There is one global light source in the Modeler, which can be assumed to be the sun since you are modeling a tree.
The light object itself sits in the top-right corner of the Tree Window. When you click it, you are presented with a gizmo to rotate it, and thus affect the directional lighting in the scene.
Selecting the light will also show the Light Properties in the Property Bar, where you can edit all the other lighting information. Right clicking on the light will also present a contextual menu with shortcuts to many lighting options.
Note: A shortcut for rotating the light gizmo relative to the current view is to hold the 'V' key and click & drag the mouse in the Tree Window.
The color and intensity of the directional light can be adjusted in the Light Properties when the light is selected. Lighting presets are also available to quickly switch between lighting scenarios.
Directional light will cast shadows. You may change the quality of these shadows in the Light Properties or contextual menu on the light object, but there is a trade-off in rendering speed.
Ambient light is light that exists in the scene but is not coming directly from the sun / light source. It is light from the environment around the subject. Without any ambient light, shadows would be completely black.
The color and intensity of the ambient light can be adjusted in the Light Properties when the light is selected. Lighting presets are also available to quickly switch between lighting scenarios.
Ambient light uses three colors to set up the environment: sky, horizon, and ground. These colors are interpolated to form the sky and ground in the background of the Tree Window.
Ambient light comes from all directions at once. But similar to how directional light can cast shadows, parts of the tree should be “shadowed” from ambient light. This technique is referred to as “ambient occlusion” or “AO”.
AO will darken crevices and the interior of a tree, giving it much more realistic lighting. You can also use an ambient occlusion map to further refine AO on a per-material basis.
Calculating AO is fairly quick, but it is not something that can be done all the time. You can compute it by clicking the “Ambient Occlusion:Render” button in the Light Properties, or by merely hitting 'O' in the Tree Window. Editing the geometry of a tree will invalidate AO until the next calculation, though AO is computed when the tree is first loaded.
With default settings, AO will most likely do a good job. However, you can further refine the effects of AO by modifying the “Brightness”, “Contrast”, “Min”, and “Max” properties. Generally you want the very outside parts of the tree to be fully white. The interior of the tree should be dark but not black. The depth of the dark interior is up to the artist, depending on how dense and enclosed the tree is.
Note: Switch to the “AO” render mode to visualize the computed AO on the tree for easy editing.