Satellite observations reveal that changes in rainfall patterns affect crops and forests

A new NASA-led study has found that how the rain falls in a given year is almost as important to the world’s vegetation as the amount of precipitation. In a Dec. 11 report in the journal Nature, researchers found that even in years with similar total precipitation, plant growth conditions were lower in years with less and more rainfall. showed that they are different.

In years when rainfall was less frequent and concentrated, plants in drier environments like the southwestern United States were more likely to grow. In humid ecosystems, such as the rainforests of Central America, vegetation tended to deteriorate, perhaps because it could not withstand long dry seasons.

Scientists previously estimated that nearly half of the world’s vegetation is driven primarily by the amount of rain that falls each year. The role of diurnal variation is poorly understood, said lead author Andrew Feldman, a hydrologist and ecosystem scientist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. Changes in precipitation patterns are resulting in stronger storms and longer dry seasons between them than they did 100 years ago.

“You can think of it this way: If you have a houseplant, what would happen if you gave it a full pitcher of water on Sunday instead of a third of a pitcher of water on Monday, Wednesday, and Friday? ” said Feldman. When you scale this to the scale of the U.S. corn belt or rainforest, the answer could impact crop yields and ultimately the amount of carbon dioxide plants remove from the atmosphere.

blooming in the desert

The team, which included researchers from the U.S. Department of Agriculture and multiple universities, analyzed 20 years of field and satellite observations spanning millions of square miles. Their study area encompassed diverse landscapes from Siberia to the southern tip of Patagonia.

They found that plants, present on 42% of the Earth’s vegetation surface, are sensitive to daily fluctuations in rainfall. Just over half of them had better results, often showing an increase, in years with fewer but more rainy days. These include cultivated land as well as arid landscapes such as grasslands and deserts.

In contrast, low- and mid-latitude hardwood forests (e.g. oak, maple, beech) and tropical rainforests tended to fare worse under these conditions. The effects were particularly pronounced in the Indo-Pacific rainforests, including the Philippines and Indonesia.

Statistically, daily precipitation variations were almost as important as annual precipitation totals in driving global growth.

red light, green light

The new research is primarily based on NASA’s suite of missions, including the Integrated Multi-satellite Retrievals for GPM (IMERG) algorithm, which uses an international satellite network to provide rainfall and snowfall estimates for much of the Earth every 30 minutes. It depends on the dataset.

To measure plant responses on a daily basis, the researchers calculated how green an area appeared in satellite images. “Greenness” is also known as the Normalized Difference Vegetation Index and is commonly used to estimate the density and health of vegetation. They also tracked the faint reddish light that plants emit during photosynthesis. When plants absorb sunlight and convert carbon dioxide and water into food, their chlorophyll “leaks” unused photons. This faint glow is called solar-induced fluorescence and is a tell-tale sign of a thriving plant.

Plant fluorescence, which is invisible to the naked eye, can be detected with instruments aboard satellites such as NASA’s Orbital Carbon Observatory 2 (OCO-2). OCO-2, launched in 2014, observed intense fluorescence in the Midwest during the growing season.

Feldman said the findings highlight the important role plants play in moving carbon around the planet, known as the carbon cycle. Vegetation such as crops, forests and grasslands form vast carbon ‘sinks’, absorbing excess carbon dioxide from the atmosphere.

“If we can better understand how plants thrive and decline from day to day with each storm, we may be able to better understand their role in that important cycle,” Feldman said.

The study also included researchers from NASA’s Jet Propulsion Laboratory in Southern California, Stanford University, Columbia University, Indiana University, and the University of Arizona.