Trees grow tall when adverse factors such as exposure, summer drought, attacks by pests and pathogens etc. do not limit height growth, particularly in woodland or sheltered valleys. The limiting factor is then the transport of water. As trees grow taller the tension in the water column increases even when there is ample soil moisture. The maximum theoretical height of a tree is said to be 122–130m, while the tallest known tree at present (a coast redwood in California) is 115.7m tall. However, this correspondence could be partly coincidental since coast redwood grows in cloud forest.