A NASA-funded study finds that changing rainfall patterns over much of the United States in the last century have allowed plants to grow more vigorously and absorb more carbon dioxide from the atmosphere.

This map shows that for the entire continental United States there was a strong correlation between rainfall and plant growth. Therefore, increases in precipitation are likely to stimulate plant growth and carbon sequestration.Credit: C. Milesi, University of Montana, School of Forestry

In the presence of water and sunlight, plants take in carbon dioxide (CO2) during photosynthesis to create fuel, glucose and other sugars, for building plant structures. Better understanding of biological and physical processes that contribute to carbon uptake by plants will help scientists predict climate change and future levels of CO2, a heat-trapping gas in the atmosphere.

"The changes in the hydrologic cycle is one of the mechanisms that is often overlooked in the recent debate over carbon sequestration in the United States," said Ramakrishna Nemani, a researcher at the University of Montana's School of Forestry, and lead author of the study that appears in an issue of Geophysical Research Letters later this month.

Scientists have noticed that the U.S. terrestrial carbon sink, an effect where carbon is drained from the air and stored in the land, has been increasing since the latter part of the 20th century. Previous research has claimed this rise may be due to an observed greening of the U.S. as a result of forest re-growth, as well as greater concentrations of atmospheric CO2 and warming temperatures.

For the first time, however, this study suggests that changing rainfall patterns may play a bigger role in plant growth and carbon absorption. Computer model results showed that on average from 1950 to 1993 higher humidity combined with an eight percent increase in precipitation has led to a 14 percent increase in plant growth in the U.S. The data over that time period also show increases in cloud cover, minimum temperatures, soil moisture and stream flows, which are all signs of a changing hydrologic cycle.

Whether or not shifting rainfall patterns result in a positive uptake of carbon by land ecosystems depends on complex interactions that include plant physiology, and both the magnitude and timing of changes that impact the water cycle.

This map shows the large increases in carbon uptake by land ecosystems for much of the continental U.S. between 1950 and 1993. Credit: C. Milesi, University of Montana, School of Forestry

Between 1950 and 1993, in general, the minimum temperatures in the spring have become warmer, and autumns have become wetter, which have combined to lengthen the growing season for plants. A longer growing season means plants pull carbon from the air for a greater period of time. In addition, the magnitude of precipitation on average has gone up in the conterminous U.S., except over the Pacific Northwest.

"Most people only think of the idea that more water means more growth, but really plants benefit from more water in a number of ways," said Steven Running, a co-author of the paper, who is also a researcher at the University of Montana's School of Forestry.

When the air is wetter, plants can open special cells in their leaves without losing much water to the air, increasing CO2 uptake while reducing the amount of water needed to grow. Additionally, wetter soils promote decomposition of dead plant materials, releasing nutrients needed for plant growth. Also, higher humidity in the spring helps maintain higher night temperatures, which makes for more frost-free days and lengthens the growing season.

The authors found that without enhanced rainfall and humidity, CO2 increases and temperature changes have a lesser effect on plant growth.

Greenhouse gases warm the air, and warmer air can hold more water, which impacts the hydrologic cycle. Changes in the hydrologic cycle may mean more rainfall in some regions and less in other places, affecting plant growth and carbon absorption, which in turn affects future concentrations of greenhouse gases, Nemani said.

The study was funded by the Vegetation Ecosystem Modeling and Analysis Project and the Moderate-resolution Imaging Spectroradiometer science team, under NASA's Earth Science Enterprise.