Variable Establishment, Growth and Health of Hardwoods Direct Seeded onto Two Former Agricultural Fields (MN DNR)

Overview

The focus of this case study is on establishment of mixed hardwood forest stands on two former agricultural fields through a technique known as direct seeding of hardwoods.

We were initially planning on separate studies on these fields because they were seeded in three different years. However, after we field-visited them and took data plots we decided they were similar enough to combine into this one study.

Some notable features of this case study include:

1. 19 years after establishment, it provides an opportunity to document current conditions to assess impacts of landform, soil differences and seed mixes on establishment and growth of various species.
2. While the seeding project overall has been largely successful for forest establishment, there has been some landform-related, environmentally-caused dieback and mortality on the site. We have analyzed forest health issues to develop useful lessons for future afforestation projects.
3. Documenting current conditions can serve as baseline data to help assess impacts of any future follow-up treatments.
4. The sites have exceptionally good documentation of silvicultural activities and costs.

Figure 1: A portion of the site in 2020 showing healthy white oak and black walnut crowns.

Silviculture Objective(s)

Improve water quality, wildlife habitat, outdoor recreation opportunities and produce forest products by establishing a mixed hardwood forest stand on former agricultural fields.

Pre-treatment stand description and condition

Stand establishment and management history: 

The sites were agricultural fields growing annual crops before project initiation.

Pre-treatment species composition: 

NA

Pre-treatment growth and stocking: 

NA

Pre-treatment forest health issues: 

NA

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 objectives for this afforestation project included improving water quality, wildlife habitat, outdoor recreation opportunities and production of forest products.

Silviculture Prescription

Explanatory Note: 2 different fields were direct seeded over a total of 3 different years, which may make the documentation a bit challenging to follow. We recommend starting by referring to Figure 2 for a map of when each area was seeded. 

Each treatment consisted of preparing the field, seeding, and tending by:

• Discing the soil
• Applying hardwood seeds
• Mowing emergent vegetative competition to a height greater than emerging hardwood seedlings
• Herbicide release from emerging vegetation (mostly grasses)

Figure 2: The study site was seeded on two fields, in three different years.

Detailed prescription

The following series of treatments were implemented on 27.4 total acres: 7.7 acres were seeded in 2001, 4.1 acres were seeded in 2002 and 15.6 acres were seeded in 2003. Recall that the acres seeded in 2002 and 2003 were on the same parcel.

What actually happened during the treatment

Things went largely as planned. The actual seed mixtures were sometimes slightly different than originally planned due to limitations of seed availability for some species. This is common for direct seeding projects.

Post-treatment assessment

In 2020, 19 growing seasons after seeding, most of the acreage is on its way to becoming a mixed hardwood stand with a strong walnut component.  Portions also have a good oak component. 

Site 1 averages about 900 black walnut, 240 white oak, 80 hackberry, 20 red oak and 20 black cherry stems/acre. There is a patch in the northern portion of site 1 with heavy cottonwood and aspen regeneration that naturally seeded in from adjacent trees. There are significant numbers of ash and boxelder almost throughout the site, and very small amounts of elm and white pine that have also seeded in from adjacent trees. 

Figure 12. Oak and walnut crowns emerging above competition on part of the site in 2020

Table 1: Stems/acre of regeneration by species, size class, and plot frequency on site 1 in 2020.

Site 2 averages about 400 black walnut, 492 red oak, 585 white and bur oak, 1870 green ash, 31 hackberry, and 308 sugar maple stems/acre. There are significant numbers of boxelder and small amounts of elm that have seeded in from adjacent trees.

Table 2: Stems/acre of regeneration by species, size class, and plot frequency on site 2 in 2020.

Figure 12. Oak and walnut crowns emerging above competition on part of the site in 2020

Figure 13. Walnuts beginning to achieve crown closure among ash and boxelder in 2020

Notes about ash and boxelder:

The objectives for including ash in the seeding mixture in early direct seedings were threefold:

1. Help establish “crown closure" and shade out grass and weed competition as early as possible.
2. Improve timber quality of high value hardwoods such as oak and walnut by providing side competition to nearby stems and forcing them to grow straighter, and with fewer low limbs.  
3. Due to general ease of establishment, serve as the tree crop in any portions of the stand where higher value species failed to become established. Note: The ash was seeded before EAB was known in this area.

There is also a significant amount of boxelder that seeded in naturally from adjacent trees. Similar to ash, some boxelder during early years of establishment is beneficial to helping achieve earlier crown closure to shade out grass. However, at high levels and left uncontrolled they can have a negative impact by outgrowing and shading out more desirable tree species. Their level of competition to more desirable trees will be monitored, and they will be controlled as needed.

While establishment of trees was good on most areas, there is considerable spatial variability in tree survival and growth in 2020 

• On some portions of the site, trees have already achieved crown closure and are healthy and growing rapidly, with only minor dieback on some specimens.
• On some portions of the sites, seedlings are growing slowly and struggling through heavy grass competition. Growth and vigor should accelerate considerably in most of these areas after achieving crown closure, which should happen on most of the site over the next 5 to 10 years.
• There is dieback and mortality across almost all species at variable levels across both sites. Most severe appears to be on black walnuts nearest Partridge Creek. 

Figure 15: Excellent walnut establishment and growth but a bit of dieback here.

Potential causes of tree survival and growth variability

The tree survival and growth variability found on this site is common on direct seedings of former agricultural fields. Several factors may have contributed to the variability, including soils, topography and landform, seed sowing rates, seed quality and genetic variability. We share our thoughts on these factors and the impacts they may have had on tree survival and growth below: 

Soils 

Subtle differences in soil factors such as compaction, structure, nutrient levels and moisture retention can impact tree survival and growth. One great strength of direct seeding vs. planting is that it enables superior adaptation to subtle variations in soils. When planting a few hundred seedlings/acre at regularly spaced intervals, small variations in soils generally are not accounted for. In contrast, direct seeding thousands of seeds of several species over the entire site enables those species best suited to take over and thrive in a particular area based on microsite conditions.

When setting up and administering direct seeding projects, Foresters typically use soil survey information to determine which species to seed. They also sometimes vary the seed mixture within the site to some extent if soil and topography differences are great enough to justify that. For example on this site, more walnuts and less seed of other species were probably sown on the lower, occasionally flooded portions of the site nearest Partridge Creek.

In Figure 3 below, one can see spatial differences in vegetation, tree densities and height in the 2017 color infrared aerial photo (top image). The Natural Resources Conservation Service (NRCS) soils map (bottom image) shows that there are different soils across the site. The NRCS soil map provides very useful information, but it is not a refined enough tool to show very small soil variations at the microsite level. An idea for a future case study to see if more refined soils data and analysis might be valuable in planning direct seedings is to probe and analyze soils in areas with superior survival and growth and compare to areas with poorer survival and growth.

Figure 3: 2017 color IR aerial photo (top) and NRCS soils map (bottom) of sites 1 and 2.

Topography and Landform

Topography and landform are factors that can impact tree survival and growth by impacting air and soil temperatures, and moisture retention and availability. Northerly and easterly aspects on even subtle slopes tend to be cooler and moister than places with westerly or southerly slopes. In addition to topography of the site itself, adjacent hills and valleys can affect conditions on a site by impacting air temperatures and moisture levels.

Topography and landform-related dieback and mortality was observed on all species

While most portions of these sites are populated with adequate numbers of desirable trees to achieve forested condition, there is dieback and mortality across almost all species, at variable levels, across both sites that appears largely attributable to impacts of landform. Three landform-related causes were identified as likely causes of the variable amounts of dieback and mortality we observed:

• Extreme winter cold
• Seasonally saturated soils
• Leaf and shoot disease

Figure 4 below is a LIDAR image the study sites and the surrounding landscape, which shows that the sites are lower than surrounding topography. This makes the sites something of a “wet bowl”. On sites like this, winter and late spring tree-damaging cold air settles in, and in the growing season, moist cool air that is perfect for leaf and shoot disease development and spread settles there. The convergence of streams near the sites suggests the soil could become saturated for long periods after extreme precipitation. Walnut is particularly sensitive to extreme winter cold, and it does not do well when soils are saturated for a lengthy period. The observation of dieback and mortality across multiple species with similar symptoms is a solid indicator of an environmental cause. The affected trees had either top dieback, lower canopy dieback, or both, and sometimes there was a healthy tree right next to a sick one. It was difficult to see obvious death/dieback patterns within the landscape. Some of the lower canopy dieback on the walnuts may have been caused by late spring frost. Some of the top dieback may have been caused by the extreme winter low temperatures in winter 2018/2019. However, extremely wet growing seasons could also promote lower and upper canopy dieback.

Figure 16. Heavier dieback and some mortality on black walnut here

Figure 4: Lidar image of study sites and surrounding area

Seed Quality and Genetic Variability 

Genetic variability and quality of seed is always a factor that impacts tree survival and growth. Some genetic seed variability is beneficial, since it is an important factor in achieving the goal of establishing a genetically diverse forest. One great strength of direct seeding versus planting is that it enables superior adaptation to subtle variations in site conditions. When planting a few hundred seedlings/acre at regularly spaced intervals, small variations in site conditions generally are not accounted for. In contrast, direct seeding thousands of seeds of several species over the entire site enables those species and specimens best suited to take over and thrive in a particular area, based on microsite conditions. Some genetic diversity is a given on direct seedings, when one considers the volume of seed required – it has to come from many trees. 

We don’t have records on seed sources for this site that tell us details of exactly where the seed came from. However, we do know that procurement and handling procedures ensured that the seed used came from the either the same seed zone (or at most 50 miles to the south), and that it was viable. 

Herbaceous Plants 

For herbaceous plants in 2020, we observed mostly high cover of sun-loving native weeds, including goldenrod (Solidago sp.) along with some grasses, asters, raspberries, gooseberries, clematis, and wild parsnip. 

Plans for future treatments

Monitor the stand through periodic inventory surveys. 

Design and execute further planting and tending efforts such as release from competition as needed.

Costs and economic considerations

Refer to Figure 10 below.

Figure 10: Annual and total Partridge Creek direct seeding project costs.

Other notes

Historical regeneration check data 

One of the objectives of this study is to document conditions over time for the reference of current and future foresters. For that reason, we include regeneration check data and notes for several early years in Figures 5 through 9 below. These study sites have exceptionally good regeneration check records.

Figure 5. May 30 2002 regeneration check results from site 2

Figure 6. September 29 2003 regeneration check results for sites 1 and 2

Figure 7. June 3 2004 regeneration check results for sites 1 and 2

Figure 8. June 3 2005 regeneration check results for sites 1 and 2

Figure 9. May 17 2006 regeneration check results for sites 1 and 2

We gratefully acknowledge:

The rigorous analysis of causes of dieback and mortality for this study provided by Central Region Forest Health Specialist Brian Schwingle.

The valuable review and editing assistance of MNDNR’s Silviculture Program Leader Mike Reinikainen.

This case study was developed with support from the United States Department of Agriculture's National Institute for Food and Agriculture, Renewable Resources Extension Act. Project #2021-46401-35956, principal investigator Eli Sagor, University of Minnesota.

Summary / lessons learned / additional thoughts

In 2020, 19 growing seasons after seeding, most of the acreage is on its way to becoming a mixed hardwood stand with a strong walnut component.  Portions also have a good oak component. 

Overall, direct seeding worked well for afforestation of these former agricultural fields. 

While establishment of trees was good on most areas, there is considerable spatial variability in tree survival and growth in 2020 

The variability found on this site is common on direct seedings of former agricultural fields. Several factors probably contributed to the variability, including soils, topography and landform, and seed sowing rates, quality and genetic variability. While we can’t know the exact impacts of each on this site, we include related findings and recommendations for several items below. 

There is topography and landform-related dieback and mortality across almost all species at variable levels, across both sites 

While the large majority of these sites are populated with adequate numbers of desirable trees to achieve forested condition, there is dieback and mortality across almost all species at variable levels across both sites that appears largely attributable to impacts of landform. The observation of dieback and mortality across multiple species with similar symptoms is a solid indicator of an environmental cause.

On sites like this that are lower than surrounding topography, winter and late spring tree-damaging-cold-air settles in, and in the growing season, moist cool air that is perfect for leaf and shoot disease development and spread settles there.

Soils on the lowest portions of the sites are also prone to flooding, resulting in saturated conditions for long periods after extreme precipitation.

Black walnut is particularly sensitive to damage from spring frosts, extreme winter cold, and soil saturation for lengthy periods.

We suggest a greater diversity of species for future direct seedings in places with similar landform: frequently flooded sites near streams, and surrounded on several sides with higher topography 

We would diversify the species for future seedings on sites like these. Swamp white oak, river birch, bur oak, bitternut hickory, silver maple and cottonwood would all be good species to add. Silver maple is a spring seeder, so our suggestion is to seed silver maple and perhaps cottonwood on lower areas the spring following the fall seeding of most species. It would also be worthwhile to interplant some species that are harder to establish using direct seedling: cottonwood cuttings/plugs, silver maple and white oak since it needs to be seeded immediately after acorn collection or they will die.

If climate adaptation is a goal, interplanting some lowland species common to the next growing zone to the south may be worth a try.

We would still include the original species in future seedings for site like these, but at lower levels for walnut and higher for hackberry. Seeded species included black walnut, red and white oak, and smaller amounts of black cherry, sugar maple, basswood and hackberry. While there was mortality and dieback at varying levels across these sites on all of these species, enough of them are surviving in good condition so that we would recommend still including them in the mix for sites like these. We would probably adjust seeding rates to be lower for black walnut and higher for hackberry. Recognize that there will be some variable dieback and mortality for these species, but based on our observations, enough of them will survive in good condition to be a part of a future diverse stand.

More detailed soils and topography information would be a valuable subject for future case studies 

It is highly likely that spatial variability in soils is partially responsible for different tree survival and growth rates across this site. While NRCS soils maps are useful for generally matching species to site, they are not a refined enough tool to show some subtle soil variations at a scale below an acre.  It might therefore be valuable to do additional soil data collection and analysis work in the future on this and/or other former agricultural field direct seeding sites. Soils in areas with good tree growth and survival could be probed and analyzed, and then compared to soils in areas with poor results.

Supplemental content