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Herbaceous plant species have been the focus of extensive, long-term research into climate change responses, but there has been little effort to synthesize results and predicted outlooks from different model species. We summarize research on climate change responses for eight intensively-studied herbaceous plant species. We establish generalities across species, examine limitations, interrogate biases, and propose a path forward. All six forb species exhibit reduced fitness, maladaptation, and/or population declines in at least part of the range. Plasticity alone is likely not sufficient to allow adjustment to shifting climates. Most model species also have spatially-restricted dispersal that may limit genetic and evolutionary rescue. These results are surprising, given that these species are widespread, span large elevation ranges, and generally have substantial levels of genetic and phenotypic variation. The focal species have diverse life histories, reproductive strategies, and habitats, but most are native to North America. Thus, these species may poorly represent rare species, habitat specialists, or species endemic to other parts of the world. We encourage researchers to design demographic and field experiments that evaluate plant traits and fitness in contemporary and potential future conditions across the full life cycle, and that consider the effects of climate change on biotic interactions. 

Abstract PremiseSeed germination involves risk; post‐germination conditions might not allow survival and reproduction. Variable, stressful environments favor seeds with germination that avoids risk (e.g., germination in conditions predicting success), spreads risk (e.g., dormancy), or escapes risk (e.g., rapid germination). Germination studies often investigate trait correlations with climate features linked to variation in post‐germination reproductive success. Rarely are long‐term records of population reproductive success available. MethodsSupported by demographic and climate monitoring, we analyzed germination in the California winter‐annualClarkia xantianasubsp.xantiana. Sowing seeds of 10 populations across controlled levels of water potential and temperature, we estimated temperature‐specific base water potential for 20% germination, germination time weighted by water potential above base (hydrotime), and a dormancy index (frequency of viable, ungerminated seeds). Mixed‐effects models analyzed responses to (1) temperature, (2) discrete variation in reproductive success (presence or absence of years with zero seed production by a population), and (3) climate covariates, mean winter precipitation and coefficient of variation (CV) of spring precipitation. For six populations, records enabled analysis with a continuous metric of variable reproduction, the CV of per‐capita reproductive success. ResultsPopulations with more variable reproductive success had higher base water potential and dormancy. Higher base water potential and faster germination occurred at warmer experimental temperatures and in seeds of populations with wetter winters. ConclusionsGeographic variation in seed germination in this species suggests local adaptation to demographic risk and rainfall. High base water potential and dormancy may concentrate germination in years likely to allow reproduction, while spreading risk among years. 

Introgression is pervasive across the tree of life but varies across taxa, geography, and genomic regions. However, the factors modulating this variation and how they may be affected by global change are not well understood. Here, we used 200 genomes and a 15-y site-specific environmental dataset to investigate the effects of environmental variation and mating system divergence on the magnitude of introgression between a recently diverged outcrosser-selfer pair of annual plants in the genusClarkia. These sister taxa diverged very recently and subsequently came into secondary sympatry where they form replicated contact zones. Consistent with observations of other outcrosser-selfer pairs, we found that introgression was asymmetric between taxa, with substantially more introgression from the selfer to the outcrosser. This asymmetry was caused by a bias in the direction of initial F1 hybrid formation and subsequent backcrossing. We also found extensive variation in the outcrosser’s admixture proportion among contact zones, which was predicted nearly entirely by interannual variance in spring precipitation. Greater fluctuations in spring precipitation resulted in higher admixture proportions, likely mediated by the effects of spring precipitation on the expression of traits that determine premating reproductive isolation. Climate-driven hybridization dynamics may be particularly affected by global change, potentially reshaping species boundaries and adaptation to novel environments. 

Bet hedging consists of life history strategies that buffer against environmental variability by trading off immediate and long-term fitness. Delayed germination in annual plants is a classic example of bet hedging and is often invoked to explain low germination fractions. We examined whether bet hedging explains low and variable germination fractions among 20 populations of the winter annual plant Clarkia xantiana ssp. xantiana that experience substantial variation in reproductive success among years. Leveraging 15 years of demographic monitoring and 3 years of field germination experiments, we assessed the fitness consequences of seed banks and compared optimal germination fractions from a density-independent bet-hedging model to observed germination fractions. We did not find consistent evidence of bet hedging or the expected trade-off between arithmetic and geometric mean fitness, although delayed germination increased long-term fitness in 7 of 20 populations. Optimal germination fractions were two to five times higher than observed germination fractions, and among-population variation in germination fractions was not correlated with risks across the life cycle. Our comprehensive test suggests that bet hedging is not sufficient to explain the observed germination patterns. Understanding variation in germination strategies will likely require integrating bet hedging with complementary forces shaping the evolution of delayed germination. 

Abstract Premise Pollen movement is a crucial component of dispersal in seed plants. Although pollen dispersal is well studied, methodological constraints have made it challenging to directly track pollen flow within multiple populations across landscapes. We labeled pollen with quantum dots, a new technique that overcomes past limitations, to evaluate the spatial scale of pollen dispersal and its relationship with conspecific density within 11 populations of Clarkia xantiana subsp. xantiana , a bee‐pollinated annual plant. Methods We used experimental arrays in two years to track pollen movement across distances of 5–35 m within nine populations and across distances of 10–70 m within two additional populations. We tested for distance decay of pollen dispersal, whether conspecific density modulated dispersal distance, and whether dispersal kernels varied among populations across an environmentally complex landscape. Results Labeled pollen receipt did not decline with distance over 35 m within eight of nine populations or over 70 m within either of two populations. Pollen receipt increased with conspecific density. Overall, dispersal kernels were consistent across populations. Conclusions The surprising uniformity in dispersal distance within different populations was likely influenced by low precipitation and plant density in our study years. This suggests that spatiotemporal variation in the abiotic environment substantially influences the extent of gene flow within and among populations. 

Populations often vary in their evolutionary responses to a shared environmental perturbation. A key hurdle in building more predictive models of rapid evolution is understanding this variation—why do some populations and traits evolve while others do not? We combined long-term demographic and environmental data, estimates of quantitative genetic variance components, a resurrection experiment and individual-based evolutionary simulations to gain mechanistic insights into contrasting evolutionary responses to a severe multi-year drought. We examined five traits in two populations of a native California plant, Clarkia xantiana , at three time points over 7 years. Earlier flowering phenology evolved in only one of the two populations, though both populations experienced similar drought severity and demographic declines and were estimated to have considerable additive genetic variance for flowering phenology. Pairing demographic and experimental data with evolutionary simulations suggested that while seed banks in both populations likely constrained evolutionary responses, a stronger seed bank in the non-evolving population resulted in evolutionary stasis. Gene flow through time via germ banks may be an important, underappreciated control on rapid evolution in response to extreme environmental perturbations. 

Abstract Much theory has focused on how a population’s selfing rate affects the ability of natural selection to remove deleterious mutations from a population. However, most such theory has focused on mutations of a given dominance and fitness effect in isolation. It remains unclear how selfing affects the purging of deleterious mutations in a genome-wide context where mutations with different selection and dominance coefficients co-segregate. Here, we use individual-based forward simulations and analytical models to investigate how mutation, selection and recombination interact with selfing rate to shape genome-wide patterns of mutation accumulation and fitness. In addition to recovering previously described results for how selfing affects the efficacy of selection against mutations of a given dominance class, we find that the interaction of purifying selection against mutations of different dominance classes changes with selfing and recombination rates. In particular, when recombination is low and recessive deleterious mutations are common, outcrossing populations transition from purifying selection to pseudo-overdominance, dramatically reducing the efficacy of selection. At these parameter combinations, the efficacy of selection remains low until populations hit a threshold selfing rate, above which it increases. In contrast, selection is more effective in outcrossing than (partial) selfing populations when recombination rates are moderate to high and recessive deleterious mutations are rare. 

Spatial patterns of adaptation provide important insights into agents of selection and expected responses of populations to climate change. Robust inference into the spatial scale of adaptation can be gained through reciprocal transplant experiments that combine multiple source populations and common gardens. Here, we examine the spatial scale of local adaptation of the North American annual plant common ragweed, Ambrosia artemisiifolia, using data from four common gardens with 22 source populations sampled from across a ∼1200 km latitudinal gradient within the native range. We found evidence of local adaptation at the northernmost common garden, but maladaptation at the two southern gardens, where more southern source populations outperformed local populations. Overall, the spatial scale of adaptation was large—at the three gardens where distance between source populations and gardens explained variation in fitness, it took an average of 820 km for fitness to decline to 50% of its predicted maximum. Taken together, these results suggest that climate change has already caused maladaptation, especially across the southern portion of the range, and may result in northward range contraction over time.