Response to heat stress of populations of two Sphagnum species from alpine bogs at different altitudes

Sphagnum mosses are a fundamental component of bog vegetation in northern regions, where these plants play a major role in controlling important ecosystem processes. As heat waves are expected to become increasingly intense and frequent, especially in cold territories, it is important to improve our knowledge of heat resistance in Sphagnum species. We investigated the response to heat stress of S. fuscum and S. magellanicum. Three populations of the two species collected at different altitudes (1090 m, 1870 m and 2100 m) were grown at three daytime temperature levels: 25 °C (AT); 36 °C (MT); 43 °C (HT). The HT treatment decreased concentrations of chlorophyll and nitrogen in the plant tissues, which resulted in lower net CO₂ exchange rates and quantum yield of PS_II. The plants recovered significantly within six days, probably because temperature in the living tissue did not reach lethal thresholds because of the high water content in the plant tissues. Contrary to our main hypothesis, that S. magellanicum had greater resistance to high temperatures because of its more southern distribution, the two species showed much the same response patterns to heat stress. Supporting our second hypothesis, populations of both species originating from the highest site suffered somewhat stronger, although still reversible, damage when grown at HT. Heat stress brought about by heat waves will unlikely have differential effects on these two Sphagnum species. We also conclude that heat waves are unlikely to exert irreversible damage to the Sphagnum layer in bog ecosystems if high temperatures are not coupled with drought.