You hear a lot of talk about invasive species. You see signs about them all over these days. Drive anywhere near a lake and you’ll find notices about zebra mussels. Drive through Napa County and pass billboards about the glassy-winged sharpshooter. Each year, the list of invasive species grows longer and after a while, it’s easy to grow a little numb.
So when I tell you about a fight against an invasive species going on right next door to Safari West, you may find yourself only mildly interested. And when I tell you that the invasive species in question is a grass, you may tune out altogether.
But what if I told you the key to combatting this invader is fire? Would that grab your attention? Then strap in because new science suggests that prescribed burning might offer a whole new way of defending the ecosystem.
We call the grass in question medusahead grass and it comes from the Mediterranean. So far as we can tell, it showed up in Oregon sometime in the late 1800’s. In all likelihood, seeds of this resilient plant arrived with a shipment of livestock. Medusahead’ sharp, barbed seeds stick easily in animal fur or human clothing.
However it arrived here, once this grass put down roots in American soil, the species took off. Today it has spread around much of the American west including down into Napa and Sonoma counties. It is such a sturdy, fast-growing plant that it has out-competed many of our native grasses and overtaken whole ecosystems.
You may wonder why this matters. One grass species has supplanted the other. What possible difference could that make? As it turns out, a significant one. Medusahead grass is remarkably well-defended. The sharp awns—needle-like structures poking up from the tightly clustered seeds—can puncture the inside of a grazing animal’s mouth. As a result, these animals only eat the young and soft medusahead shoots. Once it matures, most grazing animals including cows and deer, avoid it.
Studies have shown that grasslands, savannas, and woodlands invaded by medusahead can experience an 80% decrease in available forage. Basically, this alien grass isn’t food and where it grows, real food cannot.
What’s most important about this, at least for humans and at least in the short term, is what happens to this uneaten grass. Without animals to consume it, the grass goes to seed and then become senescent; it dries up and dies. Once dead, it just sits there. Part of what makes the plant so difficult to eat is its high silica content, and this characteristic also drastically slows its rate of decomposition. Uneaten and only slowly rotting, the dead grasses accumulate like kindling at the bottom of a stack of logs.
Fires on the Rise
When we reported on a prescribed burning program at the Pepperwood Preserve, it was hot on the tail of the famous valley fire. That fire decimated much of Lake, Sonoma, and Napa counties and raged out of control for days. By the time it was brought to heel, the valley fire had raged across over 75,000 acres, destroyed nearly 2,000 structures, and been named the third worst fire in California history.
A glut of factors helped this fire to burn so devastatingly and so long including the increasingly dry and hot California climate. Other reasons include years of a fire-suppression policy that has prevented the occasional clearing out of the accumulating fuel load. Couple those factors with an invasive species that adds heavily to the amount of fuel on the ground and you have a recipe for disaster.
Fighting Fire with Fire
As wildfires have grown more frequent and destructive, scientific attention has turned to alternative methods of battling the blaze. As it turns out, controlled burning offers one of the best methods of prevention. A wildfire can’t rage where it has nothing to burn.
As it turns out, controlled burning has taken place for hundreds, if not thousands of years. Indigenous peoples across the world have long used fire to clear and fertilize lands and the Pepperwood Preserve has picked up on the idea
Pepperwood has held a handful of prescribed burns on their property designed to reduce fuel loads and mitigate wildfire risks. When fires are frequent and the fuel load kept low, they burn cool and fast, clearing undergrowth from our oak woodlands without killing the trees.
While experimenting with prescribed burning, Pepperwood has also discovered a remarkably efficient method of combating medusahead grass. The grass, as it turns out, has an Achille’s heel; it matures later in the season than our local grasses. What this means is that as the grasslands of Sonoma County go to seed and begin to dry out, easily visible patches of green appear. These are the medusahead strongholds, still developing the seeds that will turn into next year’s crop.
In 2016 Pepperwood burned 7 acres of medusahead infested grassland. The Pepperwood team timed the burn to take place after the native grasses had gone to seed, but the medusahead had not. On a day with no wind, they lit the grass. The still air helped the CalFire crews to maintain control and ensured that the flames would move slowly, raising the temperature enough to kill the embryonic seeds.
The results of this tactic, as we’ve since learned, were impressive. While Pepperwood scientists continue to process their data, this year’s growth revealed a dramatic reduction in the amount of Medusahead across that 7-acre plot.
A little over a month ago, Pepperwood repeated their experiment, this time on a 22-acre plot. This year introduced a new wrinkle however as air quality and the availability of CalFire personnel delayed the burn long enough for much of the medusahead to mature. While we won’t know how much this delay impacted the burn, it seems likely that more of the medusahead seeds will have survived.
Even with a delayed burn, the experiment will yield useful data. A side-effect of burning is a massive release of nitrogen back into the soil, virtually guaranteeing a spike in growth the following spring. For this reason, it is often beneficial to burn the same acreage again the following year. A second pass at eradicating the invaders if you will.
Alongside this operation, it is often necessary to replant as well. While the Pepperwood burn was designed not to kill the local grass seeds, some mortality is to be expected. To compensate for this as well as to replenish some local plants, Pepperwood has been growing plugs in their nursery. These sprouts and seedlings will eventually get planted in the nitrogen-rich, competition-deficient soils of these many burned acres. In the long haul, this appears to be a good recipe for ecosystem health.
Our Fiery Future
The 2017 fire season has arrived early with several wildfires already burning across the state. As I sit here typing, the Detwiler fire—which consumed some 70,000 acres, 63 homes, and nearly took the town of Mariposa—continues to smolder. As we look to the future it seems all but certain that fire seasons will only grow worse. At least until the accumulated fuel load of the last century has been consumed.
Moving forward, we will need to adopt a more ecological philosophy of fire control. Rather than preventing everything from burning, we’ll need to adapt to natural fire regimes. If we can map out the fire ecology of an ecosystem, we can use small, controlled burns to prevent massive, catastrophic ones.
Since a freshly burned field invites new growth, this philosophy also provides a level of defense against aggressive species like medusahead. In a healthy, verdant ecosystem, invaders often struggle to find an available niche. By keeping back the medusahead we’ll also help restrict the accumulated fuel load. As it turns out, fire can, in this case, be the key to its own prevention.