Rapid climate change - a global process
It is extremely unlikely that natural climate variability contributed more than 25% to the climate warming in the past 60 years. The large part of observed warming, at least 74 %, are virtually certain attributable to human activities (Huber & Knutti, 2012). Most of the warming occurred in the last 35 years, and since the beginning of modern record-keeping (1880), 19 of the warmest years have occurred since the year 2000, with 2016 and 2020 having been the hottest (NASA, 2021). In the current perspective, measures taken to mitigate the progressing climate warming are not promising. All industrial countries are likely to fail in complying with their promise of reducing greenhouse gas emissions (Victor et al., 2017). Amplifying anthropogenic climate change will increasingly affect all life on Earth. The effects of climate change, however, differ from region to region, from one ecosystem, life form, or species to another. The detection of its ecological impacts require a variety of means ranging from remote sensing, field-based long-term surveillance to experimental studies.
Global biodiversity declines
While species richness may be increasing at regional scales in many parts of the world, mostly through the introduction of non-native species, biodiversity is decreasing on the global scale (Sax & Gaines, 2003). The sixth mass extinction event in Earth's history might have already commenced (Isbell et al., 2017), with extinction rates of vascular plants estimated to be up to 500 times above the background rate and nearly 600 extinctions since Linnaeus published Species Plantarum in the mid-18th century (Humphreys et al., 2019 ). The actual extinction of species, however, is very difficult to verify (Barnosky et al., 2011). Even if the ultimate extinction is delayed, population decline to the point where many species exist only as remnants of their former abundance will be an immediate challenge for biodiversity monitoring and biological conservation (Briggs, 2017). Climate change can become a major driver of biodiversity decline (Bellard et al., 2012), affecting both areas already strongly transformed by human activities as well as remote semi-natural and natural ecosystems. Climate change impacts may be masked or enhanced by other direct and indirect anthropogenic impacts, such as landscape fragmentation, habitat conversion and species invasions. Remote areas such as many mountain summit regions, however, are little affected by human population pressure and land use activity and, thus, can provide an ideal 'natural laboratory' for ecological research on climate change impacts.
Mountain regions - hot spots of organismic diversity
Owing to the compression of thermal zones and to orographic isolation caused by low-temperature conditions, high mountains host a high number of species, including many endemic species. Mountains host a larger proportion of the Earth's biodiversity than would be expected by area (Körner et al., 2011). In Europe, for example, alpine environments cover only 3 % of the continent's area, however, about 20 % of all native European vascular plant species may be found there (Väre et al., 2003). Tropical mountain regions such as the northern Andes and the south-eastern part of the Himalaya system are among the top biodiversity hot spots on Earth (Barthlott et al., 2007). Hence, climate change-induced threats to the biodiversity in mountain regions would impact on a significant part of the planet's natural heritage.
The alpine life zone - fragile ecosystems across the globe
Alpine environments are areas located above the low-temperature determined treeline, or its substitutes. This 'alpine life zone' is dominated by low stature vegetation being largely composed of long-lived species. The alpine life zone is globally distributed, from polar to tropical latitudes and occurs across oceanic and continental climates (Körner, 2021; Nagy & Grabherr, 2009). Many alpine regions are among the remaining most pristine environments on Earth. Even though direct human land use has affected alpine areas relatively little, today's climate warming may result in the reduction of its current extent and, in some cases, the complete disappearance of fragmented and isolated alpine areas ( Pauli & Halloy 2019).
Climate warming can impede the growth of cold-adapted species, and more importantly, may impose competition on alpine plants from species from lower elevations. Ameliorated thermal conditions are expected to enable alpine plants to migrate to higher elevations or into habitats that were previously too cold. Many mountain ranges which host a large number of endemic plants are often those which do not have extensive area for climatically suitable substitute habitats and, thus, are very likely to suffer critical biodiversity losses (Theurillat & Guisan, 2001; Halloy & Mark, 2003; Pickering & Armstrong, 2003; Dirnböck et al., 2011). Model projections suggested substantial habitat losses in mountains of limited vertical extent (Engler et al., 2011). Actual disappearance of species, however, may be delayed if long-lived alpine plants persevere even in habitats which are already climatically unsuitable, accumulating an increasing extinction debt (Dullinger et al., 2012; Cotto et al., 2017). Rapid climate warming could lead to differential migration rates of species with different life strategies and propagation abilities, leading to the formation of new plant assemblages and result in changes in ecosystem functioning (Root et al., 2003).
Mountain plants already respond to climate warming
Climate-induced upward shifts of mountain plants have already been detected. Upper treelines are mostly limited by low-temperature regimes (Körner, 2012). Thus climate warming is expected to drive advancements of the upper timberlines and treelines and the resulting narrowing of the alpine lifezone. A growing body of evidence suggests that processes of upward treeline expansion occur in response to climate change, although such shifts can be limited by a variety of non-climatic factors and climate signals can be confounded by human land use activity (Cudlin et al., 2017). Correspondingly, a global meta-analysis, thus, showed treeline advance at only 52% of the 166 sites, the remaining were stable (47 %), whereas treeline recession was very rare (1 %), (Harsch et al., 2009). Increased mountain forest growth or advances of altitudinal treelines have been reported from several mountain ranges where human land use pressure is low or absent, such as in the Urals (Moiseev & Shiyatov, 2003; Hagedorn et al., 2014), Scandes (Kullman, 2002; 2003; Hedenås et al., 2016), and in the Rocky Mountains (Sturm et al., 2001; Danby & Hik, 2007; Treml & Veblen, 2017).
Increases in alpine species richness have been reported from the Alps (Hofer, 1992; Grabherr et al., 1994, 2001; Bahn & Körner, 2003; Erschbamer et al., 2011; Wipf et al., 2013) and from Scandinavia (Odland et al., 2010) and from the eastern Himalayas ( Salick et al. 2019). A pan-European study using baseline and resurvey data of historic study summits as well as from GLORIA sites showed that the rate of increases in species richness has accelerated during recent decades and was strikingly synchronous with rising temperatures ( Steinbauer et al. 2018). A comparison of vegetation records from before 1970 with recent ones from nearby localities with the same topography and elevation showed that upper, optimum as well as lower range margins of species are shifting faster the lower they were situated historically ( Rumpf et al., 2018), which is expected to result in a rapid shrinkage of alpine land. Studies reporting population declines such as from the southern Scandes (Klanderud & Birks, 2003), Rocky Mountains (Lesica, 2014), and from the central Alps (Pauli et al., 2007, Steinbauer et al. 2020) are far less common. The latter three were based on species abundance data from permanent plots and show declines of the most cold-adapted species. The first pan-European GLORIA resurvey study showed a widespread thermophilisation process in alpine vegetation (i.e., declines of cold-adapted and/or a concurrent expansion of more warmth-demanding species) already after a period of only seven years (Gottfried et al., 2012) and observations in the central Alps over the 20-years timespan showed increasing rates of thermophilisation ( Lamprecht et al. 2018). These observations strongly suggest that alpine vegetation and the distribution limits of its species do respond rather rapidly to climate change despite the long-lived and slow-growing nature of most alpine plants.
Much uncertainty remains - the scarcity of baseline records
Assuming an average lapse rate of 0.6°C per 100 m elevation, a warming of 3°C could ultimately cause distribution limits to shift by 500 m. As a result, the alpine zone of many mountains ranges could potentially disappear. On the global level, a temperature increase of 2.2°C was estimated to leading to a loss of ~24% of lower alpine areas and of ~55% of upper alpine to nival areas (Körner, 2012). Great uncertainty, however, exists about the pace of change in vegetation composition, how much species' responses lag behind the changing climatic conditions of their habitats, and about thresholds or tipping points beyond which velocities may rapidly accelerate, especially where precipitation patterns concurrently change with the thermal regimes. Several studies showed that the process of species enrichment has recently accelerated (Walther et al., 2005; Wipf et al., 2013, Steinbauer et al. 2018). The successful colonisation of species to new habitats, however, is complicated by also depending on non-climatic factors such as dispersal abilities and safe sites for germination and seedling establishment. Species abundance changes in established populations, such as of a species' abundance or cover, in contrast, should represent more immediate responses to climatic changes. Compared to observed species enrichments, evidence of population changes of species in alpine ecosystems, of declines in particular, are far less common. This appears to mostly reflect the lack of detailed data from permanent plots from the decades before the onset of rapid climate warming, rather than the absence of actual change.
A great demand for standardised permanent observation plots
Meteorologists and glaciologists have data series spanning many decades and sometimes dating back to the 19th century (Haeberli et al., 1996; Zemp et al. 2015; Barry, 2008). No such long-term data series are available for biodiversity, apart from a few exceptions. The increasingly apparent large-scale impact of climate change on the biosphere and its biological diversity, however, makes an effective long-term observation system an indispensable requirement for biodiversity assessments and conservation measures. The low-temperature determined alpine vegetation, therefore, is highly suitable for global comparisons, since it occurs in all climate zones from topical to polar regions (Pauli & Halloy 2019). Owing to the early recognition of the urgent demand for a ground-based monitoring soon after anthropogenic climate warming came into the broader public and scientific debate (Messerli & Ives, 1997; EEA, 1999; Heal, 1999; Becker & Bugmann, 2001; Körner & Spehn, 2002; Price & Neville, 2003), the development of a standardised and operable monitoring design and method already commenced in the late 20th century. This resulted in the implementation of the site-based international GLORIA network and monitoring programme. Launched in 2001, GLORIA now has sites on six continents, including many remote regions, where no monitoring activity took place before. At many sites, resurvey data were already recorded and, given that the network keeps operating by conducting repeat surveys at 5 to 10 years intervals, it will provide increasingly valuable data series and insights on the state of the planet's alpine biota. This requires the continued commitment of dedicated ecologists/biologists around the world – without their efforts the network would never have developed, but also long-term commitments for funding of the GLORIA field survey campaigns.
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