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COSEWIC assessment and status report on the Butternut in Canada

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COSEWIC
Executive Summary

Butternut
Juglans cinerea

Species Information

Butternut (Juglans cinerea) is a small to medium-sized tree of the walnut family that seldom exceeds 30 metres in height. In deeper soils it commonly has a central taproot and numerous widespread lateral roots. The densely hairy, alternate compound leaves have 11-17 pinnately arranged leaflets; these are nearly stalkless and attached opposite to one another. The yellowish-orange twigs are stout and hairy with a central pith that is divided into chambers. The ovoid fruit is a single-seeded nut with the husk covered with a dense layer of short sticky hairs and an inner shell with jagged ridges. The species is distinguished from the similar black walnut by such characteristics as its hairy twigs and leaves, terminal leaflet that is as large as the lateral leaflets, and oval hairy fruit with jagged ridges on the shell of the nut. In contrast, black walnut has smooth or only slightly hairy twigs and leaves with the terminal leaflet missing or smaller than the lateral ones; it has a globular, nearly hairless fruit with rounded ridges on the surface of the shell.

Distribution

Globally, butternut is found in northeastern North America, from Arkansas to Alabama north to Minnesota, and east to New Brunswick. The Canadian range runs through southern Ontario and southern Quebec to New Brunswick.

Habitat

Butternut is commonly found in riparian habitats, but is also found on rich, moist, well-drained loams, and well-drained gravels, especially those of limestone origin.

Biology

Butternut is a relatively short-lived, shade intolerant, monoecious (separate male and female flowers on the same tree) angiosperm. It bears fruit around age 20, with peak production between 30 and 60 years of age, and good seed crops every 2‑3 years. Butternut is capable of vegetative propagation from stump sprouting. Evidence to date points to low levels of genetic diversity between and within populations, although further research would be required to verify the extent of genetic variation within the species in Canada.

Population Sizes and Trends

Within its Canadian range, butternut is widespread, primarily found as a minor component of hardwood stands, but also occurring as extensive pure stands on flood plains. Inventory efforts have been limited to date. Very conservative estimates of populations are 13 000 and 7 000 to 17 000 trees in Ontario and N.B. respectively. Quebec has documented 378 sites with butternut, 39 of which have butternut comprising 25% or more of the basal area. Available information in Ontario indicates high levels of incidence of butternut canker, poor health of many butternut trees and initial reports of mortality presumably due to the butternut canker.  

Limiting Factors and Threats

Butternut canker ( Sirococcus clavigignenti-juglandacearum V.M.G. Nair, Kostichka and Kuntz) is a serious threat to the species. Butternut mortality in the United States has been tracked through the Forest Inventory and Analysis (FIA) program, as well as targeted studies. Despite the potential error associated with any sampling technique, the FIA results indicate high mortality rates. For example, the estimated mortality rates of Butternut in North Carolina and Virginia is 77%. Targeted surveys in Wisconsin, where the canker was first reported, documented an increase in the percent of infected trees from 30% to 91% between 1976 and 1992.

The canker has spread north and east across the Canadian range and is present in all three Canadian provinces where butternut occurs. Accurate information on mortality rates in Canada is not available but observational data on butternut mortality and Canadian Forest Service data on the geographic extent of butternut canker throughout most of the Canadian range of butternut indicate that similar mortality rates to those experienced in the U.S. can be predicted.

No known naturally resistant strain of butternut has been identified. Canker-free individuals have been observed within infected stands, though these cases are very rare. Where the canker has been present for decades and mortality rates have been high, surviving individuals may represent some level of resistance even if they are not canker-free.

Special Significance of the Species

Butternut is known for its edible nuts, which have a high omega-3 fatty acid content. Wood is considered to be a specialty product; although not of major commercial importance, it is used for interior finishing and turnery. Butternut also has intrinsic and aesthetic value, and provides wildlife forage and cover.

Existing Protection or Other Status Designations

Global: G3G4
Ontario: S3 ?
Quebec: S4
New Brunswick: S3 and a General Status Rank of Sensitive

Natural Heritage Ranks for adjoining states are as follows:

Minnesota: S3; Wisconsin: S3?; Michigan: S3; Ohio: S3; Pennsylvania: S4; New York: S4; Vermont: SU; New Hampshire: S1S2; Maine: SU. The highest rankings occur in Alabama and Georgia, both of which rank butternut as S1. In North Carolina and Virginia where serious losses have occurred it is ranked as S2S3 and S3?, respectively.

Butternut is still listed as a species of concern in many states and in Federal Region 9.