The Mystery of Dying Rhododendron

One of the most recognizable plants along the Blue Ridge Parkway is Rhododendron maximum, especially this time of year.  While the leaves of surrounding trees make there seasonal change from dull green to bright yellows, oranges and reds, Rhododendron sits underneath just as green as an evergreen should be.

  Healthy Rhododenron shrubs along a section of the Blue Ridge Parkway.

But something strange is afoot in Floyd County, Virginia.  We were just investigating large swaths of dead and dying Rhododendron along the Blue Ridge Parkway near Mabry Mill.  If you are enjoying a drive this time of year in Floyd County you may have spotted this phenomenon.

Dead and dying Rhododendron shrubs along the Blue Ridge Parkway.

Dead Rhododendron shrubs under forest canopy.

 

We were alerted to this mortality event by retired forest pathologist Dr. Ed Barnard who, in his fashion, assembled a small team of experts that included a pathologist from VA Tech, a forest historian, along with a county forester and us.  Ed and the crew all did their homework and we found that this event is not without precedent.  Dr. Richard Baird of Mississippi State University researched similar Rhododendron mortality in the Great Smoky Mountains National Park a few years back.  A forest pathologist from the U.S. Forest Service (Dr. Steve Oak) also investigated this issue several years ago.  Rhodo samples have ended up in diagnostic labs from landowners with shrubs from their yards, but in all this no single culprit has been found.

Cross-section of healthy Rhododendron stem.

Here is a quick rundown of what we found on our short field visit.  Above is a stem from a healthy Rhodo shrub lacking any evidence of decay, disease, infection, or impairment.  Below are stems from shrubs showing various symptoms of decline, decay, and death.

 Cross-section from unhealthy Rhododendron stem.

 Cross-section from unhealthy Rhododendron stem.

Cross-section from unhealthy Rhododendron stem.

Can you see the difference?  This staining indicates that something just ain't right.

A section of Rhododendron root with staining.

We have successfully eliminated the possibility of  an abiotic cause such as windthrow, voles, or moles.  There doesn't seem to be an insect chomping away at leaves, stems, or roots. What seems most likely is that a root rot is hard at work in these areas. 

But, there are still other organisms that could be to blame.  Nematodes (microscopic soil worms) have been reported at high numbers in some of the affected areas.  Could the Rhodo simply be aging out?  Are environmental stresses to blame?  The area in question has experienced periods of flooding then drought in rapid succession over the past couple years.

The bad news, there is an awful lot of dead Rhododendron and we don't know exactly why.  There is some good news though.  This "decline" is only affecting R. maximum and not other important relatives such as Catawba Rhododendron, Mountain Laurel, and Blueberry.  The mortality is fairly limited in its range and doesn't seem to be spreading. 

A root/stem sample with fungal growth.  (Photo:S. Fraedrich)

We bagged up some samples form the field and sent them along to several pathologists to pore over.  Our hope is  to start unraveling this mystery and figure out an answer in case a similar "decline" event happens somewhere else.

Fire in The South Explained

I am excited about this!  Our SRS Center for Disturbance Science recently partnered with Untamed Science to produce a video that helps explain the complexities of forest fires in the Southern U.S. 


Enjoy!

Hi-tech redneck...

Even Foresters use technology from time to time.  This week I have been playing with various sensors and data loggers that we will utilize in our hemlock restoration study.  Assembling, disassembling, wiring, and un-wiring to figure out what works and what will not.

Weatherproof data loggers (foreground) and pendant-style light sensors (background).

After some discussion and a small scale field trial out at our Bent Creek Experimental Forest  lab we decided to go with sensors that record light intensity, temperature, relative humidity, and soil moisture.

 A data logger and sensors deployed under tree canopy in 2 of our sub plots at Dupont SF.

We are interested in teasing out just what our hemlock saplings are experiencing from season to season.  This study is concerned with getting hemlock back on the landscape after we get a handle on that pesky hemlock woolly adelgid.  What matters more to our trees?  The amount of light they are exposed to?  The competing vegetation surrounding them?  The lack (or presence) of browsing mammals?  Conditions in the soil? And so on...

A re-purposed planting pot serves as a hat to protect our precious data logger.

Our truck parked on a forest road with one of our "cut" plots in the background.

 What a great place to work and a fantastic time of year!  Which reminds me, get all your fall color updates here for optimal leaf-looking.

In the thick of it...

We have been on our hands and knees sampling vegetation in our hemlock restoration plots for the past 2 weeks.

Andy checking out the ground layer vegetation in one of our control plots.

It is amazing what you can see when you stop and look for a while.  Our treatment plots are only approximately 10 square meters in size, but contain an array of eye-catching organisms.

 Likely an eastern tiger swallowtail caterpillar. 

Medeola virginiana, also called Indian cucumber root.

Two saddleback caterpillars, Acharia stimulea (I think).

The always distinctive Praying mantis.

 This parasitic wasp is Pelecinus polyturator.

What appears to be vertebrae and ribs from a snake kill.

Adelgid in the news...

Hemlock woolly adelgid is fit to print!

Mountain Xpress, a local publication out of Asheville, recently did a story on the adelgid and featured a cooperator of ours from the Hemlock Restoration Initiative.

Read the article article here and you will also hear what our very own Dr. Albert "Bud" Mayfield III had to add to the conversation.

Additional traps have just gone up...

I was back in Tennessee this week installing additional traps for walnut twig beetle.  Our funnel traps haven't yielded any beetles thus far this summer, so we are baiting fresh walnut bolts with lures in hopes of enticing the critters.  Along with lures, we attached sticky cards on each bolt so we can get an idea of just what insects we are attracting.

A black walnut bolt loaded up with walnut twig beetle lures and a sticky card.

 

We are throwing everything we can in suspect trees, hoping to pick up on some walnut twig beetle activity.  Last summer, we observed a real spike in beetle activity during the month of August.  We are hoping to see the same this year, if not, then we will be re-thinking our planned experiments for September and October.

One of the black walnut plantations we are working in outside of Knoxville, TN.

What's cooking this month?

Black walnut branch sections will be dipped in wax to seal in moisture.

Our lab inventory often reads like a list for a camping trip... saw, stove, propane, pot, spoon, etc.  But recently what we have been cooking in the field are small vats of paraffin wax.  We are preparing for another experiment involving the, now elusive, walnut twig beetle.  After several experiments, we have figured out that the beetles find it hard to resist a fresh, baited black walnut "bolt".  These 1-foot long branch sections, which we refer to as "bolts", come from a hazard tree that was just removed from our Bent Creek property.  Waxing the ends of these bolts assures us the most fresh sample possible. 

 Cooperators at a field site in TN hoisting a funnel trap in to a tree canopy.

Early in the month of June I was at a federal property in Tennessee setting up Lindgren funnel traps.  Yep, that small black and white thing making its way into the tree-top.  Bud and I (along with another cooperator) also deployed several sets of large, fresh bolts (3-ft sections) into about a dozen trees in and around Knoxville.  The beetles have been hard to find this year, which is good news for black walnut trees and landowners who have been affected by recent infestations, but makes running field experiments difficult.  We are hoping to pick up some activity soon for experiments we would like to initiate in September.

We will keep you posted.