Shagbark Hickory (Carya ovata)

Issue: 
May-June 2010

I CONSIDER MYSELF LUCKY. Every day on the quick five minute commute between home and my equipment, I get to see a number of shagbark hickories (Carya ovata). Most of the rest of the day I spend in environments where either all native trees have been removed or shagbarks do not occur. I suppose I am fascinated by shagbarks since they seem so rare and eccentric in their distribution. My little “commuting stand” is a couple dozen trees on the edge of an old road – a mere 100 metres away, shagbarks do not grow.

In the nearby Trent River and in Prince Edward County, my familiar surroundings of sugar maple dominated forest disappears. Here shagbark hickories are suddenly common, along with regionally rare southern oaks such as chinquapin and swamp white. Why are southern species, such as shagbarks, on droughty ridge-tops and seasonally flooded verges on the Trent River (see Catling and Catling 1993)? Why are fields long cleared by settlers still ringed by the original hickories, and why are near pure stands of shagbarks disappearing? These are a few of the questions I’ll try to answer here but first, let’s tackle the basics.

Taxonomy
The hickories (Carya) are one of nine genera comprising the Juglandaceaea. The closest related genus is the walnuts (Juglans) which is distinguished primarily by its chambered pith as compared to Carya’s solid pith. The hickories are a group of approximately 17 species found in North America, northern Mexico and eastern Asia; 11 of which occur in North America. Within the genus Carya are three sections (groupings by common characteristics), Apocarya, Carya and Sinocarya. The section Carya contains C. ovata along with six other species in North America and Mexico.

Distinguishing characteristics of the hickories are: fissured/stripping bark, branchlets with solid pith and well developed bud scales, male catkins in drooping or sessile bundles of three, stamens 3 to 10-15 per flower, fruits with husks that completely or nearly split into four sections, and nuts smooth, verrucose or slightly wrinkled (Andrews 2006).

The shagbark hickory has a large range and within it are plants that have subtle but distinct differences that are consistent and breed true (i.e. varieties). The shagbark has three described varieties – ovata, australis and mexicana. The variety ovata is the only one found in Canada. Older references refer to var. australis as a full species C. carolinae-setentrionalis (Ashe) Engl. & Graebner, the Carolina hickory or southern shagbark hickory.

There are a number of horticultural and agricultural selections of the species and hybrids, which have been developed since the species’ initial cultivation in 1629. Hybrids to combine the sweet nuts of the shagbark with the thin shells of the bitternut have been the main area of hybridization.

Two naturally occurring hybrids are regularly reported with Carya ovata – with C. laciniosa to produce C. x dunbarii Sarg. And with C. cordiformis to produce C. x laneyi Sarg. Other naturally occurring unnamed hybrids with the pecan (C. illinoinensis) have been noted.

Description
Shagbark hickory is one of the most distinctive of Ontario’s native trees. The large sheets of peeling bark are obvious on older trees and the sparse branching and stout twigs give away the species even from a distance.

The compound leaves and buds are opposite in arrangement with pinnate leaves (length 30-60 cm) of 5 (rarely 7) leaflets (length 1-17 cm); the terminal leaflet is largest. Leaflets are widest at mid-point, and are finely serrate with 2-3 hairs per axial which wear away through season. The upper side of leaves are yellowish-green to green with undersides sparsely hirsute. The buds are large and distinct. The terminal bud is ovoid, 12-18 mm long, blunt-pointed, with 4-6 overlapping scales with the inner being tomentose. Auxiliary buds are well angled from the stem.

Range
Shagbarks range across a large section of eastern North America with a small disjunct population in the northeastern mountains of Mexico. They are largely absent from the coastal plains of the Gulf of Mexico and the southeast. The species extends north into southern Ontario, Quebec and Maine. It ranges westward past the Mississippi River and barely enters eastern Texas, Oklahoma, Kansas and Nebraska.

In Canada, the species occurs throughout the Carolinian zone of Ontario, northeast to Toronto. Along Georgian Bay are a few almost pure stands. In eastern Ontario, the species begins in Brighton and continues eastward along the St. Lawrence to Trois-Rivières, Quebec. It can be found inland along several rivers and in a couple of disjunct locations such as granite outcrops in the Perth area.

The Ontario distribution shows two interesting anomalies. The isolated trees on Georgian Bay and Lake Huron are likely a result of deliberate introduction by First Nation peoples (see impact of natives on species distribution and forest composition later in the article). The gap in the distribution on the north shore of Lake Ontario between Brighton and Toronto is likely due to insufficient time for the species to reestablish itself since the last ice age. The species has made its way around the eastern and western edges of the lake, but the slow spread, literally by gravity and squirrels, has not allowed the two advancing fronts to join.

Habitat
The large range of the species indicates its broad adaptive ability to climate and soils. In the southern portions of the range, the species may never see snow, while annual snow falls of 250 cm are possible in the north. Rainfall is also highly variable with yearly totals ranging from 760-2030 mm across the range. Average yearly temperatures vary from 4º C to 21º C and the average growing season can vary from 140-260 days (see http//na.fs.fed.us/pubs/silvics_manual/Volume_2/carya/ovata.htm).

As can be readily imagined, the species grows on varied soils. The northern soils are of glacial origin while those in the south are non-glaciated. Shagbarks are known to grow on seven soil orders and soils derived from a wide variety of parent materials. These soils also show a wide range of fertility.

The species can further show an interested bimodal distribution within moisture regimes. It occurs on dry-mesic upland sites and wet-mesic bottomlands. Rich mesic soils are not a common habitat. Shagbarks are also found on alvars. It is clear that the species must be on well drained sites and cannot withstand standing water, though it apparently is tolerant of seasonal drought (alvars are particularly prone to this).
Along the Trent River, the species shows a bimodal distribution. They occur on some of the alvars, and in adjacent or nearby upland forests on calcareous soils that are part of coarse till-moraine deposits along the river. They also occur only a few miles away on droughty ridge tops, but not in the intervening slopes that are generally considered rich sites and support sugar maple dominated forests (Brownell and Blaney 1996; Brownell and Riley, 2000).

Are shagbarks distributed this way because they are tolerant of the harshest of sites or are they incapable of competing with species such as sugar maples? It appears that sugar maples can outgrow shagbarks which are of low shade tolerance, limiting shagbarks to sites on which sugar maples do not prosper. It is a remarkable biology that allows a species to thrive on both wet sites and droughty soils all within a couple of miles.

Breeding Biology/Life History
Shagbarks have male and female flowers on the same tree (monoecious) and flower in spring when leaves are near full size. Male and female flowers are generally on separate branches. Male catkins are long (10-15 cm) and hang in groups of three. The fruit is a nut 3-6 cm long, oval to obovoid, enclosed in a thin husk. At maturity (September through October), the husk dries and splits into four releasing the nut. Husks which do not split are often indicative of nonviable seed.

Trees begin fruiting at as little as 10 years of age and reach commercial maturity at 40 years. Good mast production occurs once every second or third year. A tree may produce as many as 70 litres of nuts. Dissemination of seeds is primarily by gravity, but squirrels, chipmunks and presumably blue jays may extend the range.

Ripe seeds require cold moist stratification for germination. Seeds may be stored in cool, airtight containers for up to five years. If seeds are to be cold stratified it is suggested that the “rhythm” of the season method be used. Due to problems with transplanting seedlings, it is suggested that where possible, hickory nuts be sown in fall where they will be grown. They should be covered with 5 cm of mulch and rodent-proof wire.

Shagbark hickories produce a long taproot that makes transplanting very difficult. Waldron (1997) describes hickories as the most difficult native tree to transplant due to the fact that their taproot is always longer and thicker than the top growth and they lack fibrous roots.

The tops of shagbark hickories are readily killed by fire; smaller stumps will rapidly send out new shoots while older (larger) stumps sprout less but root suckering is common. This can be easily replicated. It has been a common practice in some areas to coppice plants so that the small diameter sprouts can be used for furniture, etc.

Hickories have always been renowned for their toughness. Andrew Jackson received the name “Old Hickory” as he was considered to be as tough as the legendary strength of the hickory. Born in 1767, he had an active military career, defeating the British at New Orleans in 1815 and invading Florida in 1818. He became a judge, congressman, senator and the 7th president of the United States (1829-37) as well as amassing land in the newly opening territories of Tennessee. There he built his estate called the Hermitage. In 1830 he planted a number of hickories and it was beneath these trees that he was buried in 1845. These trees would be shading his grave today but they were toppled by a tornado in 1998. The trees were 168 years old at their demise.

Most references suggest that this species lives to 200 years and some report that the species can still be producing fruit at 300 years. The oldest known tree in Canada was a wind-thrown tree in Rondeau Provincial Park that was aged as 250 years. Shagbarks are known to be remarkably slow growers; Sternberg and Wilson (2004) state that stumps of less than 45 cm diameter regularly contain 150 annual growth rings.
The largest shagbark in the US is less than 90 cm dbh but stands 45.9 m tall and is located in Sumter National Forest, South Carolina. Canada’s largest tree was a 32 m, 95 cm dbh tree in Backus Woods Conservation Area, immediately north of Port Rowan. This tree was toppled in a storm

Uses
In the world of environmental restoration, the shagbark hickory has an obvious role to play in suitable habitats. Direct sowing of seeds and rodent control is the best method to encourage growth (see Kock 2008 for a description of propagation). The species is being lost across the range due to limited regeneration. Squirrel populations have exploded in many urban areas and parks and they limit the nuts available to sprout. In rural areas, mice and voles can quickly strip the bark of seedlings found in hedge rows. A helping hand from Homo sapiens could certainly be used.

In the modern landscape, little is heard of the shagbark. Many authors suggest that the combination of the difficulty to transplant, large ultimate size, slow growth, along with the tree being “dirty,” limits use. Other authors see this as a beautiful tree with golden fall colours and an interesting, stark, outline; they suggest increased use.

In England, the species is thought to be fast growing and in need of little pruning (Brown 2004). Personally, I have found the species requires little maintenance other than occasionally formative pruning and cabling to support limbs with included bark. Young plants are often attacked by rodents and competing vegetation can limit the growth of understory trees.

Most residential shagbarks I have encountered are remnants of past forests, very few have been deliberately planted. When house construction is planned, shagbarks are well worth retaining but they are highly vulnerable to compaction and altered soil elevations. Extensive pre-construction preservation techniques should be used.

Summary
Shagbark hickory is an interesting but often overlooked species. Beginning with First Nations through to modern ecologists and restorationists, the species has been important for survival of people and animals. The study of this species clearly indicates that distribution and survival have been influenced by man as much in the past as today. Widespread, though uncommon in much of southern Ontario, shagbarks are well worth considering for preservation and planting. The trees you plant this year are likely to be revered by generations to come in 200 years.

Sidebar 1: North America's Earliest Arborists

First Nation people may have practiced arboriculture, and this, along with their agricultural practices, helps to explain the range and density of shagbark hickory. By managing individual trees and entire landscapes, these peoples have had a great influence on forests in eastern North America. First Nations were once considered to be in harmony with nature, but now many scientists believe that they were in fact a “keystone” factor in ecosystem development.

Throughout the world, traditional cultures farmed various fruit and mast trees, which made survival easier than that of the constantly moving hunters and gathers. This has been termed balanoculture (see Logan 2005 for an account). In eastern North America, shade-tolerant forests (maple-beech) could not meet the dietary requirements of First Nations. The manipulation of forests – and in fact entire landscapes – using deliberately set fires was a method to help assure sufficient food stuffs.

The use of fire, planting, tree girdling and forest thinning allowed creation of “orchards” of hickory and other mast crops. Hickory nuts were one of the main foodstuffs for many First Nations throughout eastern North America. Nuts and acorns are calorie packed and could be stored. The sweet nuts of shagbark hickory seemed to be particularly favoured and are amongst the highest caloric ratings of mast crops.

In eastern North America, the distribution of early succession forests and hickory, oak and chestnut appear to be strongly predicted by First Nation occupation. This relationship is so strong that estimates of occupation is a better predictor of oak, hickory and chestnut than a number of topographic and soil variables (see Black et. al. 2006, Abrams and Nowacki, 2008).

Shagbark hickory appears more common in today’s landscape, particularly in the vicinity of historic First Nation villages, than might be predicted by “natural” forest process. Hickory-oak woodlands are found in areas that should favour mesic maple-beech forests and the preponderance of hickory-oak is more common than predicted by succession studies that suggest that these dry open woodlands would convert to more shade-tolerant closed canopy forest.

By studying the frequency, location and timing of fires, researchers have concluded that First Nations managed their forest habitat. (They clearly had the use of fire since their emigration from Siberia.) The control of vegetation was not only for foodstuffs production but also allowed for regeneration of browse for deer and removal of cover that could be used by attackers amongst a myriad of other reasons.

In Ontario, the species is found outside of what appears to be its “normal” range. Isolated stands near Lake Huron and Georgian Bay are far north of the range and some stands are pure hickory. Are almost pure stands in areas such as Campbellford and near Long Point, Prince Edward County, vestiges of lost First Nation farmers and arborists?


Sidebar 2: Value to Wildlife

A review of available literature and web-based materials offers an interesting perspective on people’s perception of the value of shagbarks in relation to wildlife. When considering hunted mammals and birds, shagbark hickory has been traditionally considered an important mast species. Deer, squirrels, black bears and wild turkeys use the tree but many people believe they prefer acorns to hickory nuts (see Martin et. al., 1951). Some views are so narrow that they suggest shagbarks are better on the wood pile than in the woodlot.

A more all encompassing view of wildlife and their use of trees indicates that shagbark hickory may be one of the most important of trees, not only as a food source but also as shelter.

The website Bringing Nature Home has totaled the number of Lepidoptera (moth and butterfly) species that use various tree species. In a ranking of woody genera, Carya is 10th with 235 species. The most spectacular user is the hickory horned devil, the larval stage of Citheronia regalis, the royal walnut moth. A full-grown larva often exceeds 12 cm in length and sports 10 large horns just behind the head. The moth that emerges the following summer, the royal walnut moth, is one of the largest and most beautiful Lepidoptera in North America (Tallamy 2007). The highly unusual Hag moth (Phobetron pithecium) is also a hickory specialist. As a group, the underwing moths (Catocala) are largely dependent on hickories, while the hickory hairstreak butterfly is named for obvious reasons.

Moths and butterfly larvae are often distained by those believing that they are no more than “bugs and crud.” These larvae are part of a food chain and well known groups such as birds rely heavily on caterpillars not only for their own nutrition but also to feed their young.

One species of bird has a particularly interesting relationship with shagbark hickories. The brown creeper is a small bark forager, hopping up and spiraling the trunk looking in crevices for hidden insects. It nests in almost hammock-like fashion beneath sheets of exfoliating bark on dead trees and shagbark hickories. In Ontario, the species seems to be largely absent from much of the Carolinian region and is in lower abundance in much of southern Ontario than areas reporting high abundance such as Algonquin Provincial Park (McLaren 2007).

In Indiana, the shagbark is considered the ultimate tree for the Indiana bat (Myotis sodalis), an endangered species. The bats use the trees as roost and maternal sites and apparently not only use the peeling sheets to hide under but finds that shagbarks provide excellent thermal regulation. In Ontario, species such as the little brown bat (Myotis lucifugus), the silver-haired bat (Lasionycteris noctivagans), and likely the big brown bat (Eptesicus fuscus) are found using live shagbarks. It seems clear that in forests and urban situations, the loss of dead trees with sloughing bark has lead to a decline in bats using natural roost sites. Shagbark hickories provide excellent roost sites as the bark begins to shed at 10+ years and can continue to provide outstanding habitat for two centuries or more.

Sidebar 3: Article References

Abrams, M.D. and G.J. Nowacki. 2008. Native Americans as active and passive promoters of mast and fruit trees in the eastern USA. The Holocene 18, 7: 1123-1137.

Andrews, S. 2006. Tree of the Year: Carya ovata, Part 1

Black, B.A., C.M. Ruffner, and M.D. Abrams. 2007. Native American influences on the forest composition of the Allegheny Plateau, northwest Pennsylvania. Can. J. For. Res. 36: 1266-1275.

Brownell, V.R. and C.S. Blaney. 1996. Lower Trent Region Natural Areas – Volume 3: A biological inventory and evaluation of 23 areas in the Lower Trent Region, 1995. Prepared for the Lower Trent Conservation Authority, Trenton 148pp. + 23 maps at 1:10,000.

Brownell, V.R. and J.L. Riley. 2000. The Alvars of Ontario. FON. 269pps.

Brown, G.E. 2004. The Pruning of Trees and Shrubs., 2nd ed. By T. Kirkham. Timber Press. 338pps.

Catling, P.M. and V.R. Catling. 1993. Floristic composition, phytogeography and relationships of prairies, savannas and sand barrens along the Trent River, Eastern Ontario. Canadian Field-Naturalist 107(1):24-45.

Kock H, with P. Aird, J. Ambrose, and G. Waldron. 2008. Growing Trees from Seed: A practical guide to growing native trees, vines and shrubs. Firefly 280pps.

Logan, W.B. 2005. Oak: The frame of civilization. Norton. 336pps.

Martin, A.C., H. S. Zim and A. L. Nelson. 1951. American wildlife & plants, a guide to wildlife food habits; the use of trees, shrubs, weeds, and herbs by birds and mammals of the United States. Dover Publications.

McLaren, M. 2007. Brown Creeper, pp.410-11 in Cadman, M.D., D.A. Sutherland, G.G. Beck, D. Lepage, and A.R. Couturier, eds. Atlas of the Breeding Birds of Ontario. 2001-2005. Birds Studies Canada, Environment Canada, Ontario Field Ornithologists, Ontario Ministry of Natural Resources, and Ontario Nature, Toronto, xx11+706pps.

Sternberg, G. and J. Wilson. 2004. Native trees for North American Landscapes. Timber Press. 552pps.

Tallamy, D.W. 2007. Bringing Nature Home: How Native Plants Sustain Wildlife in our Gardens. Timber Press. 388pps.

Waldron, G.E. 1997. The Tree Book: Tree Species and Restoration Guide for the Windsor-Essex Region. Project Green. 220pps.

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