Gymnosperms have hollow tubes called tracheids, while Angiosperms have larger hollow tubes called vessels. The larger diameter angiosperm vessels are not especially well adapted to dry conditions. Water movement up from the roots to the leaves relies on an unbroken column of water, pulled up in part by capillary action. If that tube of water is broken, a process called cavitation, then it is very difficult for angiosperms to form a new unbroken column of water. Gymnosperms, on the other hand, do not cavitate as easily as angiosperms, and reform the rising water column more quickly when it does break.
Pines can therefore outcompete angiosperms in sandy soils.
Sandy soils pass water through very quickly. Soils with a very high clay content, with fewer and smaller pore spaces, can hold more water, and retain it for longer periods of time. We call this property of soils their field capacity. Field capacity is the amount of water the soil can hold after being completely saturated. Clay soils have a very high field capacity, but sandy soils have a very low field capacity.