Yellowstone's northern range differs from most of the park's high volcanic rhyolite plateau because it is geologically more complex, with landslides, erodible shales and sandstones, glacial till deposits of mixed rock types, and in this area soils with high clay content. These soils hold water tightly so little is available to plants, are poorly aerated, resist root growth, and contain high levels of sodium and salts, limiting which plant communities can grow here. Low-growing plants such as greasewood, prickly pear, and rubber rabbitbrush reflect the sparse, dry environment of this habitat, which ecologists call a cold desert. Annual precipitation here is 10 to 15 inches, only one-third of the moisture received in Yellowstone's southwestern corner, and only for a few brief weeks in May and June does the landscape become lush and green as plants use snowmelt and rainfall to grow and flower before the brown, arid scene returns. Although many people have questioned the health of the northern range and wondered whether large numbers of elk, deer, pronghorn, and other wild grazers are destroying habitat, many years of research show that these naturally dry grasslands remain productive and healthy, and wildlife continue to depend on the northern range, particularly for winter forage, in a cycle of growth, grazing, and rebirth that has occurred for thousands of years.