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The big, thick roots on trees are mostly just structural. The interface where trees actually absorb stuff they need to live is a thin layer under the soil made up of billions of tiny "root hairs". Even humongous, old-growth trees are dependent on this layer of roots so small and ephemeral that they may only last a few days. Where these roots meet the soil there is such a complex mixture of bacteria, fungi, sloughed off tree cells and whatnot that it's impossible to say where the soil ends and the tree begins. This region is called the "rhizosphere".

The exudates that trees secrete into this region produce an electrical gradient that pushes needed chemicals back into the roots. These secretions also support soil organisms. Over millions of years they've co-evolved to the point where trees release chemicals purely to feed them and, in return, they extend the tree's "reach" into the soil and provide chemicals the tree can't make, in some cases literally piercing and growing within the tree's own cells. There's some research now showing that trees (and other plants) can even communicate via chemicals in the rhizosphere. Kind of like how neurons communicate across a chemical synapse.

I love this fact because it inverted the way I think about trees. In a way the forest we see is like a living protrusion of the rhizosphere, with trunks and leaves only serving to deliver sugar, carbon, and other stuff down to where all the action is. I hope it lightens your mood. If you can see a tree now there's a good chance you are witnessing a fungal symbiosis that nobody has ever identified.