New BFW tool: Traffic light system facilitates tree species selection

The tree species traffic light shows the suitability of different tree species in Austria in the climate of the future (from 2080 - 2100). The recommendations assume a warming of between 2 and 2.5 °C by the year 2100 and are based on the RCP 4.5 scenario of the Intergovernmental Panel on Climate Change (IPCC). The growth area, forecasts of temperature and precipitation, and the sea level of the specified location are incorporated into the results. The tree species traffic light thus provides an initial assessment of suitable tree species, but must be supplemented on site with additional site factors.

The assessment of potential tree species distribution under climate change scenarios here is based on species distribution models, often referred to as climate envelope models. They are the most commonly used tools because they predict the potential distribution of species by exploiting the correlation between the known occurrence of a species and the corresponding environmental conditions. For tree species occurrences, a comprehensive dataset of all European forest inventories was used.

Climate envelope models were developed using the ensemble model approach BIOMOD2 (Thuiller et al. 2016ⁱ ), combining the strengths of different modeling algorithms while accounting for their uncertainties (Chakraborty et al. 2021ⁱⁱ ). This method was applied to the following species: Spruce, Scots pine, Larch, Fir, Beech, English oak, Sessile oak, Red oak, Coastal fir, and Douglas fir.  Unfortunately, such models are not yet available for all tree species, so the suitability of other tree species was assessed by experts.

The threshold values for the color transition (i.e., at which value a tree species is indicated as red, yellow, or green) differ among the tree species. They are based on statistical ratios of the models for the modeled species, and on expert estimates for the other species. For spruce, the thresholds were set somewhat more conservatively than for the other species because of the bark beetle problem that already exists.

If a tree species is in the red range at a site, it is very likely that it will not cope well with the climatic conditions that will prevail in the future. If it is in the green range, it will most likely tolerate the climatic conditions well, but it is still possible that it could still experience problems due to poor site conditions. Depending on the tree species, site conditions will modify the outcome.