As Global Temperatures Rise, Terrestrial Plant Numbers Will Fall

Scientists have discovered that global changes such as rising temperatures and higher levels of carbon dioxide in the atmosphere are causing a drop in the availability of a vital nutrient for terrestrial plants.

This could impact the ability of forests to absorb carbon dioxide from the air and decrease the number of nutrients available for the creatures that consume them.

“Even if atmospheric carbon dioxide is stabilized at low enough levels to mitigate the most serious impacts of climate change, many terrestrial ecosystems will increasingly display signs of too little nitrogen as opposed to too much,” said study co-author Andrew Elmore of the University of Maryland Center for Environmental Science.

“Preventing these declines in nitrogen availability further emphasizes the need to reduce human-caused carbon dioxide emissions.”

The emphasis on nitrogen availability is mostly on developed, coastal regions, such as the Chesapeake Bay. However, the story is quite different on less developed land, such as the mountains of western Maryland.

This idea that the world is awash in nitrogen and that nitrogen pollution is causing all these environmental effects has been the focus of conversations in the scientific literature and popular press for decades.

What we're finding is that it has hidden this long-term trend in unamended systems that is caused by rising carbon dioxide and longer growing seasons.”

Andrew Elmore, Co-Author

Scientists examined a database of leaf chemistry of numerous species that had been gathered from all over the world between 1980 and 2017 and noticed a global trend in the availability of nitrogen decreasing.

They learned that a majority of terrestrial ecosystems, such as forests and land that has not been treated with fertilizers, are turning to be more oligotrophic, meaning fewer nutrients are available.

If nitrogen is less available it has the potential to decrease the productivity of the forest. We call that oligotrophication. In the forested watershed, it's not a word used a lot for terrestrial systems, but it indicates the direction things are going.”

Andrew Elmore, Co-Author

Nitrogen is vital for the plants’ growth and development. On the forest floor, microbes break down organic matter such as fallen leaves and discharge nitrogen to the soil. The tree retrieves that nitrogen to form proteins and grow. But, as trees are exposed to more carbon, more and more microbes are becoming nitrogen limited and discharging less nutrients to the trees.

“This new study adds to a growing body of knowledge that forests will not be able to sequester as much carbon from the atmospheric as many models predict because forest growth is limited by nitrogen,” said Eric Davidson, director of the University of Maryland Center for Environmental Science's Appalachian Laboratory.

“These new insights using novel isotopic analyses provide a new line of evidence that decreases in carbon emissions are urgently needed.”

In Europe and the U.S., regulations on coal-fired power plants have decreased the amount of nitrogen deposition as a result of clean air regulations attempting to fight acid rain.

Simultaneously, increasing the levels of carbon dioxide in the air and longer growing seasons are boosting the nitrogen demand for plants to grow.

“There are now multiple lines of evidence that support the oligotrophication hypothesis,” said study co-author Joseph Craine, an ecologist with Jonah Ventures.

Beyond declines in leaf chemistry, we are seeing grazing cattle become more protein limited, pollen protein concentrations decline, and reductions of nitrogen in many streams. These dots are starting to connect into a comprehensive picture of too much carbon flowing through ecosystems.”

Joseph Craine, Ecologist