Water is stored in a number of ways in peatlands: it can be water standing at the peat surface or in pools; gravitational water held in the acrotelm; or water stored in smaller pore spaces of the catotelm. In harvested peat bogs, bare peat surfaces are often wet because water is supplied to the surface from the water table by capillary flow along peat fibres. When conditions become very dry in summer, however, water supply is not rapid enough to compensate for losses by evaporation at the surface. Capillarity is broken and the water supply stops. Then the peat surface becomes desiccated and the small quantity of water left is so strongly retained by peat that mosses cannot have access to it. The suction by which water is held to peat particles is called the water tension. This problem is restricted to mosses because they do not have roots and physiological adaptations to overcome high water tension. However, given the importance of Sphagnum mosses in peat bogs, this represents a major problem. The use of straw mulch helps to solve the problem. Straw mulch keeps the daytime temperatures lower, and provides a shield against direct radiation. This reduces evaporation that, in turn, increases the water tension at peat surfaces enabling Sphagnum to access water.