Jill T. Anderson

Summary Climate change is simultaneously increasing carbon dioxide concentrations ([CO 2 ]) and temperature. These factors could interact to influence plant physiology and performance. Alternatively, increased [CO 2 ] may offset costs associated with elevated temperatures. Furthermore, the interaction between elevated temperature and [CO 2 ] may differentially affect populations from along an elevational gradient and disrupt local adaptation. We conducted a multifactorial growth chamber experiment to examine the interactive effects of temperature and [CO 2 ] on fitness and ecophysiology of diverse accessions of Boechera stricta (Brassicaceae) sourced from a broad elevational gradient in Colorado. We tested whether increased [CO 2 ] would enhance photosynthesis across accessions, and whether warmer conditions would depress the fitness of high‐elevation accessions owing to steep reductions in temperature with increasing elevation in this system. Elevational clines in [CO 2 ] are not as evident, making it challenging to predict how locally adapted ecotypes will respond to elevated [CO 2 ]. This experiment revealed that elevated [CO 2 ] increased photosynthesis and intrinsic water use efficiency across all accessions. However, these instantaneous responses to treatments did not translate to changes in fitness. Instead, increased temperatures reduced the probability of reproduction for all accessions. Elevated [CO 2 ] and increased temperatures interacted to shift the adaptive landscape, favoring lower elevation accessions for the probability of survival and fecundity. Our results suggest that elevated temperatures and [CO 2 ] associated with climate change could have severe negative consequences, especially for high‐elevation populations.

Summary Species differ dramatically in their prevalence in the natural world, with many species characterized as rare due to restricted geographic distribution, low local abundance and/or habitat specialization. We investigated the ecoevolutionary causes and consequences of rarity with phylogenetically controlled metaanalyses of population genetic diversity, fitness and functional traits in rare and common congeneric plant species. Our syntheses included 252 rare species and 267 common congeners reported in 153 peer‐reviewed articles published from 1978 to 2020 and one manuscript in press. Rare species have reduced population genetic diversity, depressed fitness and smaller reproductive structures than common congeners. Rare species also could suffer from inbreeding depression and reduced fertilization efficiency. By limiting their capacity to adapt and migrate, these characteristics could influence contemporary patterns of rarity and increase the susceptibility of rare species to rapid environmental change. We recommend that future studies present more nuanced data on the extent of rarity in focal species, expose rare and common species to ecologically relevant treatments, including reciprocal transplants, and conduct quantitative genetic and population genomic analyses across a greater array of systems. This research could elucidate the processes that contribute to rarity and generate robust predictions of extinction risks under global change.