Step 1: Understanding the Question:
This question tests knowledge of ecological terminology.
Step 2: Detailed Explanation:
The tree line is the boundary between forest and non-forest areas at high altitudes.
Above the tree line, trees cannot grow due to harsh conditions.
Base line (B): Not a standard term.
Taiga (C): Boreal forest.
Savanna (D): Grassland with scattered trees.
Thus, the correct answer is tree line.
Step 3: Final Answer:
Thus, the boundary is called the tree line, which corresponds to option (A).
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