Mechanisms controlling the distribution of two invasive Bromus species

Abstract

The role of physiological processes and their tolerance limits in species range formation was first formulated by Ronald Good in his "Theory of Tol-erance" (Good 1931). Good suggested that a spe-cies' distribution is closely linked to its physiological responses to climate and implied that even closely related sister species can have distinct tolerance ranges. Species' tolerance, according to Good (1931) is comprised of many tolerance parameters , but only one of them limits its distribution. In reality, however, a single species might be limited by more than one factor and process since the nature of geographical ranges and niches is multi-dimensional (Hutchinson 1957). In order to predict changes in vegetative cover, one has to look at a large number of proximal variables and understand their effect on major physiological and reproductive processes (Bykova et al. 2012, Hig-gins et al. 2012, Schurr et al. 2012). My thesis contributes to this area by looking at the effects of temperature on range formation of two closely-related grass invaders, B. tectorum and B. rubens, through the assessment of their freezing tolerance , male reproductive development and photo-synthetic performance under varying temperatures. I hypothesized that temperature plays an important role in their range formation through its effect on major physiological and reproductive processes and that differences in species physiological responses may drive their range separation. The major results of my PhD thesis are presented in three chapters (I. "Winter cold tolerance and the geographic range separation of Bromus tectorum and Bromus rubens, two severe invasive species in North America" (Bykova and Sage 2012); II. "Heat sterility of reproduction in a changing global climate: Implications for the pernicious invaders Bromus tectorum and Bromus rubens in North America"; III. "Thermal acclima-tion of photosynthesis in two winter annual grass invaders from semi-arid regions of North Amer-ica"). Bromus tectorum and Bromus rubens were of particular interest to me because of their critical role in North American semi-arid biomes and Abstract. In order to predict future range shifts for invasive species it is important to explore their ability to acclimate to the new environment and understand physiological and reproductive constraints controlling their distribution. My dissertation studied mechanisms by which temperature may affect the distribution of two aggressive plant invaders in North America, Bromus tectorum and Bromus rubens. I first evaluated winter freezing tolerance of Bromus species and demonstrated that the mechanism explaining their distinct northern range limits is different acquisition time of freezing tolerance. While B. rubens has a slower rate of freezing acclimation that leads to intolerance of sudden, late-autumn drops in temperature below-12°C, B. tectorum rapidly hardens and so is not impacted by the sudden onset of severe late-autumn cold. In addition, the analysis of male reproductive development and seed production showed that neither species produces seed at or above 36°C, due to complete pollen sterility, which might trigger climate-mediated range contractions at B. tectorum and B. rubens southern margins. Finally , a detailed gas-exchange analysis combined with biochemical modelling demonstrated that both species acclimate to a broad range of temperatures and photosynthetic response to temperature does not explain their current range separation.