Harvest and Enrichment Planting to Achieve Species Diversification and Prevent Swamping in a Northern Very Wet Ash Swamp Plant Community (MN DNR)

Overview

This case study is an assessment of site conditions after initial stages of a silvicultural prescription designed to regenerate a mature black ash stand to a more diverse young forest. Early stand condition data is presented including tree regeneration species, size and quantity, as well as basic forest inventory information for the few residual overstory trees. The intent is to track the stand and update this study periodically over time, until an adequate number of desirable species reach “free-to-grow” status. If successful, the prescription used here could potentially be used on other sites.

Establishment of a diverse species mix is needed for ash stands in order to maintain forested condition in the future because of the imminent threat of emerald ash borer mortality. Without species diversification, many former ash stands will become non-forested wetlands after the ash dies. This will be due to the hydrological change brought about by the loss of ash tree’s site drying impacts through transpiration. 

Management challenges on many ash sites include low marketability of ash, access and operability for harvests and limited species options that will thrive on these very wet sites.

The native plant community silvicultural strategy utilized was to re-initiate the stand as would severe windthrow. The study site was treated with a clearcut with reserves harvest in the winter of 2018/19 and then silver maple, red maple, bur oak and swamp white oak seedlings were planted in the spring of 2019.

Figure 1: A view of the site.

Figure 2: 2019 color IR aerial photo of study site and surrounding area. Study site boundary outlined in green. Black lines show state ownership boundaries and forest inventory stand delineations.

Figure 3: A forester walks through the study site in fall 2021. The site is very wet, as shown by the cattails. There is one red maple seedling visible to the far right.

Silviculture Objective(s)

• Maintain forested condition (prevent “swamping”) in anticipation of imminent invasion by emerald ash borer and anticipated climate change impacts.
• Regenerate a mature ash stand to a young stand with diverse species that are an ecological fit for the site.

Pre-treatment stand description and condition

Stand establishment and management history: 

This trust land parcel has been in state ownership since 1858. Before treatment, the study area was a natural origin ash stand that has probably not undergone any past forest management.

Pre-treatment species composition: 

Table 1: Pre-harvest tree volume/acre, average DBH and distribution*

*Source: MN DNR forest inventory module (stand 31, Section16 T42N R29W)

Pre-treatment forest health issues: 

EAB was not yet present on the site as of 2018, but its imminent arrival was a major driver in the stand’s desired future condition and silvicultural activities.

Landowner objectives/situation: 

While specific objectives vary from parcel to parcel, lands under the administration of DNR-Forestry are managed in alignment with Section Forest Resource Management Plans (SFRMP) to ensure that state forest management activities meet statewide goals for ecological protection, timber production, wildlife habitat and cultural/recreational values. The DNR assembles teams from the Divisions of Forestry, Fish & Wildlife, and Ecological & Water Resources who work with partners and the public to develop SFRMPs.

 Specific goal and objectives for managing ash forest types are excerpted below from the

"Division of Forestry Ash Management Guidelines, August 2020":

 Goal

Maintain and protect ecosystem values associated with wet-mesic ash forests for long-term sustainable forest management including timber production, water quality, hydrology, soils, and wildlife habitat.

 Objectives

1. Capture timber volume in stands with active EAB infestation.
2. Implement timber harvest operations and regeneration methods to maintain water regulation and minimize loss of forest cover to shrub, grass or sedge dominated habitats.
3. Maintain forests by successful establishment of non-ash species to increase stand diversity and resilience in accordance with division climate adaptation and mitigation guidelines.
4. Use natural or artificial regeneration practices to successfully establish economically and ecologically appropriate replacement tree species.

Silviculture Prescription

The following series of treatments were implemented:  

What actually happened during the treatment

Due to high water from wetter than average winter and spring, the water table was higher than normal, which limited available locations for planting seedlings. Therefore, as many seedlings as possible were planted on available places not under water. Species were randomly mixed while planting.

Only about 1.5 acres of about 3 acres of the very wet ash swamp portion of the timber sale was actually harvested due to operability challenges at harvest time on this very wet site.  

Post-treatment assessment

Regeneration

The most recent regeneration survey was conducted in the summer of 2021, two full growing seasons after planting. The survey indicates that the site is off to a promising start to reaching the objective of maintaining forested condition by becoming a successfully regenerated diverse, mixed hardwood and conifer stand.

Table 2: 2021 regeneration summary: trees per acre and plot frequency by species and size class.

As seen in Table 2, and by the crop tree suitability ratings shown in Table 3 below, two growing seasons after the harvest and planting treatment there is a mixture of desirable regeneration that is a good to fair ecological fit for this WFn64 NPC site:

• Red maple* is the dominant regenerating species by far, with the greatest number of stems and a very consistent presence across almost the entire 1.5 acre site. Average height of seedlings was about 3 feet, with a few larger sapling sprouts present as well.
• Tamarack seedlings with an average height of 2 to 3 feet were present on half of our plots.
• Yellow birch seedlings with an average height of 2 to 3 feet were present on 25% of our plots. It is only present on the eastern half or so of the study site.
• Paper birch regeneration averaging 3 feet in height is present in low amounts and very low frequency.
• Balsam fir regeneration from 1 to 5 inches DBH is present only on the eastern portion of the site, at low amounts and low frequency.
• Basswood, aspen and red oak regeneration is present in low amounts and very low frequency, with average heights of about 2 feet.  
• There is a small amount of green ash and elm regeneration present, however these are not crop tree options due to Dutch elm disease and emerald ash borer impacts.

*A few of the stems tallied as red maple may be silver maple. We could not positively differentiate when we collected data.

Table 3: WFn64 NPC Crop Tree Suitability Ratings for Regenerating Species 

Observations on artificial and natural origin regeneration

Four species were planted on the site: red maple, silver maple, bur oak and swamp white oak.

We found zero surviving bur or swamp white oak on the study area itself, although forestry staff has observed some surviving seedlings in the adjacent stand. We should also note that while our survey was thorough, it is possible that we missed a few very small stems in the very heavy vegetative competition. Future surveys will help to tell the definitive story on survival of planted bur and swamp white oak in the study area.

It is impossible to conclusively discern planted or natural origin at this point for most of the smaller red/silver maple seedlings. However, we think those with multiple stems within a foot or two of each other are likely natural origin, since they would probably not be planted that close to each other.

Origin has been natural seeding or sprouting for all other regenerating species found on the site.

Residual overstory trees

We were most interested in regeneration for this study, so the residual overstory trees are not a major part of the story here. However, we want to include data on residual overstory trees to give a full picture of site conditions. The data in Table 4 shows average basal area of residual tree stems that fell on our randomized sample plots.

Table 4: Basal area in feet2/acre greater than 5 inches DBH by species and diameter class.

Ground cover vegetation

We did not do any formal vegetation surveys for ground cover vegetation but noted that the harvested stand had plants typical of a wet meadow and young wet forest. These plants include: cattails and wetland sedges, ostrich fern, sensitive fern, wetland asters, and native shrubs like alder, willows, winterberry, dogwoods, and currents.  We did find low levels of common and glossy buckthorn (Rhamnus cathartica and Frangula alnus, respectively) in places too. 

Plans for future treatments

• Monitor the site through periodic regeneration surveys until adequate amounts of regeneration reach the “free-to-grow” stage.
• We may do some underplanting if stocking appears inadequate at any point in the future.

Costs and economic considerations

Estimated Costs (2019 dollars)

Timber sale preparation and administration:       $ 375 on 1.5 acres

Tree planting and administration:                        $ 920 on 1.5 acres

Total:                                                                   $1,295 on 1.5 acres

 Estimated Revenue (2019 dollars)

 Timber sale revenue:                                         $ 250.00 on 1.5 acres

Total:                                                                   $ 250.00 on 1.5 acres

Other notes

We are grateful for the review and editing help from MNDNR’s Silviculture Program Coordinator Mike Reinikainen.

Climate Adaptation Considerations

Tree habitat response to potential climate change scenarios for regenerating species on the study site are shown in Table 5 below.

Table 5: Tree habitat response to climate change in WFn64  

Figure 4: The study site in fall 2021. One red maple seedling is visible in the center of the photo.

Summary / lessons learned / additional thoughts

Regeneration is off to a promising start

It is early (2 years after treatment), but the site is off to a promising start to reaching the objective of maintaining forested condition by regenerating an ash stand to diverse, healthy, forest stand.  Two growing seasons after the harvest and planting treatment there is a mixture of desirable regeneration that is a good to fair ecological fit for this WFn64 NPC site.

We plan to update this study over time

We plan to periodically update this study over the next 10 to 15 years as we continue to monitor regeneration and assess need for any follow-up actions to reach the objective of transitioning from ash to achieve a diverse, healthy and resilient forest stand. 

More case studies on the silviculturally challenging northern very wet ash forest plant community are needed

 Management challenges on many wet ash forest sites include:

• Limited species options that will thrive on these very wet sites.
• A relatively small amount of experience managing ash stands with the objective of transitioning to other species.
• Low marketability of ash
• Harvesting access and operability challenges for many sites.

These challenges reinforce a need for additional case studies. Best silvicultural options for these sites are yet to be determined.