Acclimation of thermal tolerance in juvenile plants from three biomes is supressed when extremes co-occur

Given the rising frequency of thermal extremes (heatwaves and cold snaps) due to climate change, comprehending how a plant's origin affects its thermal tolerance breadth becomes vital. We studied juvenile plants from three biomes: temperate coastal rainforest, desert, and alpine. In controlled settings, plants underwent hot days and cold nights in a factorial design to examine thermal tolerance acclimation. We assessed thermal thresholds (Tcrit-hot and Tcrit-cold) and thermal tolerance breadth (TTB). We hypothesised that: 1) desert species would show the highest heat tolerance, alpine the greatest cold tolerance, with temperate species intermediate; 2) all species would increase heat tolerance post hot days and cold tolerance after cold nights; 3) combined exposure would broaden TTB more than individual conditions, especially in desert and alpine species. We found that biome responses were minor compared to the responses to the extreme temperature treatments. All plants increased thermal tolerance in response to hot 40°C days (Tcrit-hot increased by ~3.5°C) but there was minimal change in Tcrit-cold in response to the cold -2°C nights. In contrast, when exposed to both hot days and cold nights, on average plants exhibited an antagonistic response in TTB, where cold tolerance decreased and heat tolerance was reduced, and so we did not see the bi-directional expansion we hypothesised. There was, however, considerable variation among species in these responses. As climate change intensifies, plant communities, especially in transitional seasons, will regularly face such temperature swings. Our results shed light on potential plant responses under these extremes, emphasizing the need for deeper species-specific thermal acclimation insights, ultimately guiding conservation efforts.