New data on the biology and ecology of Pueraria lobata
Pueraria lobata (Fabaceae, EPPO A2 List) originates from the Far East and is very invasive in South-Eastern USA. It is also recorded in southern Switzerland (Ticino). A study has been undertaken on the situation of the plant in Switzerland and on its biology.
Distribution in Switzerland: the plant is present in Ticino in 24 stands along the Lakes Maggiore and Lugano and in 5 stands bordering Italy. Among these 29 sites, 7 cover more than 1000 m², 10 cover between 100 and 1000 m², and 12 cover less than 100 m². All infested sites are located in the warmer region of southern Switzerland, with an average annual temperature close to 11°C. 75% of these sites stand below 300 m and are oriented SW and SE. Most sites are less than 15 m away from houses and roads and less than 200 m from lakes.
Mode of dispersal: because plant wastes have been found in 20 stands, it is suspected that garden wastes may be a pathway.
Habitats: invaded habitats are open, disturbed and on the edge of forests.
Environmental requirements: the plant is able to survive in frosted and shallow soils even though its roots cannot develop fully. According to soil analysis in 6 locations, the plant can grow on soils with a pH varying from 3 to 8. P. lobata can almost double the concentration of nitrogen compounds in the top soil (1 to 6 cm deep) due to its symbiosis with Rhizobium bacteria.
Biology and reproduction: In Caslano and Magliaso, the plant can grow up to 26 cm per day. An average growth rate of 8.8 cm per day was measured between April and July. In one year, a plant can reach an average length of 15 m. The vine can also climb over trees, which break under the weight of foliage, especially when wet.
The plant mainly propagates vegetatively by developing new roots at the nodes of the shoots lying on the soil, but it can also produce viable seeds. From each node forming roots, 30 new branches of several metres long can grow in all directions. From each node, further shoots can sprout.
Impacts: according to phytosociological relevés, there is a reduction of the number of species in invaded places: while 20-25 species grow in 4 m² of non-invaded meadow or forest, only 6-9 species grow in 4 m² invaded by P. lobata. Moreover, light indicators are lower under the cover of P. lobata, while temperature indicator values are higher.
A reduction of the number and diversity of arthropods have also been identified in invaded sites. Near Magliaso, while a total of 262 arthropods (corresponding to 17 taxa) were found in uninvaded forests, only 187 arthropods (corresponding to 12 taxa) were found in invaded sites. The number of mites, beetles, ants and springtails decreased in invaded sites, while the number of Glomerida and Myriapoda feeding on litter augmented.
The distribution of P. lobata is likely to increase exponentially in southern Switzerland due to global warming. The plant should therefore be eradicated whenever possible, or at least be contained, in particular by preventing its dispersal by human activities.
Sources
Pron S (2006) Ecology, distribution and evaluation of the exotic liana Pueraria lobata (Willd) Ohwi (Fabaceae) in southern Switzerland. Thesis Department of Environmental Sciences. Swiss federal Institute of Technology ETH Zurich, Switzerland.