What is a tree line?

Explanation

**Step-by-Step Solution:**

1. **Definition of Tree Line**: The tree line refers to the boundary or edge of a habitat where trees are able to grow. Beyond this line, trees are either absent or unable to thrive.

2. **Environmental Conditions**: The presence of trees is influenced by various environmental factors. Key conditions that affect tree growth include:
- **Cold Temperatures**: In regions with very low temperatures, trees struggle to survive.
- **High Altitude**: As altitude increases, the atmospheric pressure decreases, which can hinder tree growth.
- **Lack of Moisture**: Insufficient water availability can also prevent trees from growing.
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