Warm summers prepare for spruce bark beetle return

If you lived in the woods on the Kenai Peninsula in the 1990s, you may not want to read this article. Those were the years when the spruce bark beetle outbreak killed most of the mature spruce forest on the Kenai, some 3.5 million acres. 

Living out Homer’s East End Road, my wife and I spent our weekends cutting down our beautiful old-growth Sitka spruce trees and burning the slash. Our view improved dramatically, but so did the cold wind coming up from Kachemak Bay, as well as the vehicle noise from the road.  

Spruce bark beetles thrive on runs of warm summers, and the present 3-year run of sunny summers should trigger another burst of beetle activity. My research as an ecologist at the Kenai National Wildlife Refuge during the 1990s focused on the history and causes of bark beetle outbreaks.  Using tree-ring analysis (dendrochronology), my colleagues and I developed a 250-year record of bark beetle outbreaks all around Cook Inlet. The last major outbreak was in the 1880s when many (but not all) stands on the Kenai and in Katmai and Lake Clark National Parks were heavily thinned, similar to the 1990s. There were smaller regional outbreaks in the 1970s, 1910s, 1850s and 1810s, as well as local outbreaks at other times.

The peculiar ecology of spruce bark beetles allows them to kill only the larger trees; pole-sized trees are spared and are released from competition from neighboring trees to grow rapidly and rebuild a new forest. Bark beetles bore through the outer bark and lay their eggs in galleries within the sugar-rich inner bark (phloem).

Small trees, however,  produce a lot of pitch and can cement the mother beetles in their galleries, Mafia-style.  Small trees also have thin phloem that can be too tight for a beetle. Big trees however are fat city for bark beetles.  The strategy of mass attack allows thousands of beetles to completely overwhelm the tree’s pitch defense and eat all the phloem, girdling the tree just as if the bark had been stripped off with an axe.

The phloem cylinder around a tree carries the sugar produced in the needles by photosynthesis down to the roots for storage over the winter.  In the spring the sugar comes up as watery sap through the sapwood (the outer several inches of trunk wood) and the sugar feeds the new needles. If the phloem plumbing has been cut by girdling, the sugar never gets to the roots and only zero-calorie sap rises in the spring.  The old needles turn red and the tree dies. After a year in the red-needle stage, the needles fall off and only a “gray ghost” of a tree remains.

Two factors control bark beetle outbreaks: adequate host material (enough large trees) and runs of warm summers. The forest gun must be loaded with mature trees, so to speak, and warm summers must pull the trigger. The natural growth of the forest loads the gun, and the El Nino climate cycle provides the runs of warm summers to pull the trigger. A run of cool summers (La Nina) will shut outbreaks down. 

My research found that there is a distinct temperature threshold for large outbreaks; average May-August temperatures in Homer must be at least 51 degrees F for two or more summers. The last three summers have been well above this threshold (2013, 52.5 degrees; 2014, 54.4 degrees; and 2015, estimated at 55.1 degrees), and the present El Nino is predicted to be one of the strongest on record.

The climate trigger has now been pulled, but is the gun loaded? The gun was very well loaded in the 1990s; foresters considered the forest “overmature,” at least for timber harvesting. The present spruce forest has rebounded quite quickly, primarily through growth release of pre-outbreak understory poles (see Fig. 1 on page 3), as well as recruitment of new seedlings. 

Today’s forest is certainly not mature, but many trees have grown big enough, say 6 inches or more in diameter, to host bark beetles. So the gun is at least partially loaded. That said, however, it still may take several summers to build up enough beetles to the point where they can really use mass attack effectively.

Bark beetles are always present at low (endemic) levels in the forest; they can be monitored with pheromone traps, baited with the same chemicals that beetles use to attract more beetles to a tree that they have attacked. Homer sawmill operator Steve Gibson of Small Potatoes Lumber reports that his fresh-cut logs are acting in their typical role as bait logs for bark beetles and even some of their smaller cousins, Ips beetles, but this occurs every spring with endemic beetles.

The USDA Forest Service flies annual aerial surveys for all kinds of forest pests and diseases. This summer’s survey found spruce beetle mortality higher than the last couple of years, but still low, according to a preliminary report. 

Red needle acreage was mapped across Cook Inlet between the lower Susitna River and the east end of Lake Clark Pass. A slight increase was seen in the Point MacKenzie to Big Lake area and on the west side the Kenai Peninsula, but still the numbers were low compared to past outbreaks when several hundred thousand acres of fresh beetle-kill were reported every summer. (See Fig. 2 on page 2.)

The future of spruce in southern Alaska seems grim, at least for the upland species of Sitka and white spruce, and the Sitka-white hybrid Lutz spruce. As Fig. 2 shows, global climate models forecast generally rising summer temperatures.  After 2030 these models indicate that May-August mean temps on the southern Kenai will consistently be above 51 degrees and suggest that there will always be beetles attacking any trees large enough to eat. If this turns out to be true, tomorrow’s spruce trees could be harvested for pulp, but they would rarely grow to saw-timber size.  

This dismal forecast presents a conundrum for landowners who want to replant their beetle-killed spruce forest. Fortunately, the warmer climate expands the silvicultural horizon beyond spruce. We can easily plant lodgepole pine, Siberian larch, hemlock (western and mountain), yellow cedar, red alder, fast-growing poplar hybrids for pulp, as well as native hardwoods like Sitka alder, birch, aspen and cottonwood. If the 1990s spruce bark beetle outbreak has a take-home message, it would be to avoid single-species monocultures. Sooner or later every tree species has its attackers, and the only way to win the biological war is to plant a variety of species so that they don’t all go down at once.

In the early stages of the 1990s bark beetle outbreak visiting experts from Anchorage and beyond advised us to defend our threatened spruce trees with plenty of watering, fertilizer, thinning and pruning branches off the lower 10-30 feet of the trunks. As the outbreak intensified, spraying the lower 30 feet of the trunk with the insecticide Carbaryl was also recommended. These measures were designed to create healthy, vigorous trees with sunlit trunks which the beetles would avoid.  They are suitable for low-intensity outbreaks, and they may well be adequate for the next outbreak. 

When a full-scale outbreak gets underway, however, these rules all go out the window.  The strategy of mass attack allows the beetles to attack the healthiest trees pruned or unpruned, pole-sized trees, and the upper trunks. Many trees around Homer were sprayed up to 30 feet high, but the beetles simply went higher and killed the tops.

On a brighter note, I don’t expect the next outbreak to be anywhere as severe as the 1990s outbreak for the simple reason that we don’t have the available breeding habitat, i.e., unlimited tree inner bark (phloem).  Yes, we have some newly mature trees, but only a finite number of beetles can fit into that phloem. 

In the 1990s we had forests not substantially thinned since the 1880s, so there was plenty of phloem (and warm summers) to breed enough beetles for many years of mass attack.  We’ll likely have warm summers in the future, above the 51-degree threshold, but the endemic beetle population will act like a thermostat that keeps the spruce forest at a new, lower equilibrium.  In place of spruce we can expect to see lots more hardwoods, resulting from increased fire activity, which in turn should provide more winter browse for the Giant Kenai Moose, to borrow a term from the 1890s.

Ed Berg retired as an ecologist from the U.S. Fish & Wildlife Service in 2010.  He has been an adjunct instructor in geology at the Kenai Peninsula College since 1983, and will be teaching a one-credit course on Global Climate Change at the Kachemak Bay Campus starting Oct. 22.  He and his wife, Sara, live in Homer on a lot with many alders, one birch, one mountain ash and no spruce trees.