"timber line" meaning in English

{
  "forms": [
    {
      "form": "timber lines",
      "tags": [
        "plural"
      ]
    }
  ],
  "head_templates": [
    {
      "args": {},
      "expansion": "timber line (plural timber lines)",
      "name": "en-noun"
    }
  ],
  "lang": "English",
  "lang_code": "en",
  "pos": "noun",
  "senses": [
    {
      "alt_of": [
        {
          "word": "timberline"
        }
      ],
      "categories": [
        {
          "kind": "other",
          "name": "English entries with incorrect language header",
          "parents": [],
          "source": "w"
        },
        {
          "kind": "other",
          "name": "Pages with 1 entry",
          "parents": [],
          "source": "w"
        },
        {
          "kind": "other",
          "name": "Pages with entries",
          "parents": [],
          "source": "w"
        }
      ],
      "examples": [
        {
          "bold_text_offsets": [
            [
              52,
              63
            ]
          ],
          "ref": "1957 August 26, H. A. Vallance, “By Rail to the Norwegian Arctic”, in Railway Magazine, page 574:",
          "text": "The surroundings become bleak in the extreme as the timber line is approached and trees give place to thinly scattered birches that are little more than bushes.",
          "type": "quotation"
        }
      ],
      "glosses": [
        "Alternative spelling of timberline."
      ],
      "id": "en-timber_line-en-noun-fP-4818P",
      "links": [
        [
          "timberline",
          "timberline#English"
        ]
      ],
      "tags": [
        "alt-of",
        "alternative"
      ]
    }
  ],
  "word": "timber line"
}

{
  "forms": [
    {
      "form": "timber lines",
      "tags": [
        "plural"
      ]
    }
  ],
  "head_templates": [
    {
      "args": {},
      "expansion": "timber line (plural timber lines)",
      "name": "en-noun"
    }
  ],
  "lang": "English",
  "lang_code": "en",
  "pos": "noun",
  "senses": [
    {
      "alt_of": [
        {
          "word": "timberline"
        }
      ],
      "categories": [
        "English countable nouns",
        "English entries with incorrect language header",
        "English lemmas",
        "English multiword terms",
        "English nouns",
        "English terms with quotations",
        "Pages with 1 entry",
        "Pages with entries"
      ],
      "examples": [
        {
          "bold_text_offsets": [
            [
              52,
              63
            ]
          ],
          "ref": "1957 August 26, H. A. Vallance, “By Rail to the Norwegian Arctic”, in Railway Magazine, page 574:",
          "text": "The surroundings become bleak in the extreme as the timber line is approached and trees give place to thinly scattered birches that are little more than bushes.",
          "type": "quotation"
        }
      ],
      "glosses": [
        "Alternative spelling of timberline."
      ],
      "links": [
        [
          "timberline",
          "timberline#English"
        ]
      ],
      "tags": [
        "alt-of",
        "alternative"
      ]
    }
  ],
  "word": "timber line"
}