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Car Fumes, Weeds Pose Double Whammy for Fire-Loving Native Plants

Wildflower Displays Threatened

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In September 2013, a few months after the Springs Fire blazed through the Santa Monica Mountains in Southern California, a team led by Justin Valliere started laying out plots to study how invasive weeds and air pollution would impact the resurgence of native plants that usually flourish after a wildfire. Their tests tried to mimic nitrogen coming from vehicle exhaust in nearby Los Angeles. (Justin Valliere/嘿嘿视频)

Springtime brings native wildflowers to bloom in the Santa Monica Mountains, northwest of Los Angeles. These beauties provide food for insects, maintain healthy soil and filter water seeping into the ground 鈥 in addition to offering breathtaking displays of color. 

They鈥檙e also good at surviving after wildfire, having adapted to it through millennia. But new research shows wildflowers that usually would burst back after a blaze and a good rain are losing out to the long-standing, double threat of city smog and nonnative weeds.

A led by Justin Valliere, assistant professor in the 嘿嘿视频 Department of Plant Sciences, found that native wildflowers and other plants that typically flourish following a fire were, instead, replaced by invasive plants on land that received the kind of nitrogen contained in vehicle emissions.

鈥淢any native plants in fire-prone areas rely on fire, and some are entirely dependent on it. Some are even most abundant after a fire,鈥 Valliere said. 鈥淏ut we found that these fire-following species may be especially vulnerable to the combination of nitrogen pollution and invasive plants.鈥

That鈥檚 part of the reason native plants in these mountains have been declining.

ground pink, a native flower, grows in a cluster outside
Ground pink, or Linanthus dianthiflorus, is known to bloom in higher abundance following a fire and rain. (Justin Valliere/嘿嘿视频)
Purple stars, a native plants flowers in the Santa Monica Mountains
Native shooting stars, or Dodecatheon clevelandii, also tend to bloom more after a fire and good rain. (Justin Valliere/嘿嘿视频)

Seeds banked, waiting to sprout

The problem faced by native plants can be likened to a drawn-down bank account: Funds withdrawn are not being replaced.

It starts with fire, an important ecological process, Valliere said. Flames burn through plants on the surface and return their nutrients to the soil. Seeds sleeping in the ground wait for the next rain to sprout, then use those nutrients to grow.

鈥淧lant diversity is often highest in growing seasons immediately after a site burns,鈥 he said. 

But invading plants have many advantages over native ones. They often sprout earlier, grow faster and create more seeds, all while tolerating drought.

鈥淭hey鈥檙e like cheaters,鈥 Valliere said. 鈥淭hey don鈥檛 follow the same rules.鈥 

Nitrogen, too, is an important piece of every plant鈥檚 nutrition. They get a fertilizing boost from nitrogen that floats up in vehicle emissions and falls to the ground. But the invaders use nitrogen and other nutrients to grow faster, winning the race for water and sunlight. As a result, fewer native plants reach maturity, producing fewer seeds that keep their populations thriving.

When the account balance reaches zero

The 2013 Springs Fire gave Valliere a unique opportunity to study the combined impacts of wildfire and extra nitrogen. He and colleagues from UC Riverside and the National Park Service created test plots in the Santa Monica Mountains where the fire had burned. Then, they added nitrogen to the soil to mimic the amount and type that L.A.鈥檚 smog would deposit. Over the study鈥檚 three years, native plants that typically would have flourished after wildfire instead declined even faster in the plots with added nitrogen. 

burnt landscape of shrubs and brown ground in Santa Monica mountains
After the Springs Fire in 2013, scientists led by Justin Valliere of 嘿嘿视频 studied how invasive weeds and air pollution impact the resurgence of native plants that usually flourish after a wildfire in the Santa Monica Mountains. (Justin Valliere/嘿嘿视频)

Native seeds sprouted but didn鈥檛 flower. Over time, the soil鈥檚 bank of seeds drew down.

鈥淓ach seed has one chance to flower and reproduce,鈥 Valliere said. 鈥淚f a seed grows and gets outcompeted, that seed has lost its chance to replenish the seed bank.鈥

Without the chance to replenish their bank account, native plants will die out, and the ecosystem will be thrown out of balance. 

鈥淭here is inherent value in biodiversity,鈥 Valliere said. 鈥淭hese invasive weeds could prevent the re-establishment of native shrubs after fire, sometimes forever altering the plant community.鈥

The loss of native plants can have cascading effects on the larger environment. Problems can include the loss of native bees that feed on the flowers, and mudslides when rain makes hillsides unstable.

In similar areas where biodiversity flourishes after wildfires 鈥 including parts of the Mediterranean basin, southern Africa and Australia 鈥 air pollution also threatens native vegetation.

The addition of city smog 鈥渃ould have serious consequences for the biodiversity of fire-prone ecosystems worldwide,鈥 Valliere warned. 

mountain landscape in green carpet of grass, wildflowers and burned shrubs
In spring of 2015, the area burned by the 2013 Springs Fire was again in bloom. The clearwater cryptantha shown here blooms all over California, but is especially abundant in the coastal south. (Justin Valliere/嘿嘿视频)

The study was funded by the National Park Service鈥檚 Air Resources Division and the Bloom-Hayes Ecological Research Grant.

Co-authors include Irina Irvine of the National Park Service and Edith Allen of UC Riverside.

 

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