Seasonal variation in tolerance to short-term heat and freezing extremes across land-use-driven successional stages in Calluna vulgaris

Climate change is not only increasing mean temperatures but also the frequency and intensity of climatic extremes, including heat waves and freezing events. Such extremes directly affect plant physiological performance and increase stress and mortality risk in terrestrial ecosystems. Combined with land-use change such as shifts in agricultural management practices, increasing climate variability poses a growing threat to the semi-natural coastal heathlands of Europe, where the dwarf shrub Calluna vulgaris functions as a keystone species, influencing ecosystem structure and resilience.

Responses of Calluna to environmental stressors are known to vary across successional stages, which are determined by fire regimes which promote grazing, yet empirical data on how short-term temperature extremes affect physiological tolerance across life stages remain scarce. In addition, most studies address heat or cold tolerance in isolation, limiting our understanding of plant responses to the full range of thermal stress encountered throughout the seasons and under increasingly variable climatic conditions.

To address this knowledge gap, this experimental study investigates seasonal and ontogenic variation in leaf-level thermal tolerance limits of Calluna in the red-listed Norwegian coastal heathlands. The work is a contribution to an ongoing multi-season research effort at the semi-managed heathlands on Lygra in western Norway and includes four post-fire successional stages (pioneer, building, mature, and degenerative). Here, we focus on data collected so far during two key seasonal phases (autumn and winter), capturing contrasting physiological states relevant to thermal acclimation. Across these seasons, Calluna individuals are sampled from each successional stage and exposed to controlled short-term heat and freezing treatments designed to simulate extreme temperature events.

Thermal tolerance is quantified at leaf level using chlorophyll fluorescence to determine the temperature at which photosynthetic efficiency declines by 50% (T₅₀). Heat tolerance is assessed using water bath exposures across a temperature range of 20–56 °C, while freezing tolerance is measured using controlled freezing treatments down to −20 °C. In parallel, leaf functional traits are measured to examine links between seasonal shifts in key traits such as leaf area, thickness, and mass with the physiological temperature limits.

By identifying when and which successional stages are most vulnerable to thermal extremes, this work will improve our understanding of shrub-dominated ecosystem sensitivity and inform predictions of heathland resilience under an increasingly variable climate and increasing land abandonment.