Principal Investigator

At a Glance

The Pacala group’s CMI research in the last year has embraced a large number of topics, including the final report of the Princeton Net-Zero America project. The group also produced or co-produced a series of papers, including: (1) a paper on possible failure modes of President Biden’s U.S. decarbonization agenda; (2) a paper on the need for separate national targets for CO2 and methane emissions; (3) two papers (one in Science) on how fire and other disturbances work against land-based climate solutions); and (4) multiple papers (one in Science) that improve the capacity of climate models to represent the carbon cycle in tropical forests and other ecosystems. Although not directly supported by CMI, Stephen Pacala chaired the effort of the National Academies of Science, Engineering, and Medicine that produced a peer-reviewed policy manual for a U.S. transition to a net-zero economy, which extensively used the Net-Zero America report and was a primary reason that CMI initiated the Net-Zero America effort in the first place.


Research Highlight

The Pacala group’s work over the past two years illustrates how curiosity-driven research by CMI continues to produce applied dividends. Over its history, CMI has contributed to NOAA’s Earth System Model (ESM) that predicts climate. The Pacala group developed the fundamental equations that govern the terrestrial carbon and hydrologic cycles in the model, which were implemented by the team led by Elena Shevliakova at NOAA’s Geophysical Fluid Dynamics Laboratory, some of whose work was also supported by CMI. That model and other ESMs that use these equations spontaneously predict the coexistence of plant species. This is interesting because both biodiversity and climate are imperiled. These problems might be addressed within the same modeling system, including threats to biodiversity from climate change.

Matteo Detto, Jonathan Levine and Stephen Pacala developed mathematical methods to examine this question in a simplified version of the land model, which retains its critical features. This work was published in a paper in Ecological Monographs this year. One of the analyses examined the so-called shade tolerance tradeoff, which is thought to underpin successional diversity in forests. Tree species are arrayed along an axis that separates those that grow quickly in full sunlight but die quickly in shade from those that grow slowly in the sun but survive in the shade. The researchers showed that this tradeoff can maintain the coexistence of a theoretically infinite number of species.

Relevance to bp

Some scientists argue that forest carbon storage would be maximized by offsets projects that promote rapidly growing species, so that carbon is stored quickly. Others advocate trees capable of attaining very large size, which maximizes the total amount of carbon that will ultimately be stored. Giant, rapidly growing, but shade intolerant trees, like members of the tropical genus Ceiba, offer both attributes – rapid growth and large total carbon storage. The analysis shows that stands composed of both these giants and slow growing shade tolerant species will store more carbon than either type in pure stands. This is because the shade tolerant species form an extensive carbon-storing subcanopy beneath the giant species (Figure 10.1). More surprisingly, when a multispecies forest is co-managed for biodiversity and carbon storage, the shade tolerant species allow the forest to store carbon after a disturbance that destroys the canopy trees, because their high low-light survival means that many are waiting in the understory to rapidly restore the canopy. Thus, the biodiverse forest will both store more carbon and increase resilience by restoring lost carbon faster after disturbance.

Figure 10.1.
Diagram of a multilayered tropical forest with giant shade intolerant species on top and adult shade tolerant species and young trees beneath. ( contant/view/1040)



Detto, M., J. M. Levine and S. W. Pacala, 2021. Maintenance of high diversity in mechanistic forest dynamics models of competition for light. Ecological Monographs. (

National Academies of Sciences, Engineering, and Medicine. 2021. Accelerating Decarbonization of the U.S. Energy System. Washington, DC: The National Academies Press. (