Response of a Habitat-Forming Marine Plant to a Simulated Warming Event Is Delayed, Genotype Specific, and Varies with Phenology

Growing evidence shows that increasing global temperature causes population declines and latitudinal shifts in geographical distribution for plants living near their thermal limits. Yet, even populations living well within established thermal limits of a species may suffer as the frequency and intensity of warming events increase with climate change. Adaptive response to this stress at the population level depends on the presence of genetic variation in thermal tolerance in the populations in question, yet few data exist to evaluate this. In this study, we examined the immediate effects of a moderate warming event of 4.5°C lasting 5 weeks and the legacy effects after a 5 week recovery on different genotypes of the marine plant Zostera marina (eelgrass). We conducted the experiment in Bodega Bay, CA USA, where average summer water temperatures are 14–15°C, but extended warming periods of 17–18°C occur episodically. Experimental warming increased shoot production by 14% compared to controls held at ambient temperature. However, after returning temperature to ambient levels, we found strongly negative, delayed effects of warming on production: shoot production declined by 27% and total biomass decreased by 50% relative to individuals that had not been warmed. While all genotypes’ production decreased in the recovery phase, genotypes that grew the most rapidly under benign thermal conditions (control) were the most susceptible to the detrimental effects of warming. This suggests a potential tradeoff in relative performance at normal vs. elevated temperatures. Modest short-term increases in water temperature have potentially prolonged negative effects within the species’ thermal envelope, but genetic variation within these populations may allow for population persistence and adaptation. Further, intraspecific variation in phenology can result in maintenance of population diversity and lead to enhanced production in diverse stands given sufficient frequency of warming or other stress events.

越来越多的证据表明，全球升温会导致生存于热极限（thermal limits）附近的植物种群数量下降，以及地理分布的纬度偏移。然而，即便种群完全处于某一物种既定的热耐受极限（established thermal limits）范围内，随着气候变化导致升温事件的频率与强度升高，这些种群仍可能受到损害。种群层面针对该胁迫的适应性响应，取决于目标种群是否存在热耐受相关的遗传变异，但目前鲜有相关数据可用于评估这一点。本研究以海生植物大叶藻（Zostera marina，鳗草）的不同基因型为研究对象，探究了持续5周的4.5℃中度升温事件的即时效应，以及升温结束后5周恢复期的遗留效应。本实验在美国加利福尼亚州博德加湾开展，该区域夏季海水平均温度为14–15℃，但偶尔会出现17–18℃的持续升温时段。与维持在环境温度的对照组相比，实验升温使茎枝产量提升了14%。然而，当温度恢复至环境水平后，我们观测到升温对产量存在显著的负向延迟效应：与未经历升温的个体相比，经升温处理的个体茎枝产量下降了27%，总生物量减少了50%。尽管所有基因型的产量在恢复期均出现下降，但在适宜温度条件（对照组）下生长最快的基因型，对升温的有害效应最为敏感。这表明在正常温度与高温环境下的相对表现间，可能存在权衡关系。海水温度的小幅短期升高，可能会在该物种的热耐受范围（thermal envelope）内产生持久的负面效应，但这些种群内的遗传变异或可帮助种群维持存续并实现适应性演化。此外，若升温或其他胁迫事件发生频率足够高，物候（phenology）的种内变异可维持种群多样性，并提升多样种群群落的产量。