Understanding How Plants and Ecosystems Will Respond to Increasing Concentrations of Atmospheric Carbon Dioxide

Mike Tercek, Walking Shadow Ecology, Gardiner, Montana
Julie Whitbeck, University of New Orleans
Rusty Rodriguez, University of Washington
Regina Redman, University of Washington

Our research uses naturally occurring carbon dioxide (CO₂) emissions in Yellowstone to investigate the ways in which plants might respond to worldwide increases in atmospheric CO₂ during the next century. Human activity has already resulted in a 35% increase in atmospheric CO₂ since 1800, and the concentration of this atmospheric “greenhouse gas” is projected to double within the next 100 years. This increase is extremely rapid from an evolutionary perspective, and it is unclear how plants, including many important food crops, will respond.  

We have measured the carbon dioxide concentrations in and near dozens of vegetated geothermal areas within Yellowstone National Park (YNP). Many of these sites had high CO₂ concentrations, ranging from 450 to over 2,000 parts per million (ppm). (Normal atmospheric concentration is currently 380 ppm). A few of the sites are quite large (greater than 10 m2), and almost all are far removed from human disturbance. In contrast to most previous studies of natural environments with elevated levels of carbon dioxide, our surveys of Yellowstone have identified high CO₂ sites that can be paired with control sites, that is, sites with comparable vegetation, soil type, and environmental characteristics, but with background levels of CO₂.  

Using two independent methods, we found that leaves from the high-CO₂ sites consistently had less RuBisCo, the enzyme that captures carbon dioxide for photosynthesis, compared to leaves collected from plants growing at background-CO₂ sites. We also found that the leaves from plants collected at high-CO₂ sites had higher levels of sucrose, a product of photosynthesis. These findings suggest that plants growing in high-CO₂ areas of YNP make physiological adjustments similar to those observed in artificial experiments that pump carbon dioxide into forests. However, unlike plants in artificial experiments, YNP plants have likely been exposed to elevated CO₂ concentrations for many generations, and consequently offer an opportunity to study the long-term effects of CO₂ enrichment on ecological processes. It’s not unreasonable to expect that some plants growing in Yellowstone’s geothermal areas have genetically adapted to relatively high temperatures and high CO₂ environments—conditions that may be much more common on Earth in several hundred years.  

Future studies in our high CO₂ sites will include long-term carbon dioxide and temperature measurements, experiments that will determine how carbon dioxide enrichment contributes to the depletion of soil nitrogen (an important plant nutrient), and more attention to plant community structure.