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COSEWIC assessment and status report on the Butternut in Canada

Biology

General

Butternut is relatively short lived, rarely living more than 75 years. It is shade intolerant. Although young trees can tolerate shade from the side, butternut does not survive when shaded from above (Rink, 1990).

In Ontario, butternut is usually found as scattered individuals or in small groups in mixed hardwood stands, or as remnant or volunteer trees in fence lines or open fields. In N.B. butternut is scattered throughout the Grand Lake Ecoregion on flood plain soils and is a common component of field hedgerows. A number of extensive pure stands occur on several flood plain islands. Butternut is also scattered throughout the upland hardwood forest in the Meductic Ecodistrict (Valley Lowlands Ecoregion), which is underlain with rich calcareous soils (Sabine, pers. comm. 2003).


Reproduction and Genetics

Butternut flowers from April to June, depending on location. The species is monoecious (separate male and female flowers on the same tree), and wind-pollinated. The male flowers are thick, green catkins that develop from axillary buds. The female flowers are shorter than the male flowers and occur on short stems arising in the axils of new leaves. Flowers of both sexes on an individual tree usually mature at different times (Rink, 1990).

The fruit contains an oblong nut surrounded by a semi-fleshy indehiscent, pubescent husk (Harlow et al., 1979). The fruit matures in September and October in the year of pollination. Fruits occur singly or in clusters of 2 to 5. Mature fruits are 4-6 cm long, ovoid and green. Removal of the husk yields a nut containing an embryo with two large cotyledons surrounded by a seed coat and then a thick pericarp (outer husk). The cotyledons are sweet, oily and edible. The fruit usually remains on the tree until after leaf fall (Rink, 1990). Although the embryo can remain dormant for 2 years (OMNR, 2000) it usually germinates the following spring after seed fall (Rink, 1990).

Seed bearing starts at age 20 and peaks at age 30 to 60. Good seed crops occur every 2-3 years with light crops during intervening years. Low viable seed yields are usually caused by insect damage or lack of pollination (Rink, 1990). Seeds require cold stratification for 90 to 120 days to overcome dormancy (Young and Young, 1992).

The nut is considered to be intolerant of long-term storage and remains viable for 3 to 5 years if stored in sealed containers at temperatures just above freezing (Anonymous 1948; Wang 1974). Satisfactory storage can be obtained for at least 2 years if stored in closed containers at 80% to 90% relative humidity and +5 to 0°C. The nut cannot tolerate drying to low water contents (e.g. 5 % water content) and are sensitive to temperatures below -40°C (Wang et al. 1993). Stumps of young butternut trees are capable of sprouting (Rink, 1990) and can be propagated via rooted cuttings.

None of the species with which butternut hybridizes occur naturally within Canada. However, several of these species have been planted for nut production or landscaping. Butternut will hybridize with other species of Juglans, including heartnut (J. cordiformis) to produce buartnut (Millikan et al.,1991); Japanese walnut (J. ailantifolia) to produce J. x bixbyi; and with English walnut (J. regia) to produce J. x quadrangulata. Butternut has also been reported to successfully hybridize with little walnut (J. microcarpa) and Manchurian walnut (J. mandschurica) (Rink, 1990). There have not been any confirmed reports of black walnut (J. nigra) and butternut hybridization.

Busov et al. (1997) compared allozyme variation within and among several Juglans species including butternut. Ostry (1998) reported that genetic diversity of butternut is limited. Morin et al. (2000a) investigated 12 isozyme loci of 9 butternut populations from the species’ northeastern limit (7 from Quebec and one each from New Brunswick and Vermont). They observed low genetic diversity estimates compared to other species of the genus: only 3 of the 12 loci were polymorphic, and pairwise genetic distances were very low except for comparisons involving the population from Vermont. Population differentiation was estimated to be about 8%, but when the Vermont population was excluded, this estimate was reduced to 3%. One Quebec population, located over 100 km from the nearest sizable stand and thus with little to no gene flow with other populations, was completely monomorphic for those isozymes studied. The authors hypothesized that a combination of factors, including a genetic bottleneck occurring during the Pleistocene glaciation, influence of the butternut canker, and low migration distances of the gravity-dependent seed may have contributed to loss of diversity in butternut.


Movements/Dispersal

Seeds are dispersed by gravity, water, squirrels and other small rodents. There is evidence to suggest that some populations of butternut, among other nut bearing trees, were introduced into northeastern North America by the Iroquois, before the arrival of Europeans (Wykoff, 1991).


Nutrition and Interspecific Interactions

Butternut grows best on fertile sites but also is found on dry rocky infertile sites. Butternut trees produce a substance called juglone, a naphthoquinone that is selectively toxic to associated vegetation (Rink, 1990). The Eastern Chapter of Ontario Nut Growers web site (http://ecsong.ca/vol15no4.html#M) provides a listing of species of trees, shrubs and herbaceous plants categorized as negatively or neutrally affected by the presence of butternut and walnut. The information is derived from the publication Black Walnut Toxicity by Olga Piedrahita, Factsheet No. 84-050, Ontario Ministry of Agriculture and Food, November 1984, and is as follows:

"Plants reported as susceptible to black walnut toxicity include tomatoes, alfalfa, apple, pear, blackberry, blueberry, mountain laurel, azaleas, rhododendrons, shrubby cinquefoil (Potentilla fruticosa), red pine, white pine and other evergreens. Plants reported as showing toxicity symptoms occasionally include poverty grass (Danthonia), sweet peppers, common lilac, Persian lilac, viburnum, autumn crocus, peony, crabapple, magnolia, red raspberry, peach and Euonymus sp. Plants not affected or which have shown improved growth near walnut roots include Kentucky bluegrass, timothy, red top, orchard grass and other grasses, white clover, beets, snapbeans, lima beans, onions, parsnips, sweet corn, black raspberry, grapes, wild roses, forsythia, Virginia creeper, poison ivy, narcissus, salvia, impatiens, Rudbeckia sp., red cedar, oaks, maples, hickories and other native hardwoods. Other plants apparently tolerant to black walnut are anemone, jack-in-the-pulpit, lady fern, cyclamen, epimedium, dog's tooth violet, gentian, green hellebore, alumroot, plantain lily, iris, lilies, ostrich fern, forget-me-not, narcissus, lily turf, may apple, Solomon's seal, Christmas fern, primroses, pilewort, nightshade, meadow rue, toad lily, white clover, trillium, bellwort, wild oats, periwinkle, burning bush, honey suckle, mock orange, oaks, and poison ivy."