Take a look at almost any mountain in Alaska and you'll see trees growing on the lower reaches. As you look higher upslope you see that trees stop growing along a fairly distinct line - the timberline. The timberline is a visible boundary between the forest and the alpine. Trees might thrive on lower slopes, but at the boundary, even Alaska's hardy spruce and mountain hemlocks struggle to survive. What determines the timberline? The average temperature in July.
Even the hardiest evergreens need two months of warm weather to flower, pollinate and produce seeds. If the temperature in July averages 50 degrees, there will be enough warm weather for trees to reproduce. Cooler average temperatures mean trees can't complete their life cycle. Growth slows. At and above the timberline other factors conspire to make life hard or impossible for trees. Strong winter winds carry sharp ice crystals that blast and prune trees. Extreme winter cold freezes the trees, causing cracking and splitting. In winter, trees also battle dehydration. The ground around the roots freezes and water is unavailable. Dehydration kills the tops of the trees, and keeps the timberline trees low growing.
Above the timberline, alpine plants have adapted to mountain conditions and small perennials like dwarf willow and dryas make the most of the short growing season.