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Experiencing an excursion through the eyes of a forest modeler 

For the RESONATE project, my task aims at developing high resolution future forest trajectories and disturbance maps for the European continent. Continental scale modelling always comes along with trade-offs regarding the detailed processes. Taking this into account, we follow a bottom-up approach, where we use detailed information from local process-based forest simulations to train deep neural networks. For this, we collected forest simulations under different climate scenarios from hundreds of locations across Europe, covering large gradients of environmental and climatic conditions. By combining simulations from different regions, we can explore the relationship between forest dynamics and climate signals using deep neural networks. These neural networks learn to represent forest dynamics depending on environmental and climate conditions, allowing us to upscale the forest dynamics to continental scale. We believe that with this approach we will make a step towards better capturing local scale dynamics at the macroscale.  

But guess what, forest modeling means we spend most of the time in front of our screen, working on code and data that eventually allow a glimpse into the future of forest ecosystems. Although I spend a lot of my leisure time hiking, cycling and sometimes ski touring in the mountains, professionally I spend very little time in the field. Therefore, I was really happy to join the excursion as part of a conference we organized in Berchtesgaden some months ago. The occasion to go to the field with colleagues who spend a lot of time there and visit the system that I am currently modelling is very special and of course informative. And for me, coming from a macroecology background, it is also particularly important to see gradients in the mountain landscape and discuss their impact on vegetation processes as well as disturbances.

The excursion started on the top of mount Jenner in Berchtesgaden National Park, where we were walking from the top of the gondola station up to the viewing platform over a path that was covered in 20cm of fresh snow.  

Rupert Seidl and Michael Maroschek guided us to the top of mount Jenner, but not for the purpose of walking in the snow, but rather to talk about the so-called “treeline” which would start a bit higher up from there and to see the open landscape with some individuals of Mountain pine. Species up here deal with very short vegetation periods – when the climatic conditions actually allow vegetation growth – as well as low temperatures and large amounts of rainfall or snow in winter. Therefore, the treeline marks the altitude above which the climate does not allow trees to grow. When walking down towards the valley of Königsbach, couple of hundred meters below the top, the vegetation already became denser. Mountain pine was still dominant, but at the second stop we discovered individual Spruce, Fir, Mountain ash and Sycamore maple trees. We continued to hike down through the snow until we reached some high mountain pastures, where management legacy caused again a more open landscape, with spruce, Sycamore maple and Larch standing in small cohorts. Outside of the core zone, Berchtesgaden National Park covers some managed land where cattle graze in the summer, a legacy from those times before the park was established. Owners of farms that existed at the time of the foundation of the National Park are entitled to use the land in the same way as they were allowed to use it before. The ongoing land-use thus conserves these open landscapes. Continuing our descent, we slowly entered the closed forest, dominated by Spruce trees, complemented with Larch and Sycamore maple occurring in small groups.  

Figure 2: Orthophoto sequence of the disturbance patch between 1990 and 2020. After the windthrow in 2007, the patch had a size of 1.6 ha. In 2020 the patch had increased to 3.7 ha. Photo credits: LDBV and Nationalpark Berchtesgaden. 

Our next stop was in the middle of a large disturbance patch. This patch is a result from several disturbance events, originating from a windthrow during the Storm Kyrill, back in 2007. After the windthrow, bark beetle continuously damaged trees and increased the patch considerably in size from about 1.6 ha to 3.7 ha (see figure 2). Here we discussed the importance of such natural disturbance events and their impact on the local species composition. We also touched upon how important species richness is to increase the resilience of our forests to those disturbance events. For instance, Larch, which lose their needles during winter and have a stronger root system than other species, are less susceptible to strong winds. And really, we observed a few individual Larch trees who survived the windthrow and now provide an important role in building microhabitats that are beneficial for other species to recover from this event. Our next stops were dedicated to a chrono sequence, a series of sites that differ in time since disturbance, showing different stages of forest development. Those stops showed nicely how after a disturbance the composition changes from spruce dominated to mixed forest including Beech. Finally, the last stop was at about 800 meters altitude, where the climate was much warmer and drier than higher up, resulting in a Beech dominated, dense forest with intermixed Ash and Elm which are susceptible to fungal infestations from invasive pathogens.  

Figure 3: The same disturbance patch from the ground perspective. Photo credit: Marc Grünig. 

The excursion was a journey through the different vegetation zones that are found within the National Park, but what struck me most were the steep gradients in very small areas. For instance, the difference in elevation between mount Watzmann and lake Königssee is about 2000 meters within the distance of about 3 kilometers. Such steep gradients and the corresponding diverse landscape play a major role in creating different habitats within a small area and further drive disturbances such as avalanches and landslides. With this excursion, once again it became obvious to me: from small scale disturbances to snowfall in mid-September, within those steep gradients, very complex processes happen in mountain ecosystems. And even these might be extreme examples, we can still attempt to get better at incorporating small scale processes in continental scale models. 


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