Modelling the Long-Term Effects of Climate Change and Silvicultural Systems on Rockfall Protection: A Case Study from the Slovenian Alps

High in the mountains, forests do more than just grow; they act as living shields. When rocks break loose from steep cliffs, trees can stop them, slowing their fall or catching them entirely before they reach villages and roads below. This protective function is a critical service provided by mountain forests, but it is not permanent. The trees that form these shields are living things that grow, age, and die, and their ability to stop falling rocks depends on how many there are, how thick their trunks are, and what species they are. Now, a changing climate is adding a new layer of complexity. Warmer, drier conditions may stress certain trees, while human decisions about how to manage the forest—whether to cut trees, how to cut them, or to leave them alone—will determine what the forest looks like decades from now. The question facing forest managers is not just how to grow timber, but how to keep these natural barriers working effectively for centuries to come, even as the climate shifts and the trees themselves change.

In the Slovenian Alps, a team of researchers set out to answer this question by looking two hundred years into the future. They focused on a specific protection forest above the Ljubelj Pass, a vital mountain crossing between Slovenia and Austria. This forest, which covers about 75 hectares, is currently dominated by European beech trees, with a significant presence of Norway spruce. The researchers wanted to see how this forest would evolve under three different management styles: leaving it completely alone, using a selection system that removes individual trees to maintain a continuous canopy, or using a shelterwood system that clears sections of the forest in stages to encourage new growth. They also wanted to see how these outcomes would change if the climate became warmer and drier, as projected for the region. To do this, they did not wait two centuries; instead, they used powerful computer models to simulate the growth of the forest and the movement of falling rocks over that entire period.

The researchers first used a forest growth model called ForClim to simulate how the trees would develop. They fed the model data from a real forest inventory taken in 2022, including the number of trees, their sizes, and the species present. They then ran the simulation forward 200 years under different scenarios: one where the climate stayed exactly as it is today, and another where it followed a high-emissions path that makes the region significantly warmer. Within each climate scenario, they applied the three different management rules. The model calculated how the forest would change in terms of tree density, the size of the trunks, and the mix of species. For example, in the "no management" scenario, the forest was left to its own devices, growing and dying naturally. In the "selection" scenario, managers would periodically remove some trees to keep the forest structure diverse and continuous. In the "shelterwood" scenario, managers would clear larger patches of trees in a cycle to allow new seedlings to grow in the open space before removing the old trees.

Once the computer had generated the future forests, the researchers passed that data into a second model called Rockyfor3D. This model simulates the physics of falling rocks. It takes the terrain, the type of rock, and the forest structure—specifically the number of trees and their trunk thickness—and calculates how much energy a falling rock would have when it hits the ground. If a rock hits a tree, it loses energy; if it passes through a gap, it keeps its speed. By comparing the energy of the rocks in the different future scenarios, the researchers could determine which management style offered the best protection. They looked at the average protection the forest provided, but also at the worst-case spots where rocks might still hit with high force.

The simulations revealed that the choice of management style mattered more than the climate change itself for the first 40 to 60 years. In the short term, how the forest was managed dictated what the trees looked like and how well they could stop rocks. Over the long term, however, the climate began to reshape the forest composition. In all scenarios, the amount of beech wood declined. Norway spruce, which is sensitive to drought, fared differently depending on the management. In the unmanaged forest, spruce actually increased, likely because the older trees that were already there continued to grow and survive for a long time. But in the managed forests, especially under the warming climate, spruce declined significantly. Instead, the managed forests saw a rise in light-loving trees, such as larch and pine, and other broadleaf species that thrive in warmer conditions.

When it came to protecting the road below, the results showed a clear trade-off between average protection and consistency. The shelterwood system, which involves clearing large patches of forest, produced the strongest average protection after 200 years. This is because the model showed that the forest would eventually become very dense with young trees. However, this system had a major flaw: it created periods of very weak protection. During the regeneration phase, when the old trees are removed and the new ones are small, the forest offers almost no defense, leaving the road exposed to high-energy rocks. The "no management" approach started strong but slowly lost its effectiveness over two centuries as the forest aged and tree density dropped. The researchers noted that this decline might be even worse in reality, as the model did not account for sudden disasters like windstorms or insect outbreaks that could wipe out large sections of an unmanaged forest.

The selection management system emerged as the most balanced and reliable strategy. While it did not provide the absolute highest average protection in the final year of the simulation, it offered the most stable protection over time. Because this method keeps a continuous canopy and avoids clearing large areas, the forest never goes through a phase where it is dangerously thin. The protection remains steady, with fewer extreme spikes in rock energy hitting the road. The study found that climate change did not change the ranking of these strategies; the selection system remained the most stable option regardless of whether the climate stayed the same or warmed up.

The researchers concluded that for mountain forests where the primary goal is to protect infrastructure from rockfall, the selection system offers the best long-term solution. It avoids the dangerous gaps created by other methods and maintains a steady shield against falling rocks. While the shelterwood system might look good on average after two centuries, its periods of vulnerability make it risky for critical infrastructure. The study also highlighted that while climate change will alter which trees grow best, the way humans manage the forest has a more immediate and powerful effect on the forest's structure and its ability to protect us. By choosing to manage the forest in a way that maintains continuous cover, forest managers can ensure that these natural barriers remain effective for generations, even as the climate around them changes.