BY PATSY MILLER, PH.D. | SEPTEMBER 14, 2011
How Plants Keep Their Cool
They can’t move out of the sun; how do they survive?
It has been a long hot summer. All the desert creatures, and we human desert dwellers, would seek the shade or the air conditioning as temperatures climb into the upper 90s or top the 100ºF mark. But what about the plants? They cannot get out of the sun; they just have to tough it out. Once a seed germinates and a plant becomes established, it is “rooted” in that location, and there it stays through heat and cold, rain or drought, until it dies. So just how do plants in the Upper Sonoran Desert cope with temperatures over 100ºF and prolonged periods without rain?
Cactus spines serve to provide protection for the cactus from the sun. At first, it is hard to believe that tiny cactus spines offer any real protection from the intense desert sun. But when you consider that a cactus can have thousands of spines and each spine provides a small amount of shade and multiply that by 1,000 – well it becomes easier to believe that spines really do offer some protection from the sun for the skin of the cactus.
The undisturbed desert and xeriscape gardens, favored by water conscientious residents, contain many different native and nonnative, desert adapted species. Each of these species has its own method of coping with heat and drought, but they share many common traits that adapt them to desert environments. The high solar radiation from our clear blue summer skies is the major challenge that desert plants must deal with to survive.
Part of the solar radiation (wavelengths of 400-700 nm) is used to provide the energy for photosynthesis, the process by which plants make sugars and carbohydrates. But leaves also receive short wave solar radiation directly from the sun and long wave infrared radiation reflected from the plant’s surroundings; absorbing these types of energy is what makes a leaf (and a person) hot when out in the sun. People can reduce the amount of solar radiation they receive by standing in the shade and staying away from the walls of dark colored buildings. One way that plants reduce solar radiation is by reflecting it away.
Color, hairs and spines, salt glands, shape, size and orientation, and growth form are some of the ways that plants deal with excess thermal energy. The color of a plant affects the amount of solar radiation absorbed by leaves. Leaves that are shiny, light green or gray/silver in color have a high albedo or reflectivity and absorb less. Many desert shrubs including brittle bush (Encelia farinosa) and white bursage (Ambrosia dumosa) use color to stay cool.
Other desert shrubs such as woolly butterfly bush (Buddleia marrubifolia) have leaves covered with white hairs, while many cacti have white or light yellow spines, which are highly reflective and reduce their surface temperature. Experiments document that removing the reflective spines from a barrel cactus (Ferocactus cylindraceus) increased its average daytime surface temperature by 7.2ºF. The saltbushes (Atriplex species) get their common name from the salt glands on the leaves that are also reflective.
Vertical plants such as saguaros (Carnegiea gigantea) and most chollas and those with steeply inclined leaves (Agave species) decrease energy loads by reducing the amount of horizontal surface exposed to the rays of the sun, and thus lower leaf temperatures during stressful midday conditions. Many desert shrubs and trees have compound leaves made up of many small leaflets that are each less than 1/8” in diameter. These compound leaves have the ability to fold in half, changing from a horizontal to a vertical orientation as temperatures climb. When leaflets are that narrow, they also lose heat by convection (the loss of heat due to air movement across the leaf surface) which helps them stay at or even slightly below air temperature.
Of all the plant growth forms in the desert, perennial shrubs have the greatest ability to reduce leaf absorptance of solar radiation; that is one of the reasons shrubs are so common in the Upper Sonoran Desert. A contrasting life form is that of desert winter annuals. These brief visitors to our landscape often have large, dark green leaves, with little ability to reduce the absorptance of solar radiation. Winter annuals only grow where or when water is readily available, and they need all of the solar energy they can get because they must photosynthesize at a high rate, grow fast, and produce seeds before they die when water disappears.
Losing water is another way plants stay cool in the desert. Evaporation from the surface of a leaf cools the leaf just as a swamp cooler cools a room. However, water is usually in short supply in a desert. Plants use water loss as a cooling mechanism only as a cost of carbon acquisition for photosynthesis. To live and grow plants use carbon dioxide from the air as a source for carbon to build sugars and carbohydrates through the process of photosynthesis.
The carbon dioxide gets into the leaf through tiny pores called stomata. When these pores open to let the carbon dioxide diffuse into the leaf down a concentration gradient, water escapes out the pores because the air outside the leaf is drier than the air inside the cavity below the pore opening. This process of transpiration is costly in water because water molecules are smaller than carbon dioxide molecules, and the plant loses about six water molecules for every carbon dioxide molecule it gains.
Most perennial desert plants can only “afford” to grow when there is enough water in the soil to support the water loss that goes along with the uptake of carbon dioxide, and they certainly do not want to waste water by using it just to stay cool. So long-lived desert plants have adapted by using color, hairs, spines, small leaves, and an upright stature to reflect away solar radiation and stay cool.
However, there are tradeoffs to being highly reflective. When a desert plant reflects away solar energy, it is also reflecting away energy needed for photosynthesis. Dark green leaves typically absorb 85 percent of the incident solar radiation; a desert shrub with highly developed surface modifications may absorb only 30 to 40 percent. What the plant loses in its ability to photosynthesize, it gains in lower leaf temperatures; decreases in leaf temperatures of 8 to 18ºF as a result of increased solar reflection are common.
Native and desert adapted plant species do well in our gardens. Many of the other plants available in local nurseries can survive only with large inputs of water. Water is a limited and costly commodity in the Upper Sonoran Desert; 80 to 90 percent of household water consumption is related to landscape irrigation. What kinds of plants do you have in your yard?
SEPTEMBER 14, 2011
Red Carpet Screening of “Cave Creek Uncovered” Sept. 30 and Oct. 1 to benefit Cave Creek Museum
CAVE CREEK – Cave Creek Museum is kicking off its new season with a special red carpet screening of the new documentary “Cave Creek Uncovered” from 7 p.m. to 9 p.m. on Fri. and Sat., Sept. 30 and Oct. 1 at Good Shepherd of the Hills Episcopal Church, 6502 E. Cave Creek Rd., in Cave Creek. Tickets are $10 per person, with all proceeds benefiting Cave Creek Museum.
Guests will be the first to view this special documentary written, directed, and produced by long-time Cave Creek resident and film maker Suzanne Johnson. The Museum’s executive director, Evelyn Johnson (no relation), is the film’s executive producer. The film is underwritten by the Cave Creek Film & Arts Festival, which allocated funds for the project after the non-profit organization dissolved.
Capturing the romance and quirkiness of Cave Creek, the film chronicles how settlers from the ancient Hohokam and the pioneers to present day made use of the area’s land to survive and build their communities. As a historical piece, the film also includes highlighted snippets taken from more than 30 old-time “Creekers” who shared their stories during taped oral histories. Cave Creek author and historian Bob Boze Bell narrates the film.
The film’s production team also includes David Devoucoux, director of photography and editor, and Jay Allen, second camera and editor.
Because seats are limited for the screening of “Cave Creek Uncovered,” reservations are requested. For more information, call 480-488-2764, or visit www.cavecreekmuseum.org.
SEPTEMBER 14, 2011
MacDonald’s Ranch Pumpkin Festival
Come out and enjoy a fun filled day at MacDonald’s Ranch Annual Pumpkin Festival, conveniently located in north Scottsdale. Adults and children alike will be delighted with hayrides to and from the pumpkin patch pulled by real horses and mules. But that’s not all! A hay maze, pony rides, sack races, a variety of new western games and much more will be on hand for everyone! The miniature donkeys, Pete and Repeat, lion head bunnies, sheep, chickens, a baby horse named Bullseye and a walk in petting zoo – all are included in your admission price. This is a great experience for all ages, so please bring the whole family for your October treat. The pumpkins come in all shapes and sizes to choose from. If you get hungry you are welcome to purchase food and drinks from The Grill.
MacDonald’s Ranch is committed to serving their customers and providing an authentic western experience. MacDonald’s Ranch offers anything and everything to accommodate groups of all sizes. They can really do it all, trail rides, cowboy cookouts, hayrides to private sites, weddings, bar mitzvahs, stagecoach rides, birthday parties, or just a family reunion.
The MacDonald’s Ranch Pumpkin Patch, 26540 North Scottsdale Rd., will be open the entire month of October from 9 a.m. – 6 p.m. (closed after 1 p.m. on Halloween). Admission: $8 per person (children 12 months and younger are free). Pumpkin prices vary by size. Notice is appreciated for groups over 30. Call 480-585-0239 or visit www.macdonaldsranch.com.
SEPTEMBER 14, 2011
Volunteer Training opportunities with Desert Foothills Land Trust
Cave Creek/Carefree/Scottsdale – Americans dedicated 8.1 billion hours of volunteer service in 2010. Like most nonprofits, the Desert Foothills Land Trust relies on the time and talent of volunteers who support the mission of the organization.
The Land Trust is hosting a volunteer orientation session on September 22. The session serves as both a review for current volunteers and a great overview for those considering volunteering.
For those particularly interested in land stewardship and management activities, the Land Trust is also offering six in-depth Steward and Docent Training classes. Stewards and Docents must complete all six courses, plus the volunteer orientation session. Upon completion of the training, Stewards and Docents have the opportunity to adopt a preserve to assist the Land Trust with monitoring and management activities, leading guided hikes and providing preserve visitors with information.
Volunteer Orientation Session
Thursday, September 22, 2011, 5:30 – 7 p.m.
Desert Foothills Land Trust Office
7518 E. Elbow Bend, Ste. B6
Carefree, AZ 85377
Please RSVP to Mary Warren at 480-488-6131 or [email protected] by Sept. 19.
Steward and Docent Training
Every Thursday, 9 a.m. – Noon
September 29 through November 3
Space is limited and reservations are required
Please RSVP to Stacy Fischer at 480-488-6131 or [email protected] by Sept. 26.
More information is available at www.dflt.org.
Please donate to Pack 134’s Scouting for Stuff Rummage Sale!