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COSEWIC assessment and status report on the American Chestnut in Canada

Biology

General

American chestnut is potentially a long-lived, shade-tolerant forest tree; it requires a canopy cover for effective seedling establishment. While other members of the family are wind-pollinated, this species is insect-pollinated. Seedlings establish in the shade of mature forests by seeds that were likely dispersed and buried by squirrels. Soils tend to be light (i.e., sandy) and acidic. Its natural distribution is limited to the long growing season of the Carolinian Zone but it has successfully been grown further north.


Reproduction

Whereas other members of the family are wind-pollinated, this species is insect-pollinated. It is monoecious (both male and female flowers occur on the same tree), but self-incompatible, requiring cross-pollination for fruit set (Ambrose & Kevan, 1990). It is insect-pollinated and easily crosses with exotic species of chestnut such as C. mollissima. Flowering occurs in late spring to early summer and nuts mature in autumn of the same year. There is evidence that some individuals in natural habitats are the result of hybridization with Chinese or other chestnuts (Boland et al., 2000). As populations become smaller and more isolated, some large healthy trees are no longer within cross-pollination range and thus do not set fruit, restricting their reproductive potential. Seeds remain viable when they are kept moist and have been successfully stored for 3.5 years under controlled conditions (USDA Forest Service website) but are normally only stored overwinter because they often begin germinating at the end of the stratification period. Seeds readily dry out and have reduced viability in nature unless soon covered by falling leaves or are buried by squirrels. In storage, seeds will dry out and lose viability if not kept moist. Chilling, through direct planting in the fall, or cold moist stratification, is necessary for good germination.

Although blighted or cut trees frequently re-sprout from the root collar, there is no evidence of clonal sprouting from the root systems beyond the collar region.


Survival

Despite the devastation that the chestnut blight has brought about, there are survivors throughout this species’ range, including southern Ontario. Part of this can be explained by the frequent survival of root systems after the fungus has caused a girdling and death of the trunk; typically new growth arises from the root crown. These sprouts are also subject to blight cankers and trunk death, but often they grow large enough for flowering and fruiting before they also succumb to the blight.


Dispersal

Pollination is by insects but many isolated trees are beyond the range of pollinators for cross-pollination.

As is the case with many nut-bearing trees and shrubs, squirrels and chipmunks actively collect and bury or cache chestnuts, some of which are overlooked and successfully germinate and produce seedlings. Some native birds, such as wild turkeys and blue jays, may also be important in the dispersal of chestnuts.


Nutrition and Interspecific Interactions

This species typically occurs in mature forests with often dry but otherwise well-developed soils.

Although it is dependent on insects for pollination and squirrels and other animals for effective dispersal beyond the parent tree, neither appears to be limiting.

The chestnut blight fungus has had the most severe impact on American chestnut, reducing it from a once forest dominant in some areas of eastern North America to minor remnant stump sprouts and only occasionally a full sized tree. The forest animals that depended on the annual chestnut crop have lost a major food source.


Adaptability

American chestnut is a species of mature forest but does tolerate and thrive under moderate forest disturbance, such as selective forest harvesting. Seedlings can be successfully grown in cultivation and out-planted into appropriate soils and sites. However, it appears less tolerant of different soil types, doing poorly in alkaline clay-loam soils, such as are found in Guelph.