Surviving the Heat - The Noon Nap

July 16, 2006

By Clare Nunamaker

One of the wonders of the natural world is how plants survive the seemingly endless hot days of summer, days when we humans head for the lake or ocean, or cloister ourselves inside air conditioned buildings. There are a few mechanisms that help plants survive those hot days. One strategy is sometimes called the "noon nap" or the "midday nap."

Plants have something called stomata, small openings in their leaves, usually on the underside. (Stomata is plural for "stoma," which means "mouth" in Greek.) The stomata allow for the exchange of oxygen and carbon dioxide gases. And they also control the release of water vapor, through the process of transpiration.

Where it gets really interesting is when you understand that a plant can open or close its stomata in response to environmental conditions. Conditions such as soil moisture, humidity, carbon dioxide levels, and even light intensity can trigger a response.

On hot summer days, when temperatures are high and solar radiation intense, plants reach a point at which they can no longer move water from the soil into their leaves. For a time the plant continues to transpire water from its leaves, but soon it begins to close the stomata to prevent further water loss. In extreme cases you'll see wilted leaves, a sign of water stress and closed stomata.

The "nap" that occurs when the stomata close is so named because for that time the plant is stopping or greatly slowing two of its usual processes, photosynthesis and transpiration. With the stomata closed, not only the loss of water but also the exchange of gases is put on hold.

Of course plants have many adaptive strategies to deal with drought. The midday nap is just one, perhaps one we as humans can relate to.

You can now view Tree Tips on the web. Go to www.nwwg.org, click on Resources and click on Tree Tips.

You can contact Clare Nunamaker at forestry@nunamaker.com. Clare is a California Registered Professional Forester currently living near Edinburgh, Scotland.