Plasticity of physiological and anatomical leaf traits in Eucalyptus across a water availability gradient

Duncan D. Smith, Katherine A. McCulloh and Thomas J. Givnish, Botany, University of Wisconsin-Madison, Madison, WI, Duncan D. Smith and Mark A. Adams, Swinburne University of Technology, VIC, Australia, Thomas N. Buckley, Plant Sciences, University of California Davis, Davis, CA

Background/Question/Methods
As the primary organs for gas exchange in most plants, leaves should efficiently transport water to the sites of evaporation and they should control evaporation in a way that balances the benefit of CO2 gain against the potential detriment of H2O loss. The efficiency of water transport and the balance of gas fluxes can mean the difference between failure due to water stress at one extreme, failure due to out-competition at another extreme and success somewhere in between. Transport efficiency depends partly on vein density and conductance to CO2 and water vapor is a partly a function of stomatal anatomy. In a variable and changing climate, trait plasticity can broaden the range of tolerable conditions. We hypothesized that species adapted to drier locations would show greater plasticity in leaf anatomy and function due to the inherent fluctuations in water availability at drier locations.
We addressed this hypothesis using four common gardens established across a water availability gradient with ten species of Eucalyptus native to points along this gradient. We measured vein density and the dimensions and density of stomata, which we used to calculate maximum theoretical stomatal conductance. We further measured the minimum diffusive conductance of detached leaves in the dark.
Results/Conclusions
When grown at drier sites, species tended to have greater vein density, more stomata per area and smaller stomata. These stomatal traits combined to provide a greater theoretical maximum stomatal conductance when grown at drier sites. Within sites, species adapted to drier conditions had greater vein density, lower stomatal density and larger stomata, but these combined to give equivalent maximum stomatal conductance between species, thus suggesting different anatomical pathways to achieve the same functional result.
Minimum conductance increased when species were grown at wetter sites. This was especially true for species adapted to drier climates. Within wetter sites, dry-adapted species had substantially higher minimum conductance than wet adapted species, suggesting that when dry-adapted species receive more water they can relax their conservation while wet-adapted species have little room to tighten their conservation at drier sites. These results supported our hypothesis that these leaf traits are more plastic in drier-adapted species. But this plasticity results in greater potential for water loss at wetter locations. Whether this plasticity can translate to a competitive advantage depends on a host of other traits and plasticity in other organs and at other scales.