Tropical forests exchange more carbon dioxide with the atmosphere than any other terrestrial biome on Earth. Yet, uncertainty in the projected global carbon balance over the next century is ~3 times greater for the tropics than for any other latitude. Our poor knowledge of tropical plant physiological responses to climate change – particularly photosynthetic responses – has been identified as one of the greatest sources of uncertainty in multiple efforts to estimate and forecast the global terrestrial carbon sink. Furthermore, tropical regions are expected to experience temperatures beyond their historical climate ranges within the next two decades, and evidence suggests that tropical forest canopies are already operating beyond thermal thresholds for photosynthesis. We used a meta-analytic approach to help reduce the gap in our understanding of tropical tree photosynthetic temperature sensitivity. We gathered 19 published and unpublished photosynthetic temperature response datasets from tropical biomes spanning different temperature, rainfall, elevation gradients, representing 375 species. We investigated how photosynthetic parameters, including the optimum temperature for photosynthesis, the rate of photosynthesis at that optimum temperature, and the width of the photosynthetic response curve (a proxy for photosynthetic thermal sensitivity) responded to a suite of environmental drivers, including mean monthly temperature and precipitation.
Results/Conclusions
Optimum temperatures for photosynthesis increased with both monthly mean and maximum temperatures, but showed no relationship with precipitation. Photosynthetic thermal sensitivity, on the other hand, did not respond strongly to average temperatures, but was greater in drier tropical sites than wetter sites. Hierarchical partitioning of environmental variables revealed that: 1) optimum temperatures for photosynthesis were more strongly influenced by monthly maximum temperatures than either mean monthly temperature or precipitation, and 2) optimum rates of photosynthesis were strongly influenced by precipitation. Our results suggest that growth temperature plays an important role in the temperature where maximum photosynthesis occurs; whereas, the maximum rates of photosynthesis and the temperature ranges over which photosynthesis is optimized (i.e., thermal sensitivity) are both more closely controlled by precipitation. This research will improve modeling efforts to quantify tropical ecosystem carbon cycling and provide more accurate representations of how these key ecosystems will respond to altered temperature and rainfall patterns under climate warming.