Adding a stroke under the character for 'tree' to mark its root is a way of forming characters called the indicative method.
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指事 is one of the 六书. A pictograph, 象形, draws a thing; an indicative uses an abstract mark instead, like the single stroke of 一 or the mark above or below a line in 上 and 下. Few characters were made this way, so a 指事字 is rare.
