Ask Sam: What Happened to These Hemlocks?

Aug 24, 2018

Oh, those hemlocks look just fine.
Credit Credit Flickr Creative Commons | Nicholas A. Tonelli

Every other Friday on Morning Edition NHPR’s Sam Evans-Brown tracks down answers to questions about the environment and outdoors for our listeners in a segment we call “Ask Sam.”

Gretchen from Hanover, writes, “In this most recent heat wave, I have been out mountain biking in the woods here in the Upper Valley, and have noticed a fine bright green layer blanketing the trails.  It appears to be hemlock needles, I think, but I do not recall ever seeing them on the ground like this.  My hypothesis is the trees are losing needles due to the heat. What do you think?”

It’s a who-done-it! A mustache-twirler! A mystery! I tried reaching out to this e-mailer to ask some clarifying questions but got no response (“Gretchen? Gretchen?” *crickets*) so I’m going to present the possible answers and how you would determine which is the culprit if you ever find this same situation in your local woods.

1. Drought:

It seemed like heat alone was unlikely to be the culprit. (But is a good guess!) Eastern hemlock’s range extends all the way down the Northern Georgia, where heat like we had earlier this year is pretty common.

But heat and drought? That’s another story. Vermont’s Department of Forests sent out a bulletin in July that points out that there have been reports of  “considerable numbers of green needles being shed by hemlock in Pittsford” and says that needle shedding by hemlocks is actually a response to water stress rather than heat stress, “but the two go hand-in-hand.” Trees lose water when they photosynthesize, so if they’re losing too much they drop a bunch of needles to conserve.

So that’s our first possibility.

It's worth noting that lots of different stressors can cause hemlock to drop needles. "Hemlock does this periodically because above ground biomass gets out of whack with what root system can support," writes Kyle Lombard, with the Division of Forests and Lands, in an email, "Root damage, beaver damage, and many other things trigger it."

**If these were whole needles lying on the ground inexplicably, it could have been water stress. But really we need more info to diagnose.**

2. Invasive Pests:

Many of the people I spoke with went straight to the big baddy that’s marching up the East Coast: the hemlock wooly adelgid.

This is an invasive pest introduced on Japanese hemlock nursery stock to a plantation in Virginia and was first observed in 1951. According to Robert Jattan, with North Carolina State University, it was a pretty low-key invasion until the 1980s when it hit the Blue Ridge Mountains and Southern Appalachians and started absolutely decimating hemlocks. And that sudden mortality event coincided with a severe drought.

“And so you’re bringing in this insect, which is a piercing sucking insect — so it’s sort of sucking these trees dry — at the same time they’re experiencing a severe drought. So we did in certain situations see a large number of needles blanketing the ground,” he says.

Here’s the problem: adelgid haven’t been seen in the Upper Valley. They can be difficult to detect, so it’s possible that our listener spotted an infestation early… but that’s TOTAL speculation.

**If we want to test this hypothesis, our listener would have to go back out in the fall when the adelgids start to build their little wooly nests on the underside of the branches, and look for signs of an advanced adelgid infestation.**

3. Native Pests:

I got a forest pest expert on the phone who actually lives in the Upper Valley to suggest this theory: a hemlock looper infestation

“It’s in the group that people often call inchworms, so that’s what the ‘looper’ refers to,” explained Matt Ayers, who seemed to be paging through an entomology text-book as he spoke to me. He had never seen a looper infestation before. “It seems that it’s notorious for being a wasteful feeder,” he said, which means, “they drop a lot on the ground.”

Hemlock looper is native to the U.S., meaning that the tree and the inchworm have been engaged in a co-evolutionary arms race for millennia, and any outbreaks tend to be temporary and small-scale. “I would not be worried that this is any kind of hemlock armageddon,” Matt said. (If there is a hemlock armageddon coming, it will unfold over decades, thanks to the invasive adelgid.)

**To test this hypothesis you would have to stand under the trees and check the needles. Have they been nibbled? Are the trees full of inch-worms? It’s looper.**

Oh, but wait, while you’re here, how about a quick digression for some Amazing Science Facts?

Hemlock looper is native, but doesn’t mean that it has always miraculously been “in balance” with the species it eats. According to the research of Kevin Potter, another professor at North Carolina State University, hemlock genetics contain the signature of a pretty dramatic near-extinction event. “About 5,000 to 7,000 years ago hemlock more or less disappears throughout most of its range,” he explained, “That bottleneck corresponded with what we think was a pretty intense infestation of an insect called hemlock looper throughout much of its range.”

Over the course of hundreds of years the tree slowly was able to repopulate, spreading from glacial refugia in upstate New York and New England — another example of how deep-time thinking can be comforting in an era when humans are effectively wiping out huge quantities of wild species.

“The comfort that I find is that the resistance almost always seems to be there somewhere within the species,” said Kevin, “we have several of these apocalypses going on right now in ash, in butternut, in hemlock, in red bay down here, tan oak out west, all these different things… but almost always we find trees that survive.”

So not only can species survive invasions, but if we can find the traits that let them survive, we can help speed up their recovery. So assuming we don’t wipe species out completely, many may be able to bounce back from the setbacks we’ve dealt them.

 

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