Slopes near Anderson Ranch Reservoir, Idaho.

This photo illustrates the effect of aspect (the direction a slope is facing) on plant species distribution. The north facing slopes on the left side of the hills have large evergreen trees (Douglas Fir and Ponderosa Pine), while the south facing slopes are covered in shrubs, grasses and forbs. Although both the north and south facing slopes receive the same amount of rainfall, have similar soils, and are at the same elevation, there are a number of environmental differences. North facing slopes tend to be cooler and shadier because during the fall, winter and spring the sun is predominantly shining on the southern slopes while the northern slopes have little direct sunlight. Shadier conditions mean that there is less evaporation of moisture from the slope and more water is available for plant growth for a longer amount of time. North facing slopes also tend to have more snowpack than southern facing slopes- snow melts quicker from the southern facing slopes and stays longer on the shady north facing slopes. Adequate snowpack helps to ensure that plants will have enough water in spring.

Many plant species are generalists and will grow in variety of conditions, but some plant species found on southern facing slopes are different than the plant species found on the north facing slopes. Species on the southern slopes are often more adapted to hot and dry conditions, while species on the north facing slopes need more water and a cooler climate. A rare North American orchid- Cypripedium fasciculatum (clustered ladies slipper)- only grows on north-facing slopes, and tends to be associated with a genus of lichens- Peltigera (pelt lichens)- which also only grows in shadier, moister conditions. Differences in aspect help to create a diversity of habitats available to plant and animal species, and contributes to overall species diversity.