Alpine plants grow above the altitudinal tree-line in mountains around the world. Because alpine plants grow in areas with low summer and winter temperatures, very low night temperatures, frost, short growing-season, high winds, or extended snow-lie, and grow very slowly, it is widely accepted that alpines grow in such extreme habitats because of their physiological tolerance or requirement for cold conditions and/or their intolerance to competition from taller, more rapidly growing lowland or montane plants. Some, but certainly not all, alpines can, with care, be successfully grown in lowland gardens in the absence of competition from tall plants. Alpine plants are thus potentially sensitive to climate change, especially increased summer temperature, and are thus likely to show responses to recent climate change in the last 50-100 years. In the absence of long-term permanent recording plots, studying the responses of alpine plants to recent changes requires reliable localised plant records from, say, the 1930s and repeats of the original historical survey today using the same field methods and taxonomy as the historical survey and a critical comparison of the past and past occurrences and frequencies of the different species.
Many such resampling studies have now been done on over 100 mountains in Scandinavia and the European Alps as well as on mountains on the arctic islands of Jan Mayen, Spitsbergen, and Greenland. My Bergen colleague John-Arvid Grytnes and our research associates across Europe are currently synthesising the results of these resurveys.
One general feature that emerges is that species richness on mountains summits has increased, and that there is an altitudinal ascent of many species including grasses, dwarf shrubs, and low shrubs. On all mountains surveyed, nearly 70% of the species that show a detectable change in their upper altitudinal range-limits between surveys have shifted their range-limits upwards. In the Jotunheimen mountains of central Norway, which includes the highest mountain in Scandinavia (Galdhøppigen, 2469 m), Kari Klanderud and I showed in our 1998 resurvey that total species richness in the 1600-1800 m elevation band had increased since 1930 by about 8-14 species and between 1800-2000 m elevation by 5-8 species, but that richness of areas above 2000 m showed little or no change. Plant richness had increased, on average, by 10 species per mountain. Grasses such as wavy hair-grass and viviparous sheep’s-fescue, dwarf-shrubs such as bilberry, crowberry (Empetrum nigrum and V. vitis-idaea), and mountain heath, and low-shrubs such as downy willow have expanded their altitudinal limits by 200-300 m since 1930. Our 1998 survey showed that no high-alpine species had gone extinct from the highest summits but some (e.g. glacier buttercup, dwarf buttercup, one-flowered fleabane, alpine mouse-ear, and tufted saxifrage) had decreased in their frequency since 1930, possibly as a result of a decrease in snow-beds. Several high-altitude alpines had, however, disappeared from their outlying localities at lower altitudes. These changes are summarised in the schematic diagram. The results overall suggest that plants are responding, in part at least, to climate change through changes in snow-lie along the altitudinal gradient resulting from changes and interactions between temperature and precipitation. Such changes have allowed tall, competitive species, like the low shrub downy willow to expand upwards.
These floristic changes and others detected on European mountains appear to be a response to the combined effects of changes in precipitation and temperature causing changes in the extent and duration of snow-lie on summit areas. Mountains that have experienced the largest increase in summer precipitation have the lowest proportion of species moving upwards, and many species associated with long-lasting snow-lie are ascending. These observations suggest that climate change is affecting alpine plant altitudinal ranges but that the link between range-shifts and climate change is more complex than a simple response to temperature increase.
Results from these resurveys contrast with predictions about the impact of future climate change on alpine biodiversity from species-climate ‘niche’ models. Model results suggest drastic consequences of climate change for biodiversity with many plants predicted to go extinct in the next century. As my Oxford colleagues Kathy Willis and Shonil Bhagwat have emphasised, caution is required in interpreting such model results because their coarse spatial scale (e.g. 50×50 km grid) fails to capture the small-scale shape of the land and hence microclimate that is such an important characteristic of alpine landscapes.
Why is there little or no evidence for local extinction of high-altitude species, especially at the highest altitudes where climate warming could be expected to have the greatest impact? As Daniel Scherrer and Christian Körner in Basel have shown, it is essential to assess an alpine landscape not at the coarse-scale of species-climate models (50 × 50 km) or even at the 2 m height of standard meteorological stations but at the scale and height of the plants themselves. They have used high-resolution thermal infra-red thermometry and small data-loggers that record temperature every hour over an 80-day period to measure land-surface and soil temperatures in two alpine areas (2200-2800 m) in Switzerland. The thermal imagery reveals an enormous variation is surface temperatures due to the large number of microhabitats within an alpine landscape created by the high local variation of the landscape (ridges, hollows, boulders, streams, flushes, snow-beds, etc.). They found that mean soil-temperature for the plant growing-season had a range of 7.2C, that surface-soil temperature ranged over 10.5C, and that growing-season length had a range of more than 32 days within the summit areas studied. These local-scale inherent variations exceed IPCC warming projections for the next 100 years. A 2C regional warming would lead to a 3% loss of the currently coldest microhabitats, a reduction of 75% of the current commonest thermal conditions due to competition, an increase of 22% of the currently warmest microhabitats, and the creation of 22% new warmer microhabitats. Thus the extent of the coolest microhabitats will decrease. Such habitats will not, most importantly, be lost altogether. Warm microhabitats will become more frequent and new, warmer microhabitats will develop, so microhabitat diversity will increase. The observed increases in alpine summit plant richness seen in many botanical resurveys are a likely response to this increase in microhabitat diversity. Scherrer and Körner’s study warns against uncritically accepting the predictions of alpine-plant responses to climate warming based on a broad-scale modelling approach. They suggest that rugged alpine terrain is, for many plants, a ‘safer’ place to live under conditions of climate change than flat terrain that offers no short distance ‘escapes’ from a new thermal regime.
Local landscape diversity which is so common on alpine summit areas and slopes leads to local climate variation and thus confers biological resilience to change, as it did in the past by providing local micro-refugia for plants and animals to persist locally in regionally unfavourable conditions.
What will happen to alpine plants over the long-term, say the next 500 years? To answer this, we explore the record of alpine plants preserved as fossil pollen, seeds, or leaves, preserved in lake sediments that accumulated over the rapid climate warming at the transition from the late-glacial to the current post-glacial (Holocene) about 11700 years ago. The Greenland ice-core record indicates that air temperature increased 10K in 60 years at the onset of the Holocene. At one of the most-studied late-glacial sites in the world (Kråkenes in western Norway), Hilary Birks has shown that alpines intolerant of warm temperatures and which cannot be grown in gardens today (e.g. tufted saxifrage, arctic poppy, glacier buttercup) went locally extinct in about 50 years after the onset of the Holocene and the associated rapid temperature increase, whereas other, more temperature-tolerant alpines (e.g. alpine bistort, roseroot, dwarf willow) became more abundant and flourished for about 350 years before succumbing to competition from tall grasses, ferns, dwarf-shrubs, shrubs, and eventually birch trees. Some of these alpines survive near Kråkenes today in open exposed habitats such as sea-cliffs, open unstable screes, and wind-exposed ridges and plateau.
Christian Hof and colleagues have recently encouraged global-change ecologists to rethink species’ abilities to withstand rapid climate change. They emphasise that the palaeo-ecological record, as at Kråkenes, shows that species’ abilities to survive major climate change are greater than commonly thought. This is possibly due to inherent population genetic variability or to the fine-scale variation of natural landscapes. However, they strongly emphasise that “the synergistic effects between climate change and the ongoing destruction and fragmentation of natural habitats (leaving aside further anthropogenic pressures for biodiversity) should by no means be underestimated.” Although alpine habitats are often thought of as ‘natural’ or ‘pristine’ habitats, they are being increasingly impacted, to varying intensities depending on location, by many factors, including nitrogen deposition, hydro-electric development, tourism, changes in land-use, introduced invasive species, ski development, and over-exploitation leading to erosion and landscape degradation. Given these impacts and future climate change, the long-term future of alpine plants remains very uncertain.