When walking through a forest, there are quite a few things you'll find growing among and on trees that aren't part of the tree itself. Instead, they represent part of the diverse ecosystem that can be found in a tiny space. From the roots to the tips of their branches, trees are often home to moss, lichen and fungi species. But what's the difference between them?
First, there's fungi species. The largest part of a fungi, the spider-silk-thin strings of hyphae that form the mycelium, is usually completely hidden from our eyes, growing in the soil or in or among the bark of trees. What we typically see is just the fruiting part of a fungi, or the mushroom. These take all sorts of forms depending on the species, including the typical umbrella-shaped mushrooms, or shelf-like mushrooms, or even mushrooms that look like a mass of opaque icicles.
When fungi is present, you may also find lichen. As wiseGEEK notes, "Lichens are perhaps the most amazing living things on Earth, because they represent a symbiotic relationship between a fungus and either algae or cyanobacteria." Some lichens look a bit like a crust forming on the surface of the bark, and some may look like they have a coarse, leafy structure but lichen isn't actually in the plant kingdom at all. Lichens don't have roots to absorb resources like plants but instead make their own food from the resources around them.
Moss, unlike fungi or lichen, is indeed a plant. Knowledgenuts writes, "Mosses are one of the most primitive types of plants, and their simple structures have remained largely unchanged over the course of millions of years. Thought to have evolved from green algae, mosses are characterized by their simple, basic root structures, stems, and leaves. There are around 14,500 different types of mosses, and because of their simple structure and low nutrient requirements, they will often be found thriving in places that other plants can’t grow."
The easiest way to tell lichen and moss apart is to look for stems and leaves -- no matter how miniscule, mosses have them and lichens don't.
There are so many species of fungi, lichen and moss that if you were to take the time to study those found on your next walk in the woods, you'd be sure to spend hours in just a few square feet. They're such an interesting part of an ecosystem that it would be time well spent.