Understanding variation in physiological responses of plants to freezing within natural populations is critical to assessing the physical limits to plant distribution and how they may be altered by global climate change. If populations of L. tridentata at high latitude edges of the Chihuahuan Desert are limited by freezing stress, increases in night-time winter temperatures predicted by global climate change models could release these populations from freezing stress, thereby increasing seedling establishment and population expansion into adjacent desert grasslands. However, the extent to which range expansion will occur depends on whether or not high latitude establishment is limited by freezing, and how variation is distributed within the population. If variation within maternal families is low compared to total population variation, the potential for local adaptation exists, and northward expansions of this species may occur even without increases in night-time temperatures. Conversely, if variation in freezing tolerance is high within maternal families, selection for further cold tolerance is unlikely. In order to determine if freezing limits seedling establishment we measured survival as well as population and family level variation in physiological responses to freezing. We measured the intrinsic efficiency of photosystem II (F_vF_m) and projected leaf area before and after freezing to -10^oC or -12^oC in 10 half-sibling families of three-month-old and one-year-old L. tridentata from a high latitude Chihuahuan Desert site, where temperatures of this magnitude typically occur several times over the winter.

Results/Conclusions
Total variation in F_vF_m was high for both temperature treatments (0.014 and 0.015, respectively). ANOVA revealed no significant differences between half-sib families in F_vF_m following freezing to -12^oC (p=0.5954) or -10^oC (p=0.3188), due in part to high variability within families following freezing resulting in low correlations between half-siblings. However, there were significant differences between three-month-old plants experiencing -12^oC and -10^oC in proportion of leaf area lost following freezing (p=0.0123), as well as significant differences across the two life history stages, with three-month-old plants have significantly lower F_vF_m than one-month-old plants following freezing to -12^oC (p<0.0001) or -10^oC (p<0.0001). Finally, mortality in three-month-old plants began at -10^oC, while one-year-old plants did not experience mortality until -19^oC, which is close to the long-term minimum night-time temperature for the region. These results suggest that juvenile establishment in this population may be limited by freezing, and that small increases in winter nighttime minimum temperatures could drastically alter the proportion of seedlings surviving during warmer and colder than average winters.