California’s wet season helps grow the grass, brush and vegetation that cover hillsides and forests across the state. But by summer and fall, that same vegetation becomes wildfire fuel.That is why scientists are tracking not just how much vegetation is on the landscape, but how dry it is.Researchers with San Jose State University’s Wildfire Interdisciplinary Research Center collect vegetation samples twice a month as part of a broader effort to measure live fuel moisture across California and the West.“When fuels are extremely dry: higher risk. When they’re wet: lower risk,” said Dr. Craig Clements, professor of meteorology and director of the center.This story was produced for our 2026 Wildfire Ready special. Watch the full special report on Sunday, June 21, at 8 p.m. See our Wildfire Ready guide here. Twice each month, researchers clip new growth of shrubs called chamise in two locations in the Santa Cruz Mountains. The samples are placed in cans, taken back to a lab, weighed, dried in an oven and weighed again.“So we dry it in an oven and so we dry out all the water, we evaporate all the water and we’re just left with the plant material,” Clements said.The difference between the wet and dry weight tells scientists how much moisture was in the plant at that moment. Tracking that number over time can show when vegetation is becoming more flammable.“The more moist it is, the harder it is to ignite,” Clements said.Clements said live fuel moisture becomes especially important later in the summer and fall, when vegetation continues to dry and offshore wind events become more common.“We want to know how much moisture is in those live fuels throughout the season because they do get critical,” he said. “When they reach the critical level, fire risk goes up and it stays up through the whole fall and into winter when we get our precip.”For chamise, Clements said 70% fuel moisture is considered critical, while 60% is very critical. In some parts of Northern California, he said researchers have measured chamise in the low 50s.The SJSU team is one of dozens that do this manual measuring work across California. The readings matter because California’s wildfire risk differs greatly across different climate zones.“California has so many different mountain ranges and microclimates that we have to sample all these different areas because they’re all unique,” Clements said.Clements is often asked what the upcoming fire season will look like. His answer is always cautious.“Fuels are drying right now. And it could be bad. But it’s day-to-day weather that drives the fire danger,” he said.That means heat waves, wind events, lightning and human activity can all determine whether dry fuels actually turn into a wildfire.“We won’t know what happens or what could happen in August until we get a few weeks out from there,” Clements said.Still, one area he is watching this year is the middle elevations of the Sierra.“The lack of the snowpack in the middle elevation of the Sierra I think is more concerning because there is less soil moisture, so the fuel moistures are going to drop sooner,” Clements said. “That is going to be an elevation that we need to be worried about.”Clements said spring rainfall helped some areas recover, and that based on precipitation, parts of California are doing well. But he said the low snowpack and early melting this year stands out because snow helps keep soils and vegetation moist longer into the warm season. Long term, researchers are also seeing signs that California’s vegetation is drying as the climate warms.“We’re getting drier and drier by the decade,” Clements said. “That means that the warming and climate change is impacting the plants, the shrubs, the chaparral, and so it’s getting more dangerous.”Clements said the atmosphere is also drying, which increases the demand for moisture from plants. If that continues, he said some vegetation may struggle to survive in areas where it has historically grown. When it comes to reducing risk, Clements said the answer is clear: reduce the amount of fuel available to burn.“Doing prescribed fire across the state. Reducing the fuel loads. Because that’s the big risk,” he said.See more coverage of top California stories here | Download our app | Subscribe to our morning newsletter | Find us on YouTube here and subscribe to our channel
California’s wet season helps grow the grass, brush and vegetation that cover hillsides and forests across the state. But by summer and fall, that same vegetation becomes wildfire fuel.
That is why scientists are tracking not just how much vegetation is on the landscape, but how dry it is.
Researchers with San Jose State University’s Wildfire Interdisciplinary Research Center collect vegetation samples twice a month as part of a broader effort to measure live fuel moisture across California and the West.
“When fuels are extremely dry: higher risk. When they’re wet: lower risk,” said Dr. Craig Clements, professor of meteorology and director of the center.
Twice each month, researchers clip new growth of shrubs called chamise in two locations in the Santa Cruz Mountains. The samples are placed in cans, taken back to a lab, weighed, dried in an oven and weighed again.
“So we dry it in an oven and so we dry out all the water, we evaporate all the water and we’re just left with the plant material,” Clements said.
The difference between the wet and dry weight tells scientists how much moisture was in the plant at that moment. Tracking that number over time can show when vegetation is becoming more flammable.
“The more moist it is, the harder it is to ignite,” Clements said.
Clements said live fuel moisture becomes especially important later in the summer and fall, when vegetation continues to dry and offshore wind events become more common.
“We want to know how much moisture is in those live fuels throughout the season because they do get critical,” he said. “When they reach the critical level, fire risk goes up and it stays up through the whole fall and into winter when we get our precip.”
For chamise, Clements said 70% fuel moisture is considered critical, while 60% is very critical. In some parts of Northern California, he said researchers have measured chamise in the low 50s.
The SJSU team is one of dozens that do this manual measuring work across California. The readings matter because California’s wildfire risk differs greatly across different climate zones.
“California has so many different mountain ranges and microclimates that we have to sample all these different areas because they’re all unique,” Clements said.
Clements is often asked what the upcoming fire season will look like. His answer is always cautious.
“Fuels are drying right now. And it could be bad. But it’s day-to-day weather that drives the fire danger,” he said.
That means heat waves, wind events, lightning and human activity can all determine whether dry fuels actually turn into a wildfire.
“We won’t know what happens or what could happen in August until we get a few weeks out from there,” Clements said.
Still, one area he is watching this year is the middle elevations of the Sierra.
“The lack of the snowpack in the middle elevation of the Sierra I think is more concerning because there is less soil moisture, so the fuel moistures are going to drop sooner,” Clements said. “That is going to be an elevation that we need to be worried about.”
Clements said spring rainfall helped some areas recover, and that based on precipitation, parts of California are doing well. But he said the low snowpack and early melting this year stands out because snow helps keep soils and vegetation moist longer into the warm season.
Long term, researchers are also seeing signs that California’s vegetation is drying as the climate warms.
“We’re getting drier and drier by the decade,” Clements said. “That means that the warming and climate change is impacting the plants, the shrubs, the chaparral, and so it’s getting more dangerous.”
Clements said the atmosphere is also drying, which increases the demand for moisture from plants. If that continues, he said some vegetation may struggle to survive in areas where it has historically grown.
When it comes to reducing risk, Clements said the answer is clear: reduce the amount of fuel available to burn.
“Doing prescribed fire across the state. Reducing the fuel loads. Because that’s the big risk,” he said.
See more coverage of top California stories here | Download our app | Subscribe to our morning newsletter | Find us on YouTube here and subscribe to our channel