A Life Preserver for Drowning Trees.

• Position: FLOODING

Too much water can be as bad for trees as too little. While some are suited to survive occasional floods, most are not. In addition, as a tree becomes older, its ability to adapt to abrupt environmental changes decreases.

"Trees must maintain a proper water balance," says Tchukki Andersen, staff arborist with the Tree Care Industry Association, Londonderry, N.H. "Although most trees can withstand moisture conditions from very dry to very wet for short periods of time, continued extremes can cause serious problems, depending on the species."

Certain species--such as some oaks, pines, and junipers--have adapted to survive drier conditions. Trees that grow along rivers (willows, poplars, cottonwoods, and sycamores, for instance) and trees that grow in or around swamps and ponds (red maples, gums, and pond cypress, for example) also can grow in wet conditions.

Flood injury occurs when soil becomes saturated with water. There does not need to be an actual flood to cause flood injury, which usually is expressed through changes in the foliage.

Chlorosis is the yellowing of leaves caused by a decrease in the amount of chlorophyll (green pigment) in the leaves. It can look like the symptom of a disease, but often is caused by nondisease problems, such as excessive water.

When flood conditions are prolonged, root dieback occurs. During this process, soil is so saturated that there is not enough oxygen available to the tree roots. Without the proper balance of oxygen and carbon dioxide, roots cannot survive. Eventually, the tree is not able to absorb adequate moisture...