On the processes and factors shaping the Norway spruce’s (Picea abies) forests in the Southern Swiss Alps: An analytical and modelling approach to understand the effect of climate change and Rhododendrons on the Norway spruce stands
Creators: name: Guidotti, Andrea
Publisher: name: ETH Zurich
Year: 2020

description: The Norway spruce (Picea abies (L.) H. Karst.) is one of the most important tree species in the Southern Swiss Alps, especially in the mountain forests which support an essential protective function on the infrastructures and settlements of the inner alpine valleys. Norway spruce forests are subject to a multitude of stressors threatening their stability and hence the provision of their protective function on the long term. In this master thesis, I aim to identify the main processes and factors shaping the Norway spruce’s range in study area of the Southern Swiss Alps. I combined an analytic and modelling approach based on the most recent forest data from the Swiss National Forest Inventory (NFI) with a set of high-resolution indicators.
Firstly, I analysed the current occurrence of this species in the study area. I calculated the changes measured in the Norway spruce’s stands in terms of stem density and basal area of living and dead trees, as well as the change in the number of seedlings and saplings. The reference data are the NFI2 and the NFI4. This first analysis denotes an increase in the species’ stocks but also an increase in the tree mortality, an overall uplift of the species’ range and a strong reduction in the frequency of the species’ rejuvenation.
Secondly, I assessed the ecologic connection between the Norway spruce and the alpenroses (Rhododendron spp.), which are vectors of the needle’s bladder rust fungus (Chrysomyxa rhododendri). I used a combined analytic and modelling approach to assess the impact of this connection through a broad spectrum of topographic and pedo-climatic control variables. The procedure consists on a Likelihood-Ratio Test applied to a multi-linear regression. This combinatory assessment shows that the alpenroses’ cover degree is depressing the Norway spruce’s mixture and cover degree, both of the young and in the adult trees. This relationship is also true in the opposite direction. This evidence highlights the ecologic meaning of the allelopathic association between these species. 
Thirdly, I modelled the Norway spruce’s range by approximating the species’ mixture degree assessed in the NFI4, either referring to the established rejuvenation and to the adult trees. The method applied was a Tobit multi-regression. I produced four different models’ version. All models are based on high resolution climate data. For each fitted variable, I produced an extended version which accounts for the occurrence of the rustyleaved alpenrose (Rhododendron ferrugineum) recursively predicted with an additional model. Finally, all models’ versions were projected in the second half of the 21st Century by integrating the corresponding future climate indicators based on the CLM scenario.
The models based on the current climate display a reliable reconstruction of the present Norway spruce’s range in the study area. The non-linear boundaries of the Norway spruce’s range in the upper forest’s belts are reliably reproduced by integrating the predicted occurrence of the rusty-leaved alpenrose.
The projections based on the future climate data present a dramatic retreat of the Norway spruce’s range towards its current centre of gravity in the Upper Ticino and in Val Poschiavo. The migration of the species to higher altitudes is hindered by the presence of the rhododendron. These results contrast with conventional models based on the whole of Switzerland. This shows that distinct dynamics are responsible for the absence of the Norway spruce in the study area, which could lead to a much greater decline of the species in the future than assumed in conventional scenarios. Nevertheless, any extrapolation drawn from modelling approach must be taken very cautiously and, in no circumstances, they should be considered as quantitative forecasts.
In conclusion, in this thesis I highlighted that the Norway spruce’s stands of the Southern Swiss Alps are subject to a multitude of dynamics. On the long term-scale, they are showing an overall negative tendency of development. However, the main drivers of change as the changing disturbance regime and the land-use are not quantified in the climatic indicators. The complex interactions between these multiple factors needs
therefore to be considered with a greater attention for responsibly addressing the challenges imposed by the current development of the mountain forests.