Multiple diseases impact survival of pine species planted in red pine stands harvested in spatially variable retention patterns.
Increasing the diversity of species and structure of red pine (Pinus resinosa) is often a management goal in stands simplified by practices such as fire suppression and plantation management in many areas of the Great Lakes Region. One approach to diversification is to convert predominantly even-aged, pure red pine stands to multi-cohort, mixed-species forests through variable overstory retention at harvest. Based on limited empirical evidence, pathologists have advised against this multi-cohort approach in stands where pathogens causing damaging shoot blight diseases are established. We examined disease incidence among planted red, jack (Pinus banksiana), and white pine (Pinus strobus) in a variable retention harvest and understory woody vegetation removal (brushing) experiment in northern Minnesota. The experiment included four overstory treatments (dispersed and two aggregated overstory retention treatments and a control, N=4) that were split by an understory brushing treatment (yes or no). Prior to harvest in 2003, the fungal pine pathogens Diplodia pinea, Sirococcus conigenus and Armillaria solidipes (syn. Armillaria ostoyae) were common on the study site. Within 6 years after harvest, these pathogens reduced the survival of planted red, white and jack pine, potentially interfering with long-term management objectives. Across all treatments, shoot blight incidence was generally higher in dead red and jack pine than white pine seedlings and was predominantly caused by D. pinea. The disease killing white pine seedlings was predominately Armillaria root rot. Overstory treatment affected the percentage of jack and white pine seedling mortality attributable to shoot blight, but not the more susceptible red pine, with greater overstory retention resulting in greater disease incidence. Understory brushing had no effect on the incidence of shoot blight on seedlings. We expect disease to continue to influence stand structure and composition across all treatments. Our study results highlight the need for forest managers to assess long-term risk of potentially damaging pathogens in red pine stands prior to harvest and use that information to guide decisions regarding silvicultural practices to increase age and species diversity.