Summer Research: Why is Mistletoe a Kiss Goodbye for White Spruce?
Story posted August 03, 2009
While a lip-lock may result from a chance meeting beneath a sprig of mistletoe, its encounter with a spruce tree can be the kiss of death.
The parasitic eastern dwarf mistletoe is a clingy and dangerous consort. It latches onto its host and slowly drains its life force — killing it in 10 to 20 years. Over the past quarter century, it has had a profound effect on the white spruce forests common along the coast of Maine.
This unhealthy relationship has long been studied by Barry Logan, associate professor of biology, and is the focus of a summer research project he's leading with three students.
In early July, Logan, Shem Dixon '11, Danielle Marias '10 and Stephanie Schmiege '10 traveled to Southport Island to see the damage at their field site firsthand.
Infected trees are easy to spot; they are the ones with "witch's brooms" — tangled masses of branches that stand in stark contrast to the organized, two-dimensional splays seen on healthy spruce trees.
"Branches on a typically uninfected tree array themselves in an orderly and mostly horizontal presentation to the sun, maximizing their ability to absorb sunlight," explains Logan.
Curiously, witch's brooms are often the healthiest branches on their host trees.
"The host is not recognizing its own infection as stressful," says Logan, explaining that the tree devotes its resources — water, mineral nutrients and sugars — to the infected branch, to the detriment of its other limbs.
"As the tree dies it's actually shedding healthy branches — uninfected branches — and preserving the infection when, at least to my mind, the ideal way of coping with this would be to kill off the infected branch, like taking off the infected limb to save the body. The tree is not doing that. It's doing exactly the opposite," says Logan.
"We think that it's playing out that way because the parasite is able to, in some ways, hijack the hormonal metabolism of the host because they're both plants."
Logan and his students are examining witch's brooms from a physiological standpoint to try to understand how the formation of those brooms may compromise the health of the host.
Their hypothesis is that, since the brooms are more shaded, they're not getting as much light, so they can't do much photosynthesis — a plant's process of taking carbon dioxide from the atmosphere to make sugar — as light powers photosynthesis. If there's not enough light, photosynthesis will be compromised.
"That then is a contributing factor to the costs of infection," says Logan.
"Needles are all about gathering sunlight to do photosynthesis to pay back their construction costs and to support the tree. Growing them in witch's brooms is not a very good way to do that."
Logan's students are testing the hypothesis a number of ways.
Danielle Marias, a Gibbons summer research intern, will be using Y-Plant, a computer modeling program developed by plant biologists at the University of California-Davis.
It enables researchers to input measurements of the architecture of the plant — the angle of the branches, the distance between branches, and the size of the leaves or needles.
"If you punch in all those parameters you can construct a plant model in a computer, and then with that you can move the sun over the plant and look at any given moment of the day how efficient is it at capturing light, how much self-shading is it experiencing," says Logan.
"This is the first time Y-Plant has been considered to be used for conifers, let alone using it to study the brooms of mistletoe infection, so that's an exciting aspect of my part of the project," says Marias.
"We don't know what the results will be at all, we don't know what's going to happen, which makes it intriguing and exciting."
Stephanie Schmiege, A Surdna fellow, is also examining light intensities, but by way of an empirical approach; she'll affix sensors that measure light intensities at various depths through the broom and collect, over the course of a sunny day, light penetration levels.
The project is a familiar one for Schmiege. As a first-year, she looked at the effects of mistletoe, as well as water intake and its role in the death of the spruce, as part of Logan's Plant Physiology course.
"So now I'm taking a different aspect and I'm looking at light absorption, how brooms affect light absorption and how that may also conspire to kill the spruce tree," says Schmiege.
"So taking a different aspect is really exciting."
Schmiege is also hoping to compare her results with those of Y-Plant, to see whether the program is an accurate representation of the effect of witch's brooms on self-shading.
Shem Dixon, A Rusack Coastal Studies fellow, is constructing a chamber to enclose an entire broom. Once sealed, the team can measure the rates of photosynthesis.
"It will help us evaluate the efficiency of the branch," says Dixon.
"It should yield some data we can use in conjunction with their data to further understand just exactly how the broom functions. One of the main goals is to figure out just how efficient they are compared to a branch with normal growth.
"Going through and testing various prototype chambers through trial and error and really applying my critical thinking skills is really what's exciting to me — constructing problems and finding a way to overcome them, being creative."
The pioneering work takes the team to beautiful locales along the coast of Maine, an added bonus notes Dixon, as the team hopes to make out well in its quest to embrace the mysteries of mistletoe's fatal attraction.
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