The Science | Greenbreak

The fire risk is the grass. The science is settled.

Invasive annual grasses are one of the three biggest human-driven changes to American fire regimes, and unlike climate, this one is fixable parcel by parcel. Below is the peer-reviewed record, the mechanism behind it, and a plain statement of what that evidence does and does not let us claim.

PNAS Annual Review of Ecology and Systematics BioScience Global Change Biology UC ANR

The invasive grass–fire cycle

Invasion is not just a symptom of more fire. It is a cause of it, and each fire makes the next invasion easier. That feedback loop is the reason clearing alone never ends the problem.

Why annual grasses carry fire

Dead by June, continuous by design

Annual grasses complete their life cycle and die each summer, leaving a carpet of fine, dead material with a high surface-area-to-volume ratio. Fuels of that geometry exchange moisture with the air quickly. They cure early in the season, ignite from small sources, and carry flame rapidly across the ground because there is little bare space to interrupt them.

Why native communities do not

Green longer, bunched, and rooted deep

Native perennials stay partially green and hold higher live fuel moisture later into the season. They grow in discrete bunches separated by bare ground, which breaks the horizontal continuity fire needs to spread. And they allocate a substantial share of their biomass below ground, where it does not contribute to the surface fuel bed at all.

Why the loop tightens

Each fire hands the site back to the grass

More frequent fire kills native perennials and shrubs faster than they can re-establish. The annual grasses, adapted to reseed after disturbance, recover first and occupy the opened ground. The next fire season starts with more invasive cover than the last. That is what makes this a ratchet rather than a cycle that returns to where it started.

What the research shows

Five sources, spanning the foundational theory, the national-scale test of it, the regional record in the arid West, and the practitioner manual for reversing it. Every claim below is stated as the authors stated it.

Eight invasive grass species were associated with significantly higher fire occurrence, with increases of up to 230%.

The first national-scale analysis of the question. The authors examined 12 invasive grass species across 29 US ecoregions, comparing invaded areas against comparable uninvaded habitat. Eight of the twelve species showed significantly elevated fire occurrence, and fire frequency increases reached up to 150% in invaded habitat.

This is the study that moves the grass–fire cycle from a well-supported regional theory to a documented, continent-scale pattern. That is what makes it usable as a basis for risk assessment rather than anecdote.

The paper's senior author has publicly ranked invasive species alongside climate change and human ignitions as the "big three" human drivers of changing fire regimes. Of those three, invasive cover is the one a landowner can act on directly.

Fusco, E.J., Finn, J.T., Balch, J.K., Nagy, R.C., & Bradley, B.A. (2019). "Invasive grasses increase fire occurrence and frequency across US ecoregions." Proceedings of the National Academy of Sciences 116(47): 23594–23599. doi.org/10.1073/pnas.1908253116

The grass–fire cycle is textbook ecology, not a fringe theory.

This is the foundational paper that named and defined the mechanism: exotic grasses increase fire frequency and intensity, those fires kill native species, the grasses recover first, and the process reinforces itself with each turn.

It has been cited thousands of times in the three decades since. When a utility's vegetation team or an insurer's science reviewer asks whether this is established science, this is the reference that settles it.

D'Antonio, C.M. & Vitousek, P.M. (1992). "Biological invasions by exotic grasses, the grass/fire cycle, and global change." Annual Review of Ecology and Systematics 23: 63–87. doi.org/10.1146/annurev.es.23.110192.000431

Invasive plants change fire behavior by changing the physical properties of the fuel.

A synthesis across ecosystems showing that invasive plants alter fuel load, continuity, packing, and moisture, and that those altered fuel properties in turn change fire behavior, fire intensity, and fire return intervals.

This is the causal link that matters for underwriting: the change is physical and describable, not merely correlational. Replace the fuel type and you have changed the inputs that fire behavior models actually run on.

Brooks, M.L., D'Antonio, C.M., Richardson, D.M., Grace, J.B., Keeley, J.E., DiTomaso, J.M., Hobbs, R.J., Pellant, M., & Pyke, D. (2004). "Effects of invasive alien plants on fire regimes." BioScience 54(7): 677–688. doi.org/10.1641/0006-3568(2004)054[0677:EOIAPO]2.0.CO;2

Over 30 years of satellite record, cheatgrass-dominated land burned more often and in larger fires than any native vegetation type in the Great Basin.

A regional analysis across the arid western United States from 1980 to 2009. Lands dominated by introduced annual cheatgrass burned disproportionately more frequently, and in larger fires, than any native vegetation type in the region.

The pattern is not confined to California. This is the evidence base underneath our expansion into Colorado, Montana, and Idaho, where the same annual grasses drive the same regime shift on rangeland.

Balch, J.K., Bradley, B.A., D'Antonio, C.M., & Gómez-Dans, J. (2013). "Introduced annual grass increases regional fire activity across the arid western USA (1980–2009)." Global Change Biology 19(1): 173–183. doi.org/10.1111/gcb.12046

Restoring invaded annual systems is a documented land management discipline with known techniques and costs.

The practitioner-standard restoration manual for California's invaded annual grassland systems, published by University of California Agriculture and Natural Resources.

Its existence answers the second question a procurement reviewer asks after "is the risk real?", namely, "is the fix a defined practice or an experiment?" The methods, sequencing, and cost ranges are established and published. Our work follows this discipline and adds the per-plant record on top of it.

Gornish, E.S. (2017). "Restoration Manual for Annual Grassland Systems in California." UC Agriculture and Natural Resources, Publication 8575. anrcatalog.ucanr.edu/pdf/8575.pdf

Why natives burn differently

The distinction is not aesthetic. It is a difference in the physical fuel bed: moisture, continuity, and where the biomass sits.

Fuel characteristics of invasive annual grasses (such as wild oat and cheatgrass) compared with established native perennial plant communities.
Property	Invasive annual grasses	Native perennial communities
Summer state	Fully cured dead fuel	Partially green, higher live fuel moisture held later
Fuel structure	Continuous carpet across the ground	Bunched growth with bare gaps that break continuity
Biomass allocation	Nearly all above ground	Substantial biomass below ground
After fire	Reseeds and dominates the site	Slow recovery, which is exactly why the cycle ratchets toward invasives
Between mows	Regrows into flash fuel	Stable low-fuel structure

What we claim, and what we don't.

If you are going to price risk against this work, you need to know exactly where the evidence ends. So here it is, stated plainly and up front rather than in a footnote.

What the evidence supports

Invasive annual grasses increase fire occurrence and frequency. Restoration removes that driver.

This is the strongest, national-scale finding in the literature, and it is what our work acts on. Replacing an invasive annual fuel type with an established native plant community removes the specific driver those studies identify: the continuous, early-curing fine fuel bed. That is a change in the physical inputs to fire behavior, and it is directly observable on the ground.

What we do not claim

We do not claim restored land is fireproof, and we do not claim a specific percentage risk reduction for any individual parcel.

Restored native cover is not non-flammable. Under extreme wind, drought, and ignition conditions, any vegetated landscape in California can burn. Nobody, including us, can responsibly convert the ecoregion-scale findings above into a defensible number like "this parcel is now N percent less likely to burn." Site conditions, weather, topography, ignition sources, and neighboring land all bear on that, and the published research does not resolve it at parcel resolution.

Any vendor who gives you that number for a single property is extrapolating past the evidence. We would rather lose the deal than be the source of a figure an actuary later has to defend.

Why the ledger exists

That parcel-level gap is precisely why we build the verification record.

The science establishes the direction of the effect. It cannot, on its own, tell you what happened on your ground. So we close that gap with measurement rather than with claims: a survey-grade, append-only ledger recording what was planted, exactly where, in what condition, and how it has been maintained: species and RTK GNSS position to ±30 cm, per plant, per season, with no record ever overwritten.

That turns ecological evidence into underwritable data. Instead of asking you to accept a modeled risk reduction, we give you the ground truth an underwriter, an agency reviewer, or a mitigation program can audit directly. We run the same discipline on our own 12-acre oak woodland demonstration site in Mariposa County before we bring it to yours.

Why this is not an abstract argument.

Lahaina, Maui · August 2023

In the 2023 Lahaina fire, flames spread through invasive grasses growing on former plantation land before reaching the town. The fuel that carried the fire across that ground was the legacy of agricultural land left to invasive cover: the same category of fuel, and the same underlying mechanism, described in the studies above. It is a reminder that unmanaged invaded land adjacent to a community is not neutral ground; it is the pathway.

Discussed in: Ornes, S. (2024). "Fire in the desert." Proceedings of the National Academy of Sciences news feature. doi.org/10.1073/pnas.2402794121

Full citation list

Fusco, E.J., Finn, J.T., Balch, J.K., Nagy, R.C., & Bradley, B.A. (2019). "Invasive grasses increase fire occurrence and frequency across US ecoregions." Proceedings of the National Academy of Sciences 116(47): 23594–23599. https://doi.org/10.1073/pnas.1908253116

D'Antonio, C.M. & Vitousek, P.M. (1992). "Biological invasions by exotic grasses, the grass/fire cycle, and global change." Annual Review of Ecology and Systematics 23: 63–87. https://doi.org/10.1146/annurev.es.23.110192.000431

Brooks, M.L., D'Antonio, C.M., Richardson, D.M., Grace, J.B., Keeley, J.E., DiTomaso, J.M., Hobbs, R.J., Pellant, M., & Pyke, D. (2004). "Effects of invasive alien plants on fire regimes." BioScience 54(7): 677–688. https://doi.org/10.1641/0006-3568(2004)054[0677:EOIAPO]2.0.CO;2

Balch, J.K., Bradley, B.A., D'Antonio, C.M., & Gómez-Dans, J. (2013). "Introduced annual grass increases regional fire activity across the arid western USA (1980–2009)." Global Change Biology 19(1): 173–183. https://doi.org/10.1111/gcb.12046

Gornish, E.S. (2017). "Restoration Manual for Annual Grassland Systems in California." UC Agriculture and Natural Resources, Publication 8575. https://anrcatalog.ucanr.edu/pdf/8575.pdf

Ornes, S. (2024). "Fire in the desert." Proceedings of the National Academy of Sciences news feature. https://doi.org/10.1073/pnas.2402794121

The fire risk is the grass. The science is settled.