Where Does the Rain Go?

Rainfall interception varies with forest age and structure

rain-on-leaves
Leaves are just one intermediary between precipitation and the soil. Other parts of the forest, like tree bark and forest floor litter, also acts as points of rainfall interception. Photo courtesy of Pixabay.

Millions of people depend on the forests of the Southern Appalachian Mountains for drinking water. As climate, land use, and land cover changes alter the forest structure in these mountains, they also alter water budgets.

“The Southern Appalachian Mountains are a humid montane environment – they are essentially a cooler version of the tropics,” explains USDA Forest Service project leader and research ecologist Chelcy Miniat. “With so much rain coming in, it is important to understand how the forests use the water, especially when the remainder becomes the South’s drinking water supply.”

A forest’s water budget represents the inputs and outputs of the system (think precipitation in and streamflow out). It’s influenced by several factors: climate, evapotranspiration, and soil infiltration. These factors can vary by tree species, age, and overall forest structure.

As researchers attempt to understand the forest water budget, it is necessary to consider how every step of the water cycle is affected by the variation across forests. In a recent study, Miniat and colleagues looked specifically at interception – the water from precipitation that does not make it into the soil because it is evaporated from plant or litter surfaces instead.

“Existing models treat interception estimates very simply. And though it is just one piece of the story, it is important to properly understand how the age and structure of a forest will affect the amount of water intercepted by trees, plants, and floor litter,” says Miniat.

The study was led by Steven Brantley, formerly a post-doc with both the Forest Service and the University of Minnesota and was published in the journal Ecohydrology. It is part of a larger research initiative aiming to fully model the hydrologic budget as a function of forest age and structure.

“Holding species and other compositional factors, like climate, constant, we used empirical data from forest stands of different ages to determine and model interception rates. We tested our models to see how interception could be predicted by the stand age, the basal area of the stand, and the leaf area index (LAI),” says Brantley.

Basal area is a measurement of the land area occupied by tree trunks at their base. LAI is a measurement of canopy cover. The study included stands from 2 to 200+ years old and used a series of water capture methods and models to approximate interception.

Stand age was actually the best predictor of interception; interception increased with the age of the stand. In other words, older trees saw more precipitation lost to evaporation than did younger ones.

roan-mountain
There are more than 44 canopy species in the southern Appalachian Mountains. This extensive diversity needs to be taken into account when modeling forest dynamics – whether water, nutrient, or growth cycles. Photo by Ken Lane, via Flickr.

Surprisingly, basal area and LAI were poorer predictors of rainfall interception.

“LAI is what most models use to predict interception. However, we found that you can have an order of magnitude difference in interception within one forest even when LAI is similar,” says Brantley.

Thus, modelers cannot assume that interception remains constant across forest age, nor is it best predicted by structural measurements like basal area and LAI.

The results suggest that when it comes to interception, there are certain factors related to stand age – like bark texture and bark storage capacity – that are more significant than previously understood.

“Consider the difference between old and young red maple trees,” explains Brantley. “A young red maple tree is short and has smooth bark – this makes it easy for water to flow down and reach the ground (known as stemflow). Old red maple trees, however, are tall with shaggy bark. Their stemflow, comparatively, is significantly limited.”

While climate change and increased water demand may stretch the water budgets of southern Appalachian forests, forest management prescriptions can also impact their water budgets.

“We found that net precipitation is different for forests of different ages, even if they receive the same amount of rainfall falling on the canopy. Normal management cycles affect forest age. Because younger forests return funnel more precipitation down to the soil, this is another factor for forest managers to consider,” says Miniat.

“Knowledge comes in increments. This research is one step in building the knowledge to develop species-specific models of forest water use within the southern Appalachian Mountains. And this knowledge is an important component of understanding human impacts on natural forests,” says Miniat.

Read the full text of the article here.

For more information, email Chelcy Miniat at chelcy.f.miniat@usda.gov.

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