many centuries may penetrate the bark and destroy the vascular cambium. Nearly all of the larger trees have fire scars, many of which cover a large area of the base of the tree. Older trees are rarely killed by fire alone, but the resulting structural damage may predispose a tree to collapse and fire scars also provide entry for fungi which may cause root disease and heart rot. The resulting decayed wood is then more easily consumed by subsequent fires. The result of this cycle is further structural weakening of the tree, which may eventually lead to its collapse. Fire scars are thought to be the main cause of dead tops. Although lightning strikes rarely kill mature trees, lightning sometimes knocks out large portions of crowns or ignites dead tops. The most common cause of death in mature giant sequoias is toppling, due to weakening of the roots and lower trunk by fire and decay. The extreme weight of the trees coupled with their shallow root systems contributes to this weakening. Other causative factors include wind, water-softened soils, undercutting by streams, and heavy snow loads in the crowns. The Washington tree, located in the Giant Forest Grove in Sequoia National Park provides a good example of the aforementioned phenomenon. This tree was the second-largest tree in the world (only the General Sherman tree was larger) until September 2003, when the tree lost a portion of its crown as a result of a fire caused by a lightning strike. This reduced its height from nearly to about . The structurally weakened tree partially collapsed in January 2005, as the result of a heavy snow load in the remaining portion of its crown; it is now approximately tall. Tree measurement As with other trees, measurement of giant sequoias is conducted using established dendrometric techniques. The most frequent measurements acquired in the field include the height of the tree, the horizontal dimension of its canopy, and its diameter at breast height (DBH). These measurements are then subjected to tree allometry, which employs certain mathematical and statistical principles to estimate the amount of timber volume in a tree. Calculating the volume of a standing tree is the practical equivalent of calculating the volume of an irregular cone, and is subject to error for various reasons. This is partly due to technical difficulties in measurement, and variations in the shape of trees and their trunks.