Foreign invaders are changing the very nature of our forests

 

Native insects, diseases and parasites have influenced our Wapane’kati/Acadian forests for thousands of years. My first scientific job involved studying the population dynamics of a small, native, aphid-like insect called the balsam woolly adelgid. Working out of the Maritime Forest Research Centre in Fredericton, New Brunswick, I examined balsam fir stands in New Brunswick and Nova Scotia to understand its population ecology. Balsam fir “flat-tops” are a trademark of this bug’s presence on trees, but few folks recognize it as the cause.

New diseases and pests are arriving from other parts of the planet, often by way of international trade and travel. They are rapidly changing the nature of Acadian forests in eastern North America. Many of the affected, dying tree species are foundational elements of its forest ecology, necessary habitats for  wildlife species survival. This rate of loss is accelerating quickly.

 

Unnatural forests and booming pests

Native species like the spruce budworm used to have occasional outbreaks in natural forests, when spruce were a less prominent species. The frequency and severity of their infestations has been amplified by forest industry “management” that produces more balsam fir and spruce stands. Natural, mixed species forests are much less susceptible. Trees that are capable of colonizing clear-cuts and old fields tend to be grow-fast, die-fast sorts that heal the land, preparing the site for more long-lived tree species. This natural succession can only happen when seed sources for the original trees exist nearby. That’s a rarity now.

Besides new insects and diseases, forests in parts of the United States have already become battle grounds, with exotic plant species taking over native species’ territories. Invasive species can overpower native forests and their ground cover plants.

Purple loosestrife (Lythrum salicaria) seeds from Europe, pumped out of ballast water in ship bilges, first colonized the banks of the Hudson River in New York State, and then moved north into the Maritimes. In parts of Florida, imported Brazilian pepper plants have replaced native ground cover species, while foreign trees like Casuarina pines and Melaleuca (tea tree) overwhelm local tree species. In the State of Georgia, a family member mows annually to prevent a bushy evergreen shrub, privet, from taking over as the ground cover. He uses a chainsaw to stop invasive trees from over-running the native forest overstory.

Three-quarters of the earth’s land suface has been altered by humans. That, combined with climate change, is challenging the survival abilities of many plants.

Purple loosestrife remains a problem in many ecosystems and is found throughout much of Eastern Canada. Photo by Jodi Delong.

 

The great elms and beeches

American elms are tall, graceful trees that prefer to live on floodplains. Dutch elm disease is caused by an Ascomycota fungus that clogs the elm tree’s water conducting system. The fungus is normally spread from tree to tree by bark beetles. This disease was accidentally imported into eastern Canada many years ago. Fredericton once was called “the city of stately elms.” Elms can be inoculated chemically to keep them alive, but that’s expensive, temporary relief.  

In rural areas living elms can still be found. They are either isolated or have some disease resistance. At least 651 cultivars of American elm have been identified as resistant. I planted three of these resistant kinds on an isolated floodplain, where they can cross-pollinate only with another resistant elm strain to have resistant young.

American beech were once abundant and important providers of beech nuts for many wild birds and mammals. Their populations began to decline decades ago with the arrival of a beech scale insect from Europe, Cryptococcus fagisuga. It’s a carrier of  the disease Nectria coccinea. Cankers appear on the bark, deforming and sometimes killing the trees. Another introduced pest, the beech leaf-mining weevil, (Orchestes fagi) is currently precipitating a massive dieoff of already-compromised beech stands that remain in eastern Canada.

 

More threats to our forests

Eastern hemlocks are now dying in our region, several years after they become infested with the hemlock woolly adelgid, Adelges tsugae. This aphid-like insect probably arrived on migrating birds from the northeastern US, where it first appeared as a foreign species to North America. Females lay unfertilized eggs that hatch into more females. Almost microscopic, white fuzzy spots can be seen with the naked eye on the underside and base of hemlock needles, announcing their presence. As needles begin to die, one can look up and watch the canopy cover turning brown and rapidly thinning. Dense hemlock stands provide essential winter shelter from wind and cold for wildlife. In steep ravines, their roots stabilze streambanks, minimizing erosion. 

The Asian emerald ash borer  (Agrilus planiplennis) is currently threatening the existence of white, black, green and other ash species here. Some years ago, attempts failed to stop this insect from crossing the US/Canada border near Detroit, Michigan and Windsor, Ontario. It’s been spreading rapidly through Quebec, New Brunswick and Nova Scotia.

Humans are often speeding the expansion of these insects. People move infected wood around by transporting logs to mills, lumber to stores, or firewood to a park firepit for a weekend getaway.

Beech, hemlock and ash trees in eastern Canada show little or no resistance to these bugs. In countries where the insects are native, trees have developed a natural resistance. In many cases, biological predators have evolved to control the insect populations there.

The effect of these non-native insects on our native trees can be compared to human susceptibility. Brought by European settlers, diseases like measles infected First Nations peoples, whose bodies had never before been exposed and offered no resistance. COVID-19 is another, more current example.

 

Finding possible solutions

These insects, weeds and diseases are part of a worrisome trend that hits woodland owners hard. I’ve spent a great deal of time and money encouraging hemlocks, beech and several ash species over the past four decades in my woodland. With their future now questionable, one option is to save some specimens temporarily by chemical means. This could maintain a few trees and their seed sources on the woodland while the diebacks unfold. These species may recover at some point if and when effective biological controls (predators) for the invasive species emerge, or later tree generations possibly develop natural immunities.

Clear-cutting creates opportunities for invasive species, by eliminating the forest competition, and directing the sun’s energy for newcomers like glossy buckthorn trees. Buckthorn seeds are distributed by birds. Soils on clear-cut sites have often become impoverished by repeated tree harvests, which diminishes the number and vitality of native tree species. This lessens the chances of forest wildlife species finding habitats that fulfil all of their feeding, nesting, denning, shelter and water requirements.

After Dutch elm disease struck, the city of Fredericton, which has approximately 21,000 street trees, replanted with a considerable number of ash trees. The arrival of the emerald ash borer in February, 2021, makes a strong case that future tree plantings in villages, towns or cities across eastern North America should include a broad mix of native species that can live there.

We can all learn to identify invasive plant species in Eastern Canada. Remove them whenever this is possible, being careful to not inadvertently spread them in that process. Replant with native species that are site and soil-suited.

Wild creatures and their ecosystems can use our help!