A forest plot that received variable-density retention treatment. Photo courtesy of Alexander Goreman.

Big trees, small trees, finding strengths in diversity


For Chelsea Obeidy, an environmental science graduate from Humboldt State, a small senior capstone project studying how different factors affect seedling growth became a publication.

“I approached professor Pascal Berrill with the idea to monitor seedling growths for my senior capstone project,” Obeidy said. “I was interested in seedling monitoring on the variable density retention patches that Berrill and Christa Dagley had developed.”

Variable-density retention is a method of forest restoration that originated from HSU, according to Dr. Pascal Berrill, a professor in the department of forestry and wildland resources at HSU. In variable-density retention, overpopulated trees are thinned to maintain different densities while underpopulated trees are planted at different densities to encourage a natural-looking restoration. To apply this method, an area of the forest containing these trees can be divided into small regions or patches that receive varying levels of tree thinning and planting.

Pascal Berrill, Obeidy’s research advisor, applies this method to restore the growth of conifers, specifically Douglas firs, in a forest area that has been dominantly populated by tanoaks.

“About 50-60 years ago, people came in to harvest wood from the forests in California,” Berrill said. “When they did this, they only harvested Douglas firs and left the smaller hardwood trees, or tanoaks alone. Douglas firs are much more profitable comparing to tanoaks.”

Alexander Goreman is a forestry senior at HSU on Berrill’s team.

“Douglas firs are big trees and their trunks are straight, which makes them easier to process into lumber for different usages,” Goreman said. “Tanoaks have twisted trunks, which makes them harder to process and therefore, are only profitable as firewood.”

Clearing away Douglas firs provides space for tanoaks to thrive and take over the forest. An increase in tanoaks density is a cause for concern.

“Douglas firs are very fire resistant while tanoaks are not,” Berrill said. “Having a high density of tanoaks pose a greater wildfire threat to the forest comparing to Douglas fir. Introducing Douglas fir back into the population would decrease that risk.”

In addition to decreasing wildfire, increasing Douglas fir population while decreasing tanoaks population would lead to more tree diversity in the forest system.

“Increasing tree diversity would allow the forest to better fight off diseases among trees,” Obeidy said.

“Restoring Douglas fir population to meet the area’s original Douglas fir-to-tanoaks ratio would also encourage populations of animals that are native to the area to come back,” team member Goreman said.

To test which patch in the variable-density retention treatment produced the best growth for Douglas fir, Douglas fir seedlings were planted in each of the patches, then students collected growth measurement on the seedlings. The students also recorded factors that might affect seedling growth, such as light availability to the plants and resources competition.

“We found that below-ground competition was the factor that affected seedling growth the most,” Goreman said.

Below-ground competition occurs when plants decrease one another’s growth by competing for resources within the soil. The team found that when there were more tanoaks surrounding the Douglas firs seedlings, and at closer distances, seedling growth was greatly decreased. This is an indication that below-ground competition may be contributing to the change in growth.

Conducting research with Berrill was a valuable experience for Goreman.

“This was a cool opportunity for me to learn how to apply the things I learned in my classes and to collaborate with students from other departments,” Goreman said.

Obeidy said his experience conducting research benefited his academic career.

“This project specifically helped me find a beneficial approach to the scientific method, delineate a hypothesis and present the results in a meaningful way,” Obeidy said.

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