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Maintaining Hardwood Forest Profitability Without Ash Species

Publication Number: P3009
Updated: February 10, 2017
View as PDF: P3009.pdf
This is a USDA map, Cooperative Emerald Ash Borer Project map. Figure 1. Emerald ash borer distribution as of August 1, 2016. Highlighted states have EAB infestations. Areas outlined in blue are federal quarantine areas, while red dots represent counties where EAB has been detected.
Figure 1. Emerald ash borer distribution as of August 1, 2016. Highlighted states have EAB infestations. Areas outlined in blue are federal quarantine areas, while red dots represent counties where EAB has been detected.

Mississippi is currently surrounded by states with established emerald ash borer (Agrilus planipennis) (EAB) infestations (Figure 1). For hardwood landowners, this is an unfortunate development, as EAB has killed hundreds of millions of ash trees (Fraxinus spp.) on its way to becoming the most economically damaging insect ever introduced to North America (Herms and McCullough 2014) (Figure 2). While it is impossible to predict if or when EAB will reach Mississippi, there is no reason to believe it will not invade the state, and sooner appears to be a more realistic timeframe than later. Consequently, it’s time to focus on what this means to landowners and steps that may lessen the impacts of EAB in Mississippi.

Understanding the biological background of EAB is critical for making prudent management decisions. EAB is considered an exotic invasive species because it is not native to North America and, as a result, ash has not developed a natural defense to stop its attack. Fortunately, EAB has relied almost exclusively on ash species as a food source, suggesting that other commercially valuable tree species, such as oaks (Quercus spp.) and pines (Pinus spp.), are not at risk. Although individual species of ash differ in their attractiveness to EAB, all species are eventually attacked and killed. Tree size does not seem to factor into mortality, as trees across a range of diameter sizes have been attacked. For all these reasons, we encourage landowners to consider removing ash from their future management plans.

Some landowners may be tempted to immediately harvest their ash. If mature ash sawtimber represents a large component of your stand (more than 25 percent), immediate harvesting is probably a wise option. However, in most situations, ash is not as prevalent within stands, making the harvesting decision more difficult. In these far more common situations, several factors should be considered before deciding to harvest.

One important consideration is the regeneration potential of your stand. For landowners whose primary management objective is generating income, replacing ash with another economically valuable species is important for maintaining forest value. Ideally, desirable seed sources, stump sprouts, and advance regeneration will be available to naturally regenerate the stand. In stands where this is true, removing ash will provide immediate income that could be used to shape an economically desirable future stand through natural regeneration, with timber stand improvement (TSI). Unfortunately, due to decades of high-grading (cutting the best trees and leaving those of lower genetic quality) and the low shade tolerance of many preferred economic species, desirable natural regeneration potential is often lacking. On top of these factors, ash is a prolific sprouter and frequently produces large seed crops, making it difficult to eradicate from a stand. Cumulatively, these factors result in a situation where many landowners will be forced to rely on herbicides and artificial regeneration (planting or seeding) of alternative species in order to preserve the future value of their stands.

It is important to consider species site relationships whenever artificial regeneration is involved. Hardwood seedlings are more sensitive to site conditions than pines. For this reason, landowners planning on planting alternative species to replace ash should familiarize themselves with the specific requirements of individual species. This is especially true for bottomland sites, where soil fertility and hydrology can vary greatly over relatively short distances. Below, we identify some economically valuable species that can be planted to compensate for the loss of ash from hardwood forests. In addition, we will discuss some important site requirements for these species and some common issues encountered in artificial regeneration.

Figure 2. An adult emerald ash borer emerges from an infected ash tree.
Figure 2. An adult emerald ash borer emerges from an infected ash tree.

Upland Planting Considerations

The oak group offers several options for replacing ash in upland hardwood stands. Planting in the correct light environment is critical for oak establishment. Oak seedlings are capable of surviving at light levels as low as 20–30 percent full sun. Research has shown, however, that oak will grow best on sites that receive between 30 and 50 percent full sunlight. This relates back to the growth strategy of oak, which prioritizes root growth over height growth early in life. Planting oaks in intermediate light environments, such as described above, provides oak seedlings with enough light to build their root systems, but not enough to facilitate rapid growth from competing species, which favor early height growth above all else. Providing oaks with a favorable light environment, however, is only part of the planting equation; it is equally important to plant your seedlings on a site that maximizes survival. Landowners should be aware that planting a seedling on a higher-quality growing site does not necessarily increase the odds of survival. This is because survival ultimately depends on the seedling’s ability to compete under ambient conditions, in addition to its stress tolerance. Table 1 lists information on site characteristics for ash and some suggested alternative species.

White ash (F. americana) and green ash (F. pennsylvanica) are the most common upland hardwood ash species. Both species prefer moist upland soils. However, both can also occupy dry upland sites. Thus, landowners should recognize that multiple oak species, along with two species of pine, may be needed to fill the niche left behind by ash. Landowners should also understand that stands often contain several different types of growing sites. As such, a one-size-fits-all approach, in most cases, will not be appropriate. Below are planting suggestions for different types of growing sites.

Table 1. Preferred physiographic positions and site conditions for ash and financially valued alternative species.

Species

Common Upland Site Characteristics

Soil Texture

pH Range

white ash

rich, moist, well-drained upland soils

fine–coarse

5.0–7.5

green ash

moist upland soils

medium–coarse

5.0–8.0

northern red oak

north slopes with deep soils

fine–medium

4.3–6.5

shumard oak

terraces or deep upland soils, especially loess hills

medium–coarse

5.8–7.6

black oak

dry upland sites; best on lower slopes

fine–coarse

4.5–6.0

southern red oak

dry upland sites and broad ridges

medium–coarse

4.2–6.0

scarlett oak

poor, dry soils and upland ridges

medium–coarse

4.5–6.9

white oak

all upland sites except extremely dry, shallow soil ridges

medium–coarse

4.5–6.8

loblolly pine

all but the driest soils

fine–coarse

4.5–7.0

shortleaf pine

dry, rocky upland soils

fine–coarse

4.0–6.0

Source: Burns and Honkala 1990.

 

Upland Sites

Xeric (Dry) Sites

Xeric sites are defined by low moisture and nutrient availability. Physiographic locations typically considered xeric include ridges and south- or west-facing slopes. Sites that have a thin horizon (topsoil) or sandy texture are also considered xeric. Replanting on xeric sites will probably be unnecessary because ash is fairly uncommon on such sites. If you happen to have ash on xeric sites, planting containerized seedlings may be a better option than bare-root seedlings. Although they are typically twice the cost of bare-root seedlings, containerized seedlings are better suited to survive drier planting sites because they have more developed and undisturbed root systems. For xeric sites, we recommend planting southern red oak (Q. falcata), white oak (Q. alba), or scarlett oak (Q. coccinea). Alternatively, if you would prefer to have a mixed-pine hardwood stand, you could plant loblolly pine (P. taeda), shortleaf pine (P. echinata), or a combination of oak and pine. If your site is excessively dry, shortleaf is a better option than loblolly. Regardless of the species planted, landowners should be aware that trees will grow slower on xeric sites because of their relative lack of resources (lower site index).

 

Mesic (Moist) Sites

In contrast to xeric sites, mesic sites feature higher moisture content and nutrient availability. Physiographic locations typically considered mesic include north- and east-facing slopes and mid-slope to lower-slope locations. Sites with deep, well-drained soils, like the loess hills, are also thought of as being more mesic than xeric. Ash is generally found in greater numbers on mesic sites; consequently, mesic sites are where the greatest amount of ash replacement is going to occur. Given the greater amount of moisture and nutrients available on mesic sites, bare-root seedlings should survive well. The extra costs associated with containerized seedlings are not necessary unless you plan on planting late in the season. Commercial species best suited for planting on mesic sites include northern red oak (Q. rubra), white oak, black oak (Q. velutina), Shumard oak (Q. shumardii), and loblolly pine.

 

Bottomland Planting Considerations

There are some unique factors that must be considered when attempting to artificially regenerate bottomland sites. First, species-site relationships are very important and will be discussed thoroughly in the next section. After carefully choosing species based on soil information, you should consider other site variables.

As with upland sites, light is necessary when establishing new stands. Depending on which alternate species is selected, appropriate light levels will range between 20 percent and full sunlight. Many individuals will opt to plant oak species if forced to convert former ash stands. As previously mentioned, oak natural regeneration is maximized at 50 percent full sunlight. Planted oak seedlings will actually grow best at light intensities greater than 50 percent light, but herbaceous vegetation inherent to bottomland sites often overtakes planted seedlings if shade is not present. Seedlings may need a short period of time to establish competitive root systems before you remove overshadowing canopy trees.

Green ash is by far the most common bottomland ash species; however, pumpkin ash (F. profunda) may be encountered occasionally growing on sites too wet for green ash. Bottomland forests have a range of unique sites. Consequently, each of these sites typically warrants a different planting mix as it is encountered. As with uplands, bottomland sites should not be planted using a blanket planting mix. A variety of species will be appropriate and inappropriate depending on the individual planting site. Below are suggested species to plant on bottomland sites.

 

Bottomland Sites

Site variation within bottomland systems is driven primarily by topography, which influences a multitude of other site variables, including drainage class, soil moisture, texture, structure, pH, and vegetation. Differences in elevation and parent material change as soil particles suspended in flowing water are deposited across the floodplain. Research has shown elevation differences as small as 6 inches can completely change a site’s ability to sustain a given species. There are some differences between major and minor bottoms. Major bottoms (i.e., river bottoms) will typically possess greater soil variability, and minor bottoms (i.e., creek bottoms) will typically be more uniform with less soil variability. In addition, the physical distance among topographic positions can be compressed in minor bottoms with some topographic features being completely absent. Both can be highly productive when species are matched to site capability, and both are capable of growing similar species on proper topographic positions. Figure 3 below shows the typical topographic positions and their placement within a major floodplain.

Figure 3. A typical floodplain. Source: Mississippi State University Extension Service Publication 2004 Bottomland Hardwood Management Species/Site Relationships.
Figure 3. A typical floodplain. Source: Mississippi State University Extension Service Publication 2004 Bottomland Hardwood Management Species/Site Relationships.

While some hardwood species are capable of surviving and growing on a wide range of sites, others can survive only on very narrow ranges of site conditions. Table 2 describes individual topographic positions and lists potential hardwood species to replace ash. All alternate species presented are not commercially desirable, but they are capable of surviving on these sites. Depending on your goals, commercially undesirable species may or may not be considered as viable options for planting.

Table 2. Site suitability by topographic position of major and minor bottoms.

Topographic Position

Desirable/Suitable Species

 

Major Bottoms

Minor Bottoms

bars

eastern cottonwood, black willow

river birch, black willow

fronts

eastern cottonwood, American sycamore, sweetgum, pecan, green ash, water oak, cherrybark oak, swamp chestnut oak

cherrybark oak, Shumard oak, American sycamore, sweetgum, yellow poplar

ridges

willow oak, water oak, sweetgum, American sycamore, green ash, cherrybark oak, swamp chestnut oak

cherrybark oak, Shumard oak, swamp chestnut oak

high flats

Nuttall oak, green ash, sugarberry, willow oak

cherrybark oak, water oak, willow oak, Shumard oak, swamp chestnut oak

low flats

overcup oak, water hickory, green ash, common persimmon, sugarberry

willow oak, overcup oak, common persimmon, green ash

sloughs

overcup oak, black willow, water hickory

overcup oak, common persimmon

swamps

baldcypress, water tupelo

baldcypress, swamp tupelo, water tupelo

Source: Mississippi State University Extension Service Publication 2004 Bottomland Hardwood Management Species/Site Relationships.

 

Bars, Sloughs, and Swamps

Bars are the first topographic position encountered when moving outward from a watercourse. They are typically very wet and flood regularly. Sloughs and swamps are both old stream channels that were left as the course of the creek or river moved. Drainage is inherently poor on these sites, and, because of their heavy clay soils and lower elevations, standing surface water will be encountered for much of the year. Bars, sloughs, and swamps are not considered ash-capable sites. Thus, further discussion of ash replacement is not warranted for these sites.

 

Fronts

Fronts are generally considered the best sites in the floodplain for tree growth. Fronts are formed when streams overflow, water velocity decreases, and immediate deposition of sediment starts to occur. These are the highest sites, have the best drainage, and stay flooded only in extreme flood events. Most hardwood species will grow on front sites. However, eastern cottonwood (Populus deltoides) is typically the primary species found on these sites. This is because cottonwood grows fast and has a vertical leaf display, which allows plenty of light for other tree species to regenerate in the understory. Ash is not typically considered a front species, but, in some cases, it can comprise a significant portion of the overstory. In situations where ash is 25 percent or more of the stand, suitable replacement species include sweetgum (Liquidambar styraciflua), pecan (Carya illinoinensis), water oak (Q. nigra), cherrybark oak (Q. pagoda), Shumard oak, and swamp chestnut oak (Q. michauxii).

 

Flats

Flats are flat expanses between the front and a ridge or between ridges. Soils are typically clays with somewhat poor to poor drainage. However, an important aspect of flats is the absence of standing water for most of the growing season. Depending on elevation in relation to the drainage capability of the soils, flats may be classified as high or low. These sites are not as productive and are of lower quality than the front or ridges. Subsequently, the most desirable species (for timber management) are not suited for these sites. However, there are suitable replacement species for ash. On high flats, landowners should consider Nuttall oak (Q. texana) and willow oak (Q. phellos). If the flat is dry enough (i.e., minor bottoms), cherrybark oak, water oak, Shumard oak, and swamp chestnut oak might be viable alternatives. When considering ash replacement on low flat sites, water oak, Nuttall oak, overcup oak (Q. lyrata), and common persimmon (Diospyros virginiana) are all capable of surviving increased soil moisture typically found on these sites.

 

Ridges

Ridges are simply old fronts and are excellent sites. Ridges typically rise 2 to 3 feet above the flats and have coarser soils than flats. Consequently, drainage is better, and these sites are capable of sustaining more commercially attractive species. These sites are capable of growing the same oak species as discussed in the fronts section above.

 

Potential Problems

Shade-Tolerant Midstory

A unifying characteristic of all the discussed alternative species is their relative intolerance of shade. Oak seedlings perform best at between 30 and 50 percent light, while pine species prefer higher light environments. This is why planting in a relatively open environment is important.

One factor that causes problems for light-seeking species is a shade-tolerant midstory. As a result of decades of fire exclusion and high grading, many shade-tolerant species have become established in the midstory. This is an important issue for financially driven landowners, as most shade-tolerant species have little economic value. Harvesting ash in the presence of an established midstory will do little to create the preferred light environment for your desired alternative species. Instead, in most cases, harvesting will simply release the undesirable midstory stems, setting the stage for lower timber revenues. For this reason, treating the midstory before harvesting is a good management practice.

Midstory injection is the easiest, most effective, and cheapest tool for eliminating large, undesirable stems. In most cases, the midstory size or site conditions will make fire removal impossible and mechanical removal prohibitively expensive. Moreover, midstory injection will prevent residual sprouting, which can also be problematic when competing with shade-intolerant seedlings. For more information on midstory injection, see MSU Extension Publication 2942 Tree Injection for Timber Stand Improvement.

 

Herbaceous Vegetation

Outside of properly matching species to the site, competing vegetation is possibly the most influential factor in hardwood planting failures. Both herbaceous and woody competition may pose a threat to the survival of planted seedlings, with herbaceous competition posing the greatest threat during the first years of establishment. While woody competition from resprouts, new germinants/advanced regeneration, and midstory stems may pose substantial competitive problems on some sites, significant levels of herbaceous competition will be encountered nearly every time planting is to occur on hardwood sites. Chemical site preparation can provide excellent short-term control of competing vegetation. This is especially true if control of competing woody species is needed; however, chemical site preparation should be used only to control species that cannot be eliminated by growing-season herbaceous weed control efforts. It is these herbaceous applications that typically provide longer-term control of competition if the proper herbicide is used. Chemical site preparation is of limited value if it does not control vegetation throughout the majority of the first growing season. Thus, when chemical control is deemed necessary to control existing on-site vegetation, it should be part of a herbicide regime that includes the seedlings’ first growing season.

Increased growth and survival of hardwood seedlings planted in areas treated with broad-spectrum pre-emergent herbicides is well documented. Increases in survival average 25 to 30 percent greater than in untreated areas. Multiple herbicidal treatments have been tested, and specific herbicide recommendations can be found in MSU Extension Publication 2873 Herbicide Options for Hardwood Management.

 

Invasive Species

Another potentially serious problem with replacement of ash is the possibility of invasive plant species encroachment. Harvesting operations provide prime opportunities for invasive species establishment, as they expose bare mineral soil and increase resource availability. Harvesting and logging equipment can also transport seeds and rhizomes to a site, further increasing the odds of an invasion. While not all invasive species are strong competitors on harvested sites, several invasive species already established in Mississippi prefer such conditions. Once established, species such as cogongrass (Imperata cylindrica), Chinese tallow (Triadica sebifera), and privet (Ligustrum spp.) will aggressively occupy growing space. This can be very problematic for resource-demanding oaks and pines.

Close monitoring and decisive action are key for minimizing invasive species impacts. It is a sound management practice to search for existing invasive species before and after any kind of harvesting. Treating invasive species before harvest is strongly encouraged, as most invasives are harder to control in full sun. Another advantage of treating early is that you will have a greater selection of herbicides at your disposal compared to post-planting treatments. After planting, it is still a good idea to check your property for invasive establishment, as invasives are typically easier to treat when they are young. More information on some of the more commonly encountered invasive plant species, and control recommendations for them, can be found in MSU Extension Publication 2873 Herbicide Options for Hardwood Management.

 

Summary

The impending arrival of the emerald ash borer is an important development for hardwood management in Mississippi. Due to EAB’s host preference, stands with a high concentration of ash are in the greatest danger. While it is impossible to predict when and where EAB will arrive, stands with ash representing more than 25 percent of the merchantable sawtimber carry the greatest immediate financial risk. Preemptive harvesting is one way to generate income and mitigate risk. However, there are several factors to consider before harvesting, including future regeneration potential, herbaceous vegetation, midstory competition, and invasive species. In most situations, maintaining future value through natural regeneration will not be an option. Planting or seeding can be used to overcome existing regeneration deficiencies. However, landowners need to remember that hardwoods are more sensitive to site conditions than are pine species. Consequently, site considerations need to be taken into account when selecting alternative species.

 

References

Burns, R. M., & Honkala, B. H. (1990). Silvics of North America: 1. conifers; 2. hardwoods.

Herms, D. A., & McCullough, D. G. (2014). Emerald ash borer invasion of North America: History, biology, ecology, impacts, and management. Annual review of entomology, 59, 13–30.

Rousseau, R. J., Ezell, A. W., & Hodges, J. D. (2014). Bottomland hardwood management species/site relationships. Mississippi State University Extension Service, Publication 2004.

Self, A. B. (2015). Tree injection for timber stand improvement. Mississippi State University Extension Service, Publication 2942.

Self, A. B., & Ezell, A. W. (2015). Herbicide options for hardwood management. Mississippi State University Extension Service, Publication 2873.

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This publication was sponsored by the Mississippi Forestry Commission.

Publication 3009 (POD-02-17)
By John L. Willis, Assistant Professor, and Andrew B. Self, Assistant Extension Professor, Forestry.

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