The biology and management of bark beetles in old growth pine forests of Itasca State Park
| Executive summary part IX:Interactions between fire, bark beetles, and tree mortality |
| Prescribed fires were implemented at Itasca during 1998 and 1999. We extended our research to consider fire because it appeared that
interactions between fire and bark beetles may be at least as important to forest management as interactions between windstorm and
bark beetles. Prescribed fires might reduce bark beetle abundance by killing beetles or increase local abundance by producing volatiles
that attract beetles. In fact, the 1998 fire had only limited effects on the abundance of bark beetles (slight increases in I. pini during
May and slight decreases during mid-summer; Fig. 6.1). However, fire produced a short-term reduction in the resin defenses of red
pine and triggered rapid colonization of the scorched trunks by bark beetles (Figs. 6.3 - 6.4, Table 6.1). Within 10 - 30 days, the resin
flow in scorched trees increased to higher than baseline levels (Figs. 6.2 - 6.3), which restricted the extent of beetle galleries and
probably saved many scorched trees from being killed by bark beetles (Fig. 6.4). Nonetheless, these attacks kill the phloem, interrupt
vascular connections, and partially girdle the tree (permanently). The cambium in the infested area also dies, which precludes
additional growth of bark or wood and ensures that a scar will form. This is probably the dominant process by which so-called
"cat-faced scars" have been produced on mature red pines at Itasca (Fig. 6.5 - 6.6).
Following the prescribed burns in 1998-99, we found dozens of red pines with incipient scars forming as a result of beetles attacking the scorched lower trunks (Fig. 6.4). We also found many red pines with pre-existing cat-faced scars, whose living bark was being colonized by beetles around the periphery of the old scar. This appeared to be the result of greater heat trauma to the living tissue around the region of the trunk that lacked insulating bark. Finally, the prescribed fires directly killed many red pines when the wood that was exposed at pre-existing scars was ignited by the fire (Fig. 6.5). Fires and beetles can produce a positive feedback loop in which fires promote beetle attack, which increases susceptibility to future fires and future beetles, and which eventually leads to the death of the tree (Fig. 6.7). There can be additional positive feedback at the level of the forest, because a tree that dies in one fire increases the fuel load for future fires and therefore increases the probability of fire and beetle damage to adjacent trees (Fig. 6.8). Because the relationship between fire, bark beetles, and tree mortality appears to involve positive feedbacks, the proportion of trees that succumb to fires would be expected to increase with each additional fire, and it would be easy to underestimate the consequences of future fires for tree mortality. For example, a doubling of tree mortality rates as a result of increased fire frequency could reduce the half-life of the forest from 65 years to 37 years (Fig. 6.9). We suggest that fire management practices at Itasca be developed in concert with the refinement and parameterization of demographic models of tree survivorship. Presumably, the optimal fire management strategy is one that balances the costs of reduced survivorship in mature trees against the benefits of increased regeneration. Chapter 6 concludes with a list of specific recommendations for minimizing tree mortality associated with fire and bark beetles. Appendices 1-5 provide additional raw data. Appendices 6 and 7 are two papers from this research that have been accepted for publication in peer-reviewed journals (Environmental Entomology and Forest Ecology & Management). Three additional appendices in Volume II of this report are the result of literature searches for scientific papers related to Ips bark beetles, fire and insects, and fire and pine forests, respectively; they include citations and abstracts for about 250 papers. |
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