Red Lake WMA expanding gap irregular shelterwood in red pine plantations (MN DNR)

Overview ­

This is part of a research study between MN DNR Division of Wildlife and USDA Forest Service Northern Research Station designed to restore conditions more similar to NPC classification of natural stands with wildlife, silvicultural and ecological objectives. Two treatments are studied, but only the expanding gap irregular shelterwood treatment is described in this case study. The other treatment (not further described here) was a dispersed retention harvest that resembled free thinning which is designed to achieve a uniform residual BA of 40 ft²/ac.  The treatment described here is a combined single-tree and expanding group selection approach, with residual BA ranging within stand from 0 to 90 ft²/ac and a stand average of 40 ft²/ac. All harvest activity will produce silvicultural, ecological, habitat and economic outcomes of differing management objectives including quality production, quantity production, production of specific materials, regeneration and habitat improvements, and sound ecological management.

Silviculture Objective(s)

Objectives are to restore structural heterogeneity and complexity more reflective of natural origin older age red pine for this NPC.  As a result, the treatment will lead to a two-aged or potentially multi-cohort, red pine dominated forest with increasing abundance of non-red pine species as they colonize the gaps. The treatment will have more species diversity and stand structural complexity; less hazel cover; similar vigor; multiple cohorts including red, white and jack pine and hardwood species, increased growth of residuals, and larger diameter trees than initial stand conditions.  In addition to what is described above the treated stands will have these qualities:

• The stand is red pine dominated, but red pine abundance may fluctuate over time
• Minor species including white pine, jack pine, red oak, burr oak, red maple, paper birch, aspen (bigtooth and/or trembling), balsam fir, and white spruce are present in greater amounts than current conditions
• The stand has low to moderate hazel density
• Stand and tree productivity remains high, but are allowed to deviate within a larger acceptable range compared to the dispersed treatment
• The incidence of insects and disease fluctuates, but remains within an acceptable range
• Stand development and processes may deviate from reference conditions following disturbance, with greater diversity, structure, and resilience
• The stand is two-aged in the short term and potentially multi-cohort in the long-term

Pre-treatment stand description and condition

Stand establishment and management history

The red pine stands in this case study were machine planted in rows between 1932 and 1945. The last management was the first entry of the expanding gap irregular shelterwood with thinning between the gap openings in late summer 2004.

Figure 1: Stand average starting condition pre-2004 harvest.

Figure 2: 10 years following 2004 group selection harvest showing developing gap regeneration and mostly closed canopy matrix between gaps.

Pre-treatment species composition

Prior to the initial harvest in late summer 2004 the stands were very dense, even-aged red pine plantations with a closed canopy and little diversity in the overstory or understory. Post-2004 treatment stands were dominated by red and white pine with scattered overstory components of jack pine, paper birch, balsam fir, quaking aspen, and white spruce. Gaps were regenerating with red pine, eastern white pine, jack pine, paper birch and quaking aspen, while the understory matrix areas between the gaps were sparse with red pine, white pine, balsam fir and white spruce seedlings There was also a moderate shrub layer dominated by hazel and an increase in blueberry density and fruit production.

Pre-treatment growth and stocking 

The initial treatment application included 4 stands. Their size and stocking information from 2015 prior to the expanding gap harvest is listed in the table below.

Table 1: 2015 stocking table for the four stands described in this case study. The gap treatments represent the initial entry as part of an expanding gap irregular shelterwood. 

Pre-treatment forest health issues 

Prior to initial harvest in 2004, available growing space was declining. After 2004 harvest, some indication of shoot blight on new pine regeneration and browse by deer. 

Landowner objectives/situation 

Improve wildlife habitat and restore native plant communities, diversity and stand structure more similar to natural origin stands for the NPC.

Silviculture Prescription 

The expanding gap irregular shelterwood prescription was designed to improve growth of residuals, increase structural diversity, and increase natural regeneration in the stand. It was a regeneration harvest that used gaps and thinning between gaps to reduce stocking to a stand average of 40 ft²/ac residual basal area. The existing 1/3 to 1/2 acre gaps from the 2004 harvest were enlarged 15-25% in 2017 and sometimes combined with adjacent gaps to form irregular gaps with scattered residuals for structure while the larger areas between gaps were free thinned with a variable density approach to 60-90 ft²/ac.

What actually happened during the treatment 

Drive-to-tree feller bunchers and skidders completed whole tree harvest in 2004 to create the original gaps and establish skid routes to be used in later entries. Just prior to the 2004 harvest the stands were seeded with a mixture of jack pine (3/4 oz. per ac), eastern white pine (2 oz. per acre), and paper birch (0.08 oz. per ac). In 2017 the stands were again harvested with feller bunchers and tops were piled at the landing. The first entry trees were felled into the created gaps to protect residual trees while the second entry the harvest machinery was restricted to the matrix parts of the stand to preserve established regeneration.

Figure 3: Brady Zitzmann marking for expanding gap harvest in 2017.

Figure 4: 2019 aerial photo of one stand showing 2004 gaps with advanced regeneration (outlined in yellow) and 2017 expanded gap areas in light green between the polygons.

Post-treatment assessment

Variation in tree species diameter and density is greater in expanding gap treatments compared to evenly dispersed residuals. Expanding gap treatments had significantly greater total sapling density (<5in, 12.7cm) twelve years after treatment and after the second harvest compared to the dispersed thinning treatment. The shelterwood treatment has significantly greater sapling densities of red pine, jack pine and paper birch whereas the dispersed treatments favored eastern white pine, white spruce and balsam fir.

Figure 5: Diameter distributions, aggregated for all four treatments, over time.

Figure 6: A matrix plot example 2 years following 2017 expanding gap harvest, illustrating successful natural pine regeneration under canopy of red pine and 2004 expanded gap in the background with established regeneration.

Figure 7: A matrix plot example 2 years following expanding gap harvest, illustrating successful natural pine regeneration (foreground and center) and 2004 created gap (left and far right) with diversity in established regeneration.

Figure 8: Increase in native plant community diversity and blueberry density and fruit production following expanding gap harvest.

Plans for future treatments 

No plans for future harvest at this time, but potential for individual tree release to favor desired species survival and growth. A non-commercial release could steer composition towards desired wildlife or commercial values while commercial harvest in future years could release advanced regeneration of desired species.

Costs and economic considerations 

This harvest was cut tree marked and cruised by MN DNR and USFS staff with research goals of minimizing bias and creating stand level conditions that could be statistically evaluated; consequently, marking took longer than normal. An alternative would involve GIS-defined gaps and logger-select free thinning with scaling to expedite logging efficiency and reduce sale prep.

Climate adaptation considerations

In 2004, species seeded included eastern white pine, jack pine and paper birch. White pine is projected to have stable habitat suitability with climate change, but it had low seeding success. Supplemental planting of white pine may have been more successful. Increasing diversity allows for greater options for future management given the uncertainty in climate projections. Lowered stocking will provide greater drought resiliency and promote healthy growing conditions for residual overstory.

Summary / lessons learned / additional thoughts 

The expanding gap irregular shelterwood treatment created greater variation in tree density, increased species composition and initiation of multi-cohort development. compared to pre–harvest conditions. 

Development of multi-cohort, mixed-species stands through retention of residual red pine is achievable and full stocking may not be necessary if emulating native plant communities.

Species composition and range of tree sizes in the irregular shelterwood treatment may aid in maintaining or improving habitat suitability to a range of plant and animal species.