Lichen are an important pioneer agent of mechanical weathering through root wedging. Their small roots are often the first to appear on a bare rock surface.
Shown (left) are a camp of British soldiers (Cladonia cristatella). So named for its' red coat flower, the soldiers begin to break down the shale by digging their roots into it.
Roll over photo to highlight lichen.
Here is a textbook example of root wedging on a slightly larger scale. The roots of the tree slowly break the rock as it grows downward. In this case, mechanical weathering has also increased the rate of chemical weathering by increasing the exposed surface area of the rock. The exposed area becomes subject to the influence of chemical processes such as oxidation and hydration.
Roll over photo for close-up.