Fig. 263  Detail of drought stress symptoms on needles.	Fig. 264   Symptoms move from crown top down and from branch tips inward.	Fig. 265  Foliage discolouration and necrosis caused by drought.	Fig. 266  Mortality in a young stand due to prolonged drought. Immature trees are generally more susceptible to damage.

Distribution:  Throughout B.C. in areas experiencing unusually low rainfall, especially on sites with dry aspects and shallow, well-drained soils or areas where excessive soil disturbance has occurred.

Tree Species Affected:  All conifer species are susceptible, especially seedlings or young saplings. Species or varieties adapted to dry sites are more tolerant than those adapted to moist sites. Understory or shallow-rooted trees are usually more severely affected.

Signs & Symptoms: Drought symptoms are most apparent in late summer to early fall and can occur over extensive areas. Trees growing on shallow, gravelly, or sandy soils will show symptoms early in drought conditions. Younger trees become affected earlier than older trees, since the former have a smaller root system. Trees undergoing only moderate moisture stress will shed older needles prematurely, usually on the lower crown. Seedlings show wilting, yellowing, and necrosis of the foliage. Older trees exhibit symptoms from the top down and from the outside inward. Affected foliage eventually discolours brown in all species except larch. On larch, foliage turns yellow, droops, or shrivels around the stem or branches, and falls off abnormally early. With well-advanced drought conditions, foliage discolouration and needle death become more pronounced and tip dieback may be seen. On western white pine, drought-related stem cracks are sometimes found.

Damage: Drought can cause extensive mortality for four or five years after planting. The growth of surviving trees is often retarded. Damage is a result of growth reduction, dead tops, and defoliation. However, if drought occurs over several successive years, large trees of drought-intolerant species may die. A widespread decline of western white pine from 1950 to 1960 ("poleblight") resulted from drought.  

Similar Damage:  Many conditions that affect individual tre health can resemble drought damage.  Root disease, needle cast, and insect defoliation can be distinguished by fruiting bodies and insect feeding respectively.  Other abiotic disorders, such as frost and red belt, can be distinguished by climatic or topographic conditions preceding or associated with the damage.

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Contact Tim Ebata if you have comments on the presentation of this information.

BC Ministry of Forests
Forest Practices Branch
P.O. Box 9513 Stn. Prov. Gov.
Victoria, BC
V8W 9C2

Section phone: (250) 387-8739
Section fax: (250) 387-2136

Last updated March 04, 2002