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Other than climate change, there’s a myriad of reasons and causes amounting to worsening California’s wildfires. But as LAist recently highlighted, there’s another factor we’ve been overlooking, and that’s nitrogen deposition from air pollution coming, in large parts, from our own cars.
Gas-burning cars or big diesel trucks emit pollutants including nitrogen oxides, or NOx. The particles float through the air, converting into nitrates and settling on both the leaves of the plants and on the soil, where they work their way deep down. Then, large amounts accumulate in spots close to roads — especially over the summer. And when the first rains come, the food becomes available to plants, boosting leafy growth, especially of invasive grasses, such as the yellow grass that pervades in hills and mountains. Species such as red brome and cheatgrass have been taking over California mountains because they grow easily in a variety of conditions, but most importantly, recover more quickly from fires than the native plants that should be covering our hills, like chaparral and coastal sage scrub.
These invasive grasses make California landscapes more flammable. Every year, they regrow during the wet season and dry out as soon as the weather gets hot, leaving a whole lot of matter on the ground ready to burn. Add nitrogen into the mix and they germinate earlier and grow faster than many native species. Moreover, the invasive grasses also fill the gaps between the native plants, sucking up crucial water with their shallow roots before it can percolate to the roots of some native species that are deeper down.
Edith Allen, professor emeritus at UC Riverside, who studied nitrogen deposition in soils across Southern California for decades, told LAist: “Those are the same forms of nitrogen a farmer would put on a field, or that you would purchase when you’re going to the nursery to fertilize your lawn or your garden.”
“In the last 75 years or so, with more automobiles and industry in California, there has been a lot more nitrogen deposition and those grasses have become more productive,” she continued. “More productive grasses mean that there is more fuel to burn in the following dry season.”
And this is happening all over SoCal, from L.A. to Riverside. “What we’re seeing is even in areas where the shrublands haven’t burned, we’re seeing a thinning of coastal sage scrub. And whenever a shrub dies, that space on the landscape is filled in with exotic invasive grasses,” Allen said.
Air quality laws have lessened the amount of NOx blanketing our landscapes over the years. But scientists are still documenting high levels of nitrogen in the soil in areas close to roads and downwind from urban centers, due to older cars, diesel trucks, shipping, manufacturing, and agricultural practices.
As laid out by LAist, there are problems with deposition not just in the mountains surrounding LA’s valleys, but all along the 101 and 110. SoCal’s highest mountains are also impacted, and even as far out as Joshua Tree, where invasive grasses are increasing fire risk as well.
The COVID-19 pandemic decreased traffic in Los Angeles at its start, but it has started to pick up the case nine months after the first stay-at-home orders. In early April, nationwide VMT dropped 46% from pre-pandemic levels, but rebounding to normal in late June. Moreover, the Los Angeles area reportedly saw a 21% decrease in collisions during the coronavirus pandemic, according to a new INRIX study.