There are almost as many ways to paint trees, as there are artists to paint them.
Here are 5 different tree studies, with a variety of approaches you might try:
1) BASIC TREE STUDY --
Sketch a tree in pencil, and then lay in the foliage and grass, with a light mingling of yellows & greens (quinacridone gold & sap green). . .
Wet each shape again, and add some darker/cooler greens & blues (quinacridone burnt orange/sap green and sap green/cobalt blue), to suggest volume. Keep the first light/warm wash visible at the top of the clumps. . .
When this has dried, paint the trunk, branches, and a simple cast shadow (using quinacridone bt orange, perylene maroon, and French Ultramarine). Add a few dark marks for emphasis in the leaves. . .
2) UNDERPAINTING/SALT/NEGATIVE PAINTING -- Tree in Autumn
Do a mingling (wet-in-wet) of warm colors. When the wash has just a "sheen" to it, sprinkle on some salt, and let it dry completely. . .
After it's dry, brush off the salt, and draw a simple tree and grassy foreground line. Paint the negative shapes with a mingling of colors (golds, oranges, reds, and a bit of green). . .
3) SPATTERING/MISTING -- Flowering Tree in the Spring
Draw a tree, lightly in pencil. Paint the trunk and branches first . . .
Loosely cover up the areas that you don't want spattered, and mist the exposed area with clear water . . .
While these drops are wet, spatter some pinks and reds and magentas, and a little green. Use a brush for this, tapping your finger to spatter the paint. Then, spatter on some clear water, and then touch some of these drops with a brush loaded with the paint (pink, red, or magenta). Let your brush "dance" across the paper, dropping in paint in a "lacy" way. . .
4) MASKING & MINGLING -- Palm Trees
Draw a palm tree, or two, in pencil. Then, paint a wet-in-wet underpainting, with quinacridone gold, cobalt blue, and quinacridone rose. . .
Using watered-down masking fluid and a quill pen, apply the mask to the negative shapes -- everything other than the palm trees. . . (apply this with a Q-tip to the big shapes).
When this is completely dry, mist your paper with clear water, and mingle and spatter your warm colors -- (quinacridone gold and burnt orange, sap green, for the mingling; and cadmium red and cerulean blue, for the spattering) --
Let this dry completely, and then remove all the masking (with a rubber cement eraser, or your fingers).
5) RAINBOW OF BACKGROUND COLORS -- Stylized Trees
In pencil, draw a group of stylized trees -- interweaving trunks and branches, with no leaves. Take them right off the top and sides of your paper. Then, draw a few wavy, horizontal lines, behind the trees. . .
Leave the trees white, and paint the background shapes, in a rainbow of colors. When the background is dry, erase all the pencil lines. . .