by Jared Elm | RGS & AWS Forest Wildlife Specialist – Wisconsin
Ironwood (Ostrya Virginiana,) also called hop-hornbeam, is a small, deciduous tree in the birch (Betulaceae) family. Ironwood is highly shade tolerant and usually persists in the mid-story of hardwood forests of the eastern U.S., with a range from the Minnesota/Canada border down into the panhandle of Florida. Leaves have sharply toothed edges and are commonly mistaken for birch or elm, but the fine branching on the twigs is a good indicator to differentiate it from elm and birch. The flowers on ironwood are a giveaway, as they look like hops plants and lend to the name of hop-hornbeam. Ironwood reaches heights of 20-35 feet; however, some specimens can be as tall as 50 feet. Ironwood persists more as a shrub than a tree in much of its range in the upper Great Lakes. Its other common name, ironwood, comes from the dense, hard wood commonly used for tool handles and fence posts. Ironwood has one of the highest BTU ratings of the hardwood species, only behind shagbark hickory and black locust in terms of fuelwood heat, which is a reflection of its density and hardness.
Ironwood is a species with which, like many trees and shrubs, foresters, biologists and land managers have a complicated relationship. It has many excellent characteristics for wildlife but also has downsides. With most trees, weighing the pros and cons of proliferating or reducing any one species composition in a forest is difficult. So, what are the pros and cons of ironwood?
On a positive note, ironwood provides a high-value catkin and bud for many wildlife species. Many would argue that ironwood rivals aspen and birch in terms of importance as a food source for grouse. Ironwood is monoecious, meaning it has both male and female parts on the same plant. The male buds form and remain on the tree through winter, while the female catkins develop in the spring in the “hops” form. Grouse forage both on the spring hop-like buds as well as the winter buds. Both buds and catkins are high in protein and nutritional value during the most vulnerable part of the year – early spring and winter. Additionally, the ornate bloom is an excellent early-season pollinator tree and assists in creating ample insect activity for grouse brood rearing.
On a negative note, ironwood is highly shade tolerant and can grow in the understory in even the most shaded environments. Due to its hard nature, it’s also not very palatable to selective grazers like cattle and deer that preferentially browse softer species like oak and maple. Cattle grazing, in particular, profoundly impacts forest mid and understory competition. Often you can tell if a forest was previously grazed just based on the size and prevalence of ironwood due to the lack of palatability. This shade tolerance and lack of palatability for deer and cattle can, at times, create an artificially high stem density of ironwood. The issue with this artificially high density is ironwood creates a large canopy in the mid-story, unlike other hardwood species, which generally reach for sunlight in gaps in the canopy with small tops that create minimal shade in comparison. Ironwood is very much at home in the shade. This shades out other hardwood trees that would otherwise be free to grow in the understory.
Mid-story canopy that ironwood inhabits needs to be removed to begin the oak regeneration process and can significantly inhibit Northern hardwood regeneration as well. Methods for removing ironwood vary throughout the eastern deciduous forest, ranging from mechanical means to herbicides and everything in between. The primary factors are soil type, topography, tree spacing, and land manager/owner capability. In flat topography with large overstory trees, mechanical work utilizing dozers with a shear blade or forestry mulcher on a skid-steer is commonly employed. However, on steep topography that inhibits equipment operation or in younger forests with tighter spacing of smaller, younger trees, herbicide is the method of choice.
Fire can also be a powerful tool in controlling ironwood and other mid-story trees. Many of the most common culprits in understory shading are fire-intolerant species, ironwood being one of them. In pre-colonization forest systems, frequent low-intensity fires killed ironwood and associated species and gave way to fire-tolerant species like oak and pine. Even in forest systems we don’t associate with fire, such as sugar maple and hemlock forests, less frequent low-intensity fires were still implemented and likely prevented the over-proliferation of ironwood. Fire is an incredibly effective tool for ironwood control; however, after 100 years of fire suppression in most forests, a multi-pronged approach consisting of mechanical and chemical treatment still needs to be employed, even if fire is part of the management equation.
There’s also little to no commercial value to ironwood, which results in it frequently being left standing during timber harvests; it simply doesn’t reach a large enough size to be used as a forest product beyond fuel wood. Thus, ironwood management falls into the “non-commercial management” category and isn’t something a commercial timber harvest can fix. However, forest regeneration should begin well before the timber harvest, with the removal of shade and re-introducing disturbance that has been vacant for decades often through means not involved in a commercial timber harvest. In summary, ironwood as an individual species represents some of the complicated cogs and wheels of forest management and ecology. The proliferation of the species is a result of human negligence both in land use and inaction; there are high densities due to previous woodlot cattle grazing and poor timber harvest practices, as well as lack of fire on the landscape to suppress. So, while ironwood also has some very positive characteristics, maintaining a proper balance of these species on the landscape is critically important in creating healthy forests and abundant wildlife.
