Tropical forests are important drivers of the global carbon cycle, yet we are uncertain how rising temperature will affect tropical forest carbon balance. Previous research suggests that tropical species are currently operating at or near their thermal optimum temperatures and, due to the narrow temperature ranges, tropical plant species may be unable to physiologically acclimate to predicted shifts in temperature regimes. This research investigated plant physiological acclimation to experimental warming through two experiments, both conducted at the Tropical Response to Altered Experiment (TRACE) site in Puerto Rico. First, we investigated how tropical shrub photosynthesis and respiration responded to eight months of field-level understory warming using infrared heaters arranged in six 4-m diameter plots (three +4 °C heated and three control). We tested physiological thermal acclimation of two common understory shrub species, Psychotria brachiata and Piper glabrescens. We then investigated acclimation of two mature canopy tree species, Gaurea guidonia and Ocotea sintensis, to four weeks of leaf-level within-canopy warming, at 1.8, 9.1, 11.0, 12.8, 14.6, 15.8, 18.2, and 20.1meters height. We tested for thermal acclimation in both the understory and canopy foliage by comparing the instantaneous photosynthetic and respiratory response to temperature between the heated and control plants or leaves.
Results/Conclusions
After three months of understory field-level warming, P. brachiata showed evidence of only photosynthetic acclimation, while P. glabrescens only showed evidence of respiratory acclimation. After eight months of understory warming, we found no evidence of either photosynthetic or respiratory acclimation. Instead, we found evidence of photosynthetic decline in both of our study species. We also found no evidence of photosynthetic acclimation in our canopy warming study. We did find evidence of respiratory acclimation in one of our study species, through a decreased leaf sensitivity to temperature (Q10) in the heated leaves. This Q10 respiration response was primarily driven by acclimation in the understory heated leaves, as the difference in heated and control leaf Q10 declined as canopy height increased. In conclusion, we found no evidence of long-term acclimation to 8 months of field-scale understory warming; instead, we found evidence of photosynthetic decline. We found some evidence of respiratory acclimation to one month of leaf-level warming within the canopy; however, acclimation primarily occurred in the forest understory. Much of tropical forest carbon is cycled in the upper canopy; therefore, lack of acclimation in the upper canopy could have negative implications for tropical ecosystem carbon balance.