Here’s a puzzle for you – one with a prize. Imagine you bought a paddock like this: poor soils, scattered trees, sparse grass, a few eroded patches, bounded by roadside trees.
Now sit back, close your eyes, and imagine that you remove the sheep and cattle and watch the paddock regenerate, without assistance or disturbance. Over 40 years, native trees and shrubs colonize almost the entire area. Birds, kangaroos and other wildlife become more common. From a conservation perspective, a great outcome indeed.
But there’s just one catch. For some reason, big patches of bare soil appear. The bare patches contain no trees or shrubs and few ground plants. The bare soil is crusted and compacted.
Looking at the black and white photo above, can you predict where the patches of bare soil would occur, 40 years in advance? Would they be on the ridges, in the valleys, on the steepest slopes, or would they follow some other pattern?
To find out, let’s leap forward 40 years, and see what happened.
The image below is a composite image of two overlain air photos, one taken in 1971 and one in 2010. The background image is a close-up of the top left corner of the black and white photo at the top of the blog. It was taken in 1971, before regeneration began. The dark spots are the scattered paddock trees. Each tree canopy is about 20 m wide.
The yellow shading is from an air photo taken nearly 40 years after the B&W photo. In 2010, the yellow patches had few ground plants, no trees, no shrubs, just bare, crusted soil. [For the technically minded, the yellow shading comes from a supervised classification of the geo-referenced 2010 image]. All of the areas that aren’t yellow are now covered with native plants.
As you can see, the patches of bare soil formed obvious patterns in 2010. Many patches formed wide rings that seem to encircle – but usually were a long way away from – the original paddock trees. These rings don’t seem to match any features on the original air photo, as you can see from the top photo.
This week’s quiz question is – How did these patterns form? What causes the rings? Why aren’t the bare areas covered in plants?
To give you time to solve the mystery, all will be revealed next week. To encourage lots of guesses, the most accurate and comprehensive comment will win a signed copy of the book, Plains Wandering: Exploring the Grassy Plains of South-east Australia.
The best answer has to explain two things: why do the bare patches occur where they do, and why don’t they occur in other places on the photo?
So put your thinking caps on, and submit your comments below. I’m happy to answer questions to help crack the Mystery of the Dirt Rings.
Acknowledgments: I extend a big thank you to Deanna Duffy at CSU’s Spatial Data Analysis Network, who created the composite image.