We used species-area plots, which are nested plots that measure species richness at 11 scales, each plot covering a total of 1024 square meters, to develop species lists on sites that have regenerated following clearcutting. We selected sites that are classified as FDc23 Central Dry Pine Woodland and FDc24 Central Rich Dry Pine Woodland. FDc23 is listed as an imperiled plant community. The sites have regenerated between 1988 and 2005. Six sites had herbicide application as part of their site preparation. Four had mechanical site preparation. Two sites were dominated by jack pine after natural regeneration, and two sites were dominated by aspen after natural regeneration. One of the natural jack pine sites originated after a wildfire. All sites were within the Park Rapids Sand Plain Land Type Association, within the Northern Minnesota Drift and Lake Plains Section, and within the Laurentian Mixed Forest Province. We tried to find sites within the same soil map unit (458 Menahga loamy sand), but one site fell in 869 Graycalm sand, which isn't vastly dissimilar. Pine dominated sites were either typed as jack pine or red pine. Older red pine sites had been thinned, which is essential for maintaining understory plants.
Figure 1: FDc23, herbicide site preparation, planted red pine 1988
Figure 2: FDc24, herbicide site preparation, planted to red pine in 1986
Our working hypothesis was that sites that are treated with herbicide would show a persistent reduction in plant species richness. We also anticipated a greater presence of non-native and/or invasive species. However, while they lack statistical power to refute these hypotheses with confidence, our results do not support them.
Wherever possible, we try to maintain pine dominance within these native plant communities. A dense shrub component and a presence of quaking aspen, especially on FDc24 sites, can add to the complexity of achieving that. Where planting was necessary, some sites are dominated by red pine, as it is less palatable to whitetail deer, and offers more management options (such as rotation age flexibility) than jack pine. Red pine plantations are thinned soon after trees become merchantable (age 25-30). Managing basal area allows understory plant species to persist in a red pine plantation at a level nearly equal to a natural jack pine stand of the same age. Red pine rotation age is 60-80. Jack pine stands aren't usually thinned. Jack pine rotation age is 40 years. Jack pine in FDc23 might live longer than in FDc24, because it is growing on a less productive site (grow fast, die young). The shrub component in FDc23 offers considerably less competition than in FDc24. On many FDc23 sites, herbicide would be unnecessary, but the older sites in this study were regenerated prior to our ability to distinguish FDc23 from FDc24. FDc24 stands can start to break up by age 35, admitting more light penetration through the canopy, stimulating hazel growth, and increasing the need for competition control through herbicide or mechanical means.
Pre-treatment stand description and condition
Stand establishment and management history:
The stands originated after clearcutting. The FDc23 jack pine-dominated natural regeneration site was burned in a wildfire and the timber was subsequently salvaged. The FDc24 jack pine-dominated stand origin is unclear, but it is listed as 'natural' and shows no signs of site preparation or planting. No efforts were attempted to regenerate pines on the aspen dominated sites. Site prep chemicals used on the herbicide prepared sites included Accord, Garlon, and Roundup, individually or in combination. Mechanical site preparation included furrowing, winter shearing, patch scarifying, or disc trenching. Locations that were site prepped were planted with jack pine, red pine, or a combination of the two.
We are required by the state constitution to manage school trust fund lands to maximize revenue for the school trust fund. This is generally interpreted as observing economic rotation ages, maintaining full stocking, and promoting optimum growth rates of crop trees. Jack pine woodlands are viewed by wildlife managers as important habitats, providing food and cover for a variety of terrestrial and avian species. Red pine plantations can develop dense canopies that shade out much of the understory ground vegetation, rendering them less ideal for wildlife. Maintaining plantation basal areas lower than 120 sq ft/acre greatly alleviates that problem.
Limit competition to promote survival and growth of jack and red pines. Control basal area to promote optimum growth and maintain ground flora.
In our species-area plots that have been done in a variety of Native Plant Communities and covertypes, we generally see an increase in species richness following any disturbance. Of course, that level would be expected to be lower after herbicide application or some forms of mechanical site prep (shearing). As the canopy and subcanopies develop, species richness tends to decrease as shade intolerant species are diminished or eliminated. This being a retroactive case study, we don't know what the species richness was prior to disturbance.
On FDc23 plots, we found an average of 69 species for herbicide sites, 71 species for mechanical site prep sites, 66 for the wildfire origin jack pine site, and 61 for the natural origin aspen site. Compared to the FDc23 Species Frequency and Cover table, on herbicide treatment sites, we found 97% of the high affinity species (species present in >50% of FDc23 sites). We found 89% of the high affinity species on mechanically prepped sites, 82% on the wildfire origin site, and 63% on the aspen dominant site. Non-native and invasive species were most abundant on the mechanically prepped sites (6-11 species). Herbicide sites held 2-3 non-native species. The wildfire origin jack pine site had three and the aspen site had one.
On FDc24 plots, we found an average of 78 species for herbicide sites, and average of 85 species on mechanically prepped sites, 64 species on the aspen dominated natural regeneration site, and 65 species on the jack pine dominated natural regeneration site. Compared to the FDc24 Species Frequency and Cover table, we found 96% of the high affinity species on the herbicide treatment and mechanically treated sites, 86% of them in the aspen dominated stand, and 90% in the natural origin jack pine stand. We found up to 16 non-native and invasive species on the herbicide treatment sites, 10 on the mechanically treated sites, two in the natural origin jack pine, and one in the natural origin aspen stand.
Figure 3: Percent of high affinity species found
Figure 4: Total species richness
Some differences in species composition can be explained with synecological coordinates. On a scale of 1 to 5, they rate species on their demand or tolerance of Moisture, Nutrients, Heat, and Light. The coordinates for the various treatment on FDc23 sites are fairly close. The herbicide treated sites were neither highest nor lowest for all four measurements. The FDc23 mechanical treatment sites ranked highest for Heat and Light and tied with the aspen site for the highest rank in moisture. The wildfire origin jack pine stand ranked neither highest nor lowest for every measurement. The aspen site ranked highest for Nutrients, tied with mechanical site prep for highest in Moisture, tied with natural jack pine for lowest in Heat, and ranked lowest for Light.
In FDc24, the aspen site ranked highest for Moisture and Heat, tied the natural jack pine site for highest in Nutrients, and was neither highest nor lowest for Light. The natural jack pine site tied wtih herbicide and mechanical treatments for lowest Moisture rank, tied with aspen for the highest in Nutrients, had the highest score for Light, and was in the middle for Heat. The scores for herbicide and mechanical treatments were identical, ranking low for Moisture, Nutrients, Heat, and Light.
It's interesting to consider the environmental context for these species. Although FDc24 can experience periods of high soil moisture, these sites are generally very dry. Soil descriptions classify drainage as somewhat excessive to excessive. They are also low in nutrients, partly due to soil parent material being a well-sorted glacial outwash sand, and partly due to jack pine's influence in lowering pH levels below the tolerance level for many species. Sites tend to get poorer and drier as time goes on. FDc24 sites tend to have a dark rich topsoil, reflecting their recent history as prairie (500 years before present). FDc23 sites converted from prairie to woodland earlier (~1500 years ago), and have a correspondingly poorer and drier character. With the virtual elimination of wildfire from the ecosystem, we see more deciduous trees and shrubs on these sites, which gradually alter the soil through nutrient cycling. As a deciduous component increases, jack pine's influence on soil pH is somewhat diminished.
Overall, from our limited examination of species richness after disturbance, it appears that one of the worst things land managers can do to these communities is allowing them to become aspen dominated. Such sites will experience a slow mesification process, and fire adapted species will gradually be eliminated. Using herbicides or mechanical site prep may seem preferable if it means maintaining pine dominance. These are disturbance-dependent ecosystems, so denying them disturbance (either natural or anthropogenic) is also destructive to the community.
Although it appears that our typical site preparation methods might yield results that are relatively benign in terms of environmental effects, it was beyond the scope of this case study to consider the effects of fire on these fire dependent communities. Without fire, species that are fire-adapted will gradually become less abundant and some of them may be eliminated. The addition of biochar to a site, effected through wildfire or prescribed burns that aren't extinguished to quickly, can affect soil chemistry, presence and abundance of mycorrhyzae, and site productivity. Biochar promotes long-term carbon sequestration. Charcoal remains chemically active for about 100 years, so if it's been that long since a site was burned, it may be showing some fire deprivation effects. Fire helps maintain a receptive seedbed and is an efficient opener of semi-serotinous cones. Some of the reasons that it isn't employed more often are the cost in terms of time and money, the risk of destroying healthy trees or infrastructure, and the limited effective burn window.
Paper on the Mycorrhizal effects of adding biochar to soil: https://link.springer.com/article/10.1007/s11104-007-9391-5
Climate Adaptation Considerations
Red pine is ranked sixth in terms of competitiveness in FDc23, but is expected to do somewhat better in a drier climate. Bur and northern red oak are expected to be more competitive in a warmer climate. All species that are currently suited to FDc24 (jack pine, quaking aspen, red pine, bur oak, paper birch, and northern red oak) are expected to become more competitive under drying conditions. FDc24 oaks are expected to by favored by warming temperatures.
Summary / lessons learned / additional thoughts
This case study took a lot of work and was the effort of many people. The results refute our hypothesis that herbicide site preparation would permanently reduce species richness and eliminate key elements in these woodland communities. However, we are unable to study enough sites to achieve statistic validity. Our findings might only be revealing exceptions to the rules, however significant those exceptions may appear. A study that examined species richness on several sites, sorted by Native Plant Community, over the first thirty years or more of a stand's life would be very interesting. If you're not too busy over the next several decades, we'd be glad to help you get started.
FDc23 community overview from MNDNR-Forestry
Site photos and data sheets.
Harvey was a staff forester at the MNDNR Northwest Region headquarters in Bemidji, MN. His primary focus was Ecological Land Classification and applications in silviculture. Prior to 2008, he was primarily involved in the timber program. He worked for the Minnesota DNR since he graduated from the University of Minnesota in 1978. Harvey passed away in July of 2022.