Grand Challenges: Drought and the Global Carbon Cycle
Adam Wolf, Steve Pacala, and Kelly Caylor
A research project funded by the Princeton Environmental Institute’s Grand Challenges Program
http://www.princeton.edu/grandchallenges/
Abstract Drought has profound consequences on vegetation, leading to changes in the carbon cycle on short and long time scales. These changes include decreases in instantaneous carbon uptake; damage that limits future uptake for the life of the plant; mortality that can lead to large sources of carbon to the atmosphere; and shifts in biogeography that alter future potential for carbon uptake and capacitance. These processes are largely absent from global models, for lack of understanding in how co-occurring plants compete for water, weak understanding of how plant hydraulics is coordinated to minimize risk of drought, and few empirical data to constrain superior models of these processes. We propose a set of field and lab experiments to quantify (1) the degree to which plant competition for water below ground is mean-field, using an isotopic labeling experiment and (2) hydraulic constraints to water movement in trees. The work will closely involve undergrads for lab and field work, who will go on to take a class we will offer on design of wireless sensor networks. The training in programming and electronics will empower students to produce outlier thesis work, combining skills that are rare in this field.
“I have found a flaw in the model that I perceived is the critical functioning structure that defines how the world works, so to speak.”
– Alan Greenspan, testimony to congress, October 23, 2008
Scientists pondering the impact of droughts on the carbon cycle may well learn from Alan Greenspan’s rumination on the global financial liquidity crisis. Economic planners are faced with predicting the aggregate behavior of the global marketplace, but the actors in this marketplace are individual agents making decisions on perceived scarcity. Consequently, collectively beneficial actions are elusive, because investors pursue individualistic agendas to maximize profit, or at least avoid bankruptcy. The analogy of financial markets with forests is fairly direct, because global change introduces changes in resource availability, which in turn alters the relative value of these resources in different ways for different species. As in the global financial collapse, the disconnect between the macroscopic way systems are perceived and the atomistic way they operate is relevant to improve carbon cycle models in their treatment of the water cycle and especially drought.
There are three interdependent determinants of ecosystem water balance that we wish to constrain. The first two are characteristics of individual species, namely the control at the leaf of the evaporative demand for water, and the control in the wood of the failure point when hydraulic demand is too great. Leaf control is critical to avoiding runaway embolism that occurs under water deficit. This embolism is a primary determinant of hydraulic failure leading to drought mortality. Leaf closure is an adaptation to avoid hydraulic failure, but it too can lead to death if it prevents sufficient photosynthesis. Thus, these two phenomena should be coordinated with each other and with the carbon economy of the individual tree. This leads to important differences among species to avoid or endure drought, but this tradeoff is not quantified sufficiently to parameterize a global model.
The third determinant of ecosystem water balance is the competition among neighboring trees for shared water resources. At issue is the way water is depicted in most models – as a private fiefdom belonging to each individual – and the way we currently understand it, as an open-access commons. This becomes a critical distinction when water becomes limiting, because the common understanding of plants is that they can control the efficiency of carbon gain per water lost, which is a questionable strategy when plants are stealing water from one another. We are aware of no studies that have directly examined the extent of belowground theft.
These hydraulic phenomena shape globally significant responses of the carbon cycle, particularly mortality under dought. However, there is no model that incorporates these processes to predict consequences at the global scale. The research outlined in this proposal is intended to fill this gap. Consquently, this work will be at the forefront of carbon cycle adaptation in response to drought. The work will also link with other funded and proposed research by the PI’s to NSF to improve understanding drought impacts on ecosystems, and NSF & SESYNC to create policy-focused networks to monitor and manage natural resources in light of drought.