Tropical wet-forest understory plants have limited ability to acclimate photosynthesis to higher temperatures and have lower photosynthetic capacity under experimental warming in Luquillo, Puerto Rico

Background/Question/Methods

Tropical plants perform an important ecosystem service through carbon sequestration; however, how tropical plants will respond to increasing atmospheric temperatures is poorly understood. Temperate plants, which experience broad temperature ranges, have the capacity to acclimate quickly, while tropical species, which experience a much narrower temperature range, may not have the capacity to acclimate to the same degree or speed as temperate species. Plants, as sedentary species, acclimate photosynthetic response to increasing temperature by changing the temperature at which photosynthesis is optimal (Topt), the photosynthetic rate at optimal temperature (Aopt), or the thermal niche breadth of photosynthesis (Ω). To determine the capacity and magnitude of tropical understory plant thermal acclimation potential, we tested in situ photosynthetic temperature acclimation response at the Tropical Responses to Altered Climate Experiment (TRACE), a tropical wet-forest understory warming experiment in Luquillo, Puerto Rico. At TRACE, tropical understory plants are warmed +4 °C above ambient temperature. We measured temperature response curves from 2019 to 2020 in ambient temperature and warmed plots, starting approximately 14 months after devastating wind disturbances caused by hurricanes Irma and Maria in 2017, allowing us to capture interacting acclimation effects from climate warming and secondary successional changes.

Results/Conclusions

In the two species we measured, Piper glabrescens and Psychotria brachiata, we found both species had a lower Aopt under experimental warming and that both species had lower Aopt as the canopy closed, but there was no interacting effect of succession and experimental warming on Aopt, suggesting there is no temperature acclimation, but rather these plants have a diminished capacity to photosynthesize at higher temperatures. It also suggests plants have a diminished capacity for photosynthesis as the canopy closes and plants acclimate to the altered light regime due to successional changes. For both species, there was no apparent thermal acclimation of Topt, but P. brachiata had a higher Ω in warmed plots, suggesting P. brachiata may be able to offset the loss in maximum photosynthetic rate by increasing the breadth of temperature where photosynthesis is efficient. These results suggest that an increased temperature climate scenario will result in atmospheric temperatures in excess of photosynthetic optima and decreased photosynthetic rate for tropical understory plants, but some species may have the capacity to acclimate through a broader photosynthetic thermal niche. Overall carbon sequestration capacity for these species will likely be reduced if atmospheric temperature is increased +4 °C above current temperatures.