CALIFORNIA – Experimental forests are enshrouded in a sense of mystery; places unfamiliar to many, even those familiar with national forests as a whole. The Pacific Southwest Research Station houses 12 of those mysteries, and their research is helping bring the importance of experimental forests to light.

Located in Central Sierra Nevada, the Stanislaus-Tuolumne Experimental Forest is a priority landscape of the agency’s Wildfire Crisis Strategy. PSW Research Ecologist Eric Knapp has led on site tours for environmental groups, timber company representatives and Forest Service leaders, including Chief Randy Moore.

In 2006, Knapp uncovered historic plots at the site, dotted with sugar pines and other conifers, dating back to the 1920s. “Many of the original tags were still on the trees. It enabled us to see how old growth would have looked a century ago before fire exclusion and logging,” said Knapp.

Using research plots, which are among the oldest known on Forest Service lands in California, Knapp and partners remapped the area. With the help of maps and associated data from the National Archives, they found the forest today contains nearly 2.5 times more trees than it did nearly a century ago. Most of the small gaps that were once common had disappeared. Incense cedar and white fir replaced many of the pines. The data showed that after over a century of excluding fire, from a perspective of both tree density and fuels, had made such forests highly vulnerable to drought and wildfire.  

Armed with the knowledge provided by their research, Knapp and colleagues from the Stanislaus National Forest did not do traditional forest thinning, here, which creates space between individual trees. Instead, using historical stand structure information, they developed a new method of thinning, creating diverse structures found in the original old-growth forest. They left clumps of trees with gaps in between, which benefits wildlife like spotted owls, without sacrificing overall forest health. 

Previously, UC Berkeley researchers did hydrology studies at the Stanislaus National Forest on how different tree arrangements enhance snow accumulation and reduce snowpack melt, benefiting the water cycle. Knapp also sees the potential for future forest hydrology studies on site.

And that’s right up hydrologist David Dralle’s alley. A roughly three-hour drive south from the Stanislaus-Tuolumne is the equally scenic Teakettle Experimental Forest near the mighty Kings River—one of the places Dralle works. Like the Stanislaus-Tuolumne, this is another important site for research with remnants of an old-growth forest. 

At Teakettle, and further north in Mendocino County, Dralle literally digs deep for his research. Drilling as many as 100 feet below the surface, he examines what’s percolating underground. Dralle investigates how subsurface water impacts trees in drought and how groundwater sustains streams underneath a hillslope. Thin mountain soils aren't enough to sustain California's forests, some of the most productive in the world. As a result, the trees’ root systems must extend far below the surface to slurp water like straws in the ground.

Dralle compares the water cycle to a geologic sponge. Clay can’t absorb a lot of water, whereas fractured bedrock, riddled with cracks can, recharging the groundwater supply. “People sometimes think, if there’s a lot of rainfall, then we have plenty of water, but that’s not always true. Some areas don’t need a lot of water to replenish groundwater stores, but others do.” 

Dralle is part of a long history of Forest Service scientists thinking about watersheds. Researchers at San Dimas Experimental Forest, within Southern California’s San Gabriel Mountains, have thought about water since the 1930s. Here, they study chapparal, a shrubland plant community, along with wildfire and wildlife.

Research Economist José Sánchez and Oak Ridge Institute for Science and Education fellow, Michelle Carr, are expanding that research to include recreation. “Gifford Pinchot wandered in these buildings,” Sánchez said, gleaming. He wants to connect the public to this special place.

Sánchez and Carr will restore historic buildings here using Great American Outdoors Act funding. They're surveying stakeholders and community members on their views of San Dimas. Some envision a future citizen science hub or a writers’ retreat.  All ideas are on the table.

High school students, who toured the site in April 2023, buzzed about the quiet and clean air. “San Dimas Experimental Forest is one of the few places in SoCal where you’re in chaparral and cannot hear highway traffic,” said Region 5’s Assistant Ecologist Sarah Hennessey.

The research at San Dimas and other Experimental Forests and watersheds continues to inform science. This work is even more critical now as environmental threats to our forests mount. For more information on experimental forests check out these recent presentations.