For half of its life Lepidodendron lived as a telephone pole-like plant sticking out of the forest floor. Then it began branching. Finally, the branching growth stopped and spore cones formed at the end of the branches. Growth stopped. The tree was putting all its energy into making and releasing spores.
In some species of Lepidodendron the tree died after it was finished releasing its spores, probably because they spent all their energy on doing just that, shedding and making spores. This is like salmon who die right after laying eggs because they use up all their energy swimming up rivers and jumping up waterfalls, and spend the last bit of energy laying eggs and transferring sperm to the female.
Lepidodendron and other lycopod trees had the shallowest roots I've ever heard of. The roots barely went a couple feet into the ground for an enormous 100 foot tree. One of the reasons
Lepidodendron didn't fall down was probably that, despite its enormous size, the trunk was probably pretty light. Inside the thick bark there was a cotton-like substance, which was the vascular system. Another reason Lepidodendron didn't fall down is probably that the roots were fat and also surprisingly long. But they barely went into the ground and were nearly unbranched. The bark of Lepidodendron was a couple of inches thick, which held the tree in an upright position.
Lepidodendron had bark covered in scaly leaf scars. In the "telephone pole" stage, the leaves were gradually moving up the trunk. As the tree got larger, and older leaves fell off. Finally this process stopped as Lepidodendron started to grow its first branches. The first branches it grew were forked, and then off of those forks it grew branches that looked somewhat like those of conifers.
Some people used to think that the bark was the remains of a giant snake or lizard, which turned out to be totally wrong.
Prehistoric Life: The Definitive Visual History of Life on Earth, pg. 145