The transition to the treeless condition is more commonly gradual. Initially in a closed, tightly spaced forest (forest line), the spacing between trees widens rapidly as tree height decreases (the kampf zone). This zone gives way to a region of low twisted and stunted trees called the krummholz. Together, the kampf zone and the krummholz constitute the transition zone. The end of the krummholz marks the tree line.

The same woody species may at higher elevations grow as prostrate shrubs, especially in sheltered nooks and crannies. The zones are uneven because these kinds of local shelter conditions may extend the limits of each zone. Forests may extend along ridges where squirrels and other nut gatherers have stored seed, so each situation may have endemic differences from any assumed model of tree line.

The increase in spacing after forest line is correlated with a decline in the quality of the habitat as the temperature decreases, the wind increases, and the soil becomes increasingly impoverished. As the energy content of the ecosystem decreases, the diversity of organisms in the ecosystem diminishes.

Trees that are more widely spaced have a greater chance of survival because a greater percentage of the stem is covered with foliage, and this foliage receives more light and heat. In addition, there is less competition in the roots for the available nutrients in the soil. The isolated condition, however, makes the trees more susceptible to wind damage, snow blast, and ice damage.