Favoured aliens for the future

Some species of plant will prefer a world with higher levels of atmospheric carbon dioxide. When those species are invasive pests, the invaders may well flourish at the expense of the native vegetation.

Concern about the rising levels of carbon dioxide in the atmosphere centres on global warming. But CO₂ is of course also the raw material of photosynthesis, and the growth of some plants is often limited by its relatively low ambient concentration (less than 0.04% by volume in the atmosphere). Such plants may be stimulated to grow by any increase in the gas, while others are not, resulting in a shift in the competitive balance in a plant community.

This is a worrisome prospect for conservationists, given the possibility of non-native, invasive plants being favoured at the expense of a native flora. An example of such a situation is described in Functional Ecology by Hättenschwiler and Körner¹. They have studied the response of the invasive cherry laurel (Prunus laurocerasus) to elevated CO₂, and compared it with the responses of the native shrubs and trees in the forests of Switzerland.

As is the case with many invasive plant pests, the cherry laurel has been transported by humans away from its native region, in this case from around the Black Sea in southwest Asia. It has been introduced into many parts of Europe, where it is used as a decorative garden shrub or as a hedge. Cherry laurel provides dense, evergreen cover, and its scented flowers and black, bird-dispersed fruits have a certain appeal. But the cyanide-generating, toxic properties of its leaves, so familiar to entomologists, ensure that it is well protected from vertebrate and invertebrate grazers. As a result it competes vigorously against other plants that grow in its favoured habitat of shady forest floors. Like Rhododendron ponticum², an evergreen shrub from the same region of Asia, it has become an aggressive invader in many of the regions of Europe where it has been introduced, including the deciduous forests of Switzerland³.

Several broad-leaved evergreen shrubs from more southern climes are currently proving invasive in Switzerland, possibly assisted by climatic changes that have led to milder winters and wetter summers. But it could be that rising atmospheric concentrations of CO₂ are also having a direct influence on changing competitive balances among the plants of Swiss woodlands. Hättenschwiler and Körner examined this possibility by constructing open-topped chambers within the forest in which germinating seeds of cherry laurel were planted along with seedlings of native plants, including holly (Ilex aquifolium), ivy (Hedera helix), hornbeam (Carpinus betulus) and ash (Fraxinus excelsior). Three treatments were provided, one in which ambient CO₂ levels of 365 µmol mol⁻¹ (p.p.m.) were maintained within the chambers, together with one treatment of 500 p.p.m. and another of 660 p.p.m.; the plots were then allowed to develop over three growing seasons. Despite the depredations of mice, 24 chambers survived the experiments and were harvested to supply data on relative growth rates.

Cherry laurel seedlings that had been exposed to increased levels of CO₂ showed a 56% increase in biomass accumulation over the three years compared with the control, although there was no significant difference between the two enhanced CO₂ treatments. Holly, on the other hand, showed no significant increase in biomass when given additional CO₂. Ivy doubled its biomass under raised CO₂, and increased its length of stem growth by 137% at 660 p.p.m., but the hornbeam seedlings were unaffected by CO₂ level. The strong differential responses between species suggest that the changing atmospheric concentration of CO₂ is likely to have a substantial impact on community composition in the Swiss deciduous forests. Not only will the invasive alien cherry laurel be favoured, but the native climbing ivy may also flourish under the new conditions, which could in turn affect tree growth — ivy weighs heavily on tree canopies and can render them susceptible to wind damage. Frost-sensitive ivy is close to its climatic limit in these woods, where daily mean temperatures in January are −2°C, so increasingly mild winters will also favour its growth.

In his seminal work on invasive species, published in 1958, Charles Elton⁴ wrote of an approaching ecological explosion of plant and animal pests as a consequence of increasing movements of goods and people around the world. His prophesies have proved correct, but the further influence of global change could fuel the explosion to levels beyond all expectation. Field experiments exploring the direct effects of atmospheric CO₂ concentration on invasive plants within natural communities are scarce⁵: a more intensive and internationally planned programme of research is called for.