In the Chisos as elsewhere in desert country, what lives where depends on highly local conditions of soil and climate. While daytime temperatures in the mountains average significantly cooler than the desert flats, and while rainfall in the Basin is twice as heavy as at Rio Grande Village, just as crucial are the land’s shape and the sun’s angle. You might expect that trees grow as a woodland belt around the mountains, with treeline determined by altitude. Not so. Many trees prefer canyons to exposed slopes because soils are deeper there and drainages offer more water. Even in the same canyon, more trees may grow on cooler north-facing slopes, while grasses predominate on sunny south-facing slopes. Or pinyons, junipers, and oaks may flourish at lower elevations, while most other forest species occupy the canyon’s heights. At about 1,400 meters (4,500 feet) of elevation as the mountains begin to close, a surprise delights the eye: The first of what you might call trees beside the road. Suddenly, masses of trees—junipers, little oak trees, and pinyon pines—higher up in the drainages that crease the mountainside. And multitudes of woodland birds dart in and out of the trees and bushes along the roadway. You understand now why they call this valley Green Gulch. As you approach the first water tank you are in actual woodland. Grasses still grow on the open slopes along with bushes and pricklypear and sotol, but here are the first easily distinguished little pinyons beside the road. At 1,600 meters (1 mile) of elevation they are fairly respectable pines. Tree cover increases and, after the second water barrels, the woodland comes right down to the road. The grass is less obvious now, but it covers the ground between the trees and is especially thick on south-facing slopes. Flocks of little birds fly up from the road ahead of you. At the switchbacks in Panther Pass you peer up the steep slopes at solid stands of deciduous green trees set off against darker evergreens. Way up at the top leap scarlet flames of frost-touched Grave’s oak and fragrant sumac.