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 

Current Status & Distribution in Alaska (2022 Update)

Statewide, in 2022, Alaska experienced the driest spring (April-June) in the state’s 1925-2022 record (National Oceanic and Atmospheric Administration 2022 Drought Report). This resulted in a substantial number of calls from the public regarding drought-stress symptoms on landscape trees, particularly in Southcentral Alaska. Reported symptoms primarily consisted of needle browning and premature needle drop in landscape spruce, and stunted leaves in landscape hardwoods. In most cases, landowners were not providing supplemental water, which can be essential for stressed landscape trees during drought. Landscape trees often face additional stressors compared to forest trees, including, but not limited to, competition with turf and soil compaction. Following this record-setting spring drought, the precipitation pendulum swung towards abnormally wet conditions statewide. 

In 2022, surveyors mapped approximately 37 acres with widely scattered mortality of individual Lutz spruce in the Homer area. Similar damage was also observed in this area in 2021 but was not captured in the surveys. The bulk of  the 2022 acreage (24 acres) was on Yukon Island in Kachemak Bay, and the remainder was scattered across the high country from the Homer bluffs north to near Tustumena Lake. In these areas, scattered spruce trees of varying sizes were affected, their crowns an almost golden color. This coloration was inconsistent with that associated with spruce beetle or spruce aphid activity (both of which could potentially affect spruces trees in the Homer area). Additionally, the geographic locations where this is occurring are, in many cases, outside of the range of spruce aphid in the region. The cause of this mortality is unknown, but is suspected of being abiotic, possibly drought related. Surveyors were not able to ground check any of these affected areas this season, many of which are off the road system. 

Historic Activity (2018-2021)

Negligible drought damage was recorded in 2021. Scattered individual dying white/Lutz spruce were observed along the north side of Turnagain Arm in 2020 as well as on the south side of Turnagain Arm in the vicinity of Gull Rock (150 acres). Scattered dead spruce, many of which appear to be on rocky outcrops or other sites with shallow soils, are believed to have died due to drought conditions that occurred in 2019, perhaps compounded by secondary damage agents. Spruce beetle attacks were noted on some of the affected trees on the north side of Turnagain Arm, though not with a density of attacks on stems likely to have caused mortality. In addition to the scattered dying spruce, some patches of presumed drought-related mortality affecting a mix of tree and shrub species were also observed in this area. The US Drought Monitor noted these general areas as having lengthy periods of moderate or higher drought conditions in 2019, beginning in July and continuing well into the fall (https://droughtmonitor.unl.edu/Maps/MapArchive.aspx). 

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. Drought is also thought to have facilitated outbreaks of the hemlock sawfly and western blackheaded budworm in Southeast Alaska. Additionally, western redcedar topkill and crown thinning on Prince of Wales Island and elsewhere in the Pacific Northwest has also been linked to excessive drought in 2018 and 2019. 

Symptoms, Biology & Impacts

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. 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.

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