Global warming may push tropical forests into a climate envelope not currently occupied by closed-canopy forest. Furthermore, atmospheric CO2 concentrations are higher than they have been in millions of years. To better understand the consequences of such climate- and atmospheric changes for photosynthesis of tropical trees, experiments are required that test the capacity for acclimation of photosynthesis. For such experiments to inform the sophisticated models that are used to simulate climate-vegetation interactions, acclimation will need to be expressed in terms of the factors known to control light-saturated photosynthesis at different temperatures, such as stomatal conductance, and photosynthetic biochemistry (VCMax and JMax, respectively the maximum rates of RuBP carboxylation, and of photosynthetic electron transport). We grew seedlings of the tropical tree species Tabebuia rosea in two geodesic domes in lowland Panama at current ambient temperature and CO2 concentration, and at ambient +6°C and 1000 ppm CO2. After 3 months, CO2-response curves of photosynthesis were measured across a wide range of temperatures. To evaluate short-term acclimation in addition to long term acclimation, the conditions were then switched for the two groups, and after 4–5 days the plants were re-measured. In total almost 200 CO2-response curves were measured.
Results/Conclusions
Despite frequent occurrence of leaf temperatures well above 40°C, the plants grown at elevated temperature and CO2 appeared healthy and maintained photosynthetic capacity comparable to that of control plants. Acclimation resulted in a significantly higher optimum temperature (TOpt) of photosynthesis in warmed/elevated CO2 plants than in control plants and this was paralleled by a shift in TOpt of VCMax, while TOpt of JMax did not differ significantly between treatments. There were no clear differences in the temperature response of stomatal conductance between treatments. Moving control plants to elevated CO2 and temperature resulted in a significant decrease in photosynthetic capacity (VCMax, JMax and net photosynthesis), as well as leaf nitrogen content, and a small, but non-significant increase in TOpt of these parameters. In contrast, a move from warmed/high CO2 to control conditions did not affect photosynthetic capacity, nitrogen content or the position of TOpt. Simulations with a coupled photosynthesis-stomatal conductance model will be used to explore the underlying mechanism in the observed responses to experimental warming and CO2 enrichment.