Seeing the Forest Through the Trees

Joel Biederman is a research hydrologist at USDA ARS’s Southwest Watershed Research Center in Tucson, AZ. Dr. Biederman conducts research on making our nation’s forests more resilient, preserving the water supplies flowing from these forests, and designing forests that are less prone to fueling severe wildfires. Dr. Biederman coauthored a Snowtography handbook – which provides guidance on monitoring snowmelt in forests – that is used by public land managers, water and hydropower utilities, conservation groups, farmers, ranchers, and local residents of rural communities.

Welcome Dr. Biederman, to Under the Microscope: 

UM: What is the current state of our nation’s forest?

JB: Much of our nation's forest is crowded, with too many trees growing too close together. This is particularly common in the semiarid Southwest.  When forests become crowded, the trees struggle against their neighbors for access to sunlight, water, and nutrients. This is similar to when we plant carrots in a garden; we need to thin out the young carrots so that the ones left have enough room to thrive and produce a good crop.   

UM: How did we get to this condition?

JB: One important factor is exclusion of fire from our forests. Since about 1900, American firefighters have been mostly successful in extinguishing forest fires. For thousands of years before 1900, many forests were adapted to experience fire every few years or decades. When fire recurred frequently, there was less accumulated fuel since the last fire, and the fires burned less hot. These low-severity fires could burn slowly along the forest floor, cleaning up dead grasses and branches, recycling nutrients into the soil, and naturally thinning the young saplings, without killing many mature trees.   

ARS Research Hydrologist Dr. Joel Biederman leads installation of Snowtography stations to measure
how forest management affects snow water resources in the Coconino National Forest, Arizona.

UM: How do these conditions affect prevalence and severity of wildfires?

JB: With fire excluded for the past century, many of our national forests are piled high with dead grasses, leaves, and wood, which provide ample fuel for a fire. Next, if the living trees are overcrowded, then they may struggle to extract adequate water from the soil. Water-stressed trees have lower moisture content, which makes them more likely to burn in a fire. Finally, the last 50 years have seen increasing drought and warmer weather in many forests, which further lowers tree moisture. Together, these factors are contributing to forest wildfires that are more prevalent, larger in size, and, most importantly, more severe. With abundant dry fuel, fires are more commonly burning hotter, spreading faster, and becoming more difficult to control.   

UM: How do increasingly large and severe fires affect the water supplies of farmers, ranchers, and America's rural residents?  

JB: Forested mountains are a vital source of water for over 50 million Americans and their farms, homes, and businesses. In fact, the provision of a clean, abundant water supply is part of the founding mandate of the U.S. Forest Service, our big sister within USDA. Large, severe fires can impact water supplies in two key ways. First, hot, severe fires can bake the soil like a clay pot being fired in a kiln, reducing that soil's ability to soak up water from rain or snowmelt. This can lead to severe flooding, mudslides, and pollution of rivers and reservoirs with sediment and ash for downstream farms, ranches, and residents. 

Second, a healthy forest provides shade and shelter from sunlight and the drying effects of the wind. Since much of our forest water supplies come from snowmelt, losing most of the trees in a high-severity fire can cause more of the snowpack to evaporate into the atmosphere, leaving less snowmelt to nourish the trees and replenish downstream aquifers, rivers, and reservoirs. Exposed snowpack also melts earlier in the spring, reducing water supplies in late summer, when they are often most valuable to farmers, ranchers, and anglers.   

ARS Research Hydrologist Dr. Joel Biederman collects data from a snowfall measurement gauge as part of a study addressing how forest management affects snow water resources in the Coconino National Forest, Arizona.

UM: What do we need to do to save our forests, lower the risk of wildfires, and ensure a consistent water supply?

JB: Some important things we can do to help are reduce fuel loads out there now in our forest, harvest some of the trees in overcrowded forests, and then allow frequent, low-severity fires to do the work of keeping the forests regularly cleaned and thinned, wherever it can be done safely and the tree species are historically accustomed to low-severity fire.   

UM: Describe your snowpack research. How can it help solve these problems?

JB: Our goal is to design forest thinning harvests that most effectively conserve snowpack water resources for future forests and downstream water supplies. The U.S. Forest Service and its partners are currently removing built up fuels and thinning overgrown forests across millions of acres of our nation's forests. Our research aims to reduce the snowpack evaporated to the atmosphere and conserve snowpack on the ground.  

There are two goals here. One is to enhance the water available to the remaining trees to reduce the risk of high-severity fires that kill all the trees, glaze all the trees, and cause floods and mudslides. The other goal is to enhance the water recharging the aquifers, rivers, and reservoirs critical to American farms, ranches, businesses, and more than 50 million citizens.   

ARS Research Hydrologist Dr. Joel Biederman collects a snow sample to determine
its water content in the Coconino National Forest near Flagstaff, AZ

UM: How does this tie in with what you call spatial harvesting?

JB: When we selectively harvest some trees and leave others, the resulting mosaic of forest patches and clearings can help conserve snow. The idea is to remove enough trees that snow can make it down to the ground, rather than being intercepted in the canopy and evaporating. Then the snow on the ground can be sheltered from sun and wind by patches of neighboring forest. This works in part because the sun hangs low in the winter sky, and trees cast long shadows.  

Our spatial harvesting research aims to find the optimal size and arrangement of forest patches and gaps that benefit snowpack, conserve water resources, and nourish a thriving future forest. The optimal configuration can vary a lot with factors such as where we are in the country, what species the trees are, and whether we are on level ground or a north or south facing hillslope.   

UM: This research is not new. In fact, experiments have been done since at least the 1900s. How is AI and today’s technological tools helping to implement spatial harvesting?

JB: Several key technologies are helping us turn a corner from observing the effects of tree harvest on snow at small scales to actually designing water-conserving forest harvest prescriptions. First, with our collaborators at other agencies, universities, and water utilities, we have built a network of two dozen Snowtography stations across the mountainous forests of the Colorado River Basin, which is the single most economically vital water supply in the West. Snowtography uses automated cameras to photograph the snow depth at graduated stakes strategically placed across mosaics of forest patches and clearings to learn what configurations conserve snow. We also use lasers shot from airplanes to map the snow depth. 

Second, we feed all these measurements into a complex computer simulation model. This model learns how that particular forest interacts with snowfall, sunshine, and other weather to regulate the snowpack. Since our snow model is complex, it takes a high degree of expertise and a long time to run on a computer, so we run it over only a few dozen acres of forest. Recently, we have developed AI tools that can be trained from our complex model. These AI tools can then assess how spatial harvest affects snowpack over many thousands of acres, thereby helping forest managers make water-smart harvest plans. 

ARS Research Hydrologist Dr. Joel Biederman weighs a snow sample to determine its
water content in the Coconino National Forest near Flagstaff, AZ.

UM: What other measures are needed to improve our nation’s forests?

JB: Mechanically thinning our forests is expensive. And if the trees have become badly overgrown, they are often too slender to be of high economic value. Therefore, in many forests it would be beneficial to spatially harvest them once and then let carefully managed fire do the work of keeping fuels accumulation low and thinning young trees so that the hardiest can survive and thrive. 

UM: What can farmers, ranchers, and residents do to help?

JB: Farmers, ranchers, and rural residents often have deep appreciation for our agroecosystems and their natural rhythms. They speak to us about drought, extreme weather patterns, and the ever-increasing importance of clean, abundant water supplies from our nation's forests. Together with forest and water managers, we can work together to selectively harvest our forests, improve snow water resources, and restore a more safe, economical, and sustainable balance among fire, forests, and water. 

For more information check out the Snowtography Handbook

And for the latest research on this subject, click here.