Water deficit develops as a normal phenomenon
in plants when water loss by transpiration exceeds the rate of absorption
from soil. Chronic injury results from long-term exposure to low
water supplies (e.g. in areas with poor soils) and is expressed
by growth loss and increased susceptibility to parasitic fungi and
insects. Acute injury occurs under extreme water deficiency and
is expressed by significant growth loss or death. Symptoms of drought
damage include wilting, discoloration of foliage and premature leaf
fall. Progression within the crown is from the
top down and outside in, and the roots are usually the last portions
to die. Symptoms can be similar to those of trees suffering from
root disease, except that with root diseases, the roots die before
the foliage. In the Southwest, drought symptoms have been observed
in many trees including aspen, ponderosa pine, piñon, and
soapberry.
Winter drought is particularly harmful in Southwestern upland
forests and can render large areas vulnerable to bark beetles and
wildfire.