII: Do bark beetles kill trees at Itasca? |
| Ips infestations at Itasca could be restricted to trees that are destined to die soon anyway. If so, the abundance of Ips would have no
consequences for the demography of pine forests at Itasca and could be ignored in management decisions. We tested whether or not
Ips kill red pines at Itasca by surveying for trees that were infested by bark beetles, monitoring the fate of those trees, and evaluating
whether the infested trees were in declining physiological condition prior to being infested by beetles. In 1998, we examined about
4000 mature red pines and located 41 that were infested by bark beetles (see photos in Figs. 5.1 and 5.3). In 1999, we searched the
same area and found 39 other trees that had come under attack. Ips pini and/or I. grandicollis accounted for most of the infestations.
Of 41 red pines infested during 1998, 21 were dead, or very nearly so, by the end of the next growing season (Fig. 5.3). Of the 17 trees
that were still alive, six were sustaining continuing attacks in 1999, and 11 were apparently free of new attacks (Table 5.1). In our
judgement, some of the trees in this latter group might survive for decades longer. However, all attacked trees sustained irrepairable
damage to their vascular system, lost significant portions of their crown, and were destined to be more vulnerable to future insects,
pathogens, fire, and windstorms. Thus, beetle infestations led to rapid mortality in about half of the attacked trees and increased the
probability of mortality for the remainder.
None of the trees that were infested by bark beetles showed a pattern of declining growth in the years immediately preceding the infestation nor any systematic differences from their paired control trees at any time from 1935 - 1996 (Figs. 5.4-5.5). In fact, bark beetle infestations included some of the fastest growing trees in the park (Fig. 5.6). Apparently, bark beetles at Itasca commonly kill red pines that could otherwise live for decades. With an annual mortality rate of 0.5% from bark beetles (as suggested by our surveys), and no regeneration of trees, the Itasca population of red pines would be halved by beetle attacks in 139 years. The expected half life of the forest declines to 65 years if we include a modest level of background mortality from other causes (0.25% / year) and assume that the mortality rate increases in trees that have survived past beetle attacks (to 5% / year). If the same hypothetical forest lacked bark beetles, it would have a half life of 277 years. Bark beetle attack rates such as we observed at Itasca can have surprisingly large effects on forest longevity (Fig. 5.7). |
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