Wildfires are increasing globally in severity and duration, and they are also becoming more widespread. Because of this, areas of dense vegetation and tree growth are in danger of being destroyed or damaged in high-risk areas, which include countries or regions with hot and dry climates.

Wildfires have become rife in some areas of the U.S., including California, causing destruction to forests and residential areas. Moreover, they have also recently raged in Europe and Asia, causing various degrees of destruction and loss of life.

They are also a risk to historic forests, which have thrived for centuries. Researchers from the U.S. who recently published in the Open Access journal Fire investigated how to protect the Emerald Point forest at Lake Tahoe in California from wildfires using tree thinning and prescribed burns.

Protecting old-growth forests against wildfires

Forests have defined lifespans and ages; ancient forests are classified as “old-growth”.

Mature and old-growth forests are crucial for ecosystems, as they have various roles, including preserving biodiversity and providing protection for sensitive species. In addition, they can help to offset climate change as they capture and store large amounts of carbon.

Mature and old-growth forests are invaluable to the planet, especially in the fight against climate change. We must act to protect these historic forests as they become more vulnerable to natural and human disturbances such as wildfires.

Emerald Point Forest at Lake Tahoe in the Sierra Nevada of California is one of the last remaining old-growth stands at lake level. It is at a high risk of wildfires due to its high tree density, as it has not experienced a wildfire in over 120 years. Moreover, severe wildfires have burnt the surrounding areas of Emerald Point, including the Emerald Fire in 2018 and the Caldor Fire in 2021, which destroyed old forest growth.

Emerald Point supports the largest remaining pine trees in the Lake Tahoe Basin, as well as supporting a range of other tree species.

Old-growth forests have been reduced significantly in recent years; 150 years ago, old-growth forests such as redwoods dominated the west of California, and now less than 3% of the original forest remains.

Prescribed burning to protect old growth forests

Prescribed burning by humans is an ancient practice; it has been used for generations by indigenous peoples to manage wildlands. They were also used to control pest species and to clear land for village sites to improve access and travel.

Today, prescribed burning is used as a land management tool to reduce the risk of wildfires by decreasing vegetation. It can be used to decrease the amount of fuel that may be used during hotter, drier weather conditions, which often escalates the intensity of wildfires.

“I know it sounds cliché, but we need to fight fire with fire,” explains lead author on the study JonahMaria Weeks, University of California.

“When it comes to the conservation of old-growth stands like the one at Emerald Point, prescribed fire is an essential management tool in reducing the risk of complete loss due to catastrophic wildfire.”

The researchers from the University of California and the University of Nevada, Reno, modelled fire behaviour under severe fire conditions using a network of 15 field plots in Emerald Point, first established in 2011. The plots were originally established to monitor the health of the trees and to provide the ability to monitor forest change over time. These plots were resampled in 2017, and the authors utilised these data for the current study.

The sampling was based on the USDA Forest Service Common Stand Exam (CSE) protocol, including detailed measurements of the trees, fine fuels, coarse woody debris, ground cover, and regeneration.

Assessing the risk of wildfires

The researchers assessed the potential wildfire risk to Emerald Point by modelling potential fire behaviour using data from 2017 plot data and the Fire and Fuels Extension to the Forest Vegetation Simulator version 2022 (FFE-FVS). The FFE-FVS is a model that simulates the effectiveness of proposed fire and fuel management activities in the context of potential fire effects on long- and short-term tree dynamics crucial to silviculture, wildlife habitat, and fuel hazard. They used this model to simulate four fuel management scenarios to test the efficiency of each in reducing fire risk.

The authors describe their more conservative scenario as no management (NM), which included no thinning or fuel removal.

Their most intensive management scenario (TFRX) involved targeting pre-1850 historical forest conditions as a target, removing trees that are 8–32 inches in diameter and then conducting a prescribed fire in fall season conditions, which excluded P. lambertiana, as requested by park managers. The fall prescribed burn was stimulated using a temperature of 15°C and no wind.

The researchers also trialled a hand tree thinning scenario (HTP) followed by pile burning as a fuel reduction method. This involved all trees (excluding P. lambertiana) less than 30.5 cm being cut and piled. This resulted in a total of 80% of all surface fuel being piled and burnt the following year.

Their final fuel management scenario (SRX) was a spring prescribed fire unpaired with other fuel reduction methods; the burn was stimulated using a temperature of 15.5°C and no wind.

Protecting old-growth forests using fuel management

The researchers identified that fuel management scenarios with thinning and controlled burning are most effective for protecting mature and old-growth forests. Moreover, they further explain that of the four management scenarios compared, the TFRX and HTP suffered minimal tree loss, especially from large trees, in comparison to the other methods without thinning. Additionally, they identified that the TFRX scenario based on historical conditions was most likely to protect old trees at Emerald Point.

“The conservation of old growth in dry conifer forests of the American West is impossible without due consideration and mitigation of wildfire risk. After 100+ years of fire suppression and the loss of most of the old trees on our landscapes, it is reckless and short-sighted to think that mere protection of old growth in fire-prone landscapes will conserve it.” – senior author of the study Dr Hugh Safford.

Safford further explained that conserving old-growth forest at Lake Tahoe and other fire-dependent ecosystems means actively managing the forest in ways that replicate the essential ecological roles of fire.

If you would like to read more about how scientists are working to protect against the threat of wildfires, see our recent article on how New Technology Could Help Diminish Wildfires More Effectively.

If you would like to read more research on this subject or submit research in this area, please see the Open Access journal Fire Special Issue: Effects of Fires and Possible Restoration Interventions in Mediterranean Forest Ecosystems.