American chestnut

The reintroduction of the American chestnut is a major restoration project, one that has inspired scientists and nataure lovers for decades. It is also a project fraught with potential difficulties; success if far from assured. Because of the many complex issues involved with the American chestnut, the standarad tree delineation used for other species will not be used. Instead, the United States Department of Agriculture Forest Service,
Southern Research Station General Technical Report SRS-173 will be used.

Format modifications have been made to incorporate all of the article.

Below are the AUTHORS, ABSTRACT, INTRODUCTION, AND HABITAT sections. Other sections of the article can be found at the following links.

LIFE HISTORY

SPECIAL USES, GENETICS, and RESTORATION

The Silvics of Castanea dentata (Marsh.) Borkh., American Chestnut, Fagaceae (Beech Family).

AUTHORS
G. Geoff Wang, Professor, and Benjamin O. Knapp, Research Specialist, School of Agricultural, Forest and Environmental Sciences, Clemson University, Clemson, SC 29634-0317; Stacy L. Clark, Research Forester, U.S. Department of Agriculture, Forest Service, Southern Research Station, The University of Tennessee, Knoxville, TN 37996-4563; and Bryan T. Mudder, Biological Science Technician, U.S. Department of Agriculture, Forest Service, Southern Research Station, Clemson University, Clemson, SC 29634.

ABSTRACT

This report describes how the American chestnut (Castanea dentata) was ecologically extirpated due to an exotic pathogen, the chestnut blight (Cryphonectria parasitica), and describes current restoration efforts. The habitat, life history, special uses, and genetics of the American chestnut are detailed. The American chestnut was an important and versatile tree species until its demise from the chestnut blight. Backcross breeding techniques, the use of hypovirulent blight strains, and genetic engineering programs are currently being developed and tested to produce trees resistant to the blight fungus. The first putative blight-resistant trees have been planted and silvicultural techniques to improve competitive ability of chestnut are being tested. American chestnut is a fastgrowing species with the ability to persist in shaded conditions and it responds favorably to forest management techniques that limit competition and increase available sunlight. Restoration will require advanced artificial regeneration techniques. The effects of damaging agents other than blight, including root rot caused by Phytophthora cinnamomi, Asiatic oak weevil (Cyrtepistomus castaneus), the gypsy moth (Lymantria dispar), and Asian ambrosia beetles (Coleoptera: Scolytidae) are not well understood but may represent real barriers to restoration.

INTRODUCTION

American chestnut [Castanea dentata (Marsh.) Borkh.], also called sweet chestnut, was once a widespread and dominant species throughout the deciduous forests of eastern North America. American chestnut had the fastest growth and highest abundance in oak-chestnut forest region, but the species inhabited nearly all States east of the Mississippi River. The introduction of exotic pathogens, primarily the chestnut blight fungus [Cryphonectria parasitica (Murr.) Barr], led to the extirpation of the species as a forest canopy dominant throughout its native range. Root rot caused by Phytophthora cinnamomi (Rands) also contributed to the demise of Castanea species growing in lowland or riparian areas. The effects of these exotic pests resulted in the current listing of American chestnut as a species of special concern in Maine and Tennessee and as an endangered species in Kentucky and Michigan Plant Committee on the Status of Endangered Wildlife in Canada listed American chestnut as threatened in 1987 and changed the status to endangered in 2004. American chestnut was one of the most valued and beloved eastern hardwood species due to its use as a timber tree, its abundant nut production, and its secondary wood products. American chestnut grew straight and tall, reaching up to 5 feet in diameter and 120 feet in height, and the species could live for several hundred years. It was among the most versatile trees on the continent, historically used for construction lumber, shingles, fence posts and rails, poles, paneling, trim, furniture, and firewood. The wood was important in the charcoal iron furnace industry in the northern hardwoods region and is highly resistant to decay because of its high tannin content. Extracted tannins from chestnut once drove the leather tanning industry in the Northeastern United States. American chestnut produced copious amounts of flavorful nuts that were consumed by humans and wildlife. At its greatest distribution, American chestnut constituted up to one-fourth to one-half of the canopy trees in its native range, and because of its desirable characteristics, it was widely planted in orchards and along streets, roadsides, and fencerows for shade, windbreaks, nut production, and aesthetics. In the early 1900s, American chestnut made up more than 25 percent of all timber cut in the Southern Appalachians, was the most valuable tree in southern New England, and constituted almost half of the timber cut in Connecticut. The species was described as “one of the most promising trees for forest management” in Tennessee and “one of the most promising trees for forestry planting” at the onset of professional forestry in the United States.

The demise of American chestnut followed the introduction of two exotic fungi and is regarded as the most tragic ecological event in the post-glacial history of eastern North American forests. Root rot caused by Phytophthora cinnamomi was found to affect American chestnut as early as 1824, with the greatest impact on trees in riparian areas, heavy clay soils, and low-lying valleys. The disease was initially most damaging in the Piedmont and Coastal Plain regions but was also noted in mountainous areas, moving from low to progressively higher elevations. Unlike the chestnut blight, trees infected with Phytophthora root rot cannot maintain above-ground stems for long before death. The chestnut blight is an aggressive diffuse canker disease that was introduced along with imported Castanea spp. seedlings from Asia. The disease was first detected in 1904 at the Bronx Zoological Park in New York City; by 1950, the disease had spread throughout the range of American chestnut, and by 1960 it had killed an estimated 4 billion trees and essentially extirpated the species as a canopy tree. However, American chestnut is still a common component of eastern deciduous forests, persisting as understory sprouts that originated from blight-killed trees and saplings. The cycle of sprouting, infection, and dieback continues, with sprouts generally not exceeding small tree size and rarely growing to reproductive maturity. Recent advancements in the production of putatively blight resistant American chestnut trees indicate that restoration may be imminent. Because of the unique history of this species, relatively little is known about its ecology and life history. The objective of this technical report is to synthesize the current understanding of the silvics of American chestnut in order to facilitate current and future restoration efforts.

HABITAT

Native Range

American chestnut reached its maximum distribution in the United States about 2000 years before present, when the species established in New England forests for the first time. By the late 19th century, the American chestnut range extended from central Mississippi and Alabama through the Appalachian region and into New Hampshire, Vermont, and Maine; the range extended west into Ohio and Indiana and into portions of southern Ontario, covering an area greater than 309,000 square miles. Other reports suggest the possibility of a more extensive range, for example, chestnuts have been found far south in the Gulf Coastal Plain, occurring with longleaf pine (Pinus palustris Mill.) in southern Alabama, Florida, and Louisiana. The most concentrated abundance of American chestnut in its native range falls within the Oak-Chestnut Forest and the Mixed Mesophytic Forest regions. In the Oak-Chestnut Forest region, American chestnut was often the dominant tree species, comprising an estimated 25 percent of hardwood forests. In portions of the Appalachian Mountains, chestnut was thought to have made up 40 to 50 percent of the forest canopy, and in the Southern Appalachian Mountains, it could occasionally form pure stands. The species made up 15 billion board feet or 25 percent of the total timber volume in the Southern Appalachian Mountains. Chestnut trees comprised 50 percent of the timber on the mountainous ridges of western Maryland, dominated 26 to 36 percent of the slope forests of Maryland, and were estimated to hold 3.75 billion board feet of timber in the Blue Ridge Mountains of North Carolina. Chestnut dominated stands formed 50 percent of second growth forests in New Jersey, Pennsylvania, Connecticut, and southern New England. Most of these species composition estimates were made after the disturbances associated with European settlement, such as grazing, land-clearing for agriculture, uncontrolled wildfires, and indiscriminant logging. Chestnut regeneration probably increased following repeated disturbance due to its ability to prolifically sprout and occupy abandoned agriculture fields; therefore, historical species composition estimates may overestimate chestnut abundance prior to European settlement. Nevertheless, chestnut was perhaps the most widespread and abundant species in the Eastern United States since the last glaciation, with the highest concentration of American chestnut in the Southern Appalachian Mountains.

Climate

In the native range of American chestnut, mean annual precipitation varies from about 32 inches in western New York and southern Ontario to about 80 inches in the Southern Appalachian Mountains; however, annual precip-

itation for the majority of the native range is between 40 inches and 48 inches. Annual snowfall ranges from only trace amounts in the Southern United States to 100 inches or more in the Northern United States and in Canada. Mean annual temperature is about 40 °F in the northern part of the range and 60 °F in the extreme southern part. The frost-free period averages 100 days in the Northern United States and 240 days in the Southern United States. Although American chestnut is believed to be the most cold-hardy species of the Castanea genus, it is prone to frost damage, which may have affected its northern range and upper altitudinal limits. It has long been observed a lack of American chestnut in the bottoms of ravines and valleys because frosts commonly occur on wet, protected soils. American chestnut occurs at elevations of 1,300 to 4,500 feet in the Southern Appalachian Mountains, with the possibility of occurrence at higher elevations of 4,000 to 5,500 feet on southern exposures. In North Carolina, American chestnut grows between elevations of 500 feet and 5,500 feet. American chestnut grew at lower elevations in northern latitudes, occurring at elevations < 3,000 feet in the Catskill Mountains and at elevations < 400 feet in New Hampshire.

Only one study has examined climate-tree growth relationships of American chestnut. The relationships between American chestnut growth and climate were similar to those of other hardwoods, with increasing growth associated with increasing precipitation and decreasing Palmer Drought Severity Index values during the growing season. However, American chestnut growth had strong positive relationships with August precipitation of the current year and strong negative relationships with August temperature of the previous year, suggesting it may be slightly more sensitive to late growing season climate than other hardwoods.

Soils and Topography

American chestnut naturally occurs on a wide range of soil and topographic conditions but is most commonly associated with well-drained, subxeric to mesic soils that often occur on sand plains and dry ridges. American chestnut is believed to grow poorly in very wet or very dry soils, and rarely grows in areas with high pH or on limestone-derived soils. Within a soil pH range of 3.3 to 5.5, chestnuts were more abundant at the higher end of the range (pH of 4-5) in a study conducted in southwestern Virginia. A recent survey of surviving trees in Canada found that American chestnut was most likely to occur in deciduous forest habitats with high canopy cover (> 50 percent), gentle slopes (0 to 10 percent) and acidic (pH 4–6), sandy (> 75 percent) soils. In the southern portion of the range, the occurrence of American chestnut on northern side slopes increased with topographic relief, and the species was negatively associated with the more calcium rich soils and lower relief of the Coastal Plain region. The presence of American chestnut often indicates acidic soils derived from gneiss, metamorphosed sandstone, and sandstone that is deficient in lime and potash. A recent study also reported that American chestnut height growth was negatively correlated with pH. Although the Society of American Foresters included American chestnut as a characteristic tree in the cove hardwood forest type, it is not clear to what extent American chestnut occurred in cove or ravine sites. Some researchers indicate that chestnut was not commonly found on moist cove sites, while other researchers suggest the opposite. Some reports indicate that American chestnut grew best in rich, deep coves in Tennessee, while other researchers observed that American chestnut in cove forests produced the best timber. More recent evidence supports that American chestnut contributed substantially to the canopy of ravine and cove sites in the Cumberland Plateau and Southern Appalachian Mountains, e.g., more survivors were found in ravine sites than on ridge-top sites, and chestnut potentially represented 25 to 40 percent of the basal area in pre-blight riparian stands. Additionally, observations have been made that first-year seedling growth was negatively correlated with the proportion of sand in the soil, suggesting the superior suitability of finer textured soils in cove sites for chestnut growth. The dominance of American chestnut may have affected nutrient cycling and soil chemistry, although a replicated study examining chestnut leaf litter effects on soil biogeochemical properties has not been conducted. One descriptive study found that most soil nutrients on sandy loam soils were higher under chestnut canopies compared to areas without chestnut. As soil texture became finer, this effect disappeared, suggesting that effects of American chestnut leaf litter on soil chemistry are dependent on soil texture.

Soil nutrient requirements—American chestnut grows and remains competitive over a wide range of soil nutrient regimes. It was frequently found on relatively nutrient deficient to moderately fertile soils. Although American chestnut tolerates relatively low soil nutrient levels, it responds well to increasing soil nutrients. Improved seedling growth has been observed with increasing soil nutrient availability in a greenhouse study. Furthermore, positive growth and/or biomass response reported for American chestnut seedlings has been associated with increasing availability of magnesium (leaf mass and area, root mass), potassium (diameter, specific leaf area), and nitrogen (stem, root, leaf parameters).

Soil moisture regime—American chestnut is a relatively drought tolerant species, as suggested by its historical dominance on upland sites with well-drained, sandy soils. During an early-season drought in a hardwood forest in Pennsylvania, American chestnut saplings maintained higher leaf water potential than several associated oak (Quercus) species that are known for their drought tolerance. High water use efficiency of American chestnut seedlings exposed to drought have been compared to published reports of co-occurring hardwood species. However, American chestnut is reported to require more moisture than associated oaks, including black oak (Quercus velutina Lam.), scarlet oak (Quercus coccinea Münchh.), and chestnut oak (Quercus prinus L.). Alternatively, American chestnut growth was less related to drought during the growing season of the previous year compared to other hardwoods in the same stand. The reported dominance of American chestnut in cove forest sites, as well as on well-drained upland sites, suggests that the species can compete across a wide range of soil moisture regimes.

Associated Forest Cover

American chestnut is capable of forming pure stands, but historical reports suggest that pure stands were rare. In one classification of forest communities, American chestnut was the dominant tree species in the Oak-Chestnut Forest region. The Oak-Chestnut Forest region included five sections: (1) the Southern Appalachian section; (2) the Northern Blue Ridge section; (3) the Ridge and Valley section; (4) the Piedmont section; and (5) the Glaciated section.

Common associates of American chestnut in the Southern Appalachian Mountains included buckeyes (Aesculus spp.), basswood (Tilia americana L.), yellow poplar (Liriodendron tulipifera L.), eastern hemlock [Tsuga canadensis (L.) Carrière], white pine (Pinus strobus L.), pitch pine (Pinus rigida Mill.), table mountain pine (Pinus pungens Lamb.), northern red oak (Quercus rubra L.), white oak (Q. alba L.), black oak, scarlet oak, and chestnut oak. The canopy trees were often associated with an ericaceous shrub layer consisting of Rhododendron spp., Azalea spp., Kalmia spp., Vaccinium spp., Leucothoe spp., Menziesia spp., and others depending on geographic region and elevation. In the Glaciated section, American chestnut was commonly associated with red oak, scarlet oak, white oak, chestnut oak, pignut hickory (Carya glabra Mill. Sweet), American beech (Fagus grandifolia Ehrh.), and yellow poplar with an ericaceous understory. In the Catskill Mountains of New York, American chestnut was observed in mixed deciduous forests along with American beech, northern red oak, butternut (Juglans cinerea L.), sugar maple (Acer saccharum Marsh.), and an understory of Rhododendron spp., Kalmia spp., and other ericaceous shrubs. In the Northern Blue Ridge section, American chestnut was commonly associated with yellow poplar, northern red oak, white oak, chestnut oak, sugar maple, eastern hemlock, and basswood, with Rhododendron species, mountain laurel (Kalmia latifolia L.), and blueberry (Vaccinium spp.) as common understory species. American chestnut associates in the Ridge and Valley section included northern red oak, scarlet oak, white oak, chestnut oak, white pine, eastern hemlock, and yellow poplar, with an ericaceous understory similar to that already mentioned. In the Piedmont section, chestnut is being replaced by white oak, black oak, northern red oak, post oak (Quercus stellata Wangenh.), blackjack oak (Quercus marilandica Münchh.), chestnut oak, yellow poplar, and American beech, with an understory of dogwoods (Cornus spp.) and hickories (Carya spp.) mixed with the heath layer. American chestnut was also one of the dominant trees in the Mixed Mesophytic Forest region that encompasses the Cumberland Mountains, Cumberland and Allegheny Plateau, and Allegheny Mountain sections. It was commonly associated with American beech, yellow poplar, basswood, sugar maple, yellow buckeye, northern red oak, white oak, and eastern hemlock. American chestnut was the most dominant species in the Allegheny Mountain section, where it was commonly associated with northern red oak, white oak, black oak, scarlet oak, yellow poplar, hickories, black gum (Nyssa sylvatica Marsh.), black birch (Betula lenta L.), basswood, sugar maple, and beech. Associated forest cover of American chestnut is highly dependent on disturbance history. The composition of many forests once containing a major component of chestnut are succeeding from an oak-hickory dominated association to one dominated by maple or shade-intolerant species. In a recent survey of surviving American chestnut sprouts and trees in the Coastal Plain, Highland Rim, and Cumberland Plateau regions of Kentucky and Tennessee, 66 percent of the sampled American chestnut trees (n > 2,000) occurred within 49 feet of red maple trees. Oak species were recorded in < 30 percent of plots that surrounded American chestnut. Red maple (Acer rubrum, L.) was historically not a major component of these forest ecosystems, but it has become more common with changes in disturbance dynamics.