Black Oak

Issue: 
September-October 2007

THE OAK RIDGES MORAINE is an area of great natural beauty and conservation interest in southern Ontario. It stretches across the top of Toronto as far east as Peterborough and Rice Lake. Lake Scugog is contained behind it. The raised land comprising the moraine is a well drained glacial deposit that influences the hydrology and the highly productive land to the south to the shores of Lake Ontario.

Why the name “Oak Ridges” you may ask? The rolling landscape is dominated by white (Quercus alba) and our current species of interest, black oak (Q. velutina). Within the stands of oaks, and especially black oak, are swards of grassland, remnants of tall grass prairie. The Oak Ridges Moraine can rightly be called a savannah, as can other areas of southern Ontario where soils are well drained enough to support relict or refugial areas of tall grass prairie such as on Walpole Island, Ojibway Prairie, The Pinery, and parts of Haldimand Norfolk County. Black oaks are the trees that stand in small clumps, or are spaced apart to give the characteristic savannah landscape. The forest at and around The Pinery, on the sandy shores of Lake Huron, is dominated by black oak.

I first met and got to know black oak in the Alderville First Nations savannah restoration site, hosted by Rick Beaver and in the company of my class in Applied Ecology & Environment on a field trip there. The oaks are unmistakable in this landscape!

There are eleven or so oak species in Canada, and they are difficult to tell apart. They fall into two groups, red and white. Species in the white oak group have rounded lobes to their leaves and those in the red oak group have pointed lobes with bristles at the end. The give-away characteristics that allow distinguishing black oak from other oaks in the red oak group [the wide-spread red oak proper (Q. rubra), pin or swamp oak (Q. palustris), Shumard’s or swamp red oak) (Q. shumardii), and Hill’s oak (Q. ellipsoidalis)] are habitat and its dark green and shiny leaves with furry undersides (mostly where the veins join and especially early in the season and the reason for the name velutina) and the small (12-20 cm long and much the same width) acorns, somewhat stripy and often more than half enclosed in the cup, even when mature. The acorn’s cup has a small fringe of brownish scales. If the inner bark is exposed by a knife wound, the orange to yellow inner bark is a give-away. The outer bark is grey and smooth when the trees are young, but it turns almost black and becomes deeply ridged into long, rounded, blocky rectangles as the trees mature.

Black oak in Canada tends to be a small tree, up to about 15-20 m high and usually less than 1 m in diameter. To the south, in the USA, black oaks can grow larger, up to 60 metres tall. Their life expectancy as trees can exceed 200 years. Being savannah trees, they are remarkably fire tolerant. As the tall grass prairies were burned by native peoples, and then by European settlers, the trees were killed above the ground. However, the deep roots and the tap-root seem to have allowed sprouting and re-establishment from dormant buds near the root collar of old stems. New sprouts grow rapidly; the larger the old stem, the faster new sprouts grow. How long such plants have persisted is unknown.

Fire is used as a management tool for tall grass prairie restoration today, and care is taken to protect the trees from intense heat. Nevertheless, even if the crown is damaged, the trees have a remarkable capacity to grow anew. Seedlings, germinated in spring from buried acorns, are shade intolerant, as expected for trees of open habitats.

Once trees reach about 20 years old, they start to reproduce sexually. They peak reproductively between 40 and 70 years old. About every second year, male and female flowers appear on the same tree, but separately, as the leaves start to unfold. Male (pollen producing) flowers appear in the leaf axils of the previous year as drooping catkins. The much smaller reddish females in clusters of 1 to 4 appear in the leaf axils of the current year.

The pollen is dispersed via wind (anemophilous pollination). It does not travel far. In forested areas with abundant black oak, the genetic neighbourhood extends only about 42 metres, and individual trees can expect to receive pollen from only 7 or 8 of their neighbours. In mixed oak stands, oaks, including black oak, are notorious for hybridization.

After pollination and fertilization the fruit (acorns), take two years to mature and are ripe by fall. Often, the acorns are produced in abundance once every two or three years. The confusion of “mast fruiting” (i.e. production of mast – fruits and seeds used by wildlife) and “mass fruiting” (i.e. periodic production of large amounts of fruit) can be understood if one considers black oak as a “mass mast fruiting tree.”

The parts of the black oak that can be consumed, the reddish-brown twigs and acorns, are bitter to human taste. Nevertheless, the acorns are eaten by such birds as game birds (turkeys), some song birds, bluejays and woodpeckers. Mammals, including squirrels, chipmunks, mice, raccoons and even deer, eat them if nothing tastier is available. In years of poor acorn production, wildlife may consume almost the entire crop. Nevertheless, wildlife are the dispersers of the acorns, and birds, notoriously bluejays can carry the seeds at least 1 km from the parent plants. Typically, black oaks do not live as long as other red oaks. They seem more likely to develop heart rot, but the hollow trees are rarely without inhabitant wildlife. Dead upper branches make good nesting sites for red-headed woodpeckers.

As with many oaks, black oak is host to a wide variety of pests. Oak wilt (Ceratocystis fagacearum) is spread through root grafts and over larger distances by beetles. Shoestring root rot, or honey mushroom (Armillaria mellea), kills weakened trees. It is edible and enjoyed by some for its flavour and meaty texture. Cankers damage, but seldom kill the trees. Foliage diseases are the same as those that attack other species in the red oak group. Various wood-borers attack black oak trunks and cause serious damage. The notorious gypsy moth (Lymantria dispar) is potentially the most destructive defoliating insect, but other caterpillars also eat the leaves. The acorns suffer attacks from weevils, gall-formers and borers.

As with many bitter natural products, black oak has been used for its medicinal qualities. The inner bark, containing quercitannic acid, is a mild astringent, but white oak is better because its inner bark contains less tannin. The bark itself, made into a tea to be gargled, drank, applied to the skin or eyes, or chewed, can be used as an astringent, disinfectant, emetic, and tonic and has been used for treating dysentery, fevers (febrifuge), indigestion, asthma and lost voice.

Galls that form on the trees are strongly astringent and can be used in much the same way as the bark. Although the acorns can be used, powdered, in cooking and as a coffee substitute, the processes of removing the bitterness and astringency are long and tedious.

Other practical uses of black oak and its products are many. The heavy, hard and coarse-grained is used in construction for fence posts and as fuel. Black oak wood often has defects, so the wood is not used as much as that of other oaks. In fact, it is not highly recommended for fine timber. The bark and galls have been used as sources of natural, yellow dye. Mulched, dead leaves have been noted to repel garden pests, such as slugs and grubs, but the fresh leaves on the ground inhibit the growth of some plants.

Black oak is not a highly valued tree for any of its attributes. It is not often planted as an ornamental. Nevertheless, it habitats, natural history and interactions with so many other elements of natural animal and plant life make it an important plant of our remaining Canadian savannah ecosystems. Certainly its fall color contributes greatly to our aesthetic appreciation and reminds us of its importance.

Our mission is to enhance and promote the care and benefit of trees for present and future generations in Ontario through education, research and awareness.