As rain falls on forest canopies, its journey begins. As raindrops reach Earth, they ferry creatures and contaminants to soils and streams below. Researchers have only recently begun to explore the fine details of this journey, as evidenced by a session on “Precipitation Partitioning by Vegetation” at AGU’s virtual Fall Meeting 2020.
When a raindrop falls over land, it might bounce off leaves or slide down tree trunks before reaching the ground. Depending on where it lands, that drop will eventually contribute to a river, be absorbed into a forest floor, or evaporate back into the atmosphere. This distribution of precipitation by trees and shrubs is often the first step in the terrestrial hydrologic cycle, yet fundamental data on its consequences remain relatively sparse.
“We tend to ignore canopies as an interface for water to reach the Earth’s surface,” said John Van Stan, an ecohydrologist at Georgia Southern University. “But they connect to so many aspects of an ecosystem. They’re the first thing that controls where water goes.”
More Questions Than Answers
When hydrologists consider what happens when rain filters through trees and plants, they confront a host of important ecological and societal questions: How much rainfall actually reaches an aquifer? How does clear-cutting a forest affect local weather? How do urban trees aid storm water management?
A growing set of research projects has focused on this botanical portion of the hydrologic cycle. These studies accompany the rise in popularity of critical zone science, which investigates the connectivity of the “thin living skin” that coats Earth—from treetops to bedrock. Measuring rain, fog, and snow within the convoluted texture of a forest, however, is no easy task.
“Because it’s such a challenging measurement to take, [canopy water flow] has really been overlooked,” said Ethan Gutmann, a hydrologist at the National Center for Atmospheric Research in Boulder, Colo. “But we’re finding more and more, especially in water-limited environments, that it may be a very large component of the water cycle.”
Early this year, Van Stan, Gutmann, and their colleagues published a book that synthesized past and present advances and ongoing knowledge gaps about the transport of water along the atmosphere-plant-soil continuum. The Fall Meeting session includes a number of posters related to particulate transfer by precipitation and how human disturbances, such as forest thinning and fire, change the capacity for canopies to store water.
Gutmann said he’s particularly excited about one study, led by Dominick Ciruzzi at the University of Wisconsin–Madison, in which a team attached accelerometers to street trees to measure how much rainfall they intercept on their leaves. The study showed that rainfall bound up in trees reduced the amount of water that reached the ground below. When taken together, thousands of trees in an urban area could be a sustainable tool to mitigate flooding related to heavy rains.
According to Van Stan, one of the most understudied details of precipitation partitioning is the vertical transfer of biological materials from canopies to the soil below. “Trees are really dirty, just covered in lots of organic matter,” such as feces, fungal spores, bacteria, and metazoans critical to how ecosystems function, Van Stan said.
The numerous, tiny rivulets that form on branches, trunks, and stems during storms are like miniature Amazons capable of transporting carbon and nutrients in volumes comparable to those of large streams and rivers. Yet “we know practically nothing about it,” Van Stan said. Indeed, he noted, hydrologists interested in canopy water traditionally filtered out the organic matter and tossed it in the trash.
By directing water and nutrient flow within the critical zone, plants influence numerous biogeochemical cycles. But the extent of their influence remains a mystery.
“There are always modeling papers saying we still don’t have the best handle on the carbon cycle,” said Benjamin Runkle, who studies carbon and water cycling in agricultural systems at the University of Arkansas and was not involved in the session. Understanding the subtle yet powerful ways plants shape large-scale systems like weather, erosion, and carbon transport is critical to building better predictive models.
“To really get the numbers right,” Runkle said, researchers need to pay more attention to the details, one drop—on one leaf—at a time.
—Cypress Hansen (@PollenPlankton), Science Writer