A bit more than 30 years ago, neither climate change nor its impacts on mountain regions were high on many people’s agendas.

This situation changed for the former with the First Assessment Report of the IPCC in 1990, and for the latter with Chapter 13 in Agenda 21 entitled ‘Managing Fragile Ecosystems: Sustainable Mountain Development’ in 1992, the book ‘Mountains of the world: a global priority’ in 1997 (Fig. 1), and the Mountain Research Initiative that was initiated in 1997/1998, among others. As a consequence, multiple research activities started to focus on global change impacts on the mountain cryosphere, ecosystems and societies. Below, I will focus on mountain forests, the specific field of my research.

One of the basic assumptions of many mountain researchers and probably also mountain dwellers as well as mountain visitors is that mountain systems are fragile, for a number of reasons.

Therefore, I was not surprised to see a high sensitivity of mountain forests to climate change when I started to do research on this subject in the early 1990s (Fig. 2). In the Second Assessment Report of the IPCC, which – unlike many other IPCC reports – featured a chapter on mountains, we even concluded that climate changes as small as 2 °C could lead to severe shifts in forest species composition and ecosystem services provision. Well, at that time we did not call these things ‘services’ yet, but this is what we meant. Hence mountain forests were confirmed to be fragile.

Some of these early predictions turned out to be artifacts of the simulation models that had been used to produce them and later on, improved models suggested that drastic changes at the time scale of years to perhaps a decade (which is quite fast for forests!) were rather unlikely to happen except under strong disturbances such as windthrow (evidently), insect infestations, or direct drought effects (Fig. 3).

Sure, one of the key roles of mountain systems is that they harvest the precipitation from the atmosphere; they are the ‘water towers of the world’. Hence why should drought be an issue? Drought anomalies were not thought to be important, at least not in a (central) European mountain context. Thus, it appeared that mountain forests are much more robust and resistant to climate change than initially thought, and not really fragile.

Well, with yet improved forest models, the climate scenarios of the late 2000s and the availability of a downscaled version of these scenarios for Switzerland (‘CH2011’ and its update ‘CH2018’), we were somewhat shocked to see that even a global 2° scenario was not safe for mountain forests (Fig. 4), primarily because of the disproportionally large warming that mountain regions are experiencing. BTW, the physical reasons for this disproportionate warming is now quite well understood, and unfortunately it is likely to continue into the future: those who have experienced high warming will get even more warming.

Also, it started to dawn upon us that the frequency and severity of droughts was likely to change more than we had assumed – with severe consequences for disturbance interactions, i.e. wildfires (quite obvious) or enhanced insect infestations (because these creatures profit from dry conditions, which at the same time typically exert considerable stress on trees). Therefore, we felt we were back to square one: mountain forests are quite fragile systems indeed. And this conclusion was based on much more solid evidence than in the early 1990s.

Now you’d expect my next argument to convey that we were wrong again, and the pendulum of the late 2010s and 2020s is swinging back towards low fragility of mountain forests. I am sorry this is not what I want to convey. The extremely dry summers of 2018, 2019 – also at higher elevations – and the very dry spring in the European Alps in 2020 have induced massive tree mortality in European mountains (Fig. 5), with dramatic consequences for ecosystem services – ranging from timber production to the protection from gravitiative natural hazards such as rockfall or snow avalanches.

Although the latter may disappear in the very long term due to the absence of frozen precipitation (a.k.a. snow), other hazards will remain or will even aggravate. So this time it is nature that shows us a pattern that we are still struggling to understand. It is becoming increasingly clear that the magnitude of climate change has reached a level that will definitely leave a strong imprint on forests, their dynamics, and the provisioning of what we now call “nature’s contributions to people”. Hence, to the best of my knowledge mountain forests are fragile systems indeed.

PS: I am not sure whether the evolution of scientific knowledge depicted above reflects a flaw of the scientific method, i.e. that nothing can ever be certain, or the development of a certain degree of robustness of the knowledge obtained by an individual researcher. I leave this decision up to the reader. In any case, it appears to me that there are advantages of getting old – one starts to have a broader overview of a scientific field.