The face a slope presents to the sun -- north or south -- plays a role in the local climate created on it. This climate determines the types of plants that colonize the slope and in turn which animals species that are drawn to the area seeking their preferred foods and suitable shelter. Whether in the northern hemisphere or southern hemisphere, differences exist between north- and south-facing slopes, although the differences are reversed between the hemispheres. For instance, north-facing slopes are cooler in the northern hemisphere, and warmer in the southern hemisphere.
Amount of Sunlight
In the northern hemisphere, north-facing slopes in latitudes from about 30 to 55 degrees receive less direct sunlight than south-facing slopes. During winter months, portions of north-facing slopes may remain shaded throughout the day due to the low angle of the sun. This lack of heat causes snow on north facing slopes to melt slower than on south-facing ones. The lack of direct sunlight throughout the day, whether in winter or summer, results in north-facing slopes being cooler than south-facing slopes. The scenario is just the opposite for slopes in the southern hemisphere, where north-facing slopes receive more sunlight and are warmer. Near the equator, north- and south-facing slopes receive roughly the same amount of sunlight because the sun is almost directly overhead. At the poles, north and south slopes tend to be either shrouded in darkness all winter long, or bathed in sunlight all summer long, with only slight variation between the slopes in spring and fall.
Depth of Soil
Depth of soil on a slope, whether it faces north or south, depends on the steepness of the slope. The steeper the incline, the higher the rate of soil erosion from rain runoff. Soils on steep slopes are primarily made up of rock fragments because pieces of lightweight organic matter, such as leaves, wash away before they can decompose into soil. Slopes that have a gentle incline tend to accumulate a deeper layer of soil. Soil on south-facing slopes dries out faster and is warmer than soil on north-facing slopes due to longer exposure to sunlight.
Effect of Rainfall
The amount of rain that falls on a slope and is taken up by existing vegetation is determined by how steep the slope is, rather than whether it faces north or south. Rain runs more quickly off steeper slopes and does not have time to be taken up by plants. Rain falling on less steep inclines stays in the soil longer and is utilized by plants and trees, resulting in larger plants and colonization of plants with higher hydration needs. Vegetation on south-facing slopes in the northern hemisphere, however, also has less time to take up water because of the drying effect of the sun.
Plant communities vary widely between north- and south-facing slopes. Warmer south-facing slopes green up sooner in spring, stay greener longer in the fall, and tend to be drier than north-facing slopes. Plants that tolerate these hot, dry conditions, such as chaparral and California buckwheat (Eriogonum fasciculatum), grow well on southern slopes in their native range. A few feet away, a cooler, moister north-facing slope, with a gradual incline may be dotted with an oak woodland and shade tolerant wildflowers. Trees capture indirect sunlight better than low-growing grasses.