The Mojave desert is a tough place to live by any measure. Receiving less than 13.7 cm (6 in.) of rain per year, it is the driest of all North American deserts. Spanning a range of elevations, the Mojave is prone to extreme temperatures, from sub-freezing conditions at night and in winter up to 49 °C (120 °F) or higher in the summer. Without any natural cover, a relentless sun beats down on plants and animals alike.
One species thrives in the Mojave, however: the creosote bush (Larrea tridentata). Creosote is an evergreen shrub that grows 1 to 3 m (3-10 ft) in height and boasts bright yellow flowers. It is one of the most dominant species in the Mojave landscape, and it provides most of the scenery on the drive from Los Angeles to Las Vegas.
The dark green creosote leaves are full of resins (plant oils), making the plant inedible to all but very specialized insects and contributing to the shrub’s common nickname, ‘greasewood.’ Creosote releases these oils into the air after a precipitation event, and they permeate the desert with a distinct smell that long-time desert dwellers associate with the smell of rain.
A fascinating aspect of older plants is the propensity to form rings. Creosote shrubs can live a long time, and as they age, their oldest, most central branches die off and the crown splits apart into a ring pattern. These rings appear to march, spreading out over the years; however, each new plant is actually the same genetic individual -- meaning they’re clones.
The oldest known creosote clones are in the Lucerne Valley, California. Referred to locally as the ‘King Clone,’ this ring has been estimated by scientists to be over 11,000 years old and spans over 40 ft (15 m) in average diameter.
Figure 1. The ‘King Clone’ of creosote (Larrea tridentata) in Lucerne Valley, California, is the large ring near the center of the map. Note the black car on the road shown for scale.
Recently, I headed to the Mojave with the Parrot Sequoia camera, one of the smallest, lightest multispectral sensors on the market. In a single flight, Sequoia captures images across four defined visible and non-visible spectral bands, plus RGB imagery.
I used Sequoia to help address ecological questions about these ancient desert shrubs. Specifically, I was interested in whether ancient rings vary in productivity compared to their younger counterparts? If estimates are correct, the King Clone germinated when wooly mammoths still roamed the earth. Now that’s impressive, for sure, but I also wonder what that kind of aging might mean for the plant itself.
I mounted Sequoia on a 3DR Solo drone and using the free, open source Tower app to plan a fully autonomous mapping mission, I was able to map the King Clone and surrounding shrubs in a 10-minute mission at 30 m altitude. Though it was a single, short flight, the drone-mounted Sequoia captured data for a variety of ring sizes. I then linked up with the Pix4D team in Lausanne to collaborate on data analysis.
Pix4D stitched the Sequoia imagery into a high resolution orthomosaic in both color (RGB) and multispectral data layers. Focusing on a sub-sample of 60 shrubs that ranged from 1.2 to 23.5 m in diameter,we then used the Sequoia’s multispectral data to calculate the average normalized difference vegetation index (NDVI) across the different ring sizes.
Figure 2. NDVI map showing multispectral data from the Parrot Sequoia camera of the clonal rings of creosote (including the King Clone) in Lucerne Valley, California.
The conclusion? There was no relationship between ring size and NDVI (Figure 1), without or without the King Clone included in the dataset as a statistical outlier (r = 0.005, P = 0.96).
Figure 3. Relationship between the diameter (measured at the widest point in meters) of creosote shrub rings and the average (shrub level) NDVI.
Its a a cool result, as it suggests that the physiology of ancient shrubs, or how they function in this extreme environment, may be similar to shrubs several millennia younger. Mark Twain said that age is an issue of mind over matter: If you don’t mind, it doesn’t matter. That certainly seems to be the case with creosote in the Mojave.
Drones have already transformed the way ecosystems are studied and monitored. It’s easier and faster than ever to get a large amount of rich, accurate and actionable data. And with the Sequoia camera in particular, scientists now have an incredibly powerful tool to complement traditional vegetation data collection on the ground.