Effects of warming temperatures on germination responses and trade‐offs between seed traits in an alpine plant

Abstract

Abstract Climate warming may affect multiple aspects of plant life history, including important factors such as germination responses and the key trade‐off between offspring size and number. As a case study to address these concepts, we used an alpine plant (waxy bluebell, Wahlenbergia ceracea; Campanulaceae) that shows plasticity to warming in seed traits and in which seed dormancy status regulates germination. We chose an alpine species because alpine environments are ecosystems particularly under threat by climate change. We conducted germination assays under cool and warm temperatures using seeds produced by individuals that were grown under historical (cooler) and future (warmer) temperature scenarios. We assessed the presence of a seed size versus number trade‐off, and then examined the effects of seed number and size on germination percentage, the fractions of dormant and viable seeds, and germination velocity. Further, we examined whether warming during parental growth and during germination affected these relationships. We found evidence for a seed size versus number trade‐off only under historical parental temperatures. Indeed, under future growth temperatures parental plants produced fewer and smaller seeds and there was no evidence of a trade‐off. However, the reductions in both seed traits under warming did not affect germination, despite correlations of seed size and number with germination traits. Warming increased germination, particularly of larger seeds, but overall, it resulted in more than fourfold reductions in parental fitness. Synthesis. Our study shows the importance of growth conditions when evaluating the seed size versus number trade‐off. Stressful conditions, such as warmer temperatures, can restrain the ability of plants to reach optimal investment in reproduction, masking the trade‐off. By analysing responses across the whole life cycle, we show here an overall detrimental effect of warming, highlighting the potential risk of climate change for Wahlenbergia ceracea, and, potentially, for alpine plant communities more widely.