The purpose of this field trial is to assess natural regeneration of red pine using shelterwood and expanding gap regeneration treatments. We will also monitor the health effects on seedlings associated with the fungal disease Diplodia sapinea, which is present on site. To our knowledge, naturally regenerating red pine in this part of the state, on this native plant community, has not be attempted before. That said, MN DNR is currently preparing a case study in Park Rapids, MN examining release of advance red pine regeneration following final harvest on another FDc23 site.
This case study presents pre-treatment data. Data will be added in future years after successive treatments and assessments are complete. Crop tree and vegetative competition numbers and condition will be monitored.
If successful, natural regeneration of red pine would result in a significant improvement in net revenue compared to artificial regeneration. Ultimately, only a few hundred stems per acre of red pine fully exposed to sunlight and growing into merchantable size classes are needed to meet regeneration objectives (minimum of 400 trees per acre and 75% stocking at year 10 per MN DNR Regeneration Monitoring Procedures and Standards).
Diplodia sapinea is a fungus that causes shoot blight, branch death, decreased growth, and occasionally death to several species of pine including red pine. One of the assumed reasons that multi-aged red pine stands are uncommon in Minnesota is that Diplodia shoot blight limits naturally regenerating healthy and acceptably abundant red pine in the understory. This assumption is based on several university, state agency, and U.S. Forest Service studies, as well as years of forester observation. Diplodia is easily spread to pine seedlings and saplings from nearby mature trees, therefore the current recommendation is to grow young red pine without an overstory of red or jack pine, or at a distance that is at least twice the height of adjacent red and jack pine stands. By far the most common prescription for re-establishing red pine in mature stands is to clearcut, prepare the site mechanically or chemically and then plant.
While it goes against conventional practice, we feel there are several reasons why natural regeneration has a chance of success on these sites:
- The author has observed a number of examples of successful natural red pine regeneration occurring in small openings adjacent to mature red pine stands infected with Diplodia in this area of the state.
- The study will compare two different natural regeneration techniques, not just one, to a conventional final harvest followed by site prep and planting. The prescriptions include:
- Shelterwood using selective thinning roughly every seven years to release crop trees and to open up the forest floor for regeneration, then final harvest
- An “expanding gap” shelterwood every 7 years, with the first entry initiating the gaps and subsequent entries expanding the size of the gaps
- Conventional red pine silviculture system, thinnings approximately every 7 years until rotation age, then a clearcut with reserves regeneration harvest followed by site prep and planting.
- While natural regeneration practices for red pine have often been declared a “failure” within 3 to 5 years, at least two research papers (Ahlgren, C.E. 1976 and Bergeron, Y., and Brisson, J. 1990) indicate that natural stands can take more time (up to 10 years) to recruit the next cohort following a large disturbance. With this in mind, the plan on these sites is to begin the regeneration/recruitment phase two thinning entries prior to the final harvest. By overlapping the regeneration phase with thinning harvests, we can extend seedling recruitment to mimic natural processes without letting ground sit fallow.
- We anticipate that the open growing environment of the expanding gap treatment will reduce environmental conditions favorable to Diplodia infection. The expanding gaps will also allow us to avoid logging damage to seedlings during subsequent stand entries.
- High rates of Diplodia infection do not necessarily mean that natural regeneration won’t work, because the presence of Diplodia shoot blight on red and jack pine is not always the death knell for them. This is especially true if there is no overstory and they are fully exposed to sunlight. It is possible that hardwood competition may be the more important limiting factor when attempting to regenerate to red pine naturally, as has been observed on the Chippewa National Forest in a variable retention harvest experiment in a red pine plantation.
- The NPC (FDc23) found on this study site seems to be a good candidate for natural red pine regeneration trials. According to the establishment and recruitment table in the Silviculture Strategies website, red pine is ranked as good for establishment of regenerants but only fair for seedling recruitment to larger size classes in mature forest conditions. This indicates that red pine is able to become established in the understory but struggles to recruit to larger size classes in modern forests, presumably from vegetative competition and lack of sunlight. The silvicultural strategy we are applying with this prescription is to maintain FDc23 forests as would periodic surface fire, creating medium to large canopy gaps to control the light environment. (Link to FDc23 web page).
Successful implementation of these treatments may result in a fully stocked red pine stand comprised of trees established over an extended period (up to ten years). We also expect more species diversity and a more complex stand structure than in the existing plantation. This has the potential to benefit multiple wildlife species, increase resistance to insect and disease outbreaks, and increase stand resilience to climate change.
Pre-trial data will be shared now, however this study will continue to be updated with periodic field data collection and updates for up to 30 years. It should be noted that even though there are abundant natural red pine seedlings in the stand now, the regeneration recruitment phase will not actually begin in earnest until after the next thinning.