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Popular Science: Forest protection against Impatiens glandulifera invasion

Impatiens glandulifera is one of the most successful invasive plants in Europe. In the past, its occurrence was associated mainly with riverbanks because running water facilitated further spread. In the last 20 years, however, I. glandulifera has begun to appear also in forests. What conditions influenced this sudden change during the invasion, what effect can its presence have on the forest ecosystem and how can its spread be stopped? A group of experts from the Institute of Botany of the CAS and our Faculty took a closer look at this issue.

Impatiens glandulifera was introduced to Europe from the Western Himalaya as an ornamental plant in the 19th century. It was first grown in our country in the castle garden at Červený Hrádek u Jirkova in 1846. Already in the 19th century, it began to grow wild and soon spread rapidly. Like other invasive species, it poses a significant threat to local species and eventually to the entire ecosystem. Currently, I. glandulifera is considered an invasive species in many countries around the world. The European Union has even included it on the official list of invasive species with significant environmental and socio-economic impacts.

Impatiens glandulifera spread into the forest. Photo: Jan Čuda.

A massive increase in abundance indicates a rapid invasion in recent decades. In the Czech Republic, I. glandulifera was present in 42% of mapping cells in the 12 × 11 km resolution in 1992, and it recently occupies 77% of the cells. A similar increase can be observed in other countries, such as the United Kingdom (from 21% of the territory in 1992 to 56% today). This increase is mainly explained by the spread outside of riverine habitats. The rapid invasion is mainly attributed to increased eutrophication of rivers and changes in river-bank management, especially the abandonment of grazing and mowing of riverbanks.

So what is the main danger of this invasion? It has been known for a long time that I. glandulifera has many properties that give it a competitive advantage over other plants - most notably a three-meter-high stem with massive leaves that overshadow everything that grows beneath them. Another advantage is the ability of plants to survive in a wide range of natural conditions and almost 100% germination of seeds. Thanks to fragrant blooms, which produce large amounts of nectar, it can attract a large part of pollinators from other plants. New findings show that the range of its ‘arsenal’ is even wider. Substances that are formed during the decomposition of its biomass and are excreted from the roots reduce seed germination and the intake of nutrients in other plants, which leads to a reduction in their competitiveness. At the same time, the spread of I. glandulifera does not benefit the soil either, as the places where it grows are more prone to erosion due to its shallow root system.

Grazing is one way to stop the spread of Impatiens glandulifera. Photo: Jan Čuda.

Over the last 20 years, I. glandulifera has begun to appear to a greater extent in forests farther from rivers, and with the current management, its further expansion is probable. The spread of I. glandulifera in the forests was mainly due to the existence of large populations along riverbanks, producing large numbers of seeds, extensive disturbance of forest ecosystems by both human and natural influences (windstorms, the current bark beetle calamity). The invasion is mainly supported by the increased use of heavy forest machinery, which not only facilitates the transfer of seeds but also improves the conditions for their growth.

The authors draw particular attention to the hidden threat in the form of the production of allelopathic compounds, by which I. glandulifera affects the soil fungi and arbuscular mycorrhiza, which may thus alter nutrient cycling. The invasion of I. glandulifera could thus contribute to the further weakening of recently decimated forest ecosystems.

Effective barriers against the spread of I. glandulifera include proper landscape management, especially mowing and grazing of riverbanks. An additional method of disposal is extirpation by hand pulling, including roots, in the period before seed release. In the forest, it is possible to reduce the introduction by gentle management with minimal soil disturbance by heavy machinery and its thorough cleaning after it has been used in areas affected by I. glandulifera.

Čuda, J., Skálová, H., Pyšek, P. (2020) Spread of Impatiens glandulifera from riparian habitats to forests and its associated impacts: insights from a new invasion. Weed Research 60, 8–15.

Kateřina Fraindová

Published: Aug 07, 2020 01:50 PM

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