PS 36-177 - What does it mean to be a subspecies? Quantifying physiology of big sagebrush (Artemisia tridentata) subspecies in a common garden setting

Background/Question/Methods

For many years literature revolving around big sagebrush (Artemisia tridentata) has recognized three habitat specific subspecies: Artemisia tridentata tridentata, A. t. wyomingensis, and A. t. vaseyana. However, while these subspecies are morphologically distinct, their differentiation from each other has not been proven genetically. Previously studies have explored the distinction of these subspecies physiologically, but have focused on few physiological measurements, therefore further research is needed in order to determine whether these subspecies of big sagebrush are distinct from one another. This brings to question: How can we define/quantify what it means to be a subspecies? Common gardens are useful tools that allow us to piece apart physiological differences due to seed source location (a proxy for genetic differences). Three subspecies of big sagebrush from various populations across the western United States were planted in the Orchard Common Garden located just east of Boise, Idaho. A broad range of physiological measurements were conducted at this common garden in two years (July 2019 & May 2020) in order to determine whether differences in physiological performance and hydraulic functional traits could be detected between the three perceived subspecies.

Results/Conclusions

Contrary to the assumption of previous literature and despite their distinct environmental ranges and morphology, we found that subspecies of big sagebrush do not vary significantly from each other physiologically when grown in a similar environment. When observed in the peak of summer heat (July 2019), all three subspecies showed similar midday water potentials and midday gas exchange (e.g., transpiration, stomatal conductance, photosynthesis). When observed in spring under considerably less water stress (May 2020) still no differences in midday gas exchange and hydraulic functional traits were observed. This gives evidence of potentially little physiological difference between the three subspecies despite their differences in morphology and habitats. However, it is important to consider that our measurements were only snapshots (one month/year) and differences may have been observed if a more temporally intensive campaign was conducted.