Evaluating the vulnerability of critical early life stages in plants during heat extremes.

Plants, their seeds and their gametes show remarkable resilience and responsiveness to environmental conditions. However, worsening climate change with more severe and frequent extreme climatic events, like heatwaves and hot droughts, will likely push many species towards or beyond their physiological limits. If such events occur during important points of development and reproduction-rather than mature vegetative growth-the direct impact on individual fitness can be high, with potential to bottleneck recruitment in populations. Here we take an ecophysiological perspective to discuss what is known of the effects of extreme heat on four critical early life stage transitions in wild plant development that affect fitness and recruitment. These life stages are pollen development, pollen germination to seed set, dormant-to-imbibed seeds and seed-to-seedling transition. We use the recently developed thermal load sensitivity framework to showcase how these critical points of ontogeny could be exposed to vastly different microclimate conditions and have different physiological heat tolerance. Assessing sensitivity of these life stage transitions to increasing thermal load with the additional stressors of limited soil moisture and drying atmosphere could be an effective approach to identify at-risk populations or species. We argue that vulnerable developmental stages and narrow reproductive windows that affect recruitment must be considered for effective conservation and restoration of plant populations under climate change.