For drawing or painting bark, you essentially look and layer. It takes time to draw tree bark, like it takes time for a tree to grow bark.
Have a good look at the bark. Mind you, botanists recognize winter trees by their bark, so if you go for a scientifically or botanical accurate drawing, make sure you have the right tree (bark) in front of you. Don’t say you are drawing a beech tree when, in fact, you are drawing oak bark. Collect year round pieces of bark and label them; ‘Oak wood’, ‘Pine bark’, or ‘White birch branches’.
Having said that, don’t be overconscientious; trees are very forgiving. I yet have to meet a tree that doesn’t appreciate a good hug or shows misgivings towards being portrayed.
Select all the colours you see, and I assure you there are many colours hidden in bark. Don’t be surprised to find next to many greys and browns, lots of orange, red and yellow for the lichen and mosses.
Use a solvent with coloured pencils to liquefy the first layers, medium when you use oils, or gel when you work with acrylic paints. Make sure the first layers are smooth and don’t overdo colour in the base coat.
Golden Orioles on mossy tree branches, coloured pencils, Paula Kuitenbrouwer
Once the base coat or layer shows shape, shadow and structure, add more layers to work towards its top coat, often looking very dry, mossy or covered with lichen. Study the mosses and lichen and try to copy them in great detail. To suggest dryness, stop using solvents or gel and add thick layers of coloured pencils or paint. You can also use a graphite pencil or a very thin brush to add little greyish lines to suggest depth in the bark top coat.
Remember, practise makes perfect.