Differences in the coordination of cold tolerance traits for deciduous vs. evergreen Rhododendron

Background/Question/Methods

Differences among plant life-history types in their ability to adapt to changing temperature regimes will determine the nature of future plant assemblages. Evergreen and deciduous species may differ in their response, as freezing tolerance depends on the coordination of both leaf and xylem function. Evergreens tend to grow slowly, with leaves and xylem functional year-round, even during freezing temperatures.  Deciduous plants grow more quickly, but tend to be less freezing tolerant. Larger diameter xylem vessels are more susceptible to freeze-thaw embolism, but greater water transport capacity supports faster growth. Leaves with higher specific leaf area (SLA) and higher turgor loss point (TLP) may be less stress tolerant, but may have higher rates of growth. We investigated the coordination of these leaf and xylem traits for deciduous and evergreen plants differing in cold-hardiness. With over 900 deciduous and evergreen species from sub-tropical to alpine environments, the genus Rhododendron provides the perfect system to address this question. We examined leaf traits (size, SLA and TLP) and xylem traits (vessel diameter, number and water transport capacity, or D_h) for 9 Rhododendron species grown in a common garden. We tested for the effects of habit and minimum survivable temperature (Tmin) on these traits.

Results/Conclusions

We found significant differences across species differing in Tmin and in leaf and xylem coordination for evergreen versus deciduous species. Deciduous species had larger xylem vessels, and their D_h declined with decreasing Tmin. In contrast, all evergreens had small vessels and low D_h , regardless of Tmin. For leaves, SLA was lower for evergreen than for deciduous species but, unexpectedly, we saw that SLA did not increase as Tmin increased for either. TLP showed no significant patterns with respect to habit or Tmin. Deciduous species also had significantly smaller leaves and less variability in leaf size compared to evergreens. Thus, we found evidence that deciduous species respond to changes in Tmin by altering xylem traits that impart freezing tolerance and/or rapid growth, while evergreens respond by altering leaf traits. Our results suggest differences in the coordination of cold tolerant traits among deciduous vs. evergreen Rhododendron, which may be important in future adaptive response to climate change.