Today’s human-induced atmospheric and climate change occurs so rapidly that trees may experience massive increases in [CO2] and temperature during their lifetimes. This is a fascinating scenario, especially for plant biologists studying species-rich, high-biomass tropical forests that are a major driver of the global carbon cycle. This planetary environmental shift raises a series of intriguing questions, including: Where is the tipping point? How extreme does climate change need to get before tropical vegetation becomes severely harmed? What are the temperature thresholds of growth and reproduction?
We used six naturally lit geodesic glass domes to study growth responses of the tropical pioneer tree species Ficus insipida to elevated [CO2] and temperature. In each dome, five plants grew in the ground in relatively nutrient-rich, well-watered soil (note that this was not a pot experiment). Three domes were maintained under current-ambient temperature and CO2 conditions (= control). In the three treatment domes temperature was elevated both during the day and the night, resulting in an increase of mean diel temperature of approximately 6 ºC. Furthermore, [CO2] was raised to twice the current ambient concentration during daytime. Plant height was about 40 cm at the onset of the experiment. Besides biomass increase, we determined leaf photosynthesis, leaf carbohydrate levels, leaf thermal tolerance, soil and leaf nutrient contents, stomatal indices, and the soil bacterial community.
In both treatments, plants were about 2.5 m tall at harvest time. There were no significant treatment differences in total plant biomass, root mass ratio, specific leaf area and leaf area ratio. While maintaining the same total leaf area, plants in the elevated temperature/CO2 treatment had two times more yet smaller leaves than plants in the control treatment. Furthermore, there was a major increase in the proportion of fine roots in the elevated temperature/CO2 treatment, from 16 % to 31 %, respectively. Based on these and other results we conclude that under conditions of adequate soil moisture and nutrients, early growth of Ficus insipida is relatively resilient to relatively large increases in temperature and [CO2], despite marked phenotypic changes of leaves and roots.