Non-equilibrium in Alpine Plant Assemblages: Shifts in Europe’s Summit Floras

Abstract

Climate warming has been more pronounced in Arctic and alpine areas, and changes in the mountain flora can be expected as the temperature envelope moves upslope. On the one hand, alpine habitats will shrink due to upward migration of species from lower areas, such as trees and tall plants. On the other hand, extinctions of summit plants may be slowed down considerably by the high diversity of microhabitats, the longevity of alpine plants and positive plant-plant interactions in extreme environments. This review chapter attempts to document and monitor vegetation changes on mountain summits. Vegetation surveys that repeat century-old historical vegetation records show considerable upward migration and subsequent increases in species on summits. This trend apparently has accelerated in recent decades. Detailed monitoring of the last decade in European mountain ranges, however, shows that this vegetation change may be at the cost of rare endemic species and alpine specialists in drier Mediterranean regions. This chapter furthermore reviews other factors than temperature influencing alpine vegetation, namely precipitation and snow, nutrients, atmospheric CO 2 concentrations and land use. A subsequent question is how threatened mountain flora is by the ongoing environmental changes. Finally, this chapter discusses options for conservation and land use in high-alpine areas.

Increases in thermophilus plants in an arid alpine community in response to experimental warming

“…With these spatially-decoupled dueling limitations, we predict that thermophilization in this system may be driven by mid-elevation vegetation invading both the upper and lower alpine ecotones communities, homogenizing the overall alpine community in response to changing climate (Tingley et al 2012;Suding et al 2015). This potential pattern would contrast with thermophilization being driven solely by lower-elevation communities moving uphill, as observed in other alpine systems (Rixen and Wipf 2017;Rumpf et al 2017). This combined effect of temperature and soil limitation may be exacerbated by switches from snow-dominated to rain-dominated systems in xeric, high-elevation systems, which leads to increasing climate-change exposure at the highest elevation (McCullough et al 2016).…”

“…This lack of response of cool-adapted plant species has been termed extinction debt, and is hypothesized to be driven by lags in both population and community dynamics Hylander and Ehrlén 2013). Loss of cool-adapted alpine endemics through species interactions rather than by direct effects is predicted to be a slower process because of lags in dispersal, establishment, and local extinction of alpine species (Rixen and Wipf 2017;Alexander et al 2018). These lags could lead to disequilibrium dynamics where species are currently distributed in areas with unsuitable climate, but are predicted to be extirpated in the future (Körner 2003;Svenning and Sandel 2013).…”

“…Mountain systems provide important early indicators of plant community responses to changing climate (Körner 2003;Grabherr, Gottfried, and Pauli 2010;Rixen and Wipf 2017;Steinbauer et al 2018); climate warming is more pronounced at higher elevations (Pepin et al 2015), and exposure to climate change is exacerbated in xeric mountain ranges because of shifts from snow-dominated to rain-dominated precipitation (McCullough et al 2016). Alpine species, often cool-adapted and long-lived, are particularly sensitive to temperature change mediated either directly via their physiology and demography or indirectly via competitive exclusion by faster growing species that recruit from lower elevations (Cranston et al 2015;Rumpf et al 2017;Graae et al 2018).…”

The High Mountain Conservation in a Changing World

“…Therefore, we can expect not only the displacement of the vegetation in the mountains, but also variations in the relative thickness of the belts, or even the number of belts. At present, except for the top mountain (Rixen and Wipf 2017), vegetation response to climate change may be still obscured for concurrent land use shifts. The response of invertebrates may be less ambiguous at the current stages of mountain warming (Wilson et al 2005).…”

“…This applies in particular when the elevation is in a dry region so that large differences in water availability are added to the altitudinal thermal gradient. In areas without water shortage, warming will produce an uplift of the vegetation belts with the risk of losing the high-mountain character to some extent (Rixen and Wipf 2017). An extreme situation will be the collapse of the alpine flora in a few submits and its impoverishment in many high-mountain temperate areas.…”