Juglans cinerea, commonly known as butternut or white walnut, is a species of walnut native to the eastern United States and southeast Canada.
Description J. cinerea is a deciduous tree growing to 30 metres (98 ft) tall, rarely more. Butternut is a slow-growing species, and rarely lives longer than 75 years. It has a 40–80 cm (16–31 in) stem diameter, with light gray bark. The leaves are alternate and pinnate, 40–70 cm (16–28 in) long, with 11–17 leaflets, each leaflet 5–10 cm (2–4 in) long and 3–5 cm (1+1⁄4–2 in) broad. Leaves have a terminal leaflet at the end of the leafstalk and have an odd number of leaflets. The whole leaf is downy-pubescent, and a somewhat brighter, yellower green than many other tree leaves.
Flowering and fruiting
Like other members of the family Juglandaceae, butternut's leafout in spring is tied to photoperiod rather than air temperature and occurs when daylight length reaches 14 hours. This can vary by up to a month in the northern and southernmost extents of its range. Leaf drop in fall is early and is initiated when daylight drops to 11 hours. The species is monoecious. Male (staminate) flowers are inconspicuous, yellow-green slender catkins that develop from axillary buds and female (pistillate) flowers are short terminal spikes on current year's shoots. Each female flower has a light pink stigma. The flowers of both sexes typically do not mature at the same time on an individual tree. The fruit is a lemon-shaped nut, produced in bunches of two to six together; the nut is oblong-ovoid, 3–6 cm (1+1⁄4–2+1⁄4 in) long and 2–4 cm (3⁄4–1+1⁄2 in) broad, surrounded by a green husk before maturity in midautumn. Rodents such as squirrels help distribute the seeds.
Distribution and habitat The distribution range of J. cinerea extends east to New Brunswick, and from southern Quebec west to Minnesota, south to northern Alabama and southwest to northern Arkansas. It is absent from most of the Southern United States. The species also proliferates at middle elevations (about 610 m or 2,000 ft above sea level) in the Columbia River basin/Pacific Northwest, as an off-site species. Trees with 2.1 m or 7 ft (over mature) class range diameter at breast height were noted in the Imnaha River drainage as late as January 26, 2015. Butternut favors a cooler climate than black walnut and its range does not extend into the Deep South. Its northern range extends into Wisconsin and Minnesota where the growing season is too short for black walnut.
Ecology
Soil and topography Butternut grows best on stream banks and on well-drained soils. It is seldom found on dry, compact, or infertile soils. It grows better than black walnut, however, on dry, rocky soils, especially those of limestone origin. Butternut's range includes the rocky soils of New England where black walnut is largely absent. Butternut is found most frequently in coves, on stream benches and terraces, on slopes, in the talus of rock ledges, and on other sites with good drainage. It is found up to an elevation of 1,500 m (4,900 ft) in the Virginias – much higher than black walnut. The nuts are eaten by wildlife.
Associated forest cover Butternut is found with many other tree species in several hardwood types in the mixed mesophytic forest. It is an associated species in the following four northern and central forest cover types: sugar maple–basswood, yellow poplar–white oak–northern red oak, beech–sugar maple, and river birch–sycamore. Commonly associated trees include basswood (Tilia spp.), black cherry (Prunus serotina), beech (Fagus grandifolia), black walnut (Juglans nigra), elm (Ulmus spp.), hemlock (Tsuga canadensis), hickory (Carya spp.), oak (Quercus spp.), red maple (Acer rubrum), sugar maple (Acer saccharum), yellow poplar (Liriodendron tulipifera), white ash (Fraxinus americana), and yellow birch (Betula alleghaniensis). In the northeast part of its range, it is often found with sweet birch (Betula lenta) and in the northern part of its range it is occasionally found with white pine (Pinus strobus). Forest stands seldom contain more than an occasional butternut tree, although in local areas, it may be abundant. In the past, West Virginia, Wisconsin, Indiana, and Tennessee have been the leading producers of butternut timber.
Canopy competition Although young trees may withstand competition from the side, butternut does not survive under shade from above. It must be in the overstory to thrive. Therefore, it is classed as intolerant of shade and competition.
Diseases
Butternut canker The most serious disease of J. cinerea is butternut decline or butternut canker. In the past, the causal organism of this disease was thought to be a fungus, Melanconis juglandis. Now this fungus has been associated with secondary infections and the primary causal organism of the disease has been identified as another species of fungus, Ophiognomonia clavigignenti-juglandacearum. The fungus is spread by wide-ranging vectors, so isolation of a tree offers no protection. Butternut canker first entered the U.S. around the beginning of the 20th century, when it arrived on imported nursery stock of Japanese walnut. Symptoms of the disease include dying branches and stems. Initially, cankers develop on branches in the lower crown. Spores developing on these dying branches are spread by rainwater to tree stems. Stem cankers develop 1 to 3 years after branches die. Tree tops killed by stem-girdling cankers do not resprout. Diseased trees usually die within several years. Completely free-standing trees seem better able to withstand the fungus than those growing in dense stands or forest. In some areas, 90% of the butternut trees have been killed. The disease is reported to be spreading rapidly in Wisconsin. By contrast, black walnut seems to be resistant to the disease.
Hybrid resistance Butternut hybridizes readily with Japanese walnut. The hybrid between butternut and the Japanese walnut is commonly known as the 'buartnut' and inherits Japanese walnut's resistance to the disease. Researchers are back-crossing butternut to buartnut, creating 'butter-buarts" which should have more butternut traits than buartnuts. They are selecting for resistance to the disease. In some areas, most butternuts found as landscaping trees may be buartnuts rather than the pure species.
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