Drought tolerance in the alpine dandelion, Taraxacum ceratophorum (Asteraceae), its exotic congener T. officinale, and interspecific hybrids under natural and experimental conditions.

We compared water relations and adaptations to drought stress in native and invasive exotic dandelions, Taraxacum ceratophorum and T. officinale. Photosynthesis (A), transpiration (E), and water use efficiency (WUE; carbon gained/water lost) were measured for the two species under extreme drought in the alpine tundra of Colorado, USA. We also subjected both species and F(1) hybrids to a dry-down experiment to determine how relative physiological performance varied with water availability. Photosynthesis and transpiration in the field were low and did not differ between Taraxacum congeners; however, native T. ceratophorum had higher WUE than T. officinale. After 6 days of greenhouse drought, photosynthesis and transpiration were reduced in T. officinale compared to T. ceratophorum. Taraxacum ceratophorum maintained high WUE under control and drought treatments. Conversely, WUE in T. officinale was highly plastic between watered (low WUE) and dry-down (high WUE) treatments. Hybrids did not exhibit heterosis; instead, they were similar to T. officinale in A and E and intermediate to the parental species in WUE. Overall, results suggest that native dandelions are more drought tolerant than invasive congeners or their hybrids, but have less plasticity in WUE. Arid habitats and occasional drought in mesic sites may provide native dandelions with refugia from negative interactions with invasives.