1 Community Ecology, Swiss Federal Institute for Forest, Snow and Landscape Research (WSL), a Ramèl 18, CH-6593 Cadenazzo, Switzerland
2 Swiss Federal Institute of Technology (ETH) Zurich, Department of Environmental Sciences, Forest Ecology, Universitätstrasse 16, CH-8092 Zürich, Switzerland
3 Bern University of Applied Sciences (BFH), Langgasse 85, CH-3052 Zollikofen, Switzerland
4 Forest Soils and Biogeochemistry, Swiss Federal Institute for Forest, Snow and Landscape Research (WSL), Zürcherstrasse 111, CH-8903 Birmensdorf, Switzerland
5 Forstingenieurbüro Dr. Monika Frehner, Sixerstrasse 9, CH-7320 Sargans, Switzerland
6 School of Environment, The University of Auckland, Auckland 1020, New Zealand
The tree of heaven (Ailanthus altissima (Mill.) Swingle) is considered to be an early-successional, gap-obligate pioneer species with vigorous height growth, low shade tolerance, early fecundity and large seed production. It is a highly invasive species in many temperate and Mediterranean ecosystems outside its natural range, especially after disturbance. Due to its low shade tolerance, the potential of A. altissima to colonise undisturbed forests is thought to be low. In this study we analysed the potential of juvenile A. altissima to grow and survive in sweet chestnut (Castanea sativa Mill.) forests in southern Switzerland. We used hemispherical photography to assess the light conditions of 204 individuals of A. altissima (31% generative, 69% vegetative) aged between 1 and 7 years (median: 3 years) in six sites. Generative (seed-borne) and vegetative (clonal ramet) offspring of A. altissima are able to grow in light conditions well below the requirements of shade-intolerant tree species such as European larch (L. decidua Mill.) and Scots pine (P. sylvestris L.). The relatively low light conditions found to be sufficient for the growth and survival of generative regeneration of A. altissima suggest a higher shade tolerance for this species than previously stated, at least for early regeneration. Consequently, the colonisation frontier of A. altissima should be intensively monitored in both forest openings but also in closed canopy forests in the vicinity of seed-bearing A. altissima.
Keywords: shade tolerance, light requirement, invasive tree, colonisation