Recent and projected changes in atmospheric [CO2] and temperature differentially affect leaf structure and function in Eucalyptus sideroxylon

Background/Question/Methods

A key unresolved issue in climate change research is the response of trees to alterations in CO₂ and temperature ranging from the pre-industrial age to climate model projections for the future. Similarly, few studies have addressed the linkages between environmental effects on leaf structure (stomata, leaf thickness, size and distribution of different cell types), chemistry (N, soluble protein, carbohydrates) and function (growth, physiology) in response to climate change variables. Here, we examine the interactive effects of CO₂ (280, 400 and 640 ppm) and temperature (ambient, ambient +4C) on the leaf structural and biochemical responses that underpin the photosynthetic response of E. sideroxylon grown under natural light conditions in a glasshouse.  We hypothesized that: 1) elevated temperature would increase leaf structural responses to rising [CO₂], 2) these structural responses would affect leaf biochemical responses to rising [CO₂] and temperature, and 3) both structural and biochemical responses would affect photosynthetic responses to rising [CO₂] and temperature. We focused on Eucalyptus because it is an iconic genus with ecological importance in Australia and commercial importance worldwide.

Results/Conclusions

Rising [CO₂] and elevated temperature were associated with substantial changes in leaf anatomy and physiology. For example, rising CO₂ increased leaf thickness and palisade layer numbers, and decreased stomatal frequency and the fraction of intercellular air space. Rising temperature increased intercellular airspace and decreased the fraction of palisade cells. The anatomical and physiological changes with the transition from sub-ambient to ambient [CO₂] led to reduced photosynthesis. However, the anatomical and physiological changes we observed with elevated [CO₂] and elevated temperature did not alter photosynthesis, suggesting that the suite of changes we observed under elevated [CO₂] and temperature had offsetting effects on photosynthesis. Further, as has been found in a wide range of tree species, the effects of rising [CO₂] and elevated temperature generally were additive, rather than interactive. Our results indicate that a different suite of changes may have marked past responses to increasing [CO₂] compared with responses to future [CO₂] and temperature, and highlight the importance of studying both leaf structure and function for understanding photosynthetic responses to climate change.