Trees first evolved some time in the Devonian and Carboniferous periods. Before trees, the largest organisms on land were Prototaxites. A metre wide, eight metres high … and a fungus.
At their height, during the Devonion period, Prototaxites would have easily towered over everything else on land. To be fair, everything else on land was mostly moss and insects, but still, and once trees evolved they were pretty much doomed.