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LIFE HISTORY

 

Reproduction and Early Growth

Flowering and fruiting—American chestnuts are monoecious, with male and female reproductive parts on the same tree (Brown and Kirkman 1990, Zon 1904). Flowers are staminate catkins that are borne on a central axis 5 to 10 inches long and are produced in clusters from the axils of leaves. Flowers appear from late May to June, after frosts have passed (Brown and Kirkman 1990; Zon 1904). Weak lower lateral shoots consist of all male flowers, while stronger upper vertical shoots have male flowers on upper shoots and female flowers on lower shoots. American chestnut pollen is not as abundantly dispersed as that of many other wind-pollinated tree species and often requires another tree within 328 feet to reproduce (Paillet 2002, Russell 1987). Pollen is not dispersed until after full leaf development has occurred, and only pollen escaping above the forest canopy can be dispersed for long distances (Russell 1987). The fruit of American chestnut, maturing between September and October, is a brown edible nut with a sweet taste (Brown and Kirkman 1990), and the species was sometimes referred to as sweet chestnut due to its flavorful nut (Van Fleet 1914). The nut develops within a round prickly green bur that produces two to three nuts in each bur (Saucier 1973, Zon 1904).

 

Seed production and dissemination—American chestnut is a prolific seed producer and one of the most fruitful among all American nut-producing species. It is able to produce fruit as early as age 4 when open-grown, or at about 8 to 20 years old when competing with other trees in the forest (Paillet and Rutter 1989). Reproduction starts earlier and yields more nuts in trees reproduced by coppice than in trees regenerated from seed. A mature tree can produce 1.5 to 3 bushels or up to 6,000 nuts per year (Paillet and Rutter 1989), and unlike other nut producers that display variable masting behavior, such as white oak, American chestnut is a dependable seed producer every year. The dependable seed production is probably due to its late flowering that avoids frost damage. The fruit typically drops with the first frosts of autumn, which opens the bur, and the seeds germinate the following spring (Zon 1904). Chestnuts were historically an important food for wildlife, to the point that seed predation combined with insect damage limited the success of reproduction from seed (Detwiller 1915, Hawley and Hawes 1925). Chestnut provided a more stable and more abundant source of mast for wildlife than oaks, hickory, and beech species (Diamond and others 2000). Additionally, the nuts were extensively harvested by humans as a source of food and income (Hawley and Hawes 1925, Hepting 1974, Saucier 1973). As a result of the high demand for chestnuts by wildlife and humans, Paillet and Rutter (1989) report that only 1 to 5 viable seeds germinated into seedlings that survived for more than 1 year. When seeds were not consumed by animals or damaged by insects, chestnuts generally had high germinate success, with 60 to 70 percent germination observed in the field (McCament and McCarthy 2005) and > 90 percent germination in the greenhouse (Wang and others 2006) and in commercial tree nurseries (Clark and others 2012b.). Like many other large-seeded trees, American chestnut seeds were dispersed by wildlife species that included birds and squirrels (Sciurus spp.). The American crow (Corvus brachyrhynchos), blue jay (Cyanocitta cristata), wild turkey (Meleagus gallaparo silvestris), ruffed grouse (Bonasa umbellus) and passenger pigeon (Ectopistes migratorius) were major dispersers of large fruit and probably important for chestnut dispersal (Russell 1987, Webb 1986). Large mammals such as white-tailed deer (Odocoileus virginianus) or black bear (Ursus americanus) may have also played an important role in the dispersal of American chestnut, as the bur is thought to be an evolutionary adaptation for hitchhiking on mammalian fur.

 

Seedling development—Natural regeneration from seeds was probably rare in pre-blight years. The nuts were damaged by insects (Detlefson and Ruth 1922, Russell 1987) and highly utilized as a wildlife food source (Detwiler
1915). Small seedlings were easily killed by fire or frosts (Ashe 1911). Artificial regeneration of nursery-grown seedlings will be a crucial component of restoration. Once sufficient lines of blight-resistant seedlings are developed and tested, they can be planted, cross-pollinate with one another, and sexually reproduce. However, restoration of this species will probably always involve planting, due to the loss of seedlings from nut predation and high seedling mortality. Seedlings grow rapidly in the greenhouse or in the field as nursery-grown bare-root seedlings or direct-seeded regeneration. Artificially regenerated chestnuts grow best under moderate- to high-light conditions but are capable of surviving under low-light conditions. Seedlings averaged 3 to 4 feet in height and 0.5 to 0.6 inches in diameter at the root collar after 1 year in a commercial tree nursery after receiving steady applications of fertilizer throughout the growing season (Clark and others 2009, 2011, 2012b). Large, nursery-grown, seedlings generally outperform smaller seedlings due to the ability of the larger seedlings to reach heights above browse and natural woody competitors (Clark and others 2011). After 1 year in the field, nurserygrown seedlings that were not browsed by deer averaged 7 inches in height growth and had 80-93 percent survival if not affected by root rot caused by Phytophthora cinnamomi (Clark and others 2012a). Larger nursery seedlings appear to allocate more growth to the root system after being fieldplanted than do smaller nursery seedlings and, consequently, are better suited to resist transplant shock (Clark and others 2009, 2011). Three growing seasons after planting nurserygrown seedlings in the field, seedlings averaged 0.91 inches and 0.94 inches in root collar diameter and 63 inches and 91 inches in height, respectively, when grown in 100 percent and 34 percent full sunlight (Anagnostakis 2007). Early results indicate that trees bred for blight-resistance are similar to American chestnut but differ from Chinese chestnut with respect to height growth rates (Clark and others 2011, 2012a). Direct-seeding American chestnuts may provide an alternative to out-planting nursery-grown seedlings on certain sites but will require intensive management to avoid predation by mammals and to maintain the competitive status of the seedlings. Two-year survival from direct seeding ranged from 40 to 50 percent in a field study in Ohio (McCament and McCarthy 2005). After 2 years of growth under partial forest shade (~ 25 percent open sky), direct-seeded seedlings averaged ~ 0.23 inches in root collar diameter and ~ 14 inches in height (McCament and McCarthy 2005). Under full sunlight, seedlings grown from seed in the greenhouse averaged ~ 0.20 inches in root collar diameter and ~ 12 inches in height after one growing season; seedlings allocated < 30 percent of the total carbon gain to below-ground biomass, compared to > 65 percent allocation to below-ground biomass for white oak (Wang and others 2006). Latham (1992) reported that first-year American chestnut seedlings grew faster in height, leaf area, and total dry mass under different levels of nutrients when compared to co-occurring species, including mockernut hickory [Carya alba (L.) Nutt], northern red oak, American beech, blackgum (Nyssa sylvatica) and tulip poplar. However, root-to-shoot ratios were among the lowest for American chestnut (Latham 1992), further indicating that early growth is allocated to above-ground biomass rather than root systems.

 

Vegetative reproduction—American chestnut is a tenacious sprouter, and its ability to repeatedly produce large numbers of fast-growing sprouts following dieback is largely responsible for its persistence in forests today (Hawley and Hawes 1925). Natural sprouts will probably exist in the forest for years to come, but American chestnut will never again be a significant component of the forest canopy without the introduction of blight-resistant seedlings. Sprouts that live long enough to bear fruit are valuable because they can be used to maintain genetic diversity in breeding programs (Pierson and others 2007) and for in situ tests of blight resistance and hypovirulence (Anagnostakis 2001, Griffin and others 1983). Frothingham (1912) attributes the “prodigious sprouting capacity” and rapid growth of American chestnut as reasons for the species’ great abundance in second-growth Connecticut forests prior to the blight. Prolific sprouting is considered an adaptation for long-term survival in the forest understory (Paillet 1984, 2002). Sprouts of American chestnut arise from dormant buds that may exist for several years before developing into a shoot or from adventitious buds that develop following localized stimulation (Mattoon 1909, Paillet 1984). Chestnut rarely develops stool sprouts because the narrow region of sprouting is generally in contact with the ground or just above it. However, lowcut stumps tend to have sprouts of deeper origin and show a tendency to develop new lateral roots (Mattoon 1909). In a study by Zon (1904), chestnut was found sprouting principally from the root collar, and the best sprouts were found on low stumps. Although American chestnut is reportedly able to produce sprouts from trees up to 170 years old, the optimum age for sprouting is probably < 75 years (Mattoon 1909, Zon 1904). Mattoon (1909) observed that old stumps produce a full thicket of short, spindling sprouts, while young stumps generate a smaller number of tall, stout sprouts. Paillet (1984) characterized the growth form of sprouting American chestnuts in the northeast and found it to be highly variable. Sprouts developed a shrub-like form when grown in the shade but expressed strong apical dominance and rapid growth when released, suggesting a growth strategy well suited to periodic disturbance and release from the canopy. The most vigorous sprouts are produced at periods of maximum growth in height, which occurs within the first decade in coppice trees (Zon 1904) or 20-30 years in stands regenerated from seedlings (Mattoon 1909). Coppice trees grow faster than trees regenerated from seed during the first 20 years. Because of their fast initial   growth, sprouts reach their maximum average height growth rate during the first decade of growth. Mattoon (1909) observed rapid height growth (4 to 7 feet per year) in the first year but then a sharp decrease (2 to 3 feet per year) in the second and third year. Diameter growth continues well after height growth has decreased (Zon 1904). An increase of nearly 1 inch per year in diameter during the first 8-15 years has been observed under the most favorable  conditions (Mattoon 1909). Dormant season cutting favors the production and growth of sprouts. Mattoon (1909) reportes that midwinter cutting produced sprouts of superior first-year growth when compared to May cutting: 6.2 feet versus 3.5 feet in height and 0.42 inches versus 0.23 inches in diameter,   respectively. Early recommendations for cutting include: (1) late fall to early spring cutting to avoid production of weak sprouts too tender to stand frost; (2) cutting stumps low and at a slight angle to allow water runoff; and (3) avoiding damage to bark which may in turn damage buds (Frothingham 1912, Zon 1904). Chestnut sprouts remain an important component of understory vegetation throughout the species’ former range (Paillet 1984). However, Paillet (1984, 1988, 2002) concluded that a majority of these sprouts were not associated with canopy trees killed by the blight but instead probably originated from former seedlings that had been through several cycles of fungal infection and resprouting or were from sprouts developed from canopy trees cut before the blight. Griffin and others (1991) observed that sprouts generally did not exceed 2 inches in diameter at breast height (d.b.h.) 10 years after sprouting in a clearcut before dying back from blight. Paillet (1984) alludes to sprouting as a reproductive strategy for chestnuts awaiting a crown opening because sprouts are capable of rapid transformation from small, suppressed stems to straight, vigorous saplings in a short time. Suppressed chestnut stems that have escaped blight for several decades develop a form with a single stem or a single stem with at least one weak secondary stem (Paillet 2002).

 

Sapling and Pole Stages to Maturity Growth and Yield—Early studies indicate that American chestnut is highly competitive and fast-growing during early growth (Ashe 1911, Graves 1905, Zon 1904). These historical observations have been confirmed in recent studies. Jacobs and Severeid (2004) reported that juvenile plantation growth of American chestnut on blight-free sites in Wisconsin greatly exceeded that of interplanted black walnut (Juglans nigra L.) or northern red oak. In a recent study (Jacobs and others 2009), American chestnut averaged 3.5 inches in d.b.h. and 28.5 feet in height 8 years after planting and 10 inches d.b.h. and 45 feet in height after 19 years. Paillet and Rutter (1989) described the ability of introduced American chestnut to rapidly out-compete and eventually replace native tree species (e.g., Quercus spp., Carya spp.) as the dominant canopy tree in a Wisconsin forest. Studying American chestnut development in this same forest, McEwan and others (2006) documented that the growth of American chestnut trees released from the forest canopy following a logging event exceeded that of associated hardwoods by nearly a factor of two.


 The timber yield of American chestnut varied greatly depending on the dominance of American chestnut in the stand. Buttrick (1925) estimated that pure stands of American chestnut could yield as high as 20,000 board feet per acre and yielded approximately 20 to 30 billion board feet in the United States (Buttrick 1915). In mixed stands, Buttrick (1925) reported yield at an average of 4,000 board feet per acre with an estimated maximum of 10,000 board feet per acre for cove forests. The yield on slopes was estimated at 2,000 to 3,000 board feet per acre and that on ridges was reported at 1,500 board feet per acre. A study in Tennessee reported average annual yield of approximately 500 board feet per acre per year over a 60-year rotation (Holmes 1925). In a recent study, Jacobs and others (2009) reported that biomass accumulation reached almost 80 tons per acre in a 19-year-old plantation.

 

Rooting habit—Little is known about the rooting habit of American chestnut, and inconsistent descriptions were found in the literature. Some describe American chestnut   as having a shallow root system (Buttrick and Holmes 1913, Paillet 2002), yet others report that American chestnut is a deep-rooted species with a tap root similar to oak and lower lateral roots that spread up to 3.3 feet deep in the soil (Smith 2000, Zon 1904). Chestnut tap roots may divide into many vertical roots that extend 3 to 6 feet, each of which may develop many lateral roots (Smith 2000). Studies of American chestnut growing in the nursery indicate that chestnut seedlings have a main tap root with many lateral roots, similar to that of oak species (Clark and others 2009), and the number of lateral roots is positively correlated to both height and root collar diameter (Clark and others 2009, 2012b).

 

Reaction to competition—Although American chestnut has been shown to outgrow competing hardwood species, the presence of abundant competition reduces chestnut growth (Griffin and others 1991). Insight into the competitive nature of American chestnut can be traced back to historical pollen records. Range expansion of American chestnut during the Holocene from glacial refugia was the most recent of wind pollinated trees (Paillet 1982, 2002, Russell 1987), and its rapid expansion to canopy dominance implicates American chestnut as an exceptionally competitive species. The shade tolerance of American chestnut is still under debate (Joesting and others 2009, Wang and others 2006). Early observations suggest that American chestnut is relatively intolerant (Frothingham 1912, Hawley and Hawes 1925) to moderately tolerant (Zon 1904) of shade. Recent studies have classified American chestnut as either shade tolerant (Wang and others 2006) or intermediately shade tolerant (Joesting and others 2009). In the first study to use contemporary instrumentation on American chestnut, Wang and others (2006) suggested that American chestnut is shade tolerant based on physiological characteristics that include low light compensation and saturation points measured in a greenhouse study, and these results were later confirmed in field studies (Joesting and others 2007, 2009). The shade tolerance of American chestnut is supported by other studies on morphology, survival, and growth under canopy shade. Paillet (1982) found that chestnut can survive in deep shade under the canopy for up to three decades and is more shade tolerant than co-occurring sub-canopy species. When growing in a light-limited environment, American chestnut increases its specific leaf area (Joesting and others 2009, King 2003, McCament and McCarthy 2005, Wang and others 2006) and develops canopy architecture that is optimal for harvesting light (Paillet 1982). Anagnostakis (2007) found that seedlings growing in the field under 63 percent shade cloth had more above-ground mass than those growing in full sunlight after three growing seasons. However, the shade tolerance of artificially regenerated seedlings may be different than those growing as sprouts. Seedlings planted as bare-root nursery stock may not be able to withstand shade due to the relatively small carbohydrate reserves stored in the newly developed root system, compared to a native sprout with an older, well-established root system. Joesting and others (2009) reported that natural chestnut seedlings and saplings can subsist in the understory for many years, have a low dark respiration rate, high quantum efficiency, low N percent, low leaf mass per area, and high light-induced morphological plasticity, all of which are characteristic of a shade tolerant species. However, Joesting and others (2009) classified the shade tolerance of American chestnut as intermediate based on their finding that American chestnut has a maximum photosynthesis rate comparable to shade intolerant species. To justify their classification, Joesting and others (2009) also cited previous observations that American chestnut can rapidly assume a canopy position following the creation of canopy openings (Jacobs and Severeid 2004, McEwan and others 2006) and that seedlings respond well to increasing light both in greenhouse (Latham 1992, Wang and others 2006) and field conditions (Boring and others 1981, Griffin 1989, McCament and McCarthy 2005, Tindall and others 2004). However, we argue that the ability to fill gaps in the canopy does not confer shade intolerance to a tree species. In fact, this strategy is commonly used by many shade tolerant species, e.g., red maple. Considering all the published evidence, we maintain that American chestnut should be classified as shade-tolerant. Indeed, the continuous survival of chestnut sprouts from blight-killed advanced regeneration in former American chestnut forests (Paillet 2002) is the best testament of its shade tolerance. Its strong ability to survive for prolonged periods in deep canopy shade (Paillet 1982), coupled with its ability to grow well under partial shade (Anagnostakis 2007), suggests that chestnut maintains its presence in the understory in anticipation of disturbance events (Jacobs 2007), with a strategy similar to that of other Fagaceae species, e.g., oaks. The rapid height growth of American chestnut following release or after planting in full sun (Billo 1998; Clark and others 2009, 2012a; McEwan and others 2006; Paillet 1982, 2002; Paillet and Rutter 1989) distinguishes American chestnut from cooccurring intermediately shade tolerant species such as oaks (Paillet 2002). Competition can also affect blight development on American chestnut seedlings. Field tests indicate that blight may be more common on sprouts growing in open or clear-cut sites compared to sprouts growing in the understory of mature forests (Griffin 1989, Griffin and Elkins 1986, Reynolds and Burke 2011). However, after blight has infected the site, surviving trees appear to better withstand the blight if released from competition. The causal factor may be related to hypovirulence, which may be more prevalent if competition is controlled and chestnut stems are allowed to take a dominant canopy position (Griffin 1989). Therefore, mesic sites may offer the best opportunity for biocontrol of the fungus through hypovirulence (Griffin 1992), but such sites will also have the most need for competition control.


Allelopathy—Leachate from American chestnut litter may have allelopathic properties that limit the development of some common competitors in the southern Appalachians, including eastern hemlock and rhododendron (Rhododendron maximum L.). Good (1968) found that chestnut leaf extracts (1.8 ounce leaf per 1 fluid ounce distilled water) inhibited the germination of eastern hemlock and significantly reduced shoot length of black birch, yellow birch (Betula allegheniensis Britton), tomato (Lycopersicon esculentum Mill.), and eastern hemlock in a laboratory bioassay; he also found that adding chopped chestnut leaf material (5 g dry weight) to germination pots significantly reduced root length of tomato and root, stem, and leaf biomass of black birch. Vandermast and others (2002) found that chestnut leaf extracts (0.04 ounces per 0.7 fluid ounces of distilled water) significantly reduced seed germination of lettuce (Lactuca sativa L.), rhododendron, and eastern hemlock and also reduced the length of radicles of germinating lettuce and rhododendron. Although these studies clearly demonstrate the allelopathic potential of American chestnut, the nature of these studies prevents extrapolation of the results to field conditions.

 

Damaging agents—American chestnut might be the most susceptible tree in the Eastern United States to damaging agents. The most widespread damaging agent to American chestnut is the chestnut blight. The fungus was originally believed to be a new species and was named Diaportha parasitica, but it was discovered that it likely came into the United States on imported Asian Castanea species. The fungus was renamed Endothia parasitica, and then Cryphonectria parasitica, and it is most commonly   referred to as the chestnut blight fungus (Anagnostakis 1987, 1992). Common symptoms of chestnut blight include the presence of mycelium arranged in buff-colored fans on the inner bark, sunken or raised cankers on the infected tree, the  appearance of stromata (orange fruiting bodies) on the smooth bark surrounding cankers, the development of sprouts below cankers, and small, dying leaves and nuts on affected stems (Anagnostakis 1987, Gravatt 1925, Hawley and Hawes 1925). Disease symptoms also include  yellowing or wilting leaves that tend to persist on the tree past leaf abscission in autumn (Griffin and Elkins 1986). Cankers are usually found at the base of the tree and   usually have an elliptical shape, extending up and down the trunk. Old trees susceptible to blight typically produce sunken cankers, while more vigorous or blight resistant  trees produce slightly swollen cankers (Gravatt 1925,  Griffin 2000, Griffin and Elkins 1986). The bark over the injury is typically darker red in color than the surrounding bark, and exposed wood is often at the canker center  (Griffin and Elkins 1986).

 

As an ascomycete, the fungus produces small reddishbrown fruiting bodies on the surface of cankers where two  different types of spores are formed (Gravatt 1925). Shortlived conidia are summer spores exuded in sticky masses. After summer rains these spores stick to the feet of birds, insects, and mammals and are carried long distances to infect other trees (Burnham 1988, Hepting 1974). Ascopores are winter spores that are carried long distances by wind to infect  other trees by entering at a wound or split in the bark, where they germinate and enter the inner bark, killing vital cells as they invade (Burnham 1988, Hepting 1974). American chestnut trees commonly have wounds and punctures caused by insects, woodpeckers, and natural bark cracks (Ashe 1911). The fungus enters through these bark wounds, where it grows and has a girdling effect on the cambium of the tree. Although the blight has effective dispersal mechanisms, humans helped the spread by shipping diseased nursery stock, carrying the fungus on chestnut wood, and transporting spores on clothing, shoes, and tools (Hepting 1974).
 Although the chestnut blight has certainly had the greatest impact on American chestnut populations across the native range, the species is also susceptible to other damaging agents (Jacobs 2007). Records of dying chestnuts caused by the root rot fungus Phytophthora cinnamomi can be traced back to 1824 (Crandall and others 1945). Woods (1953) reported that an epidemic of P. cinnamomi in the southern portion of the United States nearly eliminated chestnut fro
m that part of its range from 1825–75. Crandall and others (1945) attributed P. cinnamomi to the disappearance of chestnut from the Gulf and Atlantic States as well as the foothills and mountains of Mississippi, Alabama, Georgia, Tennessee, Maryland, Virginia, North Carolina, and South Carolina. Ashe (1911) reported declining populations of chestnut throughout lower elevations of the Appalachians in Tennessee, and although not recognized at the time, the probable cause was P. cinnamomi. This pathogen spreads primarily through free-moving soil water as zoospores that colonize fine-feeder roots; the fungus then can move into healthy cells of the main tap root and into the above-ground stem, forming lesions above the root collar (Crandall and others 1945, Robin and others 1992). Common symptoms of P. cinnamomi include sudden yellowing or wilt of the leaves during the growing season, followed by defoliation and death. The pathogen can also cause a gradual reduction in the size of the leaves over several years (Crandall and others 1945). Lesions of varying size with an ink-like exudate appear on the roots and root collar cankers, leading to the name “ink disease.” The disease also affects red oaks, although it usually does not cause death (Robin and others 1992); however, it has been determined to cause the death of white oak and has been found in pine stands as well (Balci and others 2007, Campbell and Hendrix 1967). Phytophthora cinnamomi is found in many forest types throughout the Southeastern United States (Campbell and Hendrix 1967, Hendrix and Campbell 1970, McLaughlin and others 2009) but is not tolerant of cold environments with annual minimum temperatures less than -20 °C (Balci and others 2007, Benson 2002). It is generally not found north of 40° latitude (Balci and others 2007) or in higher elevations at lower latitudes. In an experimental study with American chestnut seedlings, Rhoades and others (2003) found that the occurrence of P. cinnamomi infection was greater in wet soil than in dry soil, but around 25 percent of seedlings became infected regardless of soil moisture. Many observers noted the local decline and death of large numbers of American chestnut trees adjacent to one another, and such epidemics often began at poorly drained sites with heavy soils and then spread to higher, drier sites (Crandall and others 1945). However, susceptible trees on well-drained soils have also succumbed to the disease if the soil becomes infected (Balci and others 2007, Clark and others 2009, 2012a).  Clark and others (2009, 2012a)1 attributed failure in the first-year survival of pure American chestnut plantings in the Southeastern United States to P. cinnamomi that was probably brought in from nursery stock.llen cankers (Gravatt 1925,  Griffin 2000, Griffin and Elkins 1986). The bark over the injury is typically darker red in color than the surrounding bark, and exposed wood is often at the canker center  (Griffin and Elkins 1986).

 

As an ascomycete, the fungus produces small reddishbrown fruiting bodies on the surface of cankers where two  different types of spores are formed (Gravatt 1925). Shortlived conidia are summer spores exuded in sticky masses. After summer rains these spores stick to the feet of birds, insects, and mammals and are carried long distances to infect other trees (Burnham 1988, Hepting 1974). Ascopores are winter spores that are carried long distances by wind to infect  other trees by entering at a wound or split in the bark, where they germinate and enter the inner bark, killing vital cells as they invade (Burnham 1988, Hepting 1974). American chestnut trees commonly have wounds and punctures caused by insects, woodpeckers, and natural bark cracks (Ashe 1911). The fungus enters through these bark wounds, where it grows and has a girdling effect on the cambium of the tree. Although the blight has effective dispersal mechanisms, humans helped the spread by shipping diseased nursery stock, carrying the fungus on chestnut wood, and transporting spores on clothing, shoes, and tools (Hepting 1974).
 Although the chestnut blight has certainly had the greatest impact on American chestnut populations across the native range, the species is also susceptible to other damaging agents (Jacobs 2007). Records of dying chestnuts caused by the root rot fungus Phytophthora cinnamomi can be traced back to 1824 (Crandall and others 1945). Woods (1953) reported that an epidemic of P. cinnamomi in the southern portion of the United States nearly eliminated chestnut from that part of its range from 1825–75. Crandall and others (1945) attributed P. cinnamomi to the disappearance of chestnut from the Gulf and Atlantic States as well as the foothills and mountains of Mississippi, Alabama, Georgia, Tennessee, Maryland, Virginia, North Carolina, and South Carolina. Ashe (1911) reported declining populations of chestnut throughout lower elevations of the Appalachians in Tennessee, and although not recognized at the time, the probable cause was P. cinnamomi. This pathogen spreads primarily through free-moving soil water as zoospores that colonize
fine-feeder roots; the fungus then can move into healthy cells of the main tap root and into the above-ground stem, forming lesions above the root collar (Crandall and others 1945, Robin and others 1992). Common symptoms of P. cinnamomi include sudden yellowing or wilt of the leaves during the growing season, followed by defoliation and death. The pathogen can also cause a gradual reduction in the size of the leaves over several years (Crandall and others 1945). Lesions of varying size with an ink-like exudate appear on the roots and root collar cankers, leading to the name “ink disease.” The disease also affects red oaks, although it usually does not cause death (Robin and others 1992); however, it has been determined to cause the death of white oak and has been found in pine stands as well (Balci and others 2007, Campbell and Hendrix 1967). Phytophthora cinnamomi is found in many forest types throughout the Southeastern United States (Campbell and Hendrix 1967, Hendrix and Campbell 1970, McLaughlin and others 2009) but is not tolerant of cold environments with annual minimum temperatures less than -20 °C (Balci and others 2007, Benson 2002). It is generally not found north of 40° latitude (Balci and others 2007) or in higher elevations at lower latitudes. In an experimental study with American chestnut seedlings, Rhoades and others (2003) found that the occurrence of P. cinnamomi infection was greater in wet soil than in dry soil, but around 25 percent of seedlings became infected regardless of soil moisture. Many observers noted the local decline and death of large numbers of American chestnut trees adjacent to one another, and such epidemics often began at poorly drained sites with heavy soils and then spread to higher, drier sites (Crandall and others 1945). However, susceptible trees on well-drained soils have also succumbed to the disease if the soil becomes infected (Balci and others 2007, Clark and others 2009, 2012a).  Clark and others (2009, 2012a)1 attributed failure in the first-year survival of pure American chestnut plantings in the Southeastern United States to P. cinnamomi that was probably brought in from nursery stock. Phytophthora cinnamomi represents a large threat to restoration of American chestnut in the areas with climate and soil conditions favorable to the disease, including the Southern Piedmont, the Blue Ridge, and some areas within Tennessee and Kentucky. There currently exists no effective control of the disease in the nursery, in the field, or through breeding programs. Screening by the American Chestnut Foundation and the Connecticut Agriculture Experiment Station for resistance to the disease is in the beginning stages and shows promise (Anagnostakis 2002, James 2011, Jeffers and others 2009). The genes for resistance appear to be incompletely dominant but are different from genes controlling resistance in blight (Anagnostakis 2001). Therefore, it could be many years before trees resistant to both fungal pathogens are produced. The Asian chestnut gall wasp (Dryocosmus kuriphilus) is a recently introduced pest that also poses a threat to   American chestnut (Anagnostakis and others 2011, Jacobs 2007). Introduced in Georgia in 1975, the species has quickly spread through the range of American chestnut and is now found throughout Tennessee and as far north as Cleveland, OH. Both American chestnut and Chinese chestnut are susceptible to Asian chestnut gall wasp, making it unclear how this species may affect chestnut hybrids developed for restoration (Anagnostakis and others 2011). The insect has been found on blight resistant hybrids  planted in the field, but effects on seedling growth have not been yet determined.2 Other potentially harmful exotic pests include the Asiatic oak weevil (Cyrtepistomus castaneus Roelofs Coleoptera: Curculionidae), the gypsy moth (Lymantria dispar L. Lepidoptera: Lymnantriidae), and the Asian ambrosia beetles (Coleoptera: Scolytidae). The Asiatic oak weevil is an exotic pest from Japan that prefers to feed on species in the Fagaceae family, particularly oaks (Ferguson and others 1991, Frederick and Gering 2006). This species is especially damaging because larvae feed on roots and emerging radicles, while adults feed on leaves (Roling 1979, Triplehorn 1955). The Asiatic oak weevil was the most abundant insect caught in traps in a Missouri Ozark forest (Linit 1986) and is probably contributing to oak decline in those forests. The Asiatic oak weevil was found to be more prevalent on chestnut trees than oak trees, where it was found to completely defoliate young chestnut seedlings (Johnson 1956). The gypsy moth is an exotic pest that was introduced from Europe, and the first gypsy moth outbreaks began around the same time as the chestnut blight fungus. The species has devastated vast areas of oak-dominated forests in the Appalachian Mountains in Virginia and northward through eastern deciduous forests. The gypsy moth could be a potential problem in defoliation of blight-resistant chestnuts, as it has been observed feeding aggressively on native chestnuts since 1915 (Mosher 1915). The gypsy moth was found to grow larger when fed leaves from second generation backcross chestnut hybrids when compared to leaves from pure American seedlings (Rieske and others 2003); similarly, gypsy moth grew better on transgenic chestnuts when compared to native American chestnuts (Post and Parry 2011). The preference of the moth for the BC3F3 generation of chestnut hybrid is currently unknown.


The Asian ambrosia beetles were first imported into eastern North America in the 1930s, and new species have recently been discovered (Atkinson and others 1990, Schiefer and Bright 2004). The insect bores into small saplings and trees, creating tunnels and galleries and potentially introducing fungi into the wounds. Xylosandrus crassiusculus Motschulsky has been found to negatively impact Chinese chestnuts in orchards in middle Tennessee (Oliver and Mannion 2001), but there is little information on effects  these beetles will have on American chestnut seedlings, hybrid seedlings, or transgenic trees. Mortality caused by this insect was observed on the research plots of the American Chestnut Cooperators Foundation, typically on American chestnut stem < 4 in diameter.3  Another recently imported Asian ambrosia beetle (Xylosandrus mutilatus Blandford) is known to be a pest to Chinese chestnut in its native China, but this beetle is currently south of the American chestnut species range (Scheifer and Bright 2004, Six and others 2009). Two native insects that affect chestnut include the chestnut sawfly [Craesus castaneae Marshall Hymenoptera: Tenthrediniae] and the twolined chestnut borer (Agrilus bilineatus Weber. Coleoptera: Buprestidae). The chestnut sawfly feeds on leaves, and the only paper that has reported sawfly on chestnut showed that it was more damaging to trees planted in shaded environments than those planted in open conditions. In addition, the sawfly preferred hybrid (BC2F3) and American chestnut seedlings over Chinese chestnut seedlings (Pinchot and others 2011). The chestnut sawfly is a rare species and the impact of defoliation on chestnut restoration is unclear. Historically, the American chestnut was the primary host of the twolined chestnut borer, but red oaks have become the primary host since the demise of chestnut (Haack and Acciavatti 1992). The insect is known to kill trees that are already stressed by first attacking the crown and then moving down into the trunk until the tree dies after 2–3 years (Cote and Allen 1980). The chestnut borer can invade after gypsy moth infestations, but populations can be reduced by removing dying trees and trees with poor vigor (Muzika and others 2000). No information exists on the impact this species will have on chestnut restoration, but the insect will likely only attack mature trees of low vigor.

 

The effects of fire on American chestnut are not well understood. Some early observations report that chestnut is negatively affected by fire because of its thin bark and shallow root system, suggesting that anthropogenic fire in the Appalachian oak-chestnut forest may have limited natural chestnut regeneration (Ashe 1911, Buttrick and Holmes 1913, Paillet 2002, Russell 1987). Frothingham (1912) found that American chestnut was mostly absent from frequently burned forests of pitch pine and scrub oak. Fire damage to American chestnut bark may increase susceptibility to disease and insect infestation (Hawley and Hawes 1925, Russell 1987). However, the vigorous sprouting of American chestnut suggests that it may be able to persist following infrequent burning. Paillet (2002) noted evidence of fire preceding sharp increases in the proportion of American chestnut pollen in sediment. McEwan and others (2011) related patterns of fire frequency with changing forest composition from oak-chestnut forests to forests dominated by maple. To our knowledge, there exists only one experimental fire study using pure American chestnut seedlings to determine effects of fire on seedling sprouting and growth response, but this study is still in the early stages.4 The complex interactions between site conditions, fire intensity, fire frequency, and fire season make it difficult to predict how American chestnut will respond to fire without additional research. However, the contrasting pattern of below- and above-ground biomass allocation between chestnut and oaks/hickories (Wang and others 2006) suggests that American chestnut may not be adapted to fire as well as oaks and hickories. Extreme weather events, including excessive heating, low winter temperature, frost stress, and drought, may damage American chestnut. Excessive heating of the soil can injure chestnut when young (Hough 1878), and sprouts are sensitive to frost when green, tender, and close to the ground (Mattoon 1909, Zon 1904). The health and vigor of sprouts is impaired by excessive heat or cold (Zon 1904). Gurney and others (2011) reported that American chestnut saplings are approximately 5 °C less cold tolerant than red oak and sugar maple, suggesting that the northern limit of the range may be limited by cold tolerance.

 

 

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