Heat stress in alpine plants

It may sound paradoxical to discuss extreme heat effects in the alpine environment. However, the combined effect of high solar radiation, slope inclination and exposure plus poor plant cover on surface-dry substrate may indeed permit soil surface temperatures of > 80 °C - high enough to kill most organisms on earth. Hence, for short periods, heat can become a decisive survival criterion during plant recruitment on bare ground in alpine regions.

1 - Loiseleuria procumbens

Loiseleuria procumbens can grow in very exposed places by forming dense carpets of only 1-2 cm thickness. Complete ground cover and intense transpiration prevents tissue temperatures from rising above 35 °C. Mechanical damage (e.g. by skiing or trampling) can open stands and put them at great danger. Re-colonisation into damaged areas is slow and by clonal propagation.

1 - Broken stems caused by trampling have lost water supply, hence cannot cool by transpiration, get hot and die.

2 - Alpine desert

The mechanical damage to green ground cover removes evaporative cooling and exposes dark brown humus to full solar radiation. Once such surfaces are dry, heat can reach 80 °C, making seedling establishment impossible. It takes a long time to heal such wounds by clonal encroachment, provided erosion is not faster.

2 - Bare ground such as on this ski slope can heat to 80 °C under strong insolation.

3 - Climatic hotspots in the Alpine

Thin layers of substrate on rocks dry out rapidly and heat may reach the critical 52-56 °C range even in specialist plants. Such microhabitats are commonly occupied by mosses, lichens and algae (which are desiccation tolerant), succulents (which can store water) and a few specialists with either thin, vertical leaves (which are aerodynamically well coupled to the atmosphere) or leaves of high reflectance, which prevent overheating. All these life forms are shown on this photograph with the succulent Sedum alpestre in the centre.

3 - Specialists in dry, hot microhabitats: desiccating mosses, lichens and succulents (Sedum alpestre).

Dangerous situations in the Alpine are

• bare ground on equator facing slopes
• isolated seedlings in vegetation gaps
• rock communities

Alpine plants respond to this by

• rapid establishment of seedlings at the beginning of the season
• facilitation: seedlings establishing in the shelter of other plants
• clonal propagation, growth into bare areas by phalanx mode
• high rates of transpiration (evaporative cooling)
• total stand-still of life activity (in cryptogams; associated with desiccation)