Alpine plants as functional climat indicators

Changes in temperature climate at a peculiar growing site can be detected by the measurement of heat tolerance in alpine plant species. Considering the prognosticated temperature increase these plants could be employed as bio- indicators of climate change. The development of a field portable measurement system to determine heat tolerance of plants allowed to measure the temporal dynamic of heat tolerance in alpine plants at various altitudinally different growing sites. By means of computer controlled Infrared lamps long term heating experiments were conducted, where whole plant canopies were warmed in reference to an untreated canopy. In accordance to the prognosticated temperature increase until the middle of this century of +1.5 - 4.5 K, canopy temperature was increased by +3 K. All species showed a high diurnal variability of heat tolerance of the leaf tissue of 4.8 up to 9.5 K that exceeds earlier reports (max. 0.1-5 K). Diurnal heat tolerance changes higher than +/- 1.5 K occurred on 18 % of summer days and at a high velocity of 0.4-2.2 K/h. The diurnal heat tolerance amplitude exceeded even the seasonal amplitude of higher plant species of 5 - 8 K reported earlier. Our results may thus draw a new picture of the dynamic of heat tolerance in plants. The observed heat damage to leaves of the alpine cushion plant Minuartia recurva at 2600 m a.s.l. gives evidence that the fast diurnal changes in heat tolerance may be an ecologically significant mechanism of heat survival in the alpine environment. Heat tolerance of photosystem II (PS II), the most heat susceptible part of photosynthesis, showed a high variability in all species of up to 9.6 K. Diurnal changes, increases or even decreases, in PS II thermotolerance occurred frequently with a maximum increase of +3.7 K in Loiseleuria procumbens. High temperature thresholds for photoinhibition were significantly different between species and increased by 9 K from the species in the coldest microhabitat to the species in the warmest. Temperature conditions are responsible for heat hardening. Heat tolerance increased in all species under warmer micro - site conditions, warmer investigation years and under long term artificial heating (+3 K). Thus, the heat load of a peculiar growing site can be determined by the measurement of heat tolerance in plants. Nethertheless, diurnal short term changes in heat tolerance could not solely be explained by changes in temperature. Under not heat hardening effective temperature conditions irradiation intensity and water stress appeared to be other possible factors affecting heat tolerance. Single measurement may thus not suffice to yield a significant evidence of the temperature climate at a peculiar growing site - rather long-term monitoring is necessary.