Photosynthetic energy gain and biomass energy and resource investment represent trade-offs between potential enhancements and limitations to plant productivity, respectively. We compared these characteristics in the exotic invasive Berberis thunbergii DC. with that of co-occurring natives Kalmia latifolia L. and Vaccinium corymbosum L. in a northeastern United States forest. We hypothesized that invasion by B. thunbergii could be facilitated by a lower leaf construction cost (CC) and reduced leaf nitrogen content (N) relative to photosynthetic rate (A) and maximum photosynthetic capacity (Amax), which would afford it greater energy-use efficiency (EUE) and nitrogen-use efficiency (NUE), and maximums of these variables (EUEmax and NUEmax), compared with native shrubs. Although B. thunbergii and K. latifolia exhibited similar peak-season A and Amax, EUE, EUEmax, and NUEmax were greater in B. thunbergii, which exhibited lower leaf CC and density. In contrast, EUE, EUEmax, NUE, and NUEmax did not differ between B. thunbergii and V. corymbosum given their similar A, Amax, and area-based leaf CC and leaf N. Considered with leaf phenology, our results suggest two distinct physiological mechanisms could influence B. thunbergii invasion. Specifically, deciduous B. thunbergii exhibited greater cost-effectiveness than evergreen K. latifolia, while a longer payback time for photosynthetic energy gain could offset instantaneous similarities in cost-effectiveness of earlier leafing B. thunbergii and deciduous V. corymbosum.

Environmental change, accelerated by anthropogenic activities, threatens many species and can be especially challenging for rare species given their potentially limited capacity for migration and adaptation relative to more common species. The ability to acclimate via phenotypic plasticity could provide an important path to species persistence in the face of such change. We investigated the responses of an endangered plant species endemic to a highly dynamic riparian habitat in southeastern Tennessee, USA, and its most widespread congener to environmental change to elucidate their current statuses and future vulnerability. Specifically, we compared the population-and species-level plasticity of rare Pityopsis ruthii and common P. graminifolia to contrasting light, temperature, and water conditions in a growth chamber experiment to evaluate their potential to acclimate to environmental change. Contrary to our expectations, P. ruthii had greater phenotypic plasticity than its common congener in response to both altered light and water availability. But this plasticity was not associated with increased fitness, suggesting that it was not adaptive. Concurrently, we genotyped these individuals at nine putatively neutral microsatellite loci to contrast genetic diversity across the range of each species. As expected, P. ruthii exhibited reduced genetic diversity relative to its more common congener. Overall, our findings accord with the narrow range and current habitat specificity of P. ruthii, especially its tolerance of highly variable water, and highlight its potential vulnerability to future environmental change.

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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.