Backcountry Science: A Living Thermometer

Out of all the characteristic plants found across the Appalachians, rhododendrons are easily near the top of our region’s list. Rhododendrons are seemingly everywhere, lining the sides of streams and rivers in the region and even cloaking the faces of cliffs and bluffs on our mountains’ ridges. In spring and summer, these plants come alive with blooms that range from the brilliant magenta of the Catawba Rhododendron to the muted white and pink of the Rosebay Rhododendron. But out of all these traits, one of the most interesting occurs during the winter, when few eyes are usually turned onto rhododendrons and the region’s other flowering plants.

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Catawba Rhododendron (pink-purple on ridge in background) in full bloom, as viewed from a distance on the Roan Highlands of North Carolina in Tennessee.

Appalachian hikers and other outdoorsmen have known for centuries that rhododendrons are much more than a pretty summer flower: they also can act as a weather indicator of sorts in the Appalachian backcountry. That’s because rhododendron leaves work as a sort of living thermometer, curling cigar-like when temperatures drop in the cooler months, becoming more and more tightly wrapped the colder the air temperature gets. Although some variability always exists from plant to plant and place to place, the generally accepted rule of thumb regarding the shape of rhododendron leaves and air temperatures is the following (see great photos of this phenomenon here):

  • At temperatures above freezing, rhododendron leaves stand out from their central stems at roughly a right angle.
  • As temperatures dip below freezing, these leaves begin to droop, becoming more parallel with the main stem yet still mostly flat in shape.
  • Leaves begin to curl into cigar-like shapes as temperatures drop into the 20s, eventually curling into incredibly tight cylinders when the air temperature leaves the 20s for the teens and points below.

If you live in the Appalachian region, give this pattern a real-world test as you go through winter. Take a look at rhododendron leaves, guess the air temperature based only on the position of leaves and degree of curling, and then take a look at a thermometer. While likely not exact, chances are the plant’s indication of temperature won’t be too far off from reality.

So what’s going on here? This is a neat trick and a good rule of thumb for detecting weather conditions in the backcountry, but there has to be some science behind it, right? The reason for the rhododendron leaves’ changing winter shapes actually has to do with the overall plant’s physiology – that is, the plant’s use of resources to power its functions both internally and in relation to its environment. The physiological process most important to this phenomenon starts way back with something that you probably learned first in high school: photosynthesis.

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Photosynthesis takes CO2 and water from the plant’s environment and uses light energy from the sun to produce glucose, a sugar used in storing energy. (Image from the public domain in Wikimedia Commons.)

If you cringe a little bit seeing the above equation and thinking back to a past biology class, don’t freak out: the very basics of the photosynthesis equation are not really that difficult to get your head around at all. The main goal of photosynthesis, like any reaction, is to begin with a series of starting “ingredients” (called reactants) and either build or break down molecules to create ending products. Think about it like baking a cake (actually a chemical reaction in and of itself): the cake is a product that you create by combining energy (heat from your oven) with the starting ingredients, or reactants, from your recipe. In terms of photosynthesis, plant cells start with carbon dioxide and water and use light energy from the sun to create glucose – a sugar, or energy-storing molecule. While there’s a lot more (a whole lot more!) than this going on behind the scenes, this forms the biochemical process that occurs when plants “make” their own food.

But how does all this relate to a rhododendron’s curling leaves? The reason here relies on where in the plant much of that photosynthetic activity occurs. The biochemical structures that power photosynthesis, called chloroplasts, are most concentrated in a rhododendron’s leaves. (This is actually why these leaves appear green, since chlorophyll, the dominant pigment capturing light for photosynthesis, reflects mostly green-colored light.) A plant’s flat, broad leaves act very much like a solar panel does, providing a large structure with a high surface area for more sun exposure and, by extension, the capture of more light.

This means that leaves are an absolutely crucial part of the plant, since much of the activity that builds the plant’s “food” occurs here. But with this value comes a tradeoff: the tissue making up a leaf is incredibly fragile, being easily damaged when temperatures drop below freezing and thaw out repeatedly. Many plants, like our deciduous forest trees, get around this problem by simply dropping their leaves in winter. Less sunlight is available during this time of the year anyway (meaning less potential for photosynthesis), and by dropping their leaves and going dormant, these plants can avoid wasting precious energy maintaining leaves and the potential for damage from freezing.

Many rhododendrons retain their leaves year-round, though, and that’s where the curling comes in. By curling inward, rhododendron leaves protect their fragile undersides from the cold, exposing only their upper portions, which are covered in a thick, waxy layer called the cuticle. This also reduces the surface area of the leaf and helps it better survive freezing by limiting damage from the cold. Think of it as instinctively pulling your fingers and arms – one of a human’s most vulnerable parts  – inward towards your torso when you get too cold.

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Rhododendron leaves both curling inward and drooping. (Photo from Wikipedia and distributed under a CC BY 2.0 license.)

Curling isn’t the only thing that protects a rhododendron’s leaves, though; these plants also “droop” their leaves by dropping them downward at an angle more parallel to the plant’s branches. This helps face most of the leaf’s surface area away from the sun, which itself can cause damage through UV radiation when the forest canopy (which usually shades and protects the rhododendron) is gone in winter. Coupled with the curling mentioned above, this activity serves as both a “sunblock” for the plant and a sheltering mechanism to make it through the harsh cold of winter. For us, though, it forms a convenient indicator of just how cold air temperatures are across our mountains.

Although scientists are still working out the mechanisms and triggers behind this phenomenon, we do know that there’s more than a little bit of basic biology hiding behind one of the Appalachian winter’s most interesting tricks.

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