The Forest Floor’s Role in Slowing Water Loss
By: Tyjaha Steele
When rain falls in a forest, not all of it reaches streams or is taken up by trees. Some returns to the atmosphere through evaporation, while some remains in the soil, providing the moisture plants and wildlife need to survive. Although scientists have long known that forests help conserve water, it has been difficult to determine whether the shade created by the canopy or the layer of fallen leaves covering the forest floor does more to slow water loss.
One reason that question has been difficult to answer is that canopy shade and leaf litter naturally change together as forests grow, recover from disturbances, and respond to management practices. That makes it difficult to determine how much each contributes to slowing water loss on its own. To attempt to answer that question, the team developed a series of experimental simulations that separated the effects of canopy shade and leaf litter.
Using those simulations, researchers from the University of Georgia’s Savannah River Ecology Laboratory (SREL), including Doug P. Aubrey, associate professor at SREL and the UGA Warnell School of Forestry and Natural Resources, recreated different levels of canopy shade and leaf litter under controlled conditions. By changing one while holding the other constant, they were able to isolate the role each plays in slowing evaporation from the forest floor, something that is difficult to accomplish in an intact forest.
“In earlier research, our group found that the amount of water evaporating from the forest floor of very young forest stands was quite a bit higher than we had expected. We made those measurements for a few years as canopy leaf area increased and forest floor leaf litter accumulated through early stand development. We saw that the decline in forest floor evaporation through those first few years could be explained by the cumulative amount of leaf litter inputs, but the forest canopies had been damaged from a disturbance, so we couldn’t perform an inclusive analysis and attribute the response to just litter,” shares Aubrey. “Although it seemed intuitive that the litter layer would exert stronger controls than canopy shading, we didn’t have conclusive data, so we conducted this experiment. So, why were we interested in understanding the relative importance of these factors? Well, this type of fundamental knowledge regarding the relative influence of different forest structural components on hydrologic fluxes can inform forest management and perhaps lead to innovative approaches for minimizing water losses that don’t contribute to forest productivity.”
The study found that both canopy shade and leaf litter helped reduce water loss, but the layer of fallen leaves had the greater effect. As more leaf litter accumulated on the forest floor, water was retained for longer, suggesting that the leaves form a protective layer that slows moisture loss from the soil.
As forests change over time, so do the conditions that influence evaporation. Trees grow, storms open the canopy, leaf litter builds up and decomposes, and management activities alter both the forest floor and the trees above it.
“When enough fallen leaves accumulate on the soil surface, they essentially form a barrier and the environmental conditions under that barrier are different than above it. Evaporation is a physical process, and its rate is largely determined by the atmospheric demand for water,” says Aubrey. “When the air is hot and dry, evaporation is high. When the air is less hot and less dry, evaporation is lower. Forest structure, both the canopy and the leaf litter, modify the atmospheric demand for water. The leaf litter layer creates a microclimate that is less hot and less dry than the microclimate above the layer so the atmospheric demand for water is muted and evaporation is much lower.”
That understanding could have practical applications for forest management. Practices such as pine straw harvesting or clear-cutting remove much of the protective layer covering the forest floor, which the study found can increase water loss. Maintaining that natural layer may help forests retain more moisture, while the findings can also improve models used to predict how forests use water as they grow, are managed, and recover from natural disturbances.
The full study, Exploring Forest Structural Controls on Soil and Litter Evaporation via Experimental Simulations, was published in Agricultural and Forest Meteorology and authored by Tyler E. McIntosh, C. Rhett Jackson, Caren C. Mendonca, Seth E. Younger, and Doug P. Aubrey.