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.
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
Notes about ash and boxelder:
The objectives for including ash in the seeding mixture in early direct seedings were threefold:
- Help establish “crown closure" and shade out grass and weed competition as early as possible.
- 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.
- 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.
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:
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 at the link 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 at the link 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 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.
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.