Thermal stress thresholds and physiological response in seedlings of Abies hickelii (Flous & Gaussen), an endangered species

Abstract

Abies hickelii is an endemic, endangered species that is highly sensitive to climatic fluctuations. Increasing drought and extreme temperature events may threaten its long-term survival. This study evaluated the thermal sensitivity of A. hickelii seedlings under controlled conditions. Individuals were exposed to four thermal treatments (control, 28 °C, 32 °C, and 36 °C), and both the time required to reach maximum thermal stress and the rate of foliar damage were assessed. Temperature explained more than 60% of the variability in the time to maximum stress. At 36 °C, seedlings exhibited the highest stress levels and a five-fold increase in damage rate compared to the control, indicating that a critical thermal threshold had been surpassed. Additionally, the progression of damage slowed after exceeding 90% foliar loss, suggesting the presence of a physiological inflection point. Although no significant differences were found among the control, 28 °C, and 32 °C treatments, smaller seedlings showed signs of greater
resistance. Overall, these results highlight the species’ high vulnerability to extreme heat events and provide essential information for guiding conservation and reforestation strategies under climate-change scenarios.