While climate contributes strongly to fire activity in the Sierra Nevada mountains of the western U.S., human activity, starting well before European contact, has also played an important part in the severity, frequency and sheer numbers of forest fires occurring in the area, according to researchers.
"Initially, we did work to see if we could develop long-lead forecasts for fire in the area -- six to 18 months in the future -- using climate patterns such as El Nino," said Alan H. Taylor, professor of geography, Penn State. "This would be a significant help because we could place resources in the west if forecasts indicated it would be dry and the southeast would be wet. However, the climate relationships with fire did not consistently track."
Taylor, working with Valerie Trouet, associate professor of dendrochronology, University of Arizona, merged a tree-ring-based record of Sierra Nevada fire history with a 20th century record based on annual area burned to create a record of fires spanning 415 years, from 1600 to 2015. While year-to-year fire variability was influenced by climate throughout that time, they found that large decadal-scale shifts in the Sierra Nevada fire regime were related to changes in human activity.
"Large shifts in the fire record corresponded with socio-ecological change, and not climate change, and socio-ecological conditions amplified and buffered fire response to climate," the researchers report in today's (Nov. 14) issue of the Proceedings of the National Academy of Sciences.
The researchers uncovered four time periods, each possessing their own fire regime characteristics that, while impacted by climate, were also heavily influenced by human land use patterns. The earliest fire regime period, dating from 1600 to 1775, corresponded to the time before Europeans came to the California area. During this time, Native Americans used fire to improve the production of acorns, tubers, shrubs and game such as deer. Their burn regime also controlled the amount of fuel on the forest floor. Native Americans who used the Sierra Nevada forests created a mosaic of small burned areas interwoven with unburned forest.
Early fires, because they were more frequent with less fuel build-up, were "good" fires. They burned through the forest, consumed understory fuels and left the majority of trees unharmed. The Native American mosaic of burned and unburned area prevented fires from continuously spreading.
From 1776 to 1865 the second fire regime, characterized by Spanish colonialism and the depopulation of Native Americans in the area shows more land burned. European settlers brought diseases against which Native Americans had no immunity and the population suffered. The Spanish built a string of missions in California beginning in 1769 and relocated remaining Native Americans to the mission areas. In 1793, there was a ban on burning to preserve forage, disrupting the pre-colonial Native American burning practices. The incidence of fires became more sensitive to drought and the fire regime changed, creating a time when fires were largest and most closely coupled with climate.
"Before the Native American die off, fires burned 4.5 times more area than they do today," said Taylor. "After the Native American depopulation fires burned 8 times more area than they do today."
The third fire period is from 1866 to 1903 and was initiated by the California gold rush, when thousands of people poured into the area. Settlement by large numbers of new immigrants began to break up the forest fuel and the creation of large herds of animals, especially sheep, removed large amounts of understory and changed the fire regime.
The fourth fire period began in 1904 and is linked to the federal government's policy of fire suppression on government lands. The reason pre-colonial and Spanish colonial fire levels were so much higher than today is that the current fire regime is one of suppressions with an extremely low incidence of fires compared to the past. However, suppression over the last century has allowed fuel to build up on the forest floor and opened the door for "bad" fires that destroy the forest canopy and burn large areas of land.
"Fire was locked in with decadal temperature variation until about 1860, after which time the relationship decays until the 1980s, when fire tracks temperature again," said Taylor.
The decay occurred because people changed the landscape through grazing and then changed the forests by suppressing fire.
Today's fires, according to Taylor, can be "bad" fires because a century or more of fire suppression has created a vast store of fuel to accumulate on the forest floor, allowing fires to burn long enough and hot enough to kill the forest canopy. These fires are also harder to fight.
"It is important for people to understand that fires in the past were not necessarily the same as they are today," said Taylor. "They were mostly surface fires. Today we see more canopy-killing fires."
Climate is still an important part of the regional fire regime. Extremely dry times will increase fire prevalence and extremely wet periods will decrease fires occurrence. But climate alone, in an inhabited area, cannot predict the fire regime. The actions of people must also be considered.
"We did eventually develop an understanding of how climate patterns could be used to develop long-lead forecasts," said Taylor. "But there has to be a consideration of both people and climate to predict and plan for future fire activity."
Also working on this project were Carl N. Skinner, geographer, Pacific Southwest Research Station, U.S. Forest Service and Scott L. Stephens, professor of environmental science, policy and management, University of California, Berkeley.
The U.S. Forest Service supported this research.