Depressions in the Backyard. What could have caused it?

In this episode of Dendro and Codit the tree diagnostic duo, they received a request to go check out the yard of a homeowner. The yard had some pine trees and holes were appearing, which I suppose was not dug by said homeowner. The duo rushed over before it rains, as Dendro was worried that the rain would saturate the ground, and together with the holes may cause the trees fall over. 

They arrived at the house and noticed that the yard has a well-manicured lawn as well. The trees were 76cm in diameter (approx. 2.4m girth) and over 36.5m in height. There was a gust of wind and the caused the crown of the trees to sway slightly but there was no movement on the ground surface and lower trunk. The crowns of the trees looked healthy, and no obvious dieback and no yellow needles. 

Pine trees in the yard with a well-manicured lawn.

The homeowner informed the duo that several pine trees died over the years due to different reasons and she had their stumps grounded down (note that the stumps were not entirely removed). Codit then noted that the root collars of the trees cannot be seen, which suggested that the ground level in the yard had been raised. He suggested to the homeowner to excavate the ground around the trees so as to expose the root collar, and put in a wide ring of mulch 5 to 8cm thick. The mulch "circles" are to keep the turf from competing with the trees for water and nutrients, and to protect the tree base from lawnmower damage.

Close-up of tree base with root collar being covered by soil.

Dendro suspected that the holes may be sinkholes, which are caused by soil being washed away or rock dissolving below ground. He then checked a few spots around the yard to see if any spots were saturated with a Dutch auger, which is generally used for examining soil profile and collecting soil for nutrient analysis. He picked a few spots away from the holes to probe the soil but was found to be dry below the turf layer. There does not seem to be cavities or holes found in those spots too. 

Dutch auger

The few spots (that were probed away from holes) were found to have dry soil.

Codit then probed the holes near the pine trees and discovered that the holes were 0.9 to 1.2m deep and as wide. The material dug out from beneath the holes had decayed wood besides soil. The homeowner realised that the holes are the locations where a pine tree died. Dendro concluded that the holes were formed when the stumps decayed and formed cavities in the ground. Furthermore, he postulated that the holes would not affect the health and stability of the remaining pine trees, as the holes were not large, and there was no sinkhole issue. The holes in the ground could be filled in.

Reference: Arborist News, Dec 2022, pg 18-20 and 54-55