Warm, dry environments create living conditions that challenge plant growth, reproduction and survival. Plants in these environments have evolved adaptive strategies to enhance water-use efficiency and ensure reproductive success, two of which are annuality and C₄ photosynthesis. However, life history variation is rarely included in large-scale studies of plant diversity, and the extent to which these traits coevolve and are jointly selected for is not known. To address this, we used Pagel’s models of independent and correlated evolution for over 4000 species of grasses (Poaceae), while accounting for evolutionary rate heterogeneity and potential type I statistical errors. We found that there are more C₄ than C₃ annuals and that C₄ origins predate evolution of annuality, but no support for correlated evolution between the two traits. Our results indicate that any habitat or trait similarities (e.g. small seeds, fast growth) between annuals and C₄ species reflect independent adaptations to similar environmental conditions or are contingent on the two traits themselves, rather than the result of evolutionary or functional links between them. Our results further highlight the importance of appropriate null model specification for testing evolutionary hypotheses across large, old clades.

What determines where species occur and where they do not? Brightly et al. showed that cold-origin lineages are less likely to form amphitropical distributions. This raises questions about the biogeographical costs and opportunities of adapting to a particular thermal environment, with implications for species persistence or extinction under climate change. What determines where species occur and where they do not? Brightly et al. showed that cold-origin lineages are less likely to form amphitropical distributions. This raises questions about the biogeographical costs and opportunities of adapting to a particular thermal environment, with implications for species persistence or extinction under climate change.

Evidence for the ongoing biodiversity crisis rests on assessment of a small fraction of described species, with major knowledge gaps for most organisms, including plants. Here, we highlight how digitised herbarium specimens can be used to accelerate and improve estimates of recent and ongoing plant extinctions. We focus on species already considered extinct because they represent a special category for understanding biodiversity loss and a special scientific challenge, as their ‘detection’ relies on proving absence. We propose that this challenge is embodied by a neglected biodiversity shortfall, the Katuš shortfall, encompassing all facets of unquantified levels of past, present and future biodiversity loss. To address this shortfall, we review how methods for estimating the probability of species extinction can be scaled up to harness the massive amounts of digitised data being produced across the ‘global metaherbarium’ using artificial intelligence. Thus, we suggest that the Katuš shortfall can be diminished by shifting focus from proving absence to a probabilistic framework. This can contribute to increasing the accuracy of lists of extinct plants and reveal the true extent of the biodiversity crisis.

Grasses (Poaceae) comprise c. 11 800 species and are central to human livelihoods and terrestrial ecosystems. Knowing their relationships and evolutionary history is key to comparative research and crop breeding. Advances in genome-scale sequencing allow for increased breadth and depth of phylogenomic analyses, making it possible to infer a new reference species tree of the family. We inferred a comprehensive species tree of grasses by combining new and published sequences for 331 nuclear genes from genome, transcriptome, target enrichment and shotgun data. Our 1153-tip tree covers 79% of grass genera (including 21 genera sequenced for the first time) and all but two small tribes. We compared it to a newly inferred 910-tip plastome tree. We recovered most of the tribes and subfamilies previously established, despite pervasive incongruence among nuclear gene trees. The early diversification of the PACMAD clade could represent a hard polytomy. Gene tree-species tree reconciliation suggests that reticulation events occurred repeatedly. Nuclear-plastome incongruence is rare, with very few cases of supported conflict. We provide a robust framework for the grass tree of life to support research on grass evolution, including modes of reticulation, and genetic diversity for sustainable agriculture.

Accumulating evidence is suggesting more frequent tropical-to-temperate transitions than previously thought. This raises the possibility that biome transitions could be facilitated by precursor traits. A wealth of ecological, genetic, and physiological evidence suggests overlap between drought and frost stress responses, but the origin of this overlap, i.e., the evolution of these responses relative to each other, is poorly known. Here, we test whether adaptation to frost and/or severe winters in grasses (Poaceae) was facilitated by ancestral adaptation to drought. We used occurrence patterns across Köppen-Geiger climate zones to classify species as drought, frost, and/or winter tolerant, followed by comparative analyses. Ancestral state reconstructions revealed different evolutionary trajectories in different clades, suggesting both drought-first and frost-first scenarios. Explicit simultaneous modelling of drought and frost/winter tolerance provided some support for correlated evolution, but suggested higher rates of gain of frost/winter tolerance in drought-sensitive rather than drought-tolerant lineages. Overall, there is limited support across grasses as a whole that drought tolerance acted as an evolutionary precursor to frost or severe winter tolerance. Different scenarios in different clades is consistent with present-day grasses being either cold or drought specialists, possibly as a consequence of trade-offs between different stress tolerance responses.

Background  All plants are influenced by the temperatures they are exposed to and fascinating adaptations to extreme temperatures have been described for many of them. However, the extent to which adaptation to thermal extremes is associated with costs, in terms of reduced performance at less or other stressful temperatures, is poorly known, especially for plants. In Iceland, there are two lineages of Agrostis stolonifera, one that occurs exclusively on geothermally heated soils (> 50 °C) and one that is only found on non-thermal soils. Since Iceland is a subarctic island, non-thermal areas surrounding the geothermal areas can get bitterly cold. This stark contrast in temperatures over short geographic distances provides an excellent system for studying adaptations to thermal extremes and potentially associated trade-offs. To test whether the geothermal lineage is more heat tolerant and whether this heat tolerance is associated with reduced performance under cooler conditions, we compared the heat and cold stress responses of the two lineages experimentally.

Results  No plants survived the hottest treatment (56 °C), only geothermal plants survived the second hottest treatment (49 °C) and geothermal plants also outperformed the non-thermal plants following the 46 °C treatment. In contrast, there were no differences in survival between geothermal and non-thermal plants under intermediate and cold conditions (41 °C, 21 °C and − 4 °C), but non-thermal plants outperformed geothermal plants under these conditions.

Conclusions  These results suggest that there is a trade-off between tolerating extreme heat and performance under cooler conditions, possibly indicating that geothermal A. stolonifera represents a specialised thermophilic lineage in Iceland. Our findings provide new empirical data on whole-plant responses to different thermal conditions, further understanding of the consequences of adapting to high and low temperature extremes, and raise new questions about the mechanisms, benefits and costs of thermal specialisation under different climatic conditions.

We summarise evidence of extinction reported in Red List accounts of plants (170 species) and animals (816 species) listed as extinct (EX) or extinct in the wild (EW) under different versions of the Red List. We find that types of evidence and their frequency of use are remarkably consistent across plants, animals and Red List versions. The most common evidence is ‘long time missing, fewer than half of species accounts discuss search effort, and accounts of 15 % of EX/EW plants and 27 % of animals provide no reason for the EX/EW listing. We review use of extinction probability models for EX, EW and Critically Endangered species flagged as possibly extinct, and find that 155 species have been subjects of extinction probability models. Nearly all models applied to birds and mammals agree that the species are extinct. Seventeen statistical models for estimating the probability that a species is extinct based on search effort, traits and detection rate have been published since 1993 and applied to EX, EW, or CR(PE) species. A tiny fraction (0.6 %) of Red List accounts currently include published model results; thus extinction probability modelling has been largely independent of Red Listing. This is likely to change, because since 2019, Red List guidelines provide templates for quantitative estimates of extinction likelihood. Combining statistical models with assessments of species' probability of extinction based on other criteria can strengthen extinction listings of plants and animals, and add to our understanding of the scale of modern species loss.

Frost tolerance has evolved many times independently across flowering plants. However, conservation of several frost tolerance mechanisms among distant relatives suggests that apparently independent entries into freezing climates may have been facilitated by repeated modification of existing traits (‘precursor traits’). One possible precursor trait for freezing tolerance is drought tolerance, because palaeoclimatic data suggest plants were exposed to drought before frost and several studies have demonstrated shared physiological and genetic responses to drought and frost stress. Here, we combine ecophysiological experiments and comparative analyses to test the hypothesis that drought tolerance acted as a precursor to frost tolerance in cool-season grasses (Pooideae). Contrary to our predictions, we measured the highest levels of frost tolerance in species with the lowest ancestral drought tolerance, indicating that the two stress responses evolved independently in different lineages. We further show that drought tolerance is more evolutionarily labile than frost tolerance. This could limit our ability to reconstruct the order in which drought and frost responses evolved relative to each other. Further research is needed to determine whether our results are unique to Pooideae or general for flowering plants.

Efforts to understand the mechanisms explaining the relationship between abiotic stress tolerance and range size and filling have hitherto yielded contradictory results. Unlike previous studies that have focused on single stress factors, we here examine the extent to which range size and filling can be explained by tolerance of multiple abiotic stressors (cold, shade, drought and waterlogging). As range metrics, we used range size and filling (the ratio between actual and potential range) for 331 European and North American temperate woody plant species. Stress tolerance strategies were expressed as a multivariate axis reflecting a cold/waterlogging-drought tolerance trade-off. We used mixed models to evaluate the relationship between range size/filling and this multivariate stress tolerance axis, using latitude as a covariate, and phylogeny and plant functional type as random effects. Range size and stress tolerance were negatively correlated, mostly independently of latitude and continent. Thus, cold/wet-tolerant species had the largest range sizes and cold-sensitive/drought-tolerant species the smallest. In contrast, range filling mostly depended on latitude. Our results show that abiotic stress tolerance can explain interspecific differences in range size, and to a lesser extent range filling, which sets up predictions for range size variation in plants that go beyond latitude.

Despite most angiosperms being perennial, once-flowering annuals have evolved multiple times independently, making life history traits among the most labile trait syndromes in flowering plants. Much research has focused on discerning the adaptive forces driving the evolution of annual species, and in pinpointing traits that distinguish them from perennials. By contrast, little is known about how ‘annual traits’ evolve, and whether the same traits and genes have evolved in parallel to affect independent origins of the annual syndrome. Here, we review what is known about the distribution of annuals in both phylogenetic and environmental space and assess the evidence for parallel evolution of annuality through similar physiological, developmental, and/or genetic mechanisms. We then use temperate grasses as a case study for modeling the evolution of annuality and suggest future directions for understanding annual-perennial transitions in other groups of plants. Understanding how convergent life history traits evolve can help predict species responses to climate change and allows transfer of knowledge between model and agriculturally important species.