Isolated volcanic peaks rise to considerable heights in the northwest. Open forest of ponderosa pine in the montane zone, Sitgreaves National Forest, Arizona. During late spring, there is a moisture deficit until the arrival of summer rains, which appear as thunderstorms.
Rains also come in early autumn and winter.
In the mountains, most precipitation is snow. In Arizona, the pine forests of this zone are strongly infused with Mexican species, including Chihuahuan and Apache pine. Aspen is common in this zone, and limber pine grows in places that are rockier and drier. Limber pines and bristlecone pines grow in the rockier places. The Four Corners region is composed mostly of Entisols.