Trees are not cardboard cutouts with brown trunks and leaves that are simply green if it's summer, red if it's autumn, or absent if it's winter. The 'secret' to painting believable trees is an understanding of the underlying structure of trees complemented by observation of different species.
Compile a file or sketchbook with your notes, sketches, and even bits of bark and leaves. Buy yourself a tree identification guide (a comprehensive one, not a pocket one) and learn the names and characteristics of individual species. Read the descriptions in the tree guide and compare it to what you're seeing.
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Recognise the Shapes of Tree Species
One of the fundamentals to successfully painting realistic trees is learning to recognize the characteristic shapes of different species. Look at the overall appearance of the tree and identify the overall shape of the tree.
Is it shaped like a sphere, umbrella, cone, or tube, or is it simply irregular? Is it short or tall, fat or thin, straight or spread irregularly? Do the branches point upwards or downwards? Are the leaves dense or sparse? Has it spread naturally, has got broken branches, or has a gardener pruned it?
And remember to look at the tree's root system. Trees don't just stick up out of the ground.
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Tree Trunks, Branches, Leaves, Colors
Simplify what you're wanting to paint by breaking up a tree into its components. Observe these individually, rather than as a whole, before you start painting.
Taper the width of the trunk from bottom to top, as well as branches and twigs.
For more character, paint trunks sideways in small strokes rather from top to bottom in one long stroke.
Trees in dense woodland tend to be vertical; lone trees may be angled due to wind.
Branches aren't straight, aren't the same width throughout, and don't grow parallel to each other.
Avoid putting branches opposite each other on a tree trunk; trees aren't symmetrical unless they've been pruned that way.
Make branches cross over each other to create depth. Add appropriate shadows.
Leave gaps in the foliage and show branches in the gaps.
It's insane to try to attempt to paint every leaf on a tree. Look at the leaves as a mass (or groups of color), not individuals.
Trunks and branches aren't a simple, uniform brown. Study the bark of different trees up close not only for texture, but also for color, and make notes. Look at the color of new growth, old, and dead branches. For example, gum trees have silvery creams and oaks grays. Is there any moss or fungus growing on the bark?
Trees in a woodland receding into the distance aren't all the same green. Aerial perspective applies and benefits from a bit of exaggeration.
Look out for highlights reflecting the blue of the sky. Early morning sunshine will add a warm, golden glow.