How extraordinary. I thought tree roots went as deep as the top growth.

Depends on many factors. I see the same phenomenon along a nearby footpath. The trees (alders, willow, poplar) grow alongside flooded riverine systems known as 'carr - trees grow wide rootplates as the water table is so high...but this can lead to large areas of ground lifting as the. roots 'are hoplelessly intertwined into a network of shallow roots. The shallow root system does create vulnerable and unstable root systems. When trees fall, the whole rootplate will often remain intact. Some trees are naturally shallow rooted - beech for example, and their roots will extend beyond the crown but often no deeper than several feet.

Very interesting subject!!! I hope more people will contribute.

I think rosaprimula is right in saying it depends on many factors. Off the top of my head, I'd say that roots would be encouraged to grow deeply if they are forced to hunt for water by doing so,but then take a look at my Paulownia thread, where I'm thinking that my tree has grown it's roots out horizontally in it's hunt for water. Also I think it probably depends on the variety of plant. For example, a few years ago on the steep "mountain" road (I put mountain in quotes because technically we live amonst high hills,foothills of the Appenines) up to our old house, the woods were full of felled acacia trees that had been weakened by the preceding drought, and then fell as soon as fall/winter rain and wind set in.