Roadside flora can be intriguing study

ELISABETH M. BROWN

Roadway edges are unglamorous, usually weed-choked strips of

landscape.

They are familiar territory that we tend to look through as we

drive past, but that doesn’t mean they don’t have interesting

biology.

Roadway edges are a microhabitat, where conditions differ on a

small scale within the general climate of a larger biome. They are on

the edge of the landscape, but not typical of it, due to such factors

as water availability and physical disturbance of the soil.

For example, we’re driving home from the Sierra Nevada along

Highway 395. It’s early fall, so the sides of the road are lined with

rabbitbrush festooned with bright yellow flowers. But wait, the

flowers seem to be only on the shrubs closest to the road. What’s

going on?

Let’s step back. Next time you’re in the low desert, take time to

notice that the native creosote bushes are amazingly evenly spaced.

Creosote bushes are the tall, rangy shrubs with small tough leaves

that dominate the sandy desert floor. In some places they grow in a

monoculture that extends for miles in every direction.

The spacing of the shrubs is due to a silent underground war for

water waged by the roots. A network of roots permeates the ground

under and around each shrub, scavenging up any water that percolates

through the sandy soil.

Ecologists excavating the soil beneath the plants found that

equidistant between any two plants, the roots faced off against each

other, like two armies ranged across a border. The extent of the root

system determines the size of the shrub and the distance to the next

plant. No seedling survives long against those roots, so between the

creosote bushes there is often no other plant growth.

Creosote shrubs closest to the road are larger than those farther

back, due to the road runoff effect. Water falling on the asphalt is

not absorbed, but runs off onto the roadway edge vegetation. Because

of those very efficient desert roots, usually only the first row of

shrubs benefits from the extra water.

Along Highway 395, rabbitbrush shrubs that receive extra water

from road runoff bloom first; in a dry year, these plants may be the

only ones that do.

Road edges are also disturbed more by humans than the rest of the

landscape, simply because they are on the edge. Mechanical

disturbances such as mowing or discing or application of weed killer,

are regular occurrences. This discourages less aggressive native

plants, and the reduced competition allows the tough, persistent

weeds to flourish.

It’s no accident that Pampas Grass and other fountain grasses grow

right on the edge of Laguna Canyon Road, along with other

undesirables like Castor Bean and Wild Anise: native plants can hold

their own in undisturbed areas, with strategies like the creosote’s

roots, but not on road edges.

Still, a toehold on the roadway edge is often enough to allow

weedy plants to gradually invade larger areas. They spread their

hopeful seeds on the wind and await a disturbance far away from the

roadway edge.

* ELISABETH BROWN is a biologist and the president of Laguna

Greenbelt Inc.