Almanac : Sunflecks

I recently hiked a section of the Pocumtuck Trail, which meanders for about 15 miles along the eponymous ridge from the Sugarloafs in South Deerfield to Rocky Mountain in Greenfield. The deciduous woods were coming into leaf, and the forest floor was lushly carpeted everywhere with Canada mayflower. This modest little plant puts out only 2-3 leaves on a single stalk, which is tipped these days with clusters of 12–25 star-shaped white flowers.

The mayflowers, and other early-blooming understory plants, take advantage of the sunlight available before the forest canopy thickens, casting them into shadow. But even after the oaks and maples and other trees have fully leafed out, most understory plants continue to live rather than immediately dying back to dormancy. And this presents them with a problem.

Light levels on the floor of a summer forest are generally quite low and diffuse. Almost gloomy. But in the course of a sunny day, thin, laser-like rays of direct sunlight will poke through the canopy creating “sunflecks” that move across the forest floor, either because of the motion of the sun or because wind ruffles the canopy, exposing gaps.

Sunflecks, brief as they are, may contribute more than half of the total daily photon energy available to understory plants. But capturing those quick blasts of energy and putting them to use is extremely tricky and potentially dangerous.

The machinery plants use to catch photons and convert that energy into molecules for powering growth and reproduction is fascinatingly complex. The basic idea is that some parts of a leaf harvest photons to create energized electrons. Other parts use those electrons to power enzymes and other molecular machines needed to keep the cells in the leaves and the rest of the plant alive and thriving.

In low-light conditions, a leaf will maximize the photon-harvesting machinery, which is analogous to the way our irises open wide in the dark to let more photons strike our retinas. But a sunfleck landing on that kind of dim-adapted leaf is like having someone shine a high-powered flashlight into your dark-adapted eyes. Ouch! Suddenly the leaf is flooded with energetic electrons it can’t process, and that buildup of energy can short-circuit, or blow up, the delicate photosynthetic machinery.

Plants like Canada mayflower use several strategies to cope with the dramatic variations in photon energy created by sunflecks. The chloroplasts, which are the tiny sacs within leaf cells containing chlorophyll and other things needed for photosynthesis, are mobile. When exposed to the flash of a sunfleck, they roll to the edges of the cell, effectively turning their light-sensitive faces away from the light and reducing the flow of energetic electrons. Other parts of the Rube Goldberg-like photosynthetic apparatus can respond quickly to bursts of light as well, temporarily increasing the passage of the electrons to the interior of a chloroplast, for example, so that they can be processed more rapidly and avoid an overflow.

Because nature is inherently dynamic and unpredictable, most living organisms, whether one-celled or bipedal, have at least some capacity to adapt to feast-or-famine conditions. In the not-so-distant past, humans used to experience something akin to the alternations in “diet” experienced by mayflowers—except instead of sunfleck fluctuations in photonic “food” our ancestors coped with days or weeks of hunger interspersed with the bonanza of calories from a successful hunt or from the sudden ripening of fields of berries or other harvestables.

For humans, of course, the concept of “feast-or-famine” now describes something more global: much of the world struggles with the harmful effects of a never-ending “feast” of difficult-to-resist calories, while approximately 688 million others go hungry and face literal famine. Nature continues to fluctuate, but those fluctuations are now exacerbated by human-generated climate change, and the disparities in global food supply networks are also rooted in human and societal inadequacies. Mayflowers long ago found ways to thrive and find equilibrium in the face of dramatic natural fluctuations. It remains to be seen if humans will be as successful.

Almanac is a regular Indy column of observations, musings, and occasional harangues related to the woods, waters, mountains, and skies of the Pioneer Valley. Please feel free to comment on posts and add your own experiences or observations.