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Among the Iliad-sized list of things to which we can attribute California's worsening wildfires, you've probably thought of the obvious -- climate change, the need to do more prescribed burns, and building homes and infrastructure in places that easily catch on fire.
But there's another often-overlooked factor that may also be making wildfires worse: air pollution. Specifically, something called nitrogen deposition.
YOU'RE FERTILIZING INVASIVE WEEDS WITH YOUR CAR
Say you're driving down the 101, looking out the window at the golden hills of the San Fernando Valley.
They're beautiful, but a lot of that yellow grass is actually invasive and has been in a long running war with native vegetation for supremacy of our mountains. Species such as red brome and cheatgrass have been taking over because they grow easily in a variety of conditions, and recover more quickly from fires than the native chaparral and coastal sage scrub that should be covering our hills.
The invasive grasses are also making our landscapes more flammable. Every year they regrow during our wet season and dry out as soon as the weather gets hot, leaving a whole lot of matter on the ground ready to burn.
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BOOSTING LEAFY GROWTH
As you're gazing at those hills, flowing steadily out of the back of your gas-burning car or big diesel truck are emissions, including nitrogen oxides, or NOx.
The particles then float through the air, convert into nitrates and settle on both the leaves of the plants and on the soil, where they work their way deep down.
"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," said Edith Allen, professor emeritus at UC Riverside, who studied nitrogen deposition in soils across Southern California for decades.
Large amounts accumulate in spots close to roads, especially over the summer. And it's with first rains that the food becomes available to plants, boosting leafy growth, especially of invasive grasses. They already germinate earlier and grow faster than many native species, utilizing the nitrogen first.
"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," Allen said. "More productive grasses mean that there is more fuel to burn in the following dry season."
The nitrogen deposition doesn't only boost the growth rate and size of the invasive grasses, it impacts native vegetation growth as well, and can lead to premature death for some species during droughts.
One possible reason is that it forces some native plants, such as California sagebrush, to grow bigger and produce more leaves than they naturally would, increasing their demand for water. That can be a struggle to meet, especially if things are dry -- a condition that's likely to become more common due to our changing climate.
In conjunction, 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.
A PROBLEM FAR BEYOND OUR VALLEYS
This is happening all over our hills, from L.A. to Riverside, where Allen has seen some spots completely convert to grasslands.
"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," she said.
Air quality laws have lessened the amount of NOx blanketing our landscapes over the years -- 35 percent between 1975 and 2000-- especially when it comes to modern passenger vehicles. 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.
There are problems with deposition not just in the mountains surrounding our valleys, but all along the 101 and 110, some of SoCal's highest mountains, and even as far out as Joshua Tree, where invasive grasses are increasing fire risk as well.
"Nitrogen pollution. Climate warming. Invasive species. Ecosystems aren't experiencing those threats in isolation," said Justin Valliere, professor in the Department of Biology at Cal State Dominguez Hills.
"Nitrogen deposition is underappreciated as a threat."
