UC Davis

Climate change and climate instability driven by anthropogenic fossil fuel emissions areincreasing the frequency and severity of extreme weather events. One such event, heat waves, are becoming more frequent, intense and prolonged (Meehl and Tebaldi 2004; Stillman 2019; Thiery et al. 2021). Extreme heat associated with these heat waves may be particularly damaging to critical species life-history events such as reproduction and mating, and also interactions between organisms (Vasseur et al. 2014; Harvey et al. 2023). Heat may especially impact plant- pollinator interactions and the mating of flowering plants, as heat may disrupt the process of pollination and alter the stable dynamics of mating (Hedhly et al. 2009; Walters et al. 2022; Hemberger et al. 2023). Importantly, heat may disrupt plant-pollinator interactions and successful mating directly through heat’s effects on flower gamete production and performance (Hedhly et al. 2009), but also indirectly through heat’s effects on foraging pollinators (Walters et al. 2022; Hemberger et al. 2023). My dissertation is divided into three chapters with the aim of understanding methodically how heat impacts the ecology of pollination and floral reproduction. Chapter 1 used experimental pollination treatments explicitly focusing on the perspective of the plant to understand how heat affects the dynamics of pollen limitation-the reduction in potential seed production through insufficient receipt of or pollen that is incapable of fertilizing ovules. It evaluates how heat may simultaneously influence the quantity of pollen grains produced and the quality of those pollen grains in addition to pollen quality effects inherent to pollen origin (self versus outcross). I found that heat dramatically limited pollen production, and the quality of pollen grains measured as its viability and performance to grow pollen tubes and fertilize ovules. Although outcross pollen was of higher quality than self pollen under control conditions, heat diminished these differences. These results suggest that heat primes plants to have a high risk of quantitative and qualitative pollen limitation and can disrupt plant mating strategies. Chapter 2 used experiments to quantify the relative contributions of direct (through flowers) and indirect (through pollinator foraging and flower visitation) effects of extreme heat on pollination, post- pollination, and reproduction. The experiments involved bumble bee foraging in heated chambers to allow bumble bees to disperse pollen between flowers. I found that heat strongly limited pollination, the success of those dispersed pollen grains (post-pollination) and reproduction through direct and indirect effects, and that direct effects of heat in post-pollination caused outright reproductive failure. Importantly, these effects were additive. Although both direct and indirect effects are highly limiting, the presence of direct effects dominated heat’s indirect effects on pollination, post-pollination, and reproduction. Chapter 3 used mathematical modelling to explore the sensitivity of pollination and plant reproduction to heat based on models of pollination dynamics and thermal performance curves. Due to logistical challenges that emerge in experimentation, mathematical models allowed me to overcome them and fully explore the parameter space to understand which temperatures specifically would be deleterious, and whether this depended on direct and indirect effects. Furthermore, this chapter also explored whether altering the density of pollinators could offset the negative effects of heat on flowers for pollination and post-pollination processes. My results demonstrated that pollination and reproduction respond non-linearly at high temperatures as pollination and reproduction abruptly collapse. This collapse was largely driven by decreasing pollen production, and therefore increasing pollinator abundance did not compensate for how flowers respond to heat. The collection of these chapters builds upon previous work that suggests that pollination should be highly vulnerable to heat. My results underscore the broad detrimental effect that heat has at each stage of the reproductive process (pollination and post-pollination), and that there is little to no resilience in pollination systems to the effects of heat. When plants mate during periods of extreme heat, it will likely severely limit the reproduction of plants through direct and indirect pathways (Walters et al. 2022; Hemberger et al. 2023), which threatens the persistence of plant populations and the security of food production (Battisti and Naylor 2009; Lesk et al. 2016).