Buckthorn is an aggressive invasive shrub that threatens natural ecosystems across North America. The two most common species are Common Buckthorn (Rhamnus cathartica) and Glossy Buckthorn (Frangula alnus). Both are native to Europe and Western Asia and were introduced in the 1800s, primarily for use as ornamental hedging and windbreaks. These shrubs quickly escaped cultivation, establishing themselves in woodlands, savannas, and prairies. Buckthorn’s ability to spread rapidly and dominate diverse habitats has led to its classification as a noxious weed, impacting plant life, soil health, wildlife, and agriculture.
Ecological Domination and Native Plant Suppression
Buckthorn gains a competitive advantage over native plants through a prolonged growing season. It is one of the first shrubs to leaf out in the spring and one of the last to drop its leaves in the fall, extending its photosynthesis period compared to native species. This early canopy closure allows buckthorn to monopolize sunlight and soil nutrients when native plants are dormant.
The dense, dark green canopy casts deep shade on the forest floor, severely limiting the light needed by native understory plants. This shading prevents native wildflowers, shrubs, and tree seedlings from germinating or surviving, reducing biodiversity. Over time, this transforms diverse native woodlands into near-monocultures of buckthorn.
Buckthorn’s ability to form impenetrable stands also creates physical barriers that degrade forest structure. These dense thickets displace native shrubs and small trees in the forest mid-layer. The resulting lack of structural diversity contributes to poor habitat quality, making it difficult for some larger wildlife to move or forage.
Altering Soil Chemistry and Nutrient Cycling
Beyond its physical dominance, buckthorn changes the chemical composition of the soil, making it unsuitable for native species. Buckthorn leaves are rich in nitrogen, containing up to five times the nitrogen found in native trees. This high nitrogen content causes the leaf litter to decompose rapidly, releasing a pulse of nutrients into the soil.
Studies show that soil under dense buckthorn stands can have significantly higher levels of total nitrogen and organic carbon, sometimes increasing by 78 to 79 percent. This rapid nutrient cycling creates a high-fertility environment that favors fast-growing weeds and invasive plants, while discouraging native species adapted to lower-nutrient conditions. Buckthorn can also increase soil pH, inhibiting the growth of many native plants.
The fast breakdown of the leaf litter is often accelerated by invasive earthworms, which are more abundant in buckthorn areas. This loss of the protective layer exposes the bare soil, destroying beneficial soil fungi and microbes. Native trees and shrubs rely on these fungi for nutrient uptake, making restoration difficult after buckthorn removal.
Consequences for Wildlife and Agriculture
Buckthorn thickets offer poor habitat quality despite their density. The uniform structure lacks the complexity and diversity needed for optimal nesting and foraging by many forest animals. This reduction in structural diversity diminishes ecosystem health and leads to a decline in native fauna.
The dark berries are consumed by birds, aiding the plant’s spread. However, the berries have low nutritional value and contain emodin, which acts as a laxative. This causes birds to rapidly excrete viable seeds over a wide area, facilitating colonization.
The shrub also poses an economic threat to farming by hosting specific agricultural pests and diseases. Common Buckthorn is the primary overwintering host for the soybean aphid (Aphis glycines), a major pest of soybean crops across the Midwest. It is also an alternate host for the fungus causing oat crown rust (Puccinia coronata spp. avenae), which reduces oat crop yields.