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The Fire Risk Hiding Underground

Writer: Dhaani Jeevanani
Dhaani Jeevanani
10 hours ago
4 min read

In 1950, California burned roughly half a million acres. In 2020, more than 4 million acres burned (CAL FIRE, via OEHHA, 2022). Hotter temperatures and longer droughts get most of the blame, and they’ve earned it. But what if part of the story starts at ground level, in the roots and the soil, long before a spark ever shows up?


However, a fire needs fuel as much as it needs a flame, and California’s fuel has been quietly changing.

30%: The Number That Decides Whether Grass Ignites

Fire scientists track something called fuel moisture, which is how much water is sitting inside plant material, alive or dead. When dead fine fuels like grass drop below about 30% moisture, they ignite easily. For living plants, readings below roughly 60 to 80% point to faster fire spread (Wang et al., 2025).

So where does soil come in? Plants get their water from the ground. Soil moisture doesn’t control fire directly, but it’s one link in a chain: roots, then soil water, then fuel moisture, then fire behavior.


Deep Roots vs. Shallow Roots

California’s native chaparral shrubs evolved for long, dry summers. Toyon and scrub oak send roots one to two meters or more underground, tapping moisture that lingers long after the surface dries out (Chaparral hydraulic traits study, 2016).

Non-native annual grasses like ripgut brome, soft brome, and slender wild oat keep their roots in the top 20 to 30 cm. They drain the surface after spring rains and then have nothing left. Researchers at a Southern California site measured soil water down to 2 meters across several drought years. Native shrubland held significantly steadier moisture than grassland dominated by non-native grasses (Rodriguez et al., 2023).

Soil That Acts Like a Sponge

Native coastal sage scrub soils stored about 22% more carbon and nitrogen than soils under Bromus grasslands, 2.88% carbon versus 2.22% (Caspi et al., 2019). Carbon glues soil particles into clumps that hold water better. Scientists are still untangling whether the plants build the carbon or the carbon shapes the plants, but the pattern is hard to ignore.

Grass That Quits Before Summer Starts

Annual grasses live for one wet season. By early summer their entire body is standing dead fuel, right when fire weather kicks in. That creates thick, continuous mats that carry flames across the gaps between shrubs. Ripgut brome is loaded with silica, which keeps dead material around and flammable. Arabian schismus has helped fire spread into desert-edge landscapes that rarely burned before (Wang et al., 2025). In a fire-atmosphere model of Southern California, fire spread and intensity shifted as invasive grass cover grew.

Plot Twist: It’s Not Native vs. Non-Native

Here’s where it gets interesting. Black mustard, a non-native plant, stores soil carbon at 2.87%, right up there with native sage scrub (Caspi et al., 2019). Chia sage, a native annual, behaves a lot like the invaders. And native chaparral can absolutely burn in severe drought and high wind.

The real dividing line isn’t a plant’s passport. It’s how it lives: deep-rooted perennial or shallow-rooted annual. One more honest caveat: much of the fire evidence comes from computer models, not side-by-side real fires. The pattern is promising, but researchers still need field data to nail it down.

Also, research from Braasch et al. (2025) suggests that in drought years, the gap between native and non-native communities may widen, exactly when everything else is already primed to burn. Even if you don’t live near chaparral, this touches you. Smoke doesn’t stop at county lines.


What Can We Do?

  • Volunteer for native habitat restoration. Local California Native Plant Society chapters run planting and restoration days.

  • Plant deep-rooted natives like toyon in your yard, and follow your local fire agency’s guidance on spacing and defensible space.

  • Check plant lists before you buy. The California Invasive Plant Council (Cal-IPC) tracks which plants are problems, and “wildflower” seed mixes can hide some.

  • Deal with dry grass near your home the way your local fire agency recommends, and early in the season.

  • Ask your city and park agencies whether fuel break and post-fire restoration plans include deep-rooted native shrubs and target invasive annual grasses.

A journey of a thousand miles starts with one step, or in this case, one toyon.


The Bottom Line

Fire season doesn’t begin with a spark. It begins months earlier, under our feet, in whether the ground can keep plants hydrated when summer turns up the heat. We can’t control the weather, but we can influence what’s growing in the soil. That’s a lever worth pulling.

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