Spring, light, and strategy on the forest floor

Image: Trout lily, Erythronium americanum, dw_ross via flickr.com, CC-BY-2.0

Note: This is a science outreach piece belonging to a series I write for the newsletter of the Fredericton Botanic Garden. I’d be happy to see it modified for use elsewhere and so am posting the text here under a CC BY-SA 4.0 license . If you use it, though, I’d appreciate hearing where and how.

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Spring is upon us, and it’s a great time for a walk in the woods that are part of our Botanic Garden.  In a deciduous forest, spring finds the forest floor sprinkled with green and with flashes of colour from blooming “spring ephemerals”.  The trout lily pictured above is an example, as are wood anemone, trillium, bloodroot, and a bunch of my other favourites.  But if you walk the same trail in July, you’d be hard pressed to know some of these spring bloomers were ever there – not only is their flowering finished, but their green leaves and stems have withered and gone.  Why?

The forest floor is a challenging place for a plant to grow.  For most of the growing season, it’s a very dim place, with most sunlight intercepted by the leafy canopy above.  Light matters a lot to plants, of course, because they harvest light to power the making of sugars by photosynthesis (and in doing so, they produce the oxygen we all breathe).  More light, more photosynthesis; less light, less photosynthesis.  But the key insight is that as light attenuates, a plant can’t just photosynthesize slower.  That’s because plants also respire, burning sugars with oxygen to fuel their metabolism, just as you and I do. The green tissue required to photosynthesize has a respiratory cost – and that cost is there in light and in dark, in sun and in shadow.  Plants need to run at a sugar profit, and the amount and kind of green tissue that can achieve that profit changes with available light.

There seem to be two distinct strategies for plants to deal with the forest floor. Some plants (like seedling maples) are slow and steady: they remain green on the forest floor all summer, but they economize. Slow-and-steady plants grow slowly, support relatively little green tissue and, in that tissue, build relatively little of the respiration-expensive photosynthetic machinery.  With low sugar consumption, they can afford low sugar production.  Spring ephemerals play a very different game: hurry-while-you-can.  They poke their heads above ground very early in the spring, and grow rapidly to take advantage of the short period of high light before the forest canopy leafs out.  Spring ephemerals run expensively: lots of leaf tissue and lots of photosynthetic machinery to harvest light and make lots of sugar quickly.  But this expensive machinery can’t be maintained through the long, dim summer – so spring ephemerals stow their sugar profits underground in bulbs, corms, or rhizomes and spend the rest of the year waiting for the next spring’s sunlight bonanza.  Slow-and-steady and hurry-while-you-can both work, but no single plant does both. In this, as in many ecological contexts, the jack-of-all-trades would be master of none.

And the evergreen forest floor?  Without that sunlit window in early spring, our evergreen forests don’t allow the hurry-while-you-can strategy, and have little if any spring flora.  Actually, when the evergreen canopy is dense enough, the slow-but-steady strategy doesn’t work either; like the deep ocean, a dense evergreen forest isn’t a place where photosynthesis can make a sugar profit.  These forest floors belong to the fungi (and a few weird and wonderful plants that don’t photosynthesize at all).  More about those in a future Newsletter.

© Stephen Heard (sheard@unb.ca) May 5, 2016 but licensed CC BY-SA 4.0 .

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