Question

What is a tree line?

Answer 1

Tree line is simply the single row of trees on a hilltop, the elevation over which trees will not grow, such as on a mountain, The tree line is the edge of the habitat at which trees are capable of growing. It is found at high elevations and high latitudes.

Complete answer:
The tree line is the border of the ecosystem where trees are capable of growing. At the tree line, trees show limited growth. Due to unfavourable environmental conditions, they are unable to develop beyond the tree line. The change from dense to sparse cover is shown by the tree line.
The tree line is the height above which no trees can be seen. Trees are not found beyond this line due to climatic circumstances such as freezing temperatures, high altitude, or a lack of rainfall, and if discovered, show stunted growth or form low dense matted shrubs. The tree line is sometimes distinguished from a lower timberline or forest line, which is the line below which trees form a forest with a closed canopy.
The highest elevation at which trees can survive is the alpine tree line; further up, it is too cold, or the snow cover lasts for too long, for trees to survive. The tree line in a desert denotes the driest areas where trees can grow, due to insufficient rainfall in drier desert areas. These are referred to as the "lower" tree line. The arctic tree line is the northernmost latitude in the Northern Hemisphere where trees can grow; farther north, it is too cold all year round to sustain trees.

Note:-
For a variety of reasons, the environment may be too harsh for trees to grow. This can include geothermal exposure associated with hot springs or volcanoes, such as Yellowstone; high soil acidity near bogs; high salinity associated with playas or salt lakes; or saturated groundwater that prevents oxygen from entering the soil, which is required for most tree roots' growth.