Imagine you bought a cleared paddock, removed the stock and, for 40 years, you watched the trees and shrubs regenerate. All looked wonderful except that large rings of bare, scalded soil appeared. The large, bare Dirt Rings could be seen on air photos, as shown in yellow above.
How did these strange patterns appear? This is the question I posed in my last blog, and the quiz triggered lots of thoughtful suggestions. This week we solve the Mystery of the Dirt Rings. But first, let’s go back in time to see how the landscape evolved over the past 150 years.
In the mid-1800s, when Europeans first settled the region, it supported a forest or woodland dominated by large Grey Box trees (Eucalyptus microcarpa). In the late-1800s, the paddock trees were ring-barked and cleared to create farming land.
Scattered trees were left in the paddocks. Some were intentionally retained as shade trees. Others re-sprouted and survived the ring-barking. The survivors had a characteristic growth-form, with big, low branches about chest height; the height of the early axes [as on the left-hand tree below].
Unfortunately, the soils weren’t that fertile and rainfall wasn’t high (about 450 mm per year). Nevertheless, for over a century, the region supported an active agricultural industry dominated by grazing, especially by sheep, and cropping.
More recently, the regional economy changed. Since the 1970s, many farms were purchased by city-dwellers who created hobby farms, bush blocks, and recreational, retirement and investment properties. When the stock were removed, the paddock trees regenerated, forming rings of dense, young trees around the original paddock trees.
The density of young trees varied around each paddock tree, but some patches were very dense. The old paddock trees, with their wide crowns and sagging limbs, stood in the center of dense patches of young, upright regrowth.
Trees weren’t the only plants to regenerate. After the stock were removed, Drooping Cassinia (Cassinia arcuata) and other native shrubs formed large shrublands in the paddocks that had few trees. More often, the regenerating plants formed a mosaic of shrubland and forest patches.
Broadly speaking, the trees and shrubs were arranged in concentric zones, centered on the original paddock trees. The original paddock trees were encircled by regenerating trees, and the shrubs filled in the areas further away.
The roots of the regenerating trees extended many meters beyond the outer edge of the overhead tree canopy. Few plants can compete against the tree roots for water and nutrients. So the patches of regenerating trees became encircled by a wide zone of bare soil. Under the ground, the bare soil is filled with tree roots, but seen from above, it appears bare and open.
Any leaves that fell into the open zone were quickly washed and blown into the adjacent patches of shrubs and trees. Exposed to the sun and rain, the bare soil became crusted and compacted, and soil productivity declined.
Viewed from the ground, a patchy landscape of trees, shrub and open zones was created. From the air, an amazing mosaic can be seen, with regenerating trees (in green below), patches of shrubs (dark grey, and criss-crossed by narrow kangaroo trails) and open, bare zones between the trees and shrubs (white).
And thus was formed The Mystery of the Dirt Circles. The paddocks originally contained scattered old trees, just like this.
With some digital-trickery, we can overlay the locations of the bare soil rings on the original air photo, to create this compound image. Note that these bare zones didn’t exist before the regeneration began; they were created by the regenerating trees.
In the photo above, the black blobs are the original paddock trees. Around each tree there now lies a ring of dense young trees. This regrowth zone is not shown in the image above, but is situated in the grey zone surrounding each old tree. Beyond the tree regrowth lies the bare soil zone, shown in yellow. And further away are shrubs, which sit in the grey areas away from the trees. So there you have it, the Mystery of the Dirt Rings is solved.
Patterns and processes
It’s easy to tell a story in photos, but it’s important to differentiate between the patterns and the processes that generated the patterns. We’ve nearly finished writing up a paper based on Pheona Anderson’s Honours research project that documents the patterns described above. Pheona studied the vegetation and soils in the four zones in regenerating paddocks: beneath the old trees, in the dense tree regrowth, the open areas and shrub zones. So we know how the patches are arranged and what the soils are like in each zone.
We haven’t conclusively demonstrated that competition for water between the roots of the trees and other plants is what causes the open zones to form. The patterns are certainly consistent with this hypothesis, but it’d be great to do some experiments to confirm the suggestion. For example, we could dig trenches between the young trees and the open gap zones, to cut the roots of the young trees. If we did this, we’d expect that soil moisture would increase in the open zone and that other plants may colonize the gaps, after the competing tree roots were severed.
Thanks once again to Deanna Duffy for the aerial photos of the vegetation mosaics. The ring-barking pictured above is illustrative only, and is not from this region.
And the winner is…
Last week’s quiz prompted lots of fantastic suggestions about the processes that caused the bare dirt rings. Thanks to everyone who wrote in, especially those of you who sent in more than one suggestion.
The prize for the best guess was a signed copy of the book, Plains Wandering. In the end, it was so hard to pick an outright winner that I decided to award two prizes – to Deb Little and Jo – both of whom sent in a series of great suggestions. Congratulations to both of you for solving for the Mystery of the Dirt Rings!