Life Zones of Southwestern Mountains

As a result of the variety of elevations superimposed on an arid or semi-arid climate, New Mexico and the rest of the Southwestern US has about as many vegetation zones as one can imagine. Starting at low elevation desert and increasing in altitude one encounters no fewer than five different zonations and a multitude of gradations in between. The reason for this is the variation in temperature and moisture, particularly precipitation. Depending on other conditions, you'll find a decrease in temperature of about 1 degree Celsius for every 1000 feet of elevation gain. This means that it rains and snows more at higher elevations, and the precipitation evaporates less and therefore soil moisture retention is also greater.

Of course exposure plays a big role. Southern exposures produce microclimates which are both hotter and dryer than those facing north. Exposed land is hotter and dryer than an area within a canyon or under a rock ledge which shelters the soil producing a cooler, moister microclimate.

The result is that the south-facing ridges of mountains such as the Sandias have plant communities more typical of lower elevations. The arroyos and canyons support a community more typical of higher elevations, due to the sheltering effect as well as the shallow water table resulting from runoff.

In the Sandias the winter weather fronts often come down the Great Plains from the north. The winds coming out of these storms cause snow to pile up on the east facing slopes, leaving the western flanks of the mountains in a dry "rain shadow". Most rain shadows in the US occur on the eastern slopes of the mountains because the moisture tends to come from the west, derived from the Pacific. This is especially noticeable east of the Sierra Nevada and Cascade mountains where the west slopes have the greatest snowfalls seen in the US and the eastern flanks are the edge of the Great Basin desert, a desert produced by the rain shadow of these mountains. Valleys are always dryer than the mountains surrounding them because the mountains wrench the moisture out of the air flow, whichever direction it comes from.

Use the following chart to help you identify the trees. Their presence will help you identify the life zones as well.