Drought has killed junipers in Arizona and blue oaks in the San Francisco Bay Area

Soil moisture is very low across much of the West and Upper Midwest

Soil Moisture, May 24, 2021
Soil Moisture, May 24, 2021. NOAA.
Drought Monitor, May 18, 2021
Drought Monitor, May 18, 2021. NOAA.

In some areas of the Western states the 20-year long drought has reduced moisture content in plants to the lowest levels scientists have seen. Soil moisture is very low — at or near record lows across much of California. Particularly hard hit are locations in Colorado, Utah, Nevada, New Mexico, Arizona, and California.

Junipers are dying in Arizona from a lack of water. The East Bay Times reports that drought is likely responsible for dead or dying acacia, eucalyptus, and Monterey pines in East Bay Regional Parks near San Francisco.

From the an Associated Press article by Seth Borenstein:

In Arizona, junipers are succumbing to the 20-year drought and its two-year intensification, said Joel McMillin, a forest health zone leader for the U.S. Forest Service there. Officials haven’t done a precise count but anecdotally the die-off is 5% to 30% with some patches up to 60%.

Until the dead needles drop to the ground, which takes a year or so, the fire hazard increases, fire manager Steinhardt said. “So you have something that’s highly flammable and it’s … 20-, 30-, 40-foot tall and every single one of those needles on there now becomes an ember that can be launched.”

“This is probably one of the driest and potentially most challenging situations I’ve been in,” said the veteran of 32 fire seasons.

In California, normally drought-tolerant blue oaks are dying around the San Francisco Bay Area, said Scott Stephens, a fire science professor at the University of California, Berkeley. “They don’t have access to water. Soil moisture is so low. When you start to see blue oak dying, that gets your attention.”

Human-caused climate change and decades of fire suppression that increases fuel loads are aggravating fire conditions across the West, scientists said.

Global warming has contributed to the megadrought and is making plants more prone to burning.

Vegetation with a low moisture content is easier to ignite in a wildfire and contributes to a rapid rate rate of spread that is more difficult for firefighters to suppress. Dead or dying trees that still have leaves attached during the first year or so after their decline can also enhance the spread and intensity of a fire and adds to the number of burning embers lofted into the air that can ignite spot fires ahead of the blaze, putting structures at risk that are distant from the fire.

Extremely dry soil and vegetation in May does not guarantee a busier than average summer fire season in the West — the weather in the coming months is the primary factor. If it turns relatively cool and wet, the number of acres burned is not likely to be extreme. But if the drought continues into June, July, and August, normal weather, or especially warmer and drier than normal weather, could produce a busier than average Western fire season.

Precip & Temp outlook for May 1-7, 2021
Precip & Temp outlook for May 1-7, 2021. Made May 24, 2021.
Precip & Temp outlook for June, July, & August
Precip & Temp outlook for June, July, & August, 2021. Made May 20, 2021.

Using soil moisture in grassland fire danger rating systems

“…Our research findings provide scientific justification for using soil moisture data from in situ monitoring networks in fire danger rating systems. Such soil moisture data are increasingly available and are not currently being used in the context of wildfire preparedness. ”

Above: The percent of maximum soil moisture available to plants in the top 16 inches in Oklahoma, September 11, 2016.

David M. Engle, along with other scientists at Oklahoma State University, are making a case that soil moisture should be used as one of the components in determining grassland fire danger ratings.

soil moisture station
Station that measures soil water at several depths and transmits the data. This image and the one above are courtesy of the researchers.

To assess the herbaceous fuel dynamics in grasslands, they conducted 3 studies:

1) A study that used a database of large wildfires in Oklahoma to examine the relationship of fire occurrence and fire size with soil moisture;

2) An intensive field-based study to quantify and subsequently model herbaceous fuel load and moisture content in grassland patches that differed in time since fire and, therefore, proportion of live and dead herbaceous fuel load, and;

3) Modeling the influence of herbaceous fuel dynamics and weather conditions on fire behavior in tallgrass prairie.

Their final report can be read HERE.