They inhabit mesic sites between drier uplands and moister streams. The south has transitional maritime continental climates, with moister, cooler summers and warmer winters. The gray oak woodlands are found in slightly moister soils than the pinyon woodlands. Subalpine fir is abundant in cooler, moister subzones. Thus farther north deserts can be found while the moister areas are vegetated. Generally the moister the forest habitat the greater the understory density. The more water a brush holds, the moister and richer a lather will be. The higher mountain areas are cooler and moister than the rest of the island. Frogs are typically found at higher, moister altitudes. However, like other yams, purple yam tends to have a moister texture than sweet potatoes. Scattered groves of curlleaf mountain-mahogany and aspen in moister microsites grow above the shrub layer. The western (coastal) slope is generally moister than the eastern slope. Trees on moister sites and those with access to ground water tend to be evergreen. In drought years, some birds move west into Rwanda and Burundi seeking moister conditions. Various ferns are found in moister and tree-shaded areas.