The demographic crisis of the 14th century upset this arrangement. This crisis had several causes: agricultural productivity reached its technological limitations and stopped growing, bad weather led to the Great Famine of 1315–1317, and the Black Death of 1348–1350 led to a population crash. These factors led to a decline in agricultural production. In response, feudal lords sought to expand agricultural production by extending their domains through warfare; therefore they demanded more tribute from their serfs to pay for military expenses. In England, many serfs rebelled. Some moved to towns, some bought land, and some entered into favorable contracts to rent lands from lords who needed to repopulate their estates.