Climate adaption and post-fire restoration of a foundational perennial in cold desert: insights from intraspecific variation in response to weather.
The loss of foundational but fire-intolerant perennials such as sagebrush due to increases in fire size and frequency in semi-arid regions has motivated efforts to restore them, often with mixed or even no success. Seeds of sagebrush Artemisia tridentata and related species must be moved considerable distances from seed source to planting sites, but such transfers have not been guided by an understanding of local climate adaptation. Initial seedling establishment and its response to weather are a key demographic bottleneck that likely varies among subspecies and populations of sagebrush. We assessed differences in survival, growth and physiological responses of sagebrush seedlings to weather among eleven seed sources that varied in subspecies, cytotype and climates-of-origin over 18 months following outplanting. Diploid or polyploid populations of mountain, Wyoming and basin big sagebrush (A. tridentata ssp. vaseyana, A. tridentata ssp. wyomingensis and A. tridentata ssp. tridentata, respectively) were planted onto five burned sites that normally support A.t. wyomingensis with some A.t. tridentata. A.t. wyomingensis had the most growth and survival, and tetraploid populations had greater survival and height than diploids. Seasonal timing of mortality varied among the subspecies/cytotypes and was more closely related to minimum temperatures than water deficit. Temperatures required to induce ice formation were up to 6°C more negative in 4n-A.t. tridentata and A.t. wyomingensis than in other subspecies/cytotypes, indicating greater freezing avoidance. In contrast, freezing resistance of photosynthesis varied only 1°C among subspecies/cytotypes, being greatest in A.t. wyomingensis and least in the subspecies normally considered most cold-adapted, A.t. vaseyana. A large spectrum of reliance on freezing avoidance vs. freezing tolerance was observed and corresponded to differences in post-fire survivorship among subspecies/cytotypes. Differences in water deficit responses among subspecies/cytotypes were not as strong and did not relate to survival patterns. Synthesis and applications. Low-temperature responses are a key axis defining climate adaptation in young sagebrush seedlings and vary more with cytotype than with subspecies, which contrasts with the traditional emphases on (i) water limitations to explain establishment in these deserts, and (ii) subspecies in selecting restoration seedings. These important and novel insights on climate adaptation are critical for seed selection and parameterizing seed transfer zones, and were made possible by incorporating weather data with survival statistics. The survival/weather statistics used here could be applied to any restoration planting or seeding to help elucidate factors contributing to success and enable adaptive management.