Beavers have an herbivorous and a generalist diet. During the spring and summer, they mainly feed on herbaceous plant material such as leaves, roots, herbs, ferns, grasses, sedges, water lilies, water shields, rushes and cattails. During the fall and winter, they eat more bark and cambium of woody plants; tree and shrub species used include aspen, birch, oak, dogwood, willow and alder. Beavers may cache their food for the winter, piling up wood in the deepest part of their pond where it cannot be reached by other browsers. This is known as a "raft" the top of which becomes frozen creating a "cap". The animal accesses the raft by swimming under the ice. Many populations of Eurasian beaver do not make rafts but forage on land during winter.