Mechanisms controlling the distribution of two invasive Bromus species

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.

Winter cold tolerance and the geographic range separation of Bromus tectorum and Bromus rubens, two severe invasive species in North America

“…Both species greatly reduce native biodiversity by increasing wildfire frequency and severity, and by outcompeting native species for soil water (Smith et al, 1997;Brooks, 2000;Salo, 2004;Bradley et al, 2006). The two Bromus species also have distinct distributions along elevational gradients (Beatley, 1966;Brown & Rowe, 2004;Bykova, 2012). The current distribution of B. tectorum in North America lies between 29°and 65°N (Valliant et al, 2007), and its northern limit in western North America coincides with the À30°C isocline for minimum winter temperature (Mack, 1981;Environment Canada, 2007;Valliant et al, 2007).…”

“…Bromus tectorum is thought to be the more cold tolerant of the two species, as it occurs at higher latitude and altitude than B. rubens and has been shown in field trials to survive winter conditions that are lethal to B. rubens (Hulbert, 1955). For example, in the northern Mojave desert region near Las Vegas, Nevada, B. rubens is common at low-to-mid elevations of 700-1500 m, while B. tectorum predominates at 1500-2300 m (Bykova, 2012). Bromus rubens, by contrast, is restricted to landscapes south of 46°N in western North America (Smith et al, 1997).…”