Stomatal responses of herbaceous plants to climate change since the 1800s: A case study using herbarium specimens

Nia Richardson, Léa Vilna-Santos, Axel Cortes, Mia Gaughan, Adam B. Smith , Matthew Austin

Background and Aims: As climate change accelerates, plants are expected to respond in myriad ways, such as shifting phenology or altering the density or size of stomata. Nonetheless, most studies to date have focused on just a single facet at a time.

Methods: Here, we used herbarium specimens to examine stomatal responses to climate change in two perennial herbaceous species that have not shifted flowering time across the past century, Oenothera macrocarpa (Onagraceae) and Viola sororia (Violaceae). These trends were evaluated across time and in relation to three major climate drivers: temperature, precipitation, and atmospheric CO2 concentrations.

Key Results: Stomatal lengths in O. macrocarpa became smaller, significantly changing by -0.345 μm per decade (95% confidence intervals: -0.505 to -0.184), amounting to a ~17.5% reduction in size from the 1880s to the present. Otherwise, stomatal densities and lengths did not change significantly over time and were unrelated to temperature, precipitation, and atmospheric CO2, except for a marginally significant decline in O. macrocarpa stomatal length with CO2.

Conclusions: Previous research on stomatal trends obtained from herbarium specimens across time have largely focused on trees, while the stomatal response of herbaceous plants to climate change has been relatively understudied. These findings demonstrate that species lacking a phenological response to climate change may exhibit other types of response, such as decreased stomatal size. Accordingly, this study underscores the importance of simultaneously investigating the response of multiple morphological and physiological facets for a more holistic understanding of how climate change influences species.