Ecophysiological variation of big sagebrush in common gardens: Local climate adaptation, ploidy effects, and implications for restoration practice

Background/Question/Methods

The loss of big sagebrush (Artemesia tridentata) due to wildfire and other disturbances poses challenges to wildlife conservation and ecosystem functioning, and there is considerable interest in improving prospects to restore this diverse species to Great Basin landscapes.  Problems in seed supply and use for stand establishment in current and future climate conditions are increasing the need to understand climate adaptation and seed transferability among regions.  We evaluated differences in photosynthetic water-use efficiency (WUE, determined from carbon isotopes), survivorship, and growth for nearly 60 provenances (populations throughout the western US) of the 3 subspecies of big sagebrush, and their diploid and tetraploid chromosomal variants, grown in a common garden.  In a related study with reciprocal comparisons of provenances between Idaho and Utah common gardens, we directly measured photosynthesis and water-use efficiency, along with water status, at the onset of midsummer drought.  We hypothesized that provenances originating in warmer and/or drier climates would show higher WUE and that survivorship and growth would improve with greater WUE.  

Results/Conclusions

Preliminary results did not reveal differences in photosynthesis or water status between local and distant provenances, and although WUE determined from carbon isotopes (WUE-13C) also did not differ between local and distant provenances, WUE estimated via gas exchange at midsummer was greater in basin (19%) and especially Wyoming big sagebrush (25%) provenances local to the Idaho garden.  WUE-13C varied much more among provenances than between subspecies or ploidy levels.  In the Idaho garden, tetraploid provenances that originated in warmer climates had greater WUE-13C, whereas there was no such relationship for diploids (r^2=0.26, P<0.01).  Provenances with higher WUE-13C grew taller, in tetraploids but not diploids.  However, provenances with higher WUE-13C had a broad mortality range, while those with lower WUE had high survivorship.  Local adaptation in WUE-13C is more prevalent in tetraploid compared to diploid populations.  These data thus point to a hypothesis that local adaptation has increased during the diversification of big sagebrush, and they indicate the importance of seed origins in selecting seed for restoration plantings, particularly within the tetraploid Wyoming big sagebrush.  This outcome is significant considering that the greatest restoration challenges are where Wyoming big sagebrush normally occurs, with considerable area needing restoration, low restoration planting success rates, and seed availability issues that influence pressure to transfer seeds among regions.