COS 4-5
Effects of nighttime temperature on growth and physiology of tropical trees: The importance of ontogeny
Tropical forests contain a large proportion of the global terrestrial carbon pool, but ongoing climate warming will push tropical forests into a climate envelope not currently occupied by closed-canopy forest. There are conflicting sets of information regarding the fate of tropical forests with continued warming. Warmer nights have been implicated in reduced annual diameter increment of tropical forest trees in Costa Rica, suggesting that further warming will reduce the capacity for tropical forests to store carbon. Yet, experimental nighttime warming increasedgrowth of tropical tree seedlings in Panama, suggesting that more biomass can be retained under warmer conditions. To try to reconcile these results we did a series of warming experiments in Panama using tropical trees at different ontogenetic stages. First, we exposed seedlings to different nighttime temperatures in growth cabinets. Second, we grew 3-m tall saplings under different nighttime temperatures. Third, we experimentally warmed upper-canopy leaves of tall tropical trees during the night. In all these experiments we measured gas exchange and quantified leaf carbohydrate concentrations.
Results/Conclusions
Seedling growth increased with nighttime temperature, confirming previous observations. Respiration also increased (indicating incomplete acclimation), whereas photosynthesis was independent of nighttime temperature. Similarly, nighttime warming increased sapling growth, without changes in photosynthesis. Leaf respiration of canopy trees acclimated to nighttime warming, but at their respective temperatures respiration was still higher in warmed than in control leaves.
So across ontogenetic stages we see that nighttime warming results in partial acclimation of respiration and no change in photosynthesis. Yet, seedlings and saplings increase growth whereas growth in big trees is thought to decrease.
We hypothesize that ontogenetic differences in the growth response to nighttime warming are driven by differences in biomass allocation; not by differences in physiological acclimation. Warming can stimulate leaf development, but the marginal benefit of growing new leaves is much smaller for canopy trees than for seedlings. Therefore, nighttime warming does not increase leaf area in big trees, while increasing respiration (despite partial acclimation), potentially reducing growth. In contrast, we found that seedlings increase leaf area, enabling faster growth at elevated nighttime temperature despite lower leaf-level carbon balance. We explore this hypothesis using carbohydrate data, and discuss our results with respect to the fate of the tropical carbon pool.