NATURAL VIEW: Smoggy skies a horror story for shrubs

“Menace from the sky” is not the title of a science-fiction thriller, but it could be a horror story for shrub communities in the Western United States.

Acid rain is a familiar story — not well-known is that the oxides of nitrogen (NOx) from smog fall out of the sky as tiny particles and fertilize the vegetation underneath. In some areas, more nitrogen falls from the sky than farmers apply as fertilizer to agricultural fields.

This is probably not a problem for crops, as our domesticated plants are bred to take advantage of fertilizer. Thousands of years ago, the earliest grain plants may have sprouted on nitrogen-rich refuse piles close to early human settlements.

Nature is at risk, though, because some plants can take advantage of fertilizer much more than others.

Grasses (including grains) and nonnative weeds use the extra nitrogen to grow faster, but many native plants cannot.

Local coastal sage shrubs, for example, have evolved to grow in nutrient-poor soils. Fertilizer from the sky encourages the grasses and weeds to grow, so they take up the available growing space and scarce water.

If the NOx are not curtailed, western North America faces the wholesale conversion of our shrub communities into grasslands, with the loss of many unique plant and animal species.

Biologists from UC Riverside, UC Santa Cruz and UCLA have been talking and writing about this for years without attracting attention outside the conservation community. But this summer, a dangerous side effect emerged: desert fires.

In the desert, shrubs and succulent trees like Joshua trees are spaced well apart, each one defending a space that represents a water and nutrient supply. Both are in short supply, so usually there is bare ground between the perennial plants. Under the surface, these plants have fiercely competitive root systems.

In wet years, native annual flowers find enough surface moisture around the perennials to sprout and bloom, causing brief, colorful spring displays. They then set seed and die before the dry season.

Traditionally, fires caused by lightning sputter out quickly in the desert because they can’t easily spread across bare ground.

Under the influence of the airborne NOx, however, nonnative annual grasses can gain a foothold on the desert floor.

After a couple of years of adequate rain, the grasses can produce a continuous carpet of vegetation. When they die they leave behind enough dry tinder to spread the flames between the big perennial shrubs.

That is what happened this summer in the Joshua Tree/Morongo Valley fire. Apart from the damage to human structures, the long-term survival of desert vegetation is threatened.

Desert shrubs are not particularly adapted to fires, because these aren’t very common.

But annual grasses thrive on disturbances like fires. After a fire, they sprout from seeds, grow fast, and use up the nutrients and water before the slower-growing shrubs get a chance to grow large enough to compete.

Researchers think that our shrub communities — desert and coastal — could disappear within a few decades unless we clean up the air.

I certainly hope we do, because the coastal sage scrub community alone contains dozens of species of plants and wildlife that are found nowhere else.