Establishing red pine and controlling buckthorn with mechanical site prep (MN DNR)

Overview

This case study was conducted in a recently harvested oak stand with thick brush, including invasive buckthorn. Originally dominated by oak, the stand was planted with red pine to transition to a mixed oak-pine stand. Oak naturally regenerated via seed and sprouting. Before planting red pine, the stand was double disc trenched. To test the utility of mulching as a buckthorn control method, a four-acre experimental treatment area was identified, with two acres brush mulched and double disc trenched prior to planting and two acres only double disc trenched prior to planting. 

The mulched and trenched treatment best accomplished our objectives in the short term, with lower shrub density and higher tree density compared to the trenched treatment. However, treatment did not significantly reduce the presence of shade tolerant buckthorn in the long-term.

April 2022 update:

This case study when originally published in 2019 was a summary of early results, with the intention of periodic updates to stand conditions over time. We are now providing an update on pine regeneration conditions and observations based on field data gathered in April 2022.

Silviculture Objective(s)

A broad goal of silviculture treatments in this stand is to transition the recently harvested stand from an oak-dominated cover type to a mixed oak-pine cover type. The stand is located in Sand Dunes State Forest, which is experiencing tree mortality due to oak wilt. Increasing tree species diversity by planting conifers will slow the spread of oak wilt by incorporating tree species not susceptible to the disease and minimizing infection via root grafting.

To establish the stand as an oak-pine mix, mechanical site prep was used to execute three objectives:

1) to minimize buckthorn and other shrub species growth without chemical control to accommodate public perception of herbicide use;

2) to establish planted red pine seedling; and

3) to establish naturally regenerated pin and bur oak seedlings and sprouts.

Because natural regeneration of oak is typically successful in the FDs37 native plant community, objective three was less important than objectives one and two.

Pre-treatment stand description and condition

Pre-treatment species composition: 

Before harvest in 2015, the stand was dominated by bigtooth aspen, northern pin oak, and bur oak. Harvest specifications reserved red pine, white pine, and white spruce. Post-harvest and pre-treatment, the stand contained dense brush, including 11,000 stems per acre of buckthorn. Prickly ash, raspberry, and hazel were also present.

Pre-treatment forest health issues: 

Oak wilt has been present in Minnesota since the 1950s and is common in east-central and southeastern Minnesota. The Minnesota Department of Natural Resources actively monitors oak wilt and works to suppress the spread of the disease. In oak-dominated native plant communities such as the stand in this case study, one mitigation technique is increasing tree species diversity, particularly by adding conifers to oak-dominated stands. At the time of this treatment, oak wilt was not known to be present in this stand, however it is present in many locations in Sand Dunes State Forest.

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 Managment Plans (SFRMP) to ensure that state forest management activities meet statewide goals for ecological protection, timber production, 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.

Sand Dunes State Forest (SDSF) has an operational plan that further refines the goals of the Anoka Sand Plain Subsection Forest Resource Management Plan to fit the unique circumstances and landscape within SDSF.

Silviculture Prescription

The silviculture prescription was to use clearcut harvest followed by disc trenching and broadcast herbicide application of Garlon XRT (active ingredient, triclopyr) to create growing conditions favorable for planted red pine and white pine as well as natural regeneration of oak. After trenching and spraying, 900 pine seedlings per acre were to be planted, mostly red pine with a white pine buffer adjacent to mature red pine to minimize disease pressure from Diplodia. Follow up mechanical release was expected.

What actually happened during the treatment

The stand was harvested in the winter of 2014-2015 using a hot saw feller buncher with tree length skidding. All wood was chipped on site in the NW corner of the stand.

Disc trenching and broadcast herbicide application with Garlon XRT were planned for the summer of 2016 to control the competing vegetation. However, nearby residents opposed herbicide usage at this site. In public meetings, an agreement was made to not use herbicide on this site.

However, site prep was still necessary to control buckthorn and other shrub competition in order to establish red pine and transition the stand to mixed oak-pine cover. In 2016, the entire stand was double disc trenched with parallel north-south trenches every four feet. Two acres were mulched prior to trenching to experiment with mulching and trenching as a buckthorn control method.

Figure 1: The site was double disc trenched to prepare for artificial regeneration of red and white pine.

In April of 2017, mulching test plots were installed. During the same month, the entire site was double disc trenched, with trenches placed every four feet running north-south. The day after trenching, red pine and white pine seedlings were planted at a density of about 450 seedlings per acre each, for a total density of 900 seedlings per acre. A 200+ ft. buffer area of white pine was planted against the adjacent mature red pine stand.

Figure 2: Aerial photograph of treatment site at Sand Dunes State Forest.

About a year after the initial treatment, in May of 2018, brush saw release was conducted in a three-foot radius of planted red pine. 

April 2022 update:

• Pines were bud-capped in 2020, 2021 and 2022 as a deer browse reduction measure.
• Pines were mechanically released in November 2021 by using brush saws to cut down vegetation competing with pine regeneration. 

Post-treatment assessment

In October 2018, 1.5 years after the mulching and trenching treatment, we surveyed tree and shrub regeneration using 1/500^th of an acre (5.3 ft.) fixed radius plots. Data was collected in 36 plots, with 18 plots in the mulched and trenched two acres and 18 plots in the trenched two acres. The species sampled were buckthorn, red pine, aspen, pin oak, bur oak, cherry, hackberry, elm, ash, Rubus spp. (raspberry and blackberry), hazel, prickly ash, and amur maple. Any other species were grouped together as “miscellaneous native shrubs.” For pin oak and bur oak, separate counts were maintained for stump sprouts versus seed origin.

Overall, there was lower shrub density and higher tree density in the mulched and trenched treatment area compared to the trenched treatment area (Table 1). Buckthorn, hazel, and raspberry all had greater average density and height in the trenched treatment area compared to the mulched and trenched treatment area, suggesting mulching can help set back shrub growth. However, prickly ash height and density remained similar across both treatments and was slightly higher in the mulched and trenched area.

Table 1: Summary of regeneration and competition densities by treatment.

Figure 3: The site in August 2018, 1.5 years post-treatment. Raspberry and goldenrod dominate in the mulched and trenched area.

Both treatments resulted in considerable buckthorn density, with 12,000 stems per acre (22% relative density) in the mulched and trenched treatment area and 18,139 stems per acre (27% relative density) in the trenched treatment area (Table 1). These results suggest mulching and trenching can set back shrub growth better than just trenching alone. However, buckthorn still grows aggressively in each treatment, so mulching even when combined with disc trenching does not adequately control buckthorn.

Tree density was higher in the mulched and trenched treatment, with 7,167 trees per acre compared to 3,667 trees per acre in the trenched area. Trees in the trenched treatment were an average of 0.7 ft. taller than trees in the mulched and trenched treatment (Table 1). This difference in density and height might be attributed to the slightly older age of natural regeneration in the trenched-only treatment. Untrenched strips could have contained advance regeneration that was taller in height.

Looking at target species, red pine was originally planted at a density of 900 seedlings per acre. A year and a half post-treatment, there are 722 trees per acre of red pine occurring at 83% frequency in the mulched and trenched treatment and 222 trees per acre occurring at 33% frequency in the trenched site (Table 1). The difference in red pine density between each treatment suggests that the mulching treatment benefitted planted red pine seedlings, which was an unexpected result. This could also have to do with the regeneration plot size (5.3 foot radius) being smaller than the distance between rows and seedlings (7 feet). Pin oak density was fairly constant across both treatments, while bur oak density was nearly three times higher in the mulched and trenched treatment. Mulching combined with trenching appears to have suppressed competition enough to allow planted red pine, and to a lesser extent bur oak, to establish successfully.

April 2022 update

We gathered updated pine regeneration data (trees/acre and heights by species) in April 2022.  We concentrated solely on pine data in 2022, since they were the main crop tree species of interest. We did not gather data on vegetative competition or other tree species. 

Table 2: Pine density, average height and frequency on 1/100^th acre plots by treatment* in April 2022

*The entire site also had a brush saw pine crop tree release completed in November 2021.

Observations (See Table 2.)

• There was a significant difference in the species mix by treatment on our 2022 plots, with white pine dominant on the mulched area and red pine on the unmulched area. 
• There were 96% more total pine trees/acre on the mulched area compared to the unmulched (408/acre mulched vs. 208/acre unmulched). 

Possible reasons for the survival difference include:

• Superior suppression of vegetative competition in the mulched area.
• Superior moisture-holding in the mulched area. Moisture retention can be a major factor impacting early survival on sandy soils such as those on the study site. 
• It is also possible that the different pine species proportions on the different treatment areas played a role in survival differences. 
• Average height was 38% taller on the unmulched area (3.36 feet unmulched vs. 2.44 feet mulched). 

We don’t know why this is the case, but it is possible that the relative species mix by treatment area had an impact, since red pine average heights were taller on our 2022 plots across both sites, and there was significantly more red pine than white on the mulched plots. 

We can’t say, since we did not track seed vs. seedling origin, but it is also possible that more of the white pine are seed origin from adjacent trees rather than seedling origin, which would make them younger, with lower average heights in 2022. We did not think to do it in 2022, but next time we gather data, we could try to parse seed vs. seedling origin by the trees’ spacing and location. Planted trees should have pretty consistent spacing, and be near the middle of trenches. 

• Plot frequency was high for both treatments. It was nearly the same, but slightly higher on the mulched area (100% frequency mulched vs. 92% unmulched). 

Errata - 2018 data

We need to note an error in the 2018 data: apparently all pine were counted then as red pine. Because of this, we can only compare 2018 and 2022 data for total pine, not for red pine or white pine separately. We should be able to make individual pine species comparisons when we do our next update in a few years.

Plans for future treatments

Future treatments will likely include additional mechanical release by brush saw and annual budcapping if needed in both mulched and unmulched areas. Herbicide application via spot spraying is also an option, but as herbicide use at this site has been controversial among neighboring residents, future herbicide use is unlikely.

Costs and economic considerations

Shearing & mulching: $1,000 at $500/acre

Disc trenching: $3,100 at $155/acre

Brush sawing: $3,440 at $160/acre

Originally, the silviculture prescription utilized chemical rather than mechanical site prep. Herbicide application was estimated to cost $2,500. Due to social constraints on herbicide usage, the prescription was amended to use mechanical site prep in place of herbicide. At this stand, mechanical site prep was more expensive than chemical would have been. Due to thick prickly ash, buckthorn, raspberry, and blackberry, future brush saw contract bids will likely be higher than the initial treatment cost of $3,440. Although buckthorn was less dense in the mulched treatment area, there was still a considerable amount of buckthorn (12,000 stems per acre). Project staff concluded the high site prep cost only mitigated buckthorn slightly, which does not justify the expense.

April 2022 Update

Table 3: Per-acre treatment costs by treatment and date as of April 2022

Pine establishment costs were considerably higher for the mulched area compared to unmulched, and also compared to what herbicide site preparation would have cost (Table 3).

• Mulching site preparation (total cost of $1,515.08/acre) resulted in 49% ($500/acre) greater total treatment costs than the total cost of not mulched ($1,015.08). 
• Mulching site preparation (total cost of $1,515.08/acre) resulted in 64% ($590/acre) greater total treatment costs than herbicide use (total cost of $925.08/acre) would have.

Higher pine survival means that timber values are likely to be higher for the mulched area compared to unmulched. 

It is impossible at this time to know the exact financial impact, but the higher pine survival on the mulched area compared to unmulched and resultant higher pine component as the stand matures will result in greater timber value at harvest time for the mulched area. This is because there will be a lower mixed hardwoods component (which will be mostly aspen and oak of pulpwood or firewood, rather than sawtimber quality) in the mulched area. Timber values are considerably lower for these species and products than pine sawtimber.

Past experience leads us to believe that pine survival (and timber values) if broadcast herbicide had been used would have been at least as good as the mulched area, but for much lower establishment cost. 

It is impossible to know for certain for this site because we did not try it here. However, a wealth of experience with herbicide site preparation supports our opinion that pine survival likely would have been at least as high using broadcast herbicide as with mulching, but for much lower cost.

Based on data so far for this site, we conclude that the mulching site preparation option provided adequate competition control for pine establishment, but at a significantly higher cost than if a broadcast herbicide application had been used.

The main factors limiting greater use of the mulching option for site preparation are cost and site operability. 

Mulching treatment cost for this site was $590/acre (64%) greater than if broadcast herbicide had been used. 

Operability limitations are related to the need to access and effectively operate on a site with mulching equipment (See Figure 8).  Many sites would be too remote and/or steep, and/or rough to enable use of mulching equipment. It is highly likely use of a broadcast herbicide option would also require 1 fewer brush saw release than mulching, since it would have controlled the woody competition for the first several years after treatment.

The main factors influencing the decision to try mulching on this site were public concerns over environmental safety of broadcast herbicide use, and also potential negative impacts to plant diversity. 

All of these site preparation factors and options need to be weighed for all sites, of course. However, we conclude that the high cost and operability limitations of the mulching site preparation option mean that it would only be worthy of further trials on highly sensitive, accessible and operable sites in the future. And then only if budgets are available to support its much higher cost.

An analysis of Net Present Value (NPV) for the 3 site preparation treatment options showed that NPV is negative for all treatments at discount rates of either 2% or 3%. It is most negative, by far, for the mulching treatment. 

It is important to note 2 things related to NPV:

1. Reduction of some of the assumed costs we used for the analysis (such as cost to set up and administer timber sales) could result in a net positive NPV, at least for the herbicide site preparation option, at a 2% assumed discount rate.
2. Even with negative NPVs, growing pine can be a worthwhile investment for Minnesota’s citizens on public lands because of the social and environmental benefits. 

The full NPV analysis is available upon request to the Little Falls Silviculture Program Forester.

Other notes

Data used in this case study was collected 1.5 years post-treatment and should be considered short-term results. Project staff predict that the mulching slowed down, but did not halt, buckthorn growth. Long-term results are unknown. Eventually, the mulched and double disc trenched treatment area may contain as much buckthorn as the trenched-only treatment area.

This case study was reviewed by MN DNR Silviculture Program Staff Ross Meyer and Mike Reinikainen. Case study originally submitted on 08/09/2019. It was developed with support from the United States Department of Agriculture's National Institute for Food and Agriculture (USDA-NIFA), Renewable Resources Extension Act (RREA). Project #MIN-44-E02, principal investigator Eli Sagor, University of Minnesota. Update submitted October 2023.

We are grateful to Minnesota DNR Forest Economist Don Deckard for the Net Present Value analysis.

Summary / lessons learned / additional thoughts

The silviculture prescription for this stand addressed three objectives: minimize competition from buckthorn and other shrubs; establish planted red pine seedlings; and establish naturally seeded oak. We compared two treatment types: mulched and trenched, and trenched only. The mulched and trenched treatment best accomplished our objectives in the short term, with lower shrub density and higher tree density compared to the trenched treatment. However, shrub density, particularly of buckthorn, was still high in the mulched area. While the treatment was effective, it did not eradicate buckthorn and the high cost should be considered when using mechanical means alone to reduce shrub and tree density, especially when the target species is a persistent, shade tolerant, woody invasive like common buckthorn. 

We were surprised to find that red pine density was twice as high in the mulched and trenched treatment area compared to the trenched-only treatment area. This suggests that the mulching treatment provided some benefit to planted pine seedlings, but the exact mechanism is unknown. Perhaps deer found it easier to access pine seedlings for browse in the trenched site, or there was better light availability in the mulched site that contributed to higher seedling survival. Another possibility is that pine planting was executed differently in each treatment area as mulching may have changing terrain conditions. More monitoring is required, but results suggest that mulching can increase planted red pine seedling survival.

Additional thoughts as of 2022

Our opinion based on data as of 2022 is that mulching site preparation would only be a future option in extraordinary circumstances

The main factors limiting greater use of the mulching option for site preparation are cost and site operability. Costs of mulching are very high compared to broadcast herbicide application. Additionally, many sites would be too remote and/or steep, and/or rough to enable use of mulching equipment. We conclude that the high cost and operability limitations of the mulching site preparation option mean that it would only be worthy of further trials on highly sensitive, accessible and operable sites in the future. And then only if budgets are available to support its much higher cost.

Pine survival was significantly greater in the mulched area compared to unmulched 

There were 96% more total pine trees/acre on the mulched area compared to the unmulched (408/acre mulched vs. 208/acre unmulched). Possible reasons for the survival difference include: 

• Superior suppression of vegetative competition. The mulching probably did suppress some early vegetative competition growth.
• Superior moisture-holding in the mulched area. Moisture retention can be a major factor impacting early survival on sandy soils such as those on the study site. 

It does not seem likely to us, but is possible that the relative species mix differences by treatment area played a role in survival differences. It is possible that the greater preponderance of white pine in the mulched area, with a greater preponderance of red pine in the unmulched area, impacted relative survival rates. Perhaps the white pine planting stock was in better condition when planted, for example. 

Pine average height was lower in the mulched area compared to unmulched.

Average pine height was 38% taller on the unmulched area (3.36 feet unmulched vs. 2.44 feet mulched). Possible reasons for the height difference include:

The relative species mix may have had an impact, since red pine average heights were taller on our 2022 plots across both sites, and there was more red pine than white on the mulched plots. 

It is also possible that more of the white pine are seed origin from adjacent trees rather than seedling origin. We can’t say, since we did not track seed vs. seedling origin, but seed origin would mean they were younger than planted seedlings, with resultant with lower average heights in 2022. Next time we gather data we could try to parse seed vs. seedling origin by the trees’ spacing and location. Planted trees should have pretty consistent spacing, and be near the middle of trenches. 

Pine stocking was slightly better in the mulched area, but above minimum standards for both treatments.

As of 2022, the mulched area does have slightly higher stocking, but both the mulched and unmulched areas have adequate stocking (and density) to meet our goals for achieving a mixed pine-hardwood forest stand.

Pine establishment costs were considerably higher for the mulched area compared to unmulched, and also compared to what herbicide site preparation would have cost

• Mulching site preparation resulted in 49% ($500/acre) greater total treatment costs than not mulching. 
• Mulching site preparation resulted in 64% ($590/acre) greater total treatment costs than herbicide use would have.

Thoughts on impact of no herbicide use on buckthorn densities on this site

One factor limiting our use of broadcast herbicide treatments is that they often have negative impacts on post-treatment plant diversity. In this case however, we think a broadcast herbicide treatment probably would have had positive impacts on reducing density of invasives such as buckthorn. 

Supplemental content

Supplemental figure 1: Aerial photo of study site showing 2022 data plot locations 

Supplemental figure 2: Fecon deck mulcher similar to that used for this study. Source: Fecon website. 

Supplemental figure 3: Typical site conditions in April 2022. Note that pines are bud-capped and recently mechanically released from vegetative competition.