You don’t look your age: pruning young forests to mimic old-growth forest

“I’m always looking at the age of the forest, looking for fish, assessing the light levels. Once you’ve studied it, you can’t ignore it.” Allison Swartz, a PhD student in the Forest Ecosystems and Society program in the College of Forestry at Oregon State, is in the midst of a multi-year study on forest stream ecosystems. “My work focuses on canopy structure—how the forest age and structure influences life in streams,” says Allison. “People are always shocked at how many organisms live in such a small section of stream. So much life in there, but you don’t realize it when you’re walking nearby on the trail.”

Three scientists holding large nets stand in a rocky forest stream. One wears a backpack with cable coming out of it.
No, that’s not a Ghostbuster backpack! Here, Allison is using an electrofishing device that stuns fish just long enough for them to be scooped up, measured, and released. From left to right: Allison Swartz, Cedar Mackaness, Alvaro Cortes. Photo credit: Dana Warren

Following a timber harvest, there is a big increase in the amount of light reaching the forest floor. The increase in light also results in an increase in stream temperatures. Fish such as salmon and trout, which prefer cold water, are very sensitive to temperature changes. Since these fish are commercially and recreationally important, Oregon’s water quality regulations include strict requirements for maintaining stream temperatures. As a result, buffer areas of uncut forest are left around streams during timber harvests. These buffer areas, like much of the forests in the Pacific Northwest, and in the United States in general, can be characterized as being in a state of regeneration. Dense, regenerating stands of trees from 20-90 years old, are sometimes called second-growth forest. These forests tend to let less light through than an old growth forest does. Allison’s work focuses on how life in streams responds to differences in forest growth stage.

A Pacific giant salamander – a top-level stream predator and common resident of Oregon’s forest streams. Photo credit Allison Swartz.

The definition of the term old-growth forest depends on which expert you ask, and there is even less agreement on the concept of second-growth forest. Nevertheless, broadly speaking an old-growth forest has a wide range of tree species, ages, and sizes, including both living and dead trees, and a complex canopy structure. Openings in the canopy from fallen trees allow a greater variety of plant species to be established, some of which can only take root under gaps in the canopy but which can persist after the gap in the canopy is filled with new trees. The tightly-packed canopy limits the amount of light that can reach the forest floor, including the surface of the streams that Allison studies.

Forest stream near Yellowbottom Recreation Area, Oregon. Credit: Daniel Watkins

Allison’s research project is focused on six streams in the MacKenzie river basin, which includes private land owned by the Weyerhaeuser company, parts of the Willamette National Forest, and federal land. At some of these sites, after an initial survey, gaps were cut into the forest canopy to mimic light availability in an old growth forest. Sites with cut canopies were paired with uncut areas along the same stream. The daily ebb-and-flow of aquatic species is monitored by measuring the oxygen content of the water. The aquatic and terrestrial ecosystems have mainly been studied separately, she explained, but the linkages between these systems are complex. Measurements of vertebrate species are carried out using electrofishing techniques. “We do vertebrate surveys which infludes a few species of fish and Pacific giant salamanders. We measure and weight them and then return them to the stream,” Allison explained.

Measuring cutthroat trout. Photo credit Allison Swartz.

Over the last few years, Allison has spent three months of the summer living and working at one of her research sites, the HJ Andrews Experimental Forest. “We didn’t have much in terms of internet the first few years, so you connect with people and with the environment more,” Allison said. 

Allison never expected to be in a college of forestry. Her background is in hydrology, and she spent some time working for the United States Geological Survey before beginning graduate work. She has enjoyed being part of a research area with such direct policy and management impacts. “We all use wood, all the time, for everything. So we can’t deny that we need this as a resource,” says Allison. “It’s great that we’re looking at ways to manage this the best we can—to make a balance for everybody.”

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