Impacts from Northwest Heat Wave

Leaf scorch of an ornamental cherry (Prunus sp.) one day after a heat wave with temperature reaching 46.7° C. It is not yet clear if all the blasted branches are dead, or only the foliage.

Leaf scorch of Picea glauca ‘Conica’ (dwarf Alberta spruce) on the west side of the tree 16 days after a heat wave with temperature reaching 46.7° C. Most of the foliage was killed and is falling. Twigs appeared to have remained alive. Damage was less severe on the east side.

Leaf/twig scorch of Acer sp. four days after a heat wave with temperature reaching 46.7° C.

Leaf scorch of Cornus sp. (dogwood) 16 days after a heat wave with temperature reaching 46.7° C. Many peripheral leaves were killed entirely, but those more shaded in the interior of the plant were mostly undamaged.

The Pacific Northwest of North America (Oregon, Washington, southern British Columbia) experienced a heat wave far hotter than any previously recorded in late June 2021. In Vancouver, Washington, where I live, the official high temperatures on June 26-28 ranged from 41 to 46° C (106-115° F). At our home, temperatures were a bit higher, peaking at 46.7° (116° F) on June 28.

For those of us without air conditioning, the level of misery was high (needless to say, there is now a booming business installing it). But since this is a forest pathology site, I’ll focus on the misery experienced by trees and shrubs, in my narrow experience.

Our neighbor’s ornamental cherry (no supplemental water) is shown in the first gallery. Time will tell if the branches were killed, or only the leaves. Our fruit cherry was not as badly affected, as we water it. But it has been suffering terribly from bacterial canker, and now looks much worse (I can’t bear to show a photo 😞 ).

The first gallery also shows an ornamental conifer that had the majority of its foliage killed. It will be interesting to see how that tree looks in the next months and years. The real tragedy in our yard is the rhododendrons we planted several years ago. This spring was the first time they were really growing well, and we were excited to see it. Then they got blasted. They’ll survive I expect, but this will knock them back.

Leaf scorch of Rhododendron sp. one day after a heat wave with temperature reaching 46.7° C. Soil was moist throughout the heat wave.

Leaf scorch of Rhododendron sp. one day after a heat wave with temperature reaching 46.7° C. Soil was moist throughout the heat wave.

Leaf scorch of Rhododendron sp. 16 days after a heat wave with temperature reaching 46.7° C. Soil was moist throughout the heat wave. Shoot tips survived and are producing new foliage.

Leaf scorch of Rhododendron sp. 16 days after a heat wave with temperature reaching 46.7° C. Soil was moist throughout the heat wave. Shoot tips survived and are producing new foliage.

Leaf scorch of Rhododendron sp. 16 days after a heat wave with temperature reaching 46.7° C. Soil was moist throughout the heat wave. Shoot tips survived and are producing new foliage.

The damage I’ve seen thus far in Vancouver is less severe and widespread than I expected. Mainly, it is short-term, acute damage, primarily leaf and twig scorch.

In our vegetable garden, damage was surprisingly minor. Only the snow peas were voted off the island, but they fade with the heat of summer anyway.

Thuja plicata (western redcedar) and Platycladus (Thuja) orientalis (oriental arbor-vitae) experienced similar kinds of leaf and twig scorch (last two galleries). In both, it occurred not only on foliage exposed to the sun at the worst time of day, but it was certainly most severe there.

Thuja plicata in Vancouver is a bit below its natural elevation range at this latitude, so such damage is not surprising. However, the species has been experiencing crown thinning, branch dieback, top death, and mortality even in its native range over the last few decades, and this damage has been linked to summer drought and higher temperatures ​[2]​. The heat wave may increase symptom expression in some areas.

Leaf/twig scorch of Thuja plicata (western redcedar) four days after a heat wave with temperature reaching 46.7° C. Some damage occurred even on north aspects but was most severe on the west sides of trees.

Leaf/twig scorch of Thuja plicata (western redcedar) four days after a heat wave with temperature reaching 46.7° C. Some damage occurred even on north aspects but was most severe on the west sides of trees.

Leaf/twig scorch of Thuja plicata (western redcedar) four days after a heat wave with temperature reaching 46.7° C. Some damage occurred even on north aspects but was most severe on the west sides of trees.

Leaf/twig scorch of Thuja plicata (western redcedar) four days after a heat wave with temperature reaching 46.7° C. Some damage occurred even on north aspects but was most severe on the west sides of trees.

Leaf/twig scorch of Thuja plicata (western redcedar) four days after a heat wave with temperature reaching 46.7° C. Some damage occurred even on north aspects but was most severe on the west sides of trees.

Leaf/twig scorch of Platycladus orientalis (oriental arbor-vitae) four days after a heat wave with temperature reaching 46.7° C. Damage occurred on all aspects and even in shade, but was more severe on exposed south and west aspects.

Leaf/twig scorch of Platycladus orientalis (oriental arbor-vitae) four days after a heat wave with temperature reaching 46.7° C. Damage occurred on all aspects and even in shade, but was more severe on exposed south and west aspects.

Leaf/twig scorch of Platycladus orientalis (oriental arbor-vitae) four days after a heat wave with temperature reaching 46.7° C. Damage occurred on all aspects and even in shade, but was more severe on exposed south and west aspects.

There will be some growth loss, but most of the leaf and twig scorch will fade away in a year or two. However, it is possible that larger trees will suffer long-term, delayed damage from this stress event.

[Update] Similar damage was widespread in natural and planted landscapes throughout western Oregon ​[3, 4]​. Aerial survey during the summer reported 229,000 acres of scorch, and that was considered an underestimate. Western hemlock and western redcedar were most sensitive and damage was most pronounced on west aspects and west/southwest sides of trees. Some small trees were killed. Losses in some Christmas tree plantations were substantial.

Recent data has strengthened the link between summer drought/high temperature and the damage previously observed in western redcedar ​[1]​. The phenomenon appears to be a simple disease caused by summer drought and high temperature, especially when repeated.