Understory plants in forests and woodlands compete with the canopy of trees for resources such as water, nutrients and sunlight.  When these resources are in adequate supply, plants thrive by absorbing essential nutrients and converting sunlight, water and carbon dioxide into carbohydrates and oxygen through photosynthesis.  The resulting carbohydrates provide chemical energy, in the form of sugar, and structure, in the form of cellulose, for plant growth.  Plants use the photosynthetic pigment, chlorophyl to absorb light in the red and blue spectrum for photosynthesis and reflect the unused green light that we see with our eyes. Living under a canopy of trees limits the supply of water, light and nutrients.  Understory plants thrive in this environment because they have developed strategies to cope with the limited resources.

     Some of these adaptive strategies (1) include optimizing and conserving the amount of available energy via

1. thinner leaves (2)
2. higher concentration of chlorophyll b (3)
3. focusing incoming light (4)
4. adjusting growth patterns in response to the ratio of deep red to red wavelengths of light and the amount of nitrogen in the soil (5)
5. reflecting light to get a second opportunity to capture it (paragraph below) (6)

• etc.

     Highlighted in a lecture at the NC Botanical Garden on 1/26/23, Scott Zona shared an attribute of a native NC orchid, Tipularia discolor (Cranefly Orchid) that he describes in his book A Gardener's Guide to Botany.  The surface of the leaves of this plant are green and the underside of the leaves are a beautiful purple (see photo to right).  Beyond their beauty, Scott points out that chlorophyll absorbs the red and blue light as the sun strikes the leaf from above and it gets a second chance at light that it missed when the purple undersurface of the leaf reflects the red and blue wavelengths back up into the chlorophyll from below.

     In 2014, researchers in the Dept. of Biology at Duke University proposed a theory (7) that ferns evolved to survive in low light conditions when flowering plants began to dominate the landscaped by borrowing a chimeric gene called neochrome from hornworts.  This gene allowed ferns to be more efficient in absorbing light in the red and blue spectrum.  

     Sunscreen:  There is evidence that some plants grown in high light conditions can produce an orange pigment that acts as a sunscreen.  In low light conditions, a gene KEA3 may turn off the production of this pigment (8).

Ferns adapt to dry conditions (9) by having small, thin leaves that effectively reduce the surface area from which water loss can occur and having sunken stomata pits, which trap moist air and reduce water loss rates.

    Approximately one third of ferns are epiphytes and have developed the ability to absorb enough nutrients and water from the atmosphere, limited soil or even their hosts to thrive.

     There is also data suggesting that the understory plants may also help forests mitigate the adverse effects of global warming by cooling the forest floor.  

    These are just a few examples of what makes a forest understory garden possible or even beneficial to a forest.  I will share other examples of how this unique ecosystem evolved as I learn more.

References:

1. Shade-tolerant flowering plants: Adaptations and horti-cultural implications
2. Challenging plants: Plant science
3. Why do you see various shades of green in a garden?
4. Focusing of light by leaf epidermal cells
5. Phytochrome-dependent responsiveness to root-derived cytokinins enables coordinated elongation responses to combined light and nitrate cues
6. A Gardener's Guide to Botany, Scott Zona
7. Ferns borrowed genes to thrive in low light  
8. Between light and darkness: How plants optimize photosynthesis under changing light conditions
9. What Makes a Plant Drought Tolerant?
10. A meta-analysis of understory plant removal impacts on soil properties in forest ecosystems
11. Plant Strategies for Enhancing Access to Sunlight
12. How plants optimize photosynthesis under changing light conditions
13. Shade tolerance: when growing tall is not an option
14. Sensing the light environment in plants: photoreceptors and early signaling steps
15. Phytochrome Signaling: Time to Tighten up the Loose Ends
16. Mechanisms of plant competition for nutrients, water and light
17.