Understanding the long-term dynamics of vegetation since 1953 in high-mountain regions

Abstract

1 INTRODUCTION

Understanding the intricate relationship between environmental changes and vegetation distribution is a key challenge in ecological research, particularly in light of accelerating climate change (IPCC, 2023). Alpine ecosystems, with their high sensitivity to climate change, are an important starting point for analysing these effects. In general, alpine plant species are expected to undergo substantial shifts in habitat range and community structure (Engler et al., 2011; Gottfried et al., 2012; Thuiller et al., 2005). For example, there can be typically observed an upward migration of lower-elevation species and an upward shift of the treeline, which illustrate the profound ecological transformations that are currently ongoing (Gottfried et al., 2012; He et al., 2023; Malfasi & Cannone, 2020).

However, such vegetation responses to climate change are not uniform. Species range adjustments are observed, with particularly fast dynamics at the treeline, and at lower alpine and nival belts (Cannone et al., 2007; He et al., 2023; Pauli et al., 2012). Evidence suggests that the range of cold-adapted species is shrinking (Lamprecht et al., 2018), while lower-elevation species migrate more rapidly upslope, which increases local species richness but also leads to more competition for space (Steinbauer et al., 2018; Wipf et al., 2013). This thermophilisation of assemblages (Gottfried et al., 2012; Lamprecht et al., 2018; Rumpf et al., 2018) is driven especially by a prolongation of the growing season, increased energy availability, and changes in precipitation types (Filippa et al., 2019; Pauli et al., 2012; Vitasse et al., 2021).

Besides these direct climatic effects, vegetation distribution is also modulated by a complex interplay of other factors, including nutrient availability, soil development, and land use changes, while being mediated by individual adaptation and species interactions (Bektaş et al., 2021; Bellard et al., 2012; Bourgeois et al., 2019; Martinez-Almoyna et al., 2020; Rogora et al., 2006; Tasser & Tappeiner, 2002; Theurillat et al., 1998; Wipf et al., 2015). For instance, changes in land use, such as abandonment or intensification of pasture use, play a crucial role in the distribution of plant species, as they facilitate or suppress species and thus contribute to the homogenisation of communities, regardless of their natural elevation distribution (Gehrig-Fasel et al., 2007; Hülber et al., 2020; Niedrist et al., 2009; Tasser et al., 2017).

The dynamic framework emerging from this interplay of climatic and site factors suggests that vegetation shifts are anything but linear and that feedback mechanisms are intricate. As an example, climate change-induced erosion processes and permafrost degradation can cause the establishment of ecological mechanisms known as ‘colonisation barriers’, moderating vegetation adjustments speed and trajectory (Giaccone et al., 2019; Leonelli et al., 2011; Ponti et al., 2021). Microtopography, as another example, plays a significant role in modulating local microclimate and makes species responses even more complex (Graae et al., 2018; Körner & Hiltbrunner, 2021; Scherrer & Körner, 2011). The role of ‘thermal habitats’ and snow cover changes in shaping vegetation dynamics has led to new hypotheses about how increasing energy availability in areas with late or early snowmelt might influence community composition (Choler, 2018). While some species benefit from the longer growing season, others are more at risk form late frost events, which in certain cases can lead to downward migration (Cannone & Pignatti, 2014; Lenoir et al., 2010). The shifting dynamics of plant–plant interactions make predictions even more difficult. According to the stress-gradient hypothesis, plant–plant interactions evolve along environmental gradients, with facilitation playing a more prominent role in stressful environments (Bertness & Callaway, 1994; Callaway & Walker, 1997). If the different climatic and site stress factors increase or decrease to varying degrees, the dynamics of competition and facilitation should also follow and reshape the species assemblages (Anthelme et al., 2014; Choler, 2018; Losapio, Cerabolini, et al., 2021; Losapio, Schöb, et al., 2021; Nicklas et al., 2021). Henceforth, it could appear that some communities, possibly those that increase or maintain stress levels, experience comparatively less change than others and stabilise on the long term. However, there is still insufficient scientific evidence on this.

Understanding how plant species and communities react, whether they migrate upslope and/or downslope (Lenoir et al., 2010; Pauli et al., 2012) or remain in site (Anthelme et al., 2014; Scherrer & Körner, 2011) and thus remain stable in their distribution and adapt (Bellard et al., 2012) or die and decline (Rumpf et al., 2018) remains a challenge. What role the tolerance of individual species to climatic changes plays in this interplay of effects, and what additional factors influence their dispersal abilities and thus their access to new sites and refugia, are further open questions (Graae et al., 2018; Lenoir et al., 2010).

Some of these open questions are the central research object of our study, which is based on the utilisation of historical and recent vegetation data. Historical records offer an opportunity to capture such complexities by observing long-term changes, allowing to conjecture on the most important underlying processes. In alpine vegetation, long-term observations may be even more important to disentangling these dynamics, as alpine species can transiently survive under extreme conditions and accumulate an extinction debt that delays visible responses to environmental changes (Dullinger et al., 2012). Basing our work on historical vegetation surveys from 1953 (Giacomini & Pignatti, 1955) and a resurvey after 70 years, we investigated the floristic changes in subalpine to naval plant communities in the Stelvio National Park (Italy). Specifically, we derived four literature-based hypotheses:

1. Environmental changes have primarily driven vegetation response in higher elevation communities, while lower elevations are entering stages of secondary succession.
2. Climate change and reduced grazing disturbances are leading to an increasing homogenisation of alpine vegetation.
3. Stress-tolerant plant communities exhibit greater stability in composition and distribution.
4. According to the stress-gradient hypothesis, plant interactions are shifting towards increased competition under global warming.