At a Glance
Terrestrial vegetation, such as trees and shrubs, absorbs atmospheric CO2, which is a boon in the global battle against climate change. But how well vegetation will continue to do so in the future remains uncertain. Modeling results show that time lags between tree growth responses to a changing atmosphere, and a subsequent increase in mortality of the larger trees, can generate short-term carbon gain but long-term carbon loss, a worst-case scenario for containing future warming.
Terrestrial biomes store about one third of the carbon dioxide (CO2) currently emitted by anthropogenic activity, keeping it out of the atmosphere where it warms the planet. Therefore, understanding the fate of carbon stored in terrestrial vegetation is central to forecasting and managing global climate change. However, whether terrestrial vegetation can continue to absorb atmospheric CO2 at current rates is unknown. This is because scientists do not yet fully understand how forest biomass will respond to the lengthening of the growing season resulting from a warmer climate. Nor do they understand how biomass will respond to CO2 fertilization, which is the increased rate of photosynthesis in plants that results from increased levels of CO2 in the atmosphere.
In extreme scenarios for the next century, terrestrial vegetation may shift from becoming a sink – a natural reservoir that captures and stores carbon – to a source of
atmospheric CO2, amplifying warming. Whether this shift occurs is dependent on how environmental change that stimulates growth at the individual tree level scales to biomass production at the level of the forest stand.
The Levine group is building models of tree demography and competitive interactions to investigate this question. These models permit a more mechanistic understanding of key processes that generate carbon storage in terrestrial vegetation.
The researchers found that time lags inherent in the population dynamics of forest trees can cause transient carbon storage. This means that stand biomass initially increases in response to growth stimulating environmental change but ultimately reverses course, releasing the stored carbon (red line in the Figure 7.1). The mechanism involves canopy trees getting larger in response to CO2 fertilization or warming. This leads to an initial rise of stand level carbon storage. However, under the assumption that larger trees suffer greater mortality, this initial gain in carbon storage at the stand level ultimately reverses as the forest thins, causing long-term carbon loss.
Predicting the trajectories of forest carbon storage is dependent on a better understanding of the size dependence of tree mortality—data that are difficult to obtain because of the known longevity of forest trees. This research is currently extending these single species results to more diverse forests where competitive interactions, in addition to changes in growth and mortality, drive trajectories of forest carbon storage.