Research: Earth's Water System

Earth's Water Cycle, Part I: Soil to Plants...

Two CEGE faculty, Xue Feng and Ardeshir Ebtehaj, explain their research on earth’s water system and the models that help them predict what might happen when water systems change.

XUE FENG conducts research to build understanding of the movement of water and carbon within terrestrial ecosystems, like forests and peatlands. She is interested in how that movement will affect the global climate.

The global climate and terrestrial ecosystems exist within a feedback loop: ecosystems affect how much water and carbon is released into the atmosphere, but how that release happens depends on how much water and energy are available in the first place. As ecosystems continue to adapt to climate change, scientists seek to learn more about the key working members – like trees and microbes – to better predict the future vulnerability of ecosystems.

Plant Water Use Strategies

In dry climates, forests are becoming increasingly vulnerable to droughts. Scientists are concerned because drought reduces a forest’s capacity to take in and store CO₂, which affects how much CO₂ remains in the atmosphere. Trees photosynthesize CO₂ into food. Yet every tree loses 2.4 molecules of H2O for every molecule of CO₂ they take in through their stomata. This unavoidable carbon-water tradeoff forces trees to adopt a variety of water-use strategies in the hope of surviving a drought.

It is still unclear which strategies work best under different “types” of drought— is the air too dry or is the soil too dry? Is it an intense drought that lasts only a season? Or a mild but prolonged drought? Trees that try to conserve water by closing their stomata early risk starvation if the drought drags on, while trees that keep their stomata open to take in more CO₂ risk losing too much water too quickly in an intense drought.

Feng uses a range of theoretical, fieldbased, and computational approaches to see how we might separate the winners from the losers in a drought (see Figure 1). Her research group and collaborators have found that (i) In addition to CO₂ uptake, the optimal stomata response to soil water stress must account for the cost of damage inflicted on the plants’ hydraulics system (i.e., their water transport organs) under high tension and the cost of losing unused soil water to neighboring competitors. (ii) The drought response depends not only on the static traits adopted by individual plants, but also on how the plants interact with dynamic changes in their environments, (iii) The negative impacts of more widespread climatic drought can be alleviated by hydrological processes that allow plants to locally access subsurface water. (iv) Besides adjusting their stomata openings, trees can adjust leaf area (e.g., by dropping leaves) in response to rainfall variability and seasonality to increase long-term carbon gain.

Feng’s group is currently investigating other aspects of plant water use strategies and how plant hydraulics systems can be better represented within complex Earth system models to improve predictions of ecosystem carbon and water use.

Peatland Carbon Emissions

Peatlands store around one-third to one-half of all soil carbon. In wet climates, too much water can make peatlands more prone to emitting methane (CH₄)—a greenhouse gas 25 times more powerful than CO₂. Peatlands store soil carbon under saturated and low-oxygen conditions that result in extremely slow microbial decomposition of plant debris. However, as the water table fluctuates and causes oscillating oxygen availability across the peat column, changes can occur in the timing and extent of microbial CO₂ production (more favored under oxygenated conditions), CH₄ degradation (also in oxygenated conditions), and CH₄ production (only under low-oxygen conditions) (see Figure 2). Currently, we have poor understanding of the hydrological response of peatland watersheds, and how that response influences peatland emissions of CH₄ and CO₂.

Feng’s group uses the Marcell Experimental Forest in northern Minnesota in combination with hydrological and Earth system models to study how hydrological and biogeochemical factors affect carbon emissions in northern peatlands. Her work has so far revealed the following. (i) Currently, predictions of water table depths and CH₄ emissions from Earth system models can vary substantially based on snow dynamics and watershed properties. (ii) Annual CH₄ emissions can be better explained by seasonal rather than annual water table elevations. (iii) Lateral flow of water from upland forests can potentially contribute a previously unaccounted-for source of water and chemicals into the low-laying peatlands. Together, these findings and other ongoing work are expected to help us better understand how water influences carbon cycling in peatlands and to improve representation of peatland hydrology and CH₄ cycling within Earth system models.