Black ash grows in a variety of forest types across its range. In the northeastern U.S., black ash is typically found in smaller pockets of riparian forests, ephemerally wet forests, and swamps. In this region, black ash is not always the dominant tree species and may co-occur with hemlock, yellow birch, balsam fir, red maple, American basswood, and balsam poplar
Similar stands can occur in the St. Lawrence River region of New York and in northern portions of the Great Lakes states. In these areas, however, black ash can dominate wetland forests that range in size from a few acres to hundreds of acres. Associated tree species in these regions include red maple, American basswood, yellow birch, balsam fir, balsam poplar, and hemlock.1
Many tree species are not able to establish, survive, or grow in areas that are frequently or continually flooded or areas where soils remain wet for much of the year. These areas tend to be dominated by herbaceous plants or sedges that can tolerate flooded conditions and low soil oxygen levels.1,2 Black ash has evolved several adaptations that allow the species to occupy these otherwise harsh conditions. 2,3,4,5 For example, soils in forested wetlands are typically shallow and perched above high-water tables. Black ash can establish sturdy root structures that are relatively shallow and closer to the soil surface.1 Hypertrophied lenticles and adventitious roots are adaptations that allow black ash, and other bottomland woody species, to cope with the low levels of oxygen common in soils in swampy or boggy areas.6
Growth rates of black ash trees vary, depending on the site conditions and competition with other tree species. Black ash is intolerant of shade and suppressed trees that are less than 10 cm in diameter may be as much as 60-80 years old. 7 In more productive sites, black ash trees exposed to full or nearly full sun can grow relatively quickly.
Black ash trees may begin producing seeds when they are 30 to 40 years in age.1 Seeds mature by late summer and drop to the forest floor in autumn. Black ash reportedly produces large seed crops at 5-8-year intervals. Some reports indicate viable seeds can persist in the seedbank for up to 3 or 4 years.2,8 Recent research however, found little or no persistence of a black ash seedbank hydric forest sites dominated by black ash.9
Black ash regeneration, which refers to seedlings, saplings, and the larger recruits, often comprises a large component of the understory in black ash forests.10 Recruits, typically 1-4 inches in diameter, are young trees that are likely to eventually become overstory trees. Saplings, generally < 1 inch in diameter, are often moreabundant than recruits. Saplings can become recruits if they survive deer browse, competition, shade, ice, or other damage. Densities of ash seedlings can be high in some black ash stands; particularly where ample amounts of sunlight reach the ground. In other stands that are frequently flooded or deeply shaded, black ash seedlings may be absent or present at relatively low densities.
Like all ash species, black ash is ring porous, a term that refers to the way that trees transport water and nutrients from the roots up the trunk and into canopy branches and leaves. In ring porous trees, all or nearly all of this transport occurs in the outer ring of growth. Each annual ring is formed by the combination of earlywood and latewood cells in the xylem. In spring, black ash trees begin forming earlywood vessels in the xylem. These earlywood cells have large lumens (the opening in the middle) and are loosely packed together. This facilitates transport of relatively large volumes of water to the growing leaves and shoots. In mid-summer, trees stop producing earlywood and begin producing latewood. Latewood vessels are smaller, have thicker cell walls and cells are more tightly packed together than earlywood. Together, the earlywood and latewood comprise a tree ring, which is one year's worth of radial growth. Black ash trees have notably large vessels, especially in earlywood tissue. This contributes to the pliable nature of the splints that are used for weaving baskets. 8,10,11,12
The impact of EAB on black ash is likely to be profound and could lead to the functional loss of black ash as a major overstory species in wetland forests. Larvae of EAB feed on phloem in galleries that are often serpentine and that increase in width as the larvae grow. Phloem is the inner bark that transports carbohydrates produced by the leaves down the trunk to the roots. In black ash, these larval galleries usually extend further horizontally than on other ash species. This means that each larva injures more tissue and fewer larvae are required to cause mortality of black ash compared with other ash species.13, 14 This pattern, combined with the harsh site conditions where most black ash tree occur, can lead to complete or nearly complete mortality of overstory black ash trees, sometimes within 4-6 years of invasion. In forests dominated by black ash, ecological effects of this mortality may be substantial. For example, in autumn, black ash leaves provide an input of high-quality leaf litter, which drives nutrient cycles.15 Loss of overstory black ash trees is likely to affect site hydrology,16,17 indirectly impacting other plant species.5 Habitat available for various vertebrate and invertebrate animals in these systems is also affected by mortality of overstory black ash.15
Many questions are yet to be answered about the indirect and cascading effects of EAB invasion on black ash ecosystems. Ongoing research will shed light on how young black ash fare in sites where older trees have been killed. Whether black ash regeneration can co-exist with low densities of EAB and survive long enough to reach the overstory remains unknown. Potential competition between young black ash and co-occurring tree species will also influence the long-term persistence of black ash.