Car fumes and weeds are a double whammy for fire-loving native plants

Spring 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 that seeps into the soil, in addition to providing breathtaking colors.

They are also good at surviving after wildfire, having adapted to it over millennia. But new research shows that wildflowers that usually reappear after a fire and a good rain are losing ground to the long-standing twin threats of urban smog and non-native weeds.

A recent study led by Justin Valliere, an assistant professor in the Department of Plant Sciences at UC Davis, found that native wildflowers and other plants that typically bloom after a fire were instead replaced by invasive plants on land that received the type of nitrogen contained in vehicles. emissions.

The work appears in Biology of global change.

“Many native plants in fire-prone areas are dependent on fire, and some are completely dependent on fire. Some are even more abundant after a fire,” Vallière said. “But we found that these fire-following species may be particularly vulnerable to the combination of nitrogen pollution and invasive plants.”

This is part of the reason why native plants in these mountains are in decline.

Seeds banked, waiting to germinate

The problem facing native plants can be compared to a depleted bank account: funds withdrawn are not replaced.

It starts with fire, an important ecological process, Vallière said. The flames burn the plants to the surface and return their nutrients to the soil. Seeds lying dormant in the soil wait for the next rain to germinate, then use those nutrients to grow.

“Plant diversity is often highest during the growing seasons immediately after a site burns,” he said.

But invasive plants have many advantages over native plants. They often germinate earlier, grow faster and create more seeds, while being drought tolerant.

“They’re like cheaters,” Vallière said. “They don’t follow the same rules.”

Nitrogen is also an important element in the nutrition of every plant. They receive a fertilizer supply from the nitrogen that floats 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 allow their populations to thrive.

When the account balance reaches zero

The 2013 Springs Fire gave Vallière a unique opportunity to study the combined impacts of wildfire and supplemental nitrogen. He and his colleagues at 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 of smog in Los Angeles. Over the three years of the study, native plants that would typically have thrived after a wildfire declined even more quickly in plots supplemented with nitrogen.

The native seeds germinated but did not flower. Over time, the soil’s seed reserves became depleted.

“Every seed has a chance to flower and reproduce,” Vallière said. “If a seed grows and is outcompeted, it loses its chance to replenish the seed bank.”

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

“Biodiversity has inherent value,” Vallière said. “These invasive weeds could prevent the recovery of native shrubs after fire, sometimes forever altering the plant community.”

The loss of native plants can have cascading effects on the environment as a whole. Problems can include the loss of native bees that feed on flowers and mudslides when rain makes slopes unstable.

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

The addition of urban smog “could have serious consequences for the biodiversity of fire-prone ecosystems around the world,” Vallière warned.

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