Adding soil over tree roots reduces the level of oxygen in the soil, and the roots suffer because of it. On sensitive trees, the roots may die; on others, the roots will grow up into the added layer of soil. When tree roots are damaged, the branches die back over time. Other symptoms of root injury include small, discolored leaves, premature fall color, suckers near the trunk, and dead twigs or branches. Because it may take several years for these symptoms to show up, many homeowners never realize that the added soil caused the dieback and other problems. Trees most susceptible to injury from additional fill include sugar maple (Acer saccharum), beech (Fagus sylvatica), flowering dogwood (Cornus florida), and many oaks (Quercus), pines (Pinus), and spruces (Picea). Elm (Ulmus), willow (Salix), London plane tree (Platanus Yacerifolia), pin oak (Quercus palustris), and black locust (Robinia pseudoacacia) are least affected. Older trees and those weak from other stresses are more likely to be injured than younger, more vigorous trees.