A tree-ring perspective on the invasion of Ailanthus altissima in protection forests

• Ailanthus altissima invades protection forests in southern Switzerland.
• We use tree-ring data to analyse growth dynamics and decay status of these forests.
• A. altissima shows a higher drought resistance than its main competitor C. sativa.
• Observed high frequencies of decay may reduce the ability to protect from rockfall.

Since around the 1950s the invasive tree of heaven (Ailanthus altissima (Mill.) Swingle) is spreading in several forests of southern Switzerland. To estimate the implications of the invasion of A. altissima in forests protecting humans and infrastructure from rockfall (protection forests) we compared stem growth dynamics and decay frequency of A. altissima with sweet chestnut (Castanea sativa Mill.) one of its main local competitors. Comparisons were based on dendroecological analyses and phytopathological observations on 52 A. altissima and 50 C. sativa trees from three sites in southern Switzerland. Results revealed slower radial growth of A. altissima compared to C. sativa. Climate–growth relationships showed that both species weakly respond to mean monthly temperature and precipitation sums. However, growth reductions to the severe drought years 1976 and 2003 were stronger for C. sativa compared to A. altissima, confirming the generally assumed high drought resistance of A. altissima, which may promote the species with climate warming. The incidence of heart rot varied considerably across sites and species. Locally observed high frequencies of decay in A. altissima and C. sativa suggest that the analysed protection forests may have a limited ability to protect from rockfall. Our study supplies a first long-term tree-ring based perspective on growth dynamics and decay frequency of A. altissima that can be used to complement current knowledge on the invasive behaviour of the species as a basis to adapt and optimise management of protection forests in southern Switzerland and beyond.