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.
Recently, Parrot Scientific Program Director Dr. Greg Crutsinger 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. This imagery can later be processed into high resolution maps with several rich data layers.
‘We used Sequoia to help us address ecological questions about these ancient desert shrubs,’ says Greg. ‘Specifically, do 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 we also wonder what that kind of aging might mean for the plant itself.’
By mounting the Sequoia on a 3DR Solo drone and using the free, open source Tower app to plan a fully autonomous mapping mission, Greg 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. Greg then linked up with the Pix4D team in Lausanne to collaborate on data analysis.
‘Field researchers across the globe use Pix4D photogrammetry software for data processing and analysis in plenary applications, everything from coastal monitoring to archeological sites,’ states Oliver Küng, co-founder of Pix4D. ‘In fact, you could consider these ancient creosote clones a botanical archeological site.’
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, the team then used the Sequoia’s multispectral data to calculate the average normalized difference vegetation index (NDVI) across the different ring sizes.
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).
‘That’s a cool result,’ says Greg. ‘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.
To learn more about the Sequoia camera and Pix4D go to http://www.parrot.com/usa/companies/sequoia/
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