You may have seen historical photos of the Sierra Nevada forest with cars driving through the big trees, or heard how a man rode his horse from the forest to the city with little trouble. Those open forests with fewer trees were more variable in structure and more robust than today’s forests. But what does that really mean? Ryan Tompkins, Forestry and Natural Resources Consultant and Registered Professional Forester with the University of California Agriculture and Natural Resources, explains the relationship between fewer trees and a stronger forest.
Photo: USDA Forest Service
What is forest resistance? It is a measure of forest resilience. It focuses on maintaining the essential structure and composition of the forest to different strains or complex disturbances. In other words, a mature forest may lose some trees to drought, fire, or insect attack, but the rate of mortality cannot exceed the forest’s ability to grow and provide habitat. A few will die, but many will live.
Today’s forests have a significant increase in basal area and tree density compared to historical records. Historically, forests were generally low density but highly variable in structure, with open patches and tree clusters. 22 trees per acre was not uncommon in the Sierras, but 22 trees was huge! Old and large trees have additional adaptations such as dense bark, high coverage and a high level of resistance to disturbance. We have normalized forests of unprecedented high density, uniform structure and high competition. In the year A legacy of logging in the early 1900s, today’s forests are vulnerable to fire and drought-related mortality. A century of firefighting policy and action and the effects of climate change, such as the recovery of less moisture at night.
Photo: Forest Management Education Initiative Program
What about the story of the forest? The resulting mosaic of fire in historical fire regimes results from the diversity of multiple understories characterized by grasses, forbs, limited shrubs, and more bare ground. Today’s thick understory means a less diverse understory, competition for resources between shrubs and small trees, and greater fuel loads to carry wildfires. Once affected by a severe fire, shrubs are more likely to take over burned areas and reach transplanted or regenerated seedlings.
What should today’s forest owners do to increase forest resilience? Depending on your overall management goals, aiming for historic tree density can be difficult, and California’s forest practice laws may not even allow it. The ideal forest density for your forest depends in part on the productivity of your site and your goals, but many forest ecologists and managers argue for maintaining low tree densities. Consider forest structure from two perspectives: 1) how fire moves through the forest during wildfires, and 2) how trees compete for water during droughts, considering what management actions to take.
Actions to reduce forest density and increase forest density include:
Mishandling, stacking and burning
- This is a good first step for DIYers to get rid of ladder fuels and some floor fuels.
- Generally, permits are not required if you are on your own property and are not using cost-sharing funds to finance the operation. And
- If the fire in the stack starts to slide around them, it’s a great way to introduce a fire line around them!
- It’s a great way to address surface and canopy fuels, but depending on your forest structure, your forest may not remove enough competing trees to withstand drought.
- It converts vertical fuels (such as small trees and shrubs) to ground fuels but does not remove them. It is easily added to surface fuels;
- Spilled fuels have a low flame length when burned, but may continue to burn/burn for longer periods of time, increasing the fire’s impact on remaining trees;
- It provides quick results, and can work most of the year; And
- Decomposition rates vary between locations.
Mastication and Fire: Combinations of Therapeutic Techniques, Positive and Negative, with Potential Unintended Effects. Although the flame length is low, the fire has a long residence time, which can cook and kill the remaining trees. A few years after the mastic is made, it is better to wait for the material to decompose before burning.
Timber harvesting or commercial reduction;
- Sometimes you have to thin commercially to restore low density forest conditions;
- You can use sawmill / forest products to pay for other management activities; And
- It reduces the density of trees and reduces competition among the remaining trees.
Commercial Reduction and Prescribed Fire: Very effective in reducing tree density, stand fuels and ground fuels. A combination of meaningful thinning and prescribed fire can reduce fire risk and improve the growth and strength of your trees, increasing resilience and resistance to wildfires and drought-related tree mortality!
For more information on what forest resilience is and how to measure it, see Malcolm P. North, Ryan E. Tompkins, Alexis A. Bernal, Brandon M. Collins, Scott L. Stephen and Robert A. York Click here to view a virtual panel discussion and presentation with the authors.
For more information on the history of California’s forests, visit Forest Management Series 4 – Forest History. For more information on tree growth and competition and vegetation management practices, visit Forest Management Series 5 – Tree Competition and Growth. and Forest Stewardship Series 6 – Forest Plant Management.