Assessing long term effects of invasive tree species on the protective function of forests against rockfall in Switzerland and France: the case of Ailanthus altissima (tree of heaven) – ALIEN

ALIEN will achieve these goals by interdisciplinary fundamental research on A. altissima, tightly coupling field data collection and process-based simulation modelling. In the WP on forest ecology, data will be collected in both southern Switzerland, where Ailanthus altissima is invasive, and in China, the native region of the tree. Current explorative research in southern Switzerland will be expanded from individual trees to the stand scale. We will assess parameters critical for succession modelling, such as juvenile height growth, longevity or seed dispersal in both China and Switzerland. In the WP on Forest Modelling, the established forest landscape model LandClim will be used to simulate the spread of Ailanthus altissima and to identify determinants of its invasion success. The simulations will be run with four different management strategies and for both current and future climate scenarios. In the WP on Mechanical Stability, mechanical models of Ailanthus altissima stems and root systems will be identified from laboratory and field experiments. Model parameters and their related variability will also be assessed from these experiments. In the WP on Protection Function, we will develop models and tools for quantifying the protective function of Ailanthus altissima invaded protection forests. The models will be validated and calibrated based on traces of past collisions of rocks with trees.

We expect main results in the following areas: In Forest Ecology, we will gain a better understanding on the ecological properties of A. altissima, e.g. with respect to juvenile height growth, longevity or seed dispersal. In Forest modelling, we will identify species properties relevant for the invasion success of A. altissima and assess different management strategies aiming to limit its spread. In the WP on Mechanical stability, we will improve the modelling of rock impact on a tree by integrating the effects of tree crown, root system as well as specificities of Ailanthus altissima in terms of the mechanical response of both the stem and the root system. Finally, in the WP on protection function, we will develop develop models and tools for quantifying the protective function of Ailanthus altissima invaded protection forests. Results from such validated models can be used with higher confidence for further investigations.

The novel insights on the dynamics of invasive species, their impact on risk prevention related ecosystem services, and on the efficacy of alternative management options will be of high interest to the international research community and eventually also practitioners responsible for the management of protection forests.

Project outcomes will be disseminated via scientific publications, conference papers and the project website, as well as via relevant university courses. A current list of publications can be found on the project website.

Ailanthus altissima (Mill.), commonly known as tree of heaven, is a deciduous tree native to bothnortheast and central China and Taiwan. The tree grows rapidly and is capable of reaching heights of 15 metres in 25 years. However, the species is also short-lived, rarely exceeding ages of 50 years. The tree was first brought from China to Europe in the 1740s. It has become an invasive species due to its ability both to quickly colonise disturbed areas and to suppress competition with allelopathic com-pounds. The tree also resprouts vigorously when cut, making its eradication difficult and time consum-ing. As a consequence, forests, mainly in the warmer, southern parts of the Alps, are increasingly in-vaded by A. altissima trees. Many of these forests play a key role in natural hazard risk management as they have a protective function against rockfall. Decades of fundamental research on both the ecological and mechanical characteristics of forests and trees in the Alps allowed the development of tools which enable quantitative comparisons between technical protective measures and protection forests. The expansion of the invasive species A. altissi-ma in protection forests poses questions on the future development and stability of the affected protec-tion forests and their protective capacity.
To answer these questions, both new data and further model developments are needed.

The four key goals of this project are therefore:
1. To acquire fundamental ecological knowledge on the long-term development of the structure and stability of forest stands invaded by A. altissima
2. To adapt and calibrate a forest dynamics simulation model for assessing the determinants of A. altissima spread and evaluating the effects of forest management interventions on the future de-velopment of invaded forest stands
3. To develop a model for the mechanical response of A. altissima trees during dynamic impacts
4. To develop methods and models that allow quantifying the current and future protective function of forest stands invaded by A. altissima against rockfall

ALIEN will achieve these goals by interdisciplinary fundamental research on A. altissima, tightly cou-pling field data collection and process-based simulation modelling. The novel insights on the dynam-ics of invasive species, their impact on risk prevention related ecosystem services, and on the efficacy of alternative management options will be of high interest to the international research community and eventually also practitioners responsible for the management of protection forests.