Rainforest tree species may be especially vulnerable to climate warming because they have adapted to thermally stable environmments. Rainforest species will need to physiologically acclimate to warmer temperatures in future, but our understanding of their acclimation capacity is poor and constrained by the paucity of data. This study quantified acclimation of photosynthesis and respiration temperature responses across six Australian rainforest species. We hypothesized that warming would increase the optimum temperatures of photosynthesis (ToptA), of Vcmax (ToptV) and Jmax (ToptJ), and would decrease the respiration rate and photosynthetic capacity, indicated by the maximum carboxylation rate and the maximum electron transport rate at a standard temperature of 25°C (i.e., Vcmax25, Jmax25). We exposed six rainforest species to four growth temperatures in a temperature and humidity-controlled glasshouse. The three lowest growth temperatures reflected the climate across the species’ native distribution (including home and 2 increments of 3.5°C), with the highest growth temperature reflecting conditions experienced outside the native range. Instantaneous temperature responses of light saturated photosynthesis (Anet) and CO2 response curves (A-Ci) were measured for all species in each growth temperature. Temperature responses of respiration were measured on the same species at two growth temperatures (home and home+3.5°C).
Results/Conclusions
We found an increase in ToptA with increasing growth temperature. Similarly, ToptV and ToptJ increased significantly with growth temperature. Across species, Jmax25 declined significantly in response to growth temperature, while Vcmax25 remained unchanged in response to growth temperature. The temperature sensitivity of respiration (Q10) was reduced with increasing growth temperature, resulting in reduced respiration rates at high growth temperatures. Rainforest tree species had the capacity to acclimate to 3.5°C warming via increased temperature optima and reduced respiration rates. However, growth temperatures above those experienced in their native range led to reduced photosynthesis rates in most species, indicating a limited ability to physiologically adjust.
Key words: Climate warming, rainforest, photosynthesis, optimum temperature, acclimation, biochemical components.