Summer temperatures reach the thermal tolerance threshold of photosynthetic decline in temperate conifers

Abstract

Abstract Climate change‐related environmental stress has been recognized as a driving force in accelerating forest mortality over the last decades in Central Europe. Here, we aim to elucidate the thermal sensitivity of three native conifer species, namely Norway spruce ( Picea abies ), Scots pine ( Pinus sylvestris ) and silver fir ( Abies alba ), and three non‐native species, namely Austrian pine ( Pinus nigra ), Douglas fir ( Pseudotsuga menziesii ) and Atlas cedar ( Cedrus atlantica ). Thermal sensitivity, defined here as a decline of the maximum quantum yield of photosystem II (F v /F m ) with increasing temperature, was measured under varying levels of heat stress and compared with the turgor loss point (π tlp ) as a drought resistance trait. We calculated three different leaf thermotolerance traits: the temperature at the onset (5%) of the F v /F m decline (T5), the temperature at which F v /F m was half the maximum value (T50) and the temperature at which only 5% F v /F m remained (T95). T5 ranged from 38.5 ± 0.8 °C to 43.1 ± 0.6 °C across all species, while T50 values were at least 9 to 11 degrees above the maximum air temperatures on record for all species. Only Austrian pine had a notably higher T5 value than recorded maximum air temperatures. Species with higher T5 values were characterized by a less negative π tlp compared to species with lower T5. The six species could be divided into ‘drought‐tolerant heat‐sensitive’ and ‘drought‐sensitive heat‐tolerant’ groups. Exposure to short‐term high temperatures thus exhibits a considerable threat to conifer species in Central European forest production systems.