The tropics are expected to experience unprecedented warming in the near future. Tropical plants have evolved under relatively stable temperatures, so an average increase of 2°C may cause a significant decline in CO2 uptake if thermal thresholds for photosynthesis are exceeded. By increasing the stomatal density and decreasing the stomatal size, plants can maximize evaporative cooling, which could be beneficial under elevated temperatures. The objective of this study was to measure the effects of both experimental and observational changes in microclimate on stomatal size and density in tropical wet forest understory shrubs and canopy trees.
This project is a complement of the Tropical Response to Altered Climate Experiment (TRACE) in Puerto Rico, the first ever field-scale warming experiment located in a tropical forest. The effects of experimental warming on stomata were assessed on two understory species: Piper glabrescens, and Psychotria brachiata, and the effects of changes in microclimate due to canopy height were assessed in two mature tree species: Prestoea montana, and Ocotea sintensis. Understory plant stomatal density was expected to increase with a corresponding decrease in stomatal size under experimental warming, and also with increasing canopy height, because a higher density of smaller stomata helps to increase evaporative cooling.
Results/Conclusions
Contrary to expectations, stomatal size of P. glabrescens increased after nine months of warming, but the warming treatment had no effect on stomatal density. For this species, it may be a more cost-effective strategy to havefewer, larger stomata instead of a higher density of smaller stomata under warmer conditions. In support of our hypothesis, stomata of P. brachiata decreased in size after nine months of warming. Stomatal density of P. brachiata, on the other hand, decreased after three months of warming, but the treatment effect disappeared after nine months of warming. The two species showed opposite responses to experimental warming, revealing two very different strategies. Both stomatal size and density increased with height in the canopy for P. montana and O. sintensis. This was expected for density, but not for size. It could be that a greater density of larger stomata is more beneficial for evaporative cooling in the hot, dry conditions of the upper canopy.