Regeneration After Severe Wildfire: Hotter Microclimates Exceed Hydraulic and Thermal Tolerances in Sierra Nevada Conifer Seedlings

ABSTRACT Tree seedling regeneration after wildfire will determine the long‐term trajectory of forests under climate change, yet increasing drought and heat extremes can suppress regeneration and result in forest type conversion. We combined field observations and a controlled drought‐heat growth chamber experiment to investigate how these stressors independently and jointly affect drought‐ and heat‐tolerance physiology in seedlings of montane conifer species from drought‐ and wildfire‐prone regions in the southern Sierra Nevada. In both the field and growth chamber experiment, we found that exposure to elevated leaf temperatures in post fire‐like environments dwarfed the effect of physiological adjustments in heat tolerance (temperatures causing initial impairment (T crit ) or 50% loss (T 50 ) of photosystem II capacity) and drought tolerance (turgor loss point). In the field, this resulted in narrower thermal safety margins across five conifer species in burn scars with no remaining canopy, while in the growth chamber experiment three species showed narrower or even negative thermal and hydraulic safety margins. In the growth chamber experiment, leaf temperatures were also frequently decoupled from air temperature, highlighting that biophysical feedbacks between leaf temperature and transpiration may amplify seedling water loss even under stomatal closure. For both the field and growth chamber experiment, we found no evidence of a trade‐off between drought and heat tolerance within or among species. Among species, seedlings of the range‐restricted Sequioadendron giganteum (giant sequoia) were observed to be the most vulnerable to combined drought and heat stress in the growth chamber experiment. Our results highlight that direct thermal stress may strongly constrain post‐fire conifer regeneration and underscore the need to integrate seedling energy balance and microclimate into predictions of post‐fire recovery and management strategies, especially for vulnerable species such as giant sequoia.