Enhanced Roadside Growth of Creosote Bush

March 01, 2016

CreosoteFig 1 copy

Photographer: David K. Lynch
Summary Author: David K. Lynch
The edge effect is a fundamental ecological phenomena where plants and animals adapt to and often thrive at the edges of ecosystems compared to the interiors. Among the many ecological transition zones, those along roads are extremely narrow, often only a few meters wide. Their easy accessibility and small dimensions make them convenient locations for ecological studies.

A good example of the edge effect is seen along paved desert roads, such as Scotty's Castle Road in Death Valley National Park, California, where enhanced growth of creosote bush (Larrea tridentate) is seen within a few meters of the road surface. The bushes are larger and appear healthier and greener than those just 10 m from the road. Their apparent vitality is obviously related to the road, and the obvious reason is water: rainwater runs off the road and adds extra moisture to the soil adjacent to the pavement. But there’s more to it than just water. The creosote bushes are not growing in the low regions where water collects. Instead they're growing in the high regions, the elevated berms 0.2 to 1 m high. What’s going on?

The answer goes back to when the road was built. When the bulldozer first scraped the path, it piled up the debris into elevated berms paralleling the road. The berms consist of loose soil and rocks in long rows. Being loose, water and wind-borne seeds could more easily settle into the soil, the seeds germinating and take root better than in the surrounding hard desert surface. With many crevasses and air pockets the roots grew well. The lower parts of the berm were more moist than the top due to the collected rainwater, so roots were encouraged to grow fast and deep. And sticking up from the flat desert, the berms would catch the early morning and late afternoon sunlight, thus staying warmer, thereby also promoting growth. The result: enhanced creosote bushes lining desert roads, the edge effect at its best. Photo taken on February 8, 2016.