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.
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.
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.
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 was double disc trenched, with trenches placed every four feet running north-south. The day after trenching, red pine seedlings were planting with a 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.
In October 2018, 1.5 years after the mulching and trenching treatment, we surveyed tree and shrub regeneration using 1/500th 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, Supplemental Content). 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.
Plans for future treatments
Future treatments will likely include additional mechanical release by brush saw 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.
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 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.
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.