Temperaturresistenz inneralpiner Trockenrasen

Abstract

Heat and frost resistance of all organs of the steppe grasses Stipa capillata and Stipa eriocaulis , of a C 4 grass, Bothriochloa ischaemum , and of Carex humilis from semiarid grassland in a central alpine valley (the Vinschgau) were studied throughout the course of the year. The heat resistance increases from the lowest level during the growing phase in spring to a summer maximum, and remains fairly high also in autumn and winter. During periods of heat, in which the daily maxima of air temperature are between 30 and 35°C, the heat resistance (based on the temperature causing 50 % damage after 30 min) of Stipa spp. and of Carex humilis attains values of 60 (leaves) to 65°C (shoot bases). The C 4 grass Bothriochloa ischaemum has an even higher hardiness and can survive temperatures approaching 70°C. Despite this extraordinary resistance, the organs near to soil surface are protected by an only narrow margin of safety with respect to highest temperatures in the habitat: 60-65°c at soil surface are frequently measured, as well as extreme values of 70°C and above. The leaves heat up to 40-45°C under strong irradiation; when fully hardened, they are unlikely to suffer damage. However, for C 3 grasses, leaf temperatures of this magnitude fall within the range of heat inhibition of photosynthesis. The potential frost resistance , determined after cold conditioning at −5 and −10°C, is sufficient to enable Carex humilis and Stipa species to survive frost of −30 to −35°C without severe injury, whereby Stipa capillata is slightly hardier than Stipa eriocaulis . The shoot apex, the crown and the leaf sheats are the most resistant, the roots the most sensitive parts of the plants (see Table 2). The apical meristem of Carex humilis acquires unlimited frost tolerance and cannot even be killed by immersion in liquid nitrogen. The crown tissues, rhizomes and roots of Bothriochloa ischaemum also attain a remarkable degree of frost resistence (down to −25°C), the pronounced cold sensitivity typical for C 4 grasses being restricted to the permanently freezing-sensitive leaves, so that this species can only develop and retain its leaves during the frost-free period of the year. The low degree of cold stress to which the graminoids in the Vinschgau are exposed (annual minimum air temperature usually not below −10 to −16°C) induces only moderate hardening. Calculation of frost stress response indices (see page 128) reveals that under such mild winter conditions the hardiness capacity of Carex humilis is only exploited to about 1/3, that of Stipa spp: to 1/2, and that of Bothriochloa ischaemum to 2/3. Despite this, the Stipa population growing at the margins of its distribution range has not lost the high resistance potential that is a necessary attribute for surviving the severe winters of the Eurasian continental steppe regions. This means that in the central alpine valleys an ecotype differentiation with respect to frost resistance has apparently not taken place.