The study brings together an international team including Dr Rafael Rosolem, a Senior Lecturer in the Department of Civil Engineering and Cabot Institute for the Environment member at the University of Bristol.
The group studied forest photosynthesis data from the rainforest habitat at the University of Arizona’s Biosphere 2 facility and compared them to measurements taken at natural tropical forest sites. Due to being encased under a glass dome located in the Arizonan desert, the tropical forest at Biosphere 2 experiences very unusual climatic conditions making it possibly the hottest tropical forest in the world, with temperatures reaching up to 40 degrees Celsius. This means that the Biosphere 2 tropical forest already experiences temperatures within the range of those projected for natural forests by 2100, assuming no major climate change mitigation strategies are employed.
Previous studies suggest that tropical forests are already approaching their thermal limit, consequently leading to concerns about the impacts of future warming. However, when analysing the data from the Biosphere 2 rainforest and comparing them against natural forests, the group saw that, under certain conditions, the tropical trees in Biosphere 2 were functioning well beyond temperatures currently deemed to be the limit, and even higher than those predicted for the Amazon basin by 2100.
The climate can be experimentally controlled in Biosphere 2, allowing the group to observe the effects of warming and humidity separately. This is critical because temperature and humidity are tightly linked in natural systems. The authors found that in natural forests, photosynthesis declined at high temperatures with decreasing humidity. But in B2, humid conditions were maintained even at high temperatures, and the trees continued to photosynthesise at temperatures far above natural forests.
Dr Rafael Rosolem, who was instrumental to the processing and analysis of the Biosphere 2 data adds: “This work follows a previous study we published in 2010 in which we had adapted a land surface model to replicate the environment of the Biosphere 2 tropical forest.
In that study, we were only able to reproduce the way carbon dioxide is exchanged between the vegetation and the enclosed atmosphere of the Biosphere 2 by adjusting the model so that the tropical trees are much more tolerant to warmer conditions than expected in natural forests. Our new study represents a big leap in our knowledge as it relies solely on observational evidence from the data we collected at Biosphere 2 over the years”.
The new, empirical study, found that photosynthesis was able to continue up to much higher temperatures in Biosphere 2 than natural tropical forests due to the more humid conditions within the experimental forest. Trees in Biosphere 2 may also have certain characteristics which make them more tolerant to higher temperatures, as indicated by the 2010 modelling study and from a 2018 paper by the research team. Though real world tropical forests are not as humid as Biosphere 2, the authors speculate they may be able to
function at higher temperatures in the future because the process of photosynthesis is expected to get more efficient under elevated atmospheric carbon dioxide (CO2) concentrations. Specifically, plant leaves should lose less water every time they open their stomata (tiny pores in the leaf) to allow CO2 to enter.
The research suggests tropical forests may have a margin of resilience to future warming. But, these globally important ecosystems are facing many additional threats including from drought, deforestation, and fire, making their protection a high priority.