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.

Environmental change is multidimensional, with local anthropogenic stressors and global climate change interacting to differentially impact populations throughout a species’ geographic range. Within species, the spatial distribution of phenotypic variation and its causes (i.e., local adaptation or plasticity) will determine species’ adaptive capacity to respond to a changing environment. However, comparatively less is known about the spatial scale of adaptive differentiation among populations and how patterns of local adaptation might drive vulnerability to global change stressors. To test whether fine‐scale (2–12 km) mosaics of environmental stress can cause adaptive differentiation in a marine foundation species, eelgrass (Zostera marina), we conducted a three‐way reciprocal transplant experiment spanning the length of Tomales Bay, CA. Our results revealed strong home‐site advantage in growth and survival for all three populations. In subsequent common garden experiments and feeding assays, we showed that countergradients in temperature, light availability, and grazing pressure from an introduced herbivore contribute to differential performance among populations consistent with local adaptation. Our findings highlight how local‐scale mosaics in environmental stressors can increase phenotypic variation among neighboring populations, potentially increasing species resilience to future global change. More specifically, we identified a range‐center eelgrass population that is pre‐adapted to extremely warm temperatures similar to those experienced by low‐latitude range‐edge populations of eelgrass, demonstrating how reservoirs of heat‐tolerant phenotypes may already exist throughout a species range. Future work on predicting species resilience to global change should incorporate potential buffering effects of local‐scale population differentiation and promote a phenotypic management approach to species conservation.

Mortality and shifts in species distributions are among the most obvious consequences of extreme climatic events. However, the sublethal effects of an extreme event can have persistent impacts throughout an individual's lifetime and into future generations via withingeneration and transgenerational phenotypic plasticity. These changes can either confer resilience or increase susceptibility to subsequent stressful events, with impacts on population, community, and potentially ecosystem processes. Here, we show how a simulated extreme warming event causes persistent changes in the morphology and growth of a foundation species (eelgrass, Zostera marina) across multiple clonal generations and multiple years. The effect of previous parental exposure to warming increased aboveground biomass, shoot length, and aboveground-belowground biomass ratios while also greatly decreasing leaf growth rates. Long-term increases in aboveground-belowground biomass ratios could indicate an adaptive clonal transgenerational response to warmer climates that reduces the burden of increased respiration in belowground biomass. These transgenerational responses were likely decoupled from clonal parent provisioning as rhizome size of clonal offspring was standardized at planting and rhizome starch reserves were not impacted by warming treatments. Future investigations into potential epigenetic mechanisms underpinning such clonal transgenerational plasticity will be necessary to understand the resilience of asexual foundation species to repeated extreme climatic events.