Forest & Grassland Health

Drought & Winter Drying Damage to Conifers

Host(s) in Alaska:

All vegetation can be affected by drought stress; this page is focused on conifers in environments that typically have plentiful precipitation year-round

Damage(s): discoloration of older needle cohorts and excessive premature needle drop (in the absence of other pests); stress may increase susceptibility to biotic damage agents; foliage loss and drought may reduce tree growth 

Photos

Click on image for larger version.

Pronounced fall color in Sitka spruce near Juneau following a dry growing season.

Sitka spruce near Juneau with pronounced fall color after an abnormally dry growing season in 2018.

Pronounced fall color in Sitka spruce near Juneau following a dry growing season.

Sitka spruce near Juneau with pronounced fall color after an abnormally dry growing season in 2018.

Pronounced fall color in Sitka spruce near Juneau following a dry growing season.

Sitka spruce near Juneau with pronounced fall color after an abnormally dry growing season in 2018.

Pronounced fall color in Sitka spruce near Juneau following a dry growing season.

Sitka spruce near Juneau with pronounced fall color after an abnormally dry growing season in 2018.

Fall color in western redcedar near Ketchikan

Western redcedar near Ketchikan with red older foliage in fall, an indicator of drought stress that trees typically survive (Photo by: Mary Bolshakoff). 

Current Status & Distribution in Alaska (2018 Update)

It is normal for conifers to lose older foliage (discoloration followed by needle/foliage shed) in fall as they approach winter dormancy. Excessively warm, dry conditions can increase needle shed as trees partition limited resources to more productive, younger foliage.

In 2018, Southeast Alaska experienced abnormally dry conditions for the entire growing season. The dry weather is thought to have caused significant needle discoloration and shed of Sitka spruce and pronounced fall color in the interior crowns of western redcedar. Foliar pathogens were uncommon on discolored spruce foliage in Southeast Alaska. Spruce discoloration was also reported in Southcentral Alaska, where the specific causes may be more varied (including the spruce needle cast/blight fungi, Rhizosphaera pini and Lirula macrospora) considering normal amounts of summer rainfall. 

In spring of 2017, excessive green needle drop on Sitka spruce and western hemlock was reported at many locations in Southeast Alaska, but did not cause lasting damage. The 2017 needle drop was likely triggered by rapid warm-up and dry conditions in late-March as conifers were exiting dormancy.

Symptoms, Biology & Impacts

Late-summer/Fall drought can causes notable color change in the older needle age classes followed by heavy needle drop of discolored foliage. Winter drying may cause needle drop of both green and brown needles. There are several spruce cast/blight fungi and insect defoliators (e.g., spruce aphid) that cause needle discoloration and shed, making it important to closely examine affected foliage for signs and symptoms to verify the cause of damage.

In general, the conditions required for winter drying are warm, sunny conditions while soil water remains unavailable in frozen soil. This results in water loss from foliage (transpiration) that is not replaced through the roots, damaging foliage. In mountainous terrain, this type of damage can occur near temperature inversion boundaries when tree crowns begin to transpire while water in soil remains unavailable, resulting in a characteristic 'red belt' of affected trees, as this phenomenon is sometimes called.

Drought conditions occur whenever soil moisture drops below the level required by trees. Chronic drought stress results in growth loss and increased susceptibility to parasitic fungi and insects. Acute injury occurs when an extreme water deficiency occurs, causing sudden symptom onset, growth loss or tree death. Symptoms may include discoloration of foliage, premature foliage loss, death of the tree crown from top to bottom, or from the exterior to the interior of the crown. 
 

Content prepared by Robin Mulvey, Forest Health Protection, robin.mulvey@usda.gov, some content adapted from Insects and Diseases of Alaskan Forests, available here.

Return to Damage Agent Info/Homepage
Return to top





https://www.fs.usda.gov/detailfull/r10/forest-grasslandhealth/?cid=FSEPRD609270&width=full