Tree Physiology shines a light on the understorey

Trees dominate forests, but not all the plants there are trees. They can flourish in clearings, but what about the places in shadow?
Sunlight through the forest.
Photo: fatboyke /Flickr

It’s common knowledge plants need light. Each spring shoot thrust up to reach for the sun, but what happens to the plants that cannot reach sunlight? Trees dominate forests, but not all the plants there are trees. They can flourish in clearings, but what about the places in shadow?

Tree Physiology has a virtual issue on Sunfleck Photosynthesis. Sunflecks are what the understory species live in away from direct sunlight. Smith and Berry have a piece available free (at the time of writing) explaining exactly what they mean by sunflecks. Surprisingly when dealing with light and shade it’s more than a black and white issue.

The first word that puzzled me was numbra which you can find associated with a penumbra. I’m familiar with penumbra from astronomy. An umbra is a full shadow and an penumbra is a hazy half-shadow where a body is partly-lit when something is only partly in the way of the sun. A numbra is a negative umbra. Imagine a think tree canopy blocking out the light. Now punch a hole in it so that direct sunlight can hit the understory. That hole of direct sunlight is the numbra and the hazy light shadow around it before the deep shadow is the penumbra.

Not every hole in the canopy will produce a numbra. It’s partly down to the distance between the gap in the canopy and the understory, so there’s likely to be a mix of numbral and penumbral light in the lower reaches of a forest.

Some places will get numbral light for long periods of time. Smith and Berry call these clearings, which makes sense when you think about the kind of opening that would give you more than a couple of hours of direct light.

In places there’ll be periods of direct light combined with shading. Smith and Berry propose calling these sunpatches. For the places that only get penumbral light they reserve the word sunfleck.

Does it matter if a zone is a sunpatch or a sunfleck? Quite possibly. There is a difference between direct sunlight and second hand sunlight. It’s not just a matter of intensity it’s also a matter of quality. There’s research needed on how wavelength varies between sunpatches and sunflecks. Smith and Berry also raise other problems like leaf-heating and water stress. In sunflecks plants are protected from direct heat. In sunpatches plants may have to cope with quick variation in heating from the sun.

Each plant is a complex biochemical factory that has to keep processing gases into carbohydrates and respiring to emit waste products. Tree Physiology’s Sunfleck photosynthesis issue is helpful guide to a difficult problem. You can read Smith and Berry’s paper Sunflecks? for free at Tree Physiology.

Smith W.K. & Berry Z.C. Sunflecks?, Tree Physiology, DOI:

Photo: Magic! between the trees by fatboyke / Flickr. This image licensed under a Creative Commons by-nc-sa licence.