Two Mountain Pine Beetle Stories: Winter Weather and Germination of Old Cones

We’ve been light on biology stories lately…

This one is from the Cowboy State Daily.  Don Day is a Wyoming meteorologist who, among other things, does forecasts in the Cowboy State Daily.

The story is about how people perceive bad winters through time.

Pine Beetles Aren’t Evidence Of Worst Winters Past

In addition to grafting selective memories of bad winters into supposed long-term trends, people also can mistakenly apply regional patters to other areas, Day said.

That’s the case with pine beetle infestation, he noted. During the 2000s and 2010s, pine beetles cut a swatch of death through forests across Wyoming and the West. To this day, large stands of dead pine trees are reminders of the havoc they wreaked.

A common narrative is that a lack of prolonged winter deep freezes failed to kill of beetle larva, allowing them to hatch during the spring and invade areas they’d never been in before, Day said.

People will use that as evidence that winters used to be a lot harder, he said. As the story goes, sub-zero cold snaps would go on for weeks and kill off pine beetle larva, but that quit happening because our winters got milder.

But that’s based on assumptions that conditions in the Canadian Rockies, where the beetles originated, apply across the American Rockies as well, Day said.

In truth, the Canadian Rockies are actually lower than the American Rockies and can be susceptible to polar air sinking down into them, driving temperatures to 30 below zero or so for extended periods. That’s kept the beetles in check there Day said.

But when the beetles managed to spread southward, they were able to survive winters in the American Rockies in much greater numbers, Day said.

“In southern Alberta and British Columbia, the arctic air masses can go deeper and the tree lines are lower,” Day said. “It can’t get that cold for that long in the central and northern American Rockies, it just can’t.”

So, people have “imprinted” conditions in the Canadian Rockies to mountains in Wyoming, creating the false impression that winters here used to get colder and stay cold for longer than they do now, Day said.

The up-and-down cycles of snowfall in the Cheyenne area. (Courtesy Don Day)

This one is from Chuck Rhoades of the Rocky Mountain Research Station. It’s one of their “Science You Can Use (in 5 minutes)” series. Check out the new RMRS website.

Of course, as he says, if desirable lodgepole stocking levels are 150 per acre, it doesn’t take many cones with 26% germination to provide that number, depending on other site conditions. At this point, I’d still bet on the lodgepole.. unless there’s a reburn before the next generation produces seed. Some of us will be around long enough to find out…

Compared to seeds from live or recently killed trees, seeds from these long-dead trees germinated about half as often. Germination varied across the study sites, ranging from 26 percent to 41 percent. Germination also declined with cone age, and 35 percent of the oldest cones produced no germinants at all. The team also tested germination of seeds in cones stored in the soil seedbank under snowpack. These seeds averaged 36 percent germination, which was comparable to seeds released from the canopy seedbank. Though this is good news, cones in the soil seedbank are susceptible to burning by surface and ground fires and predation by rodents.

For many lodgepole pine forests burned in the recent fires, postfire tree densities will meet or surpass acceptable stocking levels (i.e., 150 seedlings per acre). However, based on these RMRS findings, stands with high bark beetle mortality may not provide enough viable seed to reach the minimum density of seedlings needed for recovery. Thus, the double disturbance of bark beetle outbreaks and wildfire may translate into costly reforestation projects.

5 thoughts on “Two Mountain Pine Beetle Stories: Winter Weather and Germination of Old Cones”

  1. As Wuerthner often explains, the industry treats every type of mortality as something that’s bad an needs to be entirely prevented with chainsaws which is in itself the most common form of tree mortality on the planet right now. But there’s a great deal of disassociation & cognitive dissonance when it comes to protecting the forest from all forms of mortality as long as that mortality is done with $chainsaws$. Meanwhile one of my favorite references on pine beetle comes by way of the Canadian government:

    “Lodgepole pine stands heavily hit by mountain pine beetles then left unmanaged for 25 years have become forests comprising an array of tree ages interspersed with standing and fallen snags. With their complex structure, these forests now provide valuable wildlife habitat and in some cases are also growing harvestable volumes of timber.

    Mountain pine beetles killed 70 percent of the mature pine in 1979 at sites studied near Quesnel, British Columbia, in the dry Sub-boreal Pine Spruce biogeoclimatic zone. Of the trees that survived, 60 percent were lost in 2005 when beetles revisited. Altogether, 85 percent of the pine overstory succumbed to beetles.

    The proportion of dead trees left standing varies considerably among stands, ranging from 0 to 80 percent of the snags. With up to 166 m³/ha of decomposing logs lying on the ground, the debris is not considered a fire hazard. Most of the mature overstory pines that survived the beetles boosted their diameter growth rate by an average of 44 percent during the following 25 years.

    The understory of pine saplings remaining after the beetle infestations were augmented with more vigorous pine regeneration once the canopy opened. Although the new trees sprouted in scattered clusters, one-third of the sites examined meet a forestry restocking standard of 1000 stems per ha. Regrowth is enough in 30 percent of the stands to provide harvestable volumes of wood for the mid-term.”
    Forest Practices Board. 2007. Lodgepole Pine Stand Structure 25 Years after Mountain Pine Beetle Attack. Forest Practices Board Special Report FPB/SR/32. Victoria, BC.

    • Deane, thank you for this interesting post. Lodgepole regeneration looks very different in different places. I took these photos in Yellowstone National Park in 2016, 28 years after the huge fires of 1988, posted here for continued discussion.

      A wetland:

      Scattered snags and young trees:

      Very dense regeneration (that’s me in the middle of it):

      And this is what is looks like under that dense regen. Ripe for a very intense wildfire:

      • Yes, those photos you share quite literally are a snap shot in time… And yes, the reference I offered was in a much more wet and more productive site than beetle kill forests further inland.

        However the important point is that a snap shot in time, often does not accurately reflect what a forests needs in the long run to still be capable of thriving as a forest for a millenia or more.

        As in what most foresters would consider an overstocked stand that needs to be thinned is what many of us who embrace the natural sciences see as a forest that has much needed redundancy and essential competition between trees that allows the most efficient long lived trees to naturally emerge in the same way they’ve emerged for hundreds of millions of years.

        As in the human notion that we know better and we need to ‘accelerate’ this evolutionary / natural selection process with thinning could potentially eliminate the necessary redundancy for a forest to still be a forest 500 years or more from now.

        I especially dwell on this thought when I’m east of the cascades and see vast landscapes of once healthy forests littered with tiny leave trees that all died off and no sign of understory forest regeneration. As in the carrying capacity of specific sites needs to be accounted for because what works west of the Cascades for rapid regeneration is entirely different than what works east of the Cascades.

        • Deane, I’ve seen a few areas with the conditions you describe. But I’ve also seen large areas in eastern Oregon that have been thinned and Rx-burned, and many areas that need to be treated — dense, overstocked stands that are ripe for high-intensity fire.

  2. In my opinion, the story about the pine beetle, the harshness of winter and peoples’ perceptions of the severity of winters from past decades is not quite covering the topic right. The writer looks at snow levels over the years rather than temperatures. Regardless, there seems to be some disagreement about the impact of cold winters on pine beetle and the spruce beetle populations. I’m more familiar with the spruce beetle because the Rio Grande NF was ground zero for the spruce beetle epidemic that took hold in southern Colorado in the 2000s and has spread to many other parts of the state. The spruce beetle epidemic seemed to have started due to the 2000-2002 drought, with 2002 being the driest year on record. The Forest experienced one of the coldest Januarys on record while the outbreak was raging and it didn’t slow the spread of the beetles one bit. It seemed it took a few severely dry years to kick start the epidemic and then the populations were so high, it didn’t matter what the weather did. There is evidence that extremely cold temperatures may stop a spruce beetle outbreak though. In the 1940s-early 1950s, the spruce beetle was killing 100s of thousands of Engelmann spruce in the Flat Tops of Colorado, but seemed to be stopped cold after a multi-day cold snap where temperatures dropped down to -50F.

    Part of the reason the mountain pine beetle spread so quickly in Colorado may be tied to longer, warmer summer temperatures. The pine beetle generally has a one year reproductive cycle, but there was evidence that this accelerated to two reproductive cycles in a year. I would argue that fire suppression also played a role as maybe there was a much larger, uniform area of mature lodgepole pine stands for the beetles to move through than if fires were not being suppressed.

    As for faulty memories about weather severity: I think most of us suffer from some of that. We had a pretty good snow year this year in the San Juan Mountains, similar to what we used to see in the ’80s (a very snowy decade the records show), but the low temperatures seemed to be about 10 degrees F warmer this year. But, maybe that is just my faulty memory….


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