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Mystery defoliation at treeline in the Kenai Mountains

Posted: September 20, 2012 - 4:24pm
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Calamagrostis canadensis, a native grass, thrives under an aspen canopy opened up after defoliation by larvae of the aspen leaf-miner.
Calamagrostis canadensis, a native grass, thrives under an aspen canopy opened up after defoliation by larvae of the aspen leaf-miner.

About this time last year, I wrote a story about how subalpine willows in the Kenai Mountains were being defoliated by an outbreak of both native and exotic species of Geometrid moths. The caterpillars of these insects were grazing on willow leaves that grow at high elevation near treeline. Large areas of the Tustumena Benchlands, in the heart of the Kenai National Wildlife Refuge, lost much of their productive moose browse.

The Refuge has continued to get reports this summer of defoliation at treeline in areas like Summit Pass, Skyline Trail, and Cottonwood Creek. We assumed the gray band of vegetation that’s visible as you drive through the mountains on the Sterling Highway was just remnant defoliation from last year’s outbreak.

In July, we discovered that much more is going on than meets the eye — at least from afar. Approaching the top of Skyline Trail with Matt Bowser and Libby Bella, the Refuge’s entomologist and botanist, we found that the gray band is actually composed of several woody species that are being defoliated and, in some cases, killed as a result of multiple insect attacks and some unknown agent.

Quaking aspens were being defoliated, their remaining leaves turning silver from the leaf-miners (a moth) that eat the chlorophyll. Black cottonwoods were under attack from the larvae of leafrollers, a Tortricid moth, that literally rolls leaves and ties them together with silk threads. As we emerged above the treeline, it was gratifying to see subalpine stands of Barclay’s willow, a favorite winter food for moose, recovering from last year’s infestation of Geometrid moths. Bruce spanworms, one of the culprits responsible for last summer’s defoliation, were still around, feeding on highbush cranberry.

But something strange was going on just a tiny bit higher upslope. Stands of mostly dwarf birch and crowberry, with some blueberry, were clearly dead, representing the upper edge of the gray band of vegetation. Unlike shrubs and trees further downslope, which were either recovering or would likely recover from insect defoliation, this was definitely a dieback. The twigs of these shrubs were already brittle, breaking off easily in the hand.

As we looked around at other areas at treeline on both mountain slopes that form the Kenai River valley, it was obvious that this gray subalpine zone amidst otherwise green foliage was extensive. What would cause such a dieback? Was it unique to these shrub species or was it unique to this elevation?

Being a bunch of scientifically-trained, ecology-minded, occasionally-creative individuals, we came up with three working hypotheses pretty quickly: loss of mycorrhizal symbionts, wind scouring, or airborne contaminants.

Mycorrhizas are symbiotic relationships between a fungus and roots of a vascular plant. This association provides the fungus with a relatively constant source of carbohydrates produced by the plant. In return, the plant benefits from the fungus’ higher absorptive capacity for water and mineral nutrients. Mycorrhizal symbionts are sometimes shared among vascular plant species so our initial thought was that if one species was being hammered by insect defoliators, all would suffer.

But, as it turns out, dwarf birch’s main mycorrhizal symbionts are not only different than those of blueberry and crowberry, but actually competitors.

We would expect these berry-producing shrubs to do better if insects were harming dwarf birch (or visa-versa). So we discarded this idea.

Alternatively, was the agent of change simply restricted to this elevational zone? That would be the case if it was caused by a severe wind-scouring event, or perhaps rain or fog that was laced with airbourne-contaminants such as acidic sulfates and nitrates. Despite the protestations of my colleagues, I lean a little more towards the latter. It seemed to me that blueberries and crowberries that died were UNDER dwarf birch, as if the birch was intercepting precipitation and the dripping continued long after the rain or fog event had passed. And this observation certainly argues against a wind-scouring event as these understory berry plants were protected. In any event, it’s all speculative but something that does warrant further study.

We had a couple of other interesting observations that don’t bode well for the ecosystem as we know it on Skyline Trail. Under defoliated trees and shrubs, even those that were not permanently harmed, the canopy has opened up enough that Calamagrostis canadensis, a native grass more typical of lower elevations, had crowded out what was recently a vegetation community more typical of alpine tundra.

Closer to the trail, timothy was flourishing, an exotic grass commonly used for livestock forage. We find timothy on the Hansen Horse Trail, long used for horse packing into the backcountry of the Refuge. But the Skyline Trail is too steep for horses. It’s almost certainly being brought up the trail by hikers and runners from the parking area far below.

It’s difficult to put these unusual events into a larger context. It could be one of those things that occasionally happens in nature when weather, insects and plants collide. Or it could be anecdotal evidence of an ecological threshold that is being breached as our ecological systems change in response to a rapidly warming climate.

John Morton is the supervisory biologist at Kenai National Wildlife Refuge. You can find more information about the refuge at http://kenai.fws.gov or http://www.facebook.com/kenainationalwildliferefuge.

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rschwanke
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rschwanke 09/24/12 - 12:19 pm
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may not be isolated to the Kenai

Leafless dwarf birch have been observed through the Talkeetna mountains north of Chickaloon east through the Alphabet Hills near Paxson for the past 3 years during our wildlife surveys (as well as other areas I've heard) - following several years of random defoliations of a variety of deciduous trees/shrubs in the same general areas. While the cause is unknown in our area, I have observed perfectly healthy stands adjacent to these otherwise defoliated stands, and have found myself covered in small geometrid-type caterpillars (no other dominant shrub species in the area besides Salix pulchra with has been thus far unaffected). There are no leaf remnants left to clearly indicate defoliation, so I don't know what the specific cause is - your hypotheses are intriguing. Insects have been my suspicion, perhaps attacking buds? Regardless, I don't think enough time has passed to determine whether or not the shrubs in our area are dead. Considering upland shrub stands are naturally regenerating, unlike scouring or fire regenerated lower elevation willow and mixed-forest stands, there's always a mix of young, old, and decadent dwarf birch which could lead one to believe the plants have died.

I certainly appreciate this article, and hope that with additional interest in the issue, we will be able to pin point what is causing these otherwise leafless dwarf birch. Until now, all the deciduous defoliations I've observed in central and interior AK have passed in time. I can only hope this is one of those cases.

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