The secret: layers of depth, not individual trees
A forest reads through overlapping planes of foliage at different distances, each a different value, with light filtering through. You suggest detail with brushes rather than rendering every leaf. Build it back-to-front in depth layers and it comes together quickly.
1. Set the value plan
Decide the lighting — misty morning, dappled noon, golden evening — and block the scene in value planes from light (distant) to dark (foreground). The background is pale and hazy; the foreground is dark and detailed. This depth gradient is the foundation.
2. Build back to front
Work in separate layers: distant trees (light, low-contrast, simple silhouettes), midground trees (more form and value), foreground (darkest, most detail). Overlap them. Each nearer layer is darker and crisper. Use foliage brushes from the nature category — see the best free nature brushes.
3. Add foliage with scatter brushes
Lay canopies and undergrowth with foliage scatter brushes, dark to light, leaving sky holes and gaps. Vary brush size between layers. For individual trees within the scene, apply the structure-and-light approach from how to draw trees.
4. Trunks and the forest floor
Add vertical trunks in the midground and foreground — lighter where light catches them, darker in shadow. Suggest a forest floor with leaf litter, roots and ground texture. Stamp foliage and ferns from the stamps category for quick undergrowth.
5. Light shafts and atmosphere
The magic of a forest is light filtering through the canopy. On an Add or Screen layer, paint soft diagonal light shafts (god rays) coming through gaps, and add haze between depth planes. This atmosphere ties everything together and creates mood. For glow technique, see glow effects (same Add/Screen idea).
6. Dappled light and detail
Add dappled highlights on the foreground floor and leaves where sun breaks through. Concentrate detail at the focal point and keep the rest suggested. Resist over-detailing — a forest is about impression and depth, not every leaf.
Brushes and next steps
Paint a forest with free foliage and texture brushes from the nature and stamps categories, or any free brushset. Place your forest in a full landscape and deepen it with atmospheric perspective.