We all know what tree line is in Colorado: the point above which trees don’t grow. But what determines tree line: Cold? Soil conditions? Precipitation? And is tree line different in various parts of the world?
Although tree line looks like a sharp delineation from a distance, you can see that it actually consists of three different zones upon closer inspection. The area above which trees cannot grow thick enough to form a canopy is called timberline or forest line. Above timberline there is usually an area where small groups of trees can survive but not enough to form a canopy, and the limit of these trees is called tree line. Individual, though often stunted, trees may grow above tree line up to what is called the species line. These three zones usually occur within a hundred vertical feet of one another.
There are also different kinds of tree lines depending on the main factors that inhibit tree growth. When we refer to tree line in the Colorado Rockies, we are normally referring to the Alpine tree line, where cold, snowpack, soil quality and high winds are among the limiting factors. In other regions, there are desert tree lines, where moisture is the key limiting factor. There are also desert-Alpine tree lines, where cold and moisture are the key limiting factors.
Current research is directed at how climate change affects tree line. And locals have noted trees growing on some of our formerly tree-free summits. There are complex and interesting dynamics involved here. For instance, because seedlings have different limitations than trees, sometimes seedlings will survive when trees can’t, or vice versa. So the current tree line might reflect the climate from decades or centuries ago. Or there might be lags for when tree line moves because seedlings struggle to get established.
Alpine tree lines vary greatly across the world, especially in areas north of 30 degrees north or south of 20 degrees south in latitude. In Colorado, tree line is around 11,500 feet. In contrast, tree line in the Swiss Alps is around 7,200 feet.
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