Weighing the Risks and Benefits of Flowering Early for the Woody Perennial Prunus pumila (Rosaceae)

Abstract

As global weather patterns become increasingly unpredictable, the need to study the impact of this phenomenon on natural and anthropocentric ecosystems alike is increasingly urgent. Based on climate change predictions for the rest of this century, much previous research has been conducted to reveal plants’ response to stress from heat and drought. However, fewer studies have focused on the response of perennial plants to false spring, or freezing after de-acclimation, another possible result of climate change. Plant phenology has already begun to advance by weeks since the middle of the twentieth century, which makes plants that flower and leaf out early particularly vulnerable to freezing damage from late spring frosts. Temperature during anthesis (when open flowers are present on a plant) has a significant impact on fertility, floral metabolism, and the production and quality of floral rewards that attract pollinators. Therefore, there may be advantages to flowering early that outstrip the potential risks, especially if pollinators also adjust their phenology to warming conditions. To determine whether earlier flowering plants were at risk for freezing damage, we first monitored a population of Prunus pumila with artificially delayed phenology over the course of the 2020 growing season. We measured flower and fruit number and mass, pollinator visitations, the effects of floral age on hand pollination, and seed germination. Consistent with previous studies, our results suggest that flowering time and temperature at anthesis affect reproductive success, with fewer fruits produced in warmer conditions. To build on this work, in 2021, we examined the effects of floral freezing on P. pumila pollinator attraction and reproductive success. Field-grown potted plants were exposed to one of two sub zero (°C) temperatures or a near-freezing control temperature. Flower samples were assessed for tissue damage through electrolyte leakage and stereo microscopy. In addition, a subset of flowers was hand pollinated or bagged to exclude insects and to gauge the effects of pollen limitation and selfing, respectively. Plants were then returned to the field and observed over the course of flowering for pollinator visitation and monitored for signs of successful fertilization and reproduction. This study showed that, in general, P. pumila buds and flowers are resistant to light freezing but not to hard freezing, but this difference did not affect overall pollinator visitation rate. Our findings will contribute to the current knowledge of Prunus, a global genus with high economic and ecologic value that could be greatly impacted by climate change.