A site at the limit of tree growth: the bed of the Maggia torrent in Ticino Canton, Switzerland. A dendroecological study.
Based on a dissertation (by the senior author) for the University of Basle. A study was conducted in the natural riverbed of the Maggia torrent near Cevio and Giumaglio. The course of the torrent has not been deliberately altered by man. Riparian vegetation dynamics were reconstructed from records of yearly maximum river flow, seven aerial photographs taken since 1935, observations on soil profiles, scars on the stems of trees, the presence of tension wood, and the shapes and ages of trees and shrubs growing along the river. Periodically, floods destroy the tree sites, or sedimentation or erosion affect the ecology of the sites. Even though the trees and shrubs are adapted to extreme site conditions, they rarely survive more than 20 years. Species characteristics leading to survival are: resistance to long periods of flooding, rapid vegetative regeneration of roots and branches, fast germination of seeds, rapid shoot growth, rapid healing of injuries, and effective compartmentalization of rot within the tree. Alnus incana, Salix elaeagnos, and S. purpurea are the species most likely to survive in the river bed. Dendrochronologically dated scars and observations on the beginning of tension wood in the stems were especially useful in assessing the mechanical stress of floods on trees growing in the riverbed and along the floodplain.