Impacts of drought and bark beetles on red pine forests of the Anoka Plains, Minnesota

Executive Summary
Red pine, the state tree of Minnesota, was an important element of pre-settlement forests, and a significant timber species during the early 20th century. Red pine has been widely propagated since the late 1950s and has proven its value in terms of growth, pest resistance, soil conservation, wildlife, biodiversity, aesthetics, recreation, and timber value. Also, it facilitates a silvicultural system with regular selective harvests over a long rotation time, which allows for growing economic returns to landowners and the forest products industry while simultaneously favoring forested landscapes that include more large trees and old-growth stands than the region has known since the early 1900s. Because it is only now that the first rotation of plantings are maturing into forests, land managers have surprisingly limited experience on which to base management decisions. For example, there is limited ability to know when, or where, there will be consequential mortality of adult trees from drought and bark beetles, and what, if anything, can be done to mitigate the undesirable impacts. Motivated by the most recent drought, we conducted studies to address the following questions: (1) Why is red pine mortality associated with drought and bark beetles more common in the Anoka plains of east central Minnesota than in nearby, apparently similar, forests; e.g., the Colfax region of west central Wisconsin? (2) Are there predictable patterns in tree mortality within a region that are related to soil type? (3) Do bark beetle infestations tend to become self-perpetuating eruptions following a drought?
       The Anoka Plains and the Colfax region, separated by ~ 90 miles, both contain extensive red pine forests that experience similar climates and grow on sandy soils derived from glacial outwash. The Anoka Plains have received an average of 6% less annual precipitation, which seems inadequate by itself to explain the history of recurrent droughts and forest disturbance. However, analyses of river discharge data indicated that the sandy soils of Anoka are different enough from the loamy sands of Colfax to strongly affect the probability of droughts. In ~20 years since 1930, the median weekly discharge rate from rivers that drain the Anoka plains have dropped below 30% of the long-term average. This compares to only 1 year that rivers draining the Colfax region have been so low. Droughts in 2000, 1988-89, 1976, 1964, and the 1930s were plainly evident in the river discharge data, which suggests that real-time, online, discharge data provided by the USGS can be used to recognize droughts at the time when tree water stress is maximal. The recent drought at Anoka apparently reached its nadir in October of 2000. Some tree deaths became evident the following winter. More extensive tree mortality during the next two growing seasons was apparently due to an epidemic of the bark beetle, Ips pini, that was triggered by the drought. Regional differences in the rates of red pine growth and mortality are so large that optimal silvicultural practices (e.g., harvest schedules) must be different, but we know of no regionally customized guidelines that are presently available to land owners and forest managers.
       Results supported the hypothesis that populations of I. pini are normally regulated at endemic levels by resource limitations, but can switch to an epidemic state following a resource pulse from drought-killed trees. Endemic populations of I. pini may function chiefly as scavengers of trees that are dying for other reasons, while epidemic populations seem to escape resource limitations by attacking, killing, and reproducing within healthy trees. Under this model, bark beetles amplify climatic effects into self-perpetuating episodes of forest disturbance. Direct death of trees from drought may be less important than drought as a factor that triggers state changes of bark beetles from endemic to epidemic. Pest systems with multiple equilibria are ideal candidates for cost-effective control, because occasional intervention can drive eruptions back to endemic levels where they will tend to be maintained by natural regulatory forces. In the case of I. pini, it is relatively cheap and easy to monitor local populations with pheromone-baited funnel traps to identify potential eruptions before they occur (our sampling suggested an action threshold of ~ 500 I. pini / stand). To our knowledge, this was the first test for eruptive population dynamics in I. pini. Conclusions should be regarded as tentative, but indicate the possibility for cost-effective mitigation of forest disturbance in regions such as the Anoka Plains. We suggest that a plan be developed and implemented to evaluate different possible strategies for pest monitoring and control. This could probably be done in a way that: (1) permits validation or refinement of bark beetle population models; (2) tests for possible undesirable side effects, e.g., removal of natural controls by predators; and (3) provides immediate reductions in tree mortality from beetles.