When illustrating trees, the single most significant aspect is lighting. Study the direction of the light and quality of shadows in the tree you are sketching and start a fixed pattern of shading the leafy areas.
When drawing, shadowed areas will be darker and will involve heavier lines; moderately shaded areas require somewhat lighter hand; and regions left white portray highlighted leaves.
Keep in mind to use negative space (the sections between branches where there are no leaves) to aid for stressing the leafy parts. Slowly deepen the shadowed regions and adjust shading to produce the illusion of individual leaves and the realistic quality of the tree.
By means of the flat side of a pencil to create broader strokes of different darkness is another procedure used to draw trees and leaves. This method produces a less definite leaf pattern.
Put emphasis on zones by using an eraser to build areas of light. You may also desire to apply a disorganized scribble method, drawing squiggly lines of diverse thicknesses to design a less defined tree.
Keep in mind that all of these techniques rely on using shadows and light to expose the idea of clusters of leaves.
How to sketch full forests of trees
Sketching a complete woodland of trees is a little bit more complex, as you don't want to draw each tree individually, but too much evenness will also be not right for the drawing.
The problem: We may perceive trees of similar type as nearly identical but this is not the case. While alike, they are all unique and should be viewed and drawn in a way that demonstrates this.
Reviewing the direction and quality of the light and how it gives character to each tree is the starting point. Even in a crowded forest, single trees can be identified. This is the aspect that you must render into your drawing.
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