Heat tolerance and canopy temperatures of Larix sibirica under highly continental climate in Mongolia's boreal forest

Compared to drought stress, direct heat damage has been considered secondary as a cause of climate change-induced tree mortality and productivity declines in forests. However, evidence from temperate, subtropical and tropical forests is accumulating that direct heat damage in photosystem II (PS II) is also a realistic scenario under climate change. We analyzed PS II heat tolerance in Larix sibirica, which represents a dominant boreal tree species in Siberia and northern Central Asia in cold environments with subzero or near-zero mean annual temperatures, but nevertheless warm summers. Thermal imaging was applied to relate heat thresholds found in the laboratory to canopy temperatures. Measurements were repeated in three months during the short growing season to test for the occurrence of heat acclimation. After 4 h of heat exposure, the temperature of the most rapid decline of the maximum fluorescence yield (Fv/Fm) due to heat (TIP), which is thought to indicate irreversible damage, was 41.2{+/-}0.0{degrees}C. The critical temperature (Tcrit) indicating initial reversible heat stress in PS II was 39.8{+/-}0.1{degrees}C. L. sibirica had large thermal safety margins, when TIP and Tcrit were compared to the canopy temperatures measured during the study period, but not compared to record heat maxima since 2000. The trees acclimated to heat over summer by increasing TIP and thus increasing the tolerance to severe heat stress. However, tolerance to moderate heat (indicated by Tcrit) was simultaneously weakened, suggesting the reallocation of resources during acclimation. In most summers, L. sibirica forests are not threatened by direct heat damage, but heat extremes as previously recorded from our study region in Mongolias boreal forest could increase mortality due to PS II damage. Heat can thus be seen as one out of several stressors that reduces the vitality and growth of southern boreal forests with L. sibirica under climate change.