Lecture Desert plant adaptations, physiology, and ecological convergence.

Transcription

1 Lecture Desert plant adaptations, physiology, and ecological convergence. NOTE to students who may have downloaded these notes: These notes were meant for me, not you, and have references that may/may not have been mentioned in class. Still, they should be useful for studying. Open Google Earth and Powerpoint. Syllabus error: FOR THE ECOL 514 STUDENT In addition to the Ecol 414 activities, Ecol 514 students will write a 3-5 pp. paper expanding the Plant Talk topic, which you are obligated to restrict to the topic of desert plant biogeography. In particular, you should pick a Sonoran Desert species, explain its present distribution, and provide evidence to this effect. This paper, with references and in appropriate, typed format, demonstrates your graduate level competence. You must make an appointment no later than July 18 to agree on your topic with the instructor. The first draft is due by August 1, and will be returned with critique if revision is needed. The final draft will be due August 8. Well this doesn t agree with the calendar, which says to submit your proposal by Monday - a short description, one page, of your presentation. Title, two references that indicate some level of research. I will look these over and hand them back Wednesday. 25 M Keying out plants. ECOL 414 students submit a short description of the presentation, as well as its title and at least two references. ECOL 514 students submit gradlevel project proposal. ALL Students must have chosen date for their Plant Talk at end of class. I suggest that the all students talk with me after class, or via , before Monday. We ll end a little early today so you ll have time. Monday, have your proposal in hand. ECOL 514 students, here s an example of a proposed project I ve already approved: the biogeography of Yucca and yucca moths. EVERYBODY BE HERE AND READY TO LEAVE AT 5 PM, Saturday Desert Museum. My cell: Gonna be miserably hot unless we re saved by rain.

2 TODAY: Start highlighting the Big 30. Palo Verde, Ironwood, Mesquite. We'll have a break today, during which you can check these out. Begin drawing them, if you can. Also: (1) Coping with aridity adaptations. Like plants that are canteens. (2) Convergent evolution. Canteens again. If we're going to the desert museum, we can revisit the question: What s a desert? Depends on your perspective. If a desert is a place where lack of water limits perennial plant cover to less than 50%, then water is the primary limiting factor in growth and reproduction. So what. Big deal. Everyone thinks the desert is SO tough, when actually it s just fatal if you run out of water. Otherwise, it s pretty nice most of the year. ANY biome is tough on plants think of how many seeds they make, and how many survive to make their own seeds. (In a stable pop, one plant produces one replacement over its lifetime). When I m in a very green place, I m thinking: Help. Remember: if you re a plant, light is like oxygen. You re being strangled by the dark. And it is dark. Ever been in the jungle? You won't know how dark it is until you try to take a photo. And a jungle in the night? Like a cave. In the desert, you can see. Hike in the night, by starlight. READ: Joseph Wood Krutch, The Desert Year, p. 23.

3 So What is a desert? It s light. Bright. Open. Can walk around with ease, camp where you please. Not crowded because Limited water is limiting plant growth. Let's take a look at SW Arizona (powerpoint) OK let return to Limited water is limiting plant growth. Water use cannot exceed water uptake over the long term. This could be a period of months to even a year or more but only if you re a canteen, and store water. All plants however, have the same sources for uptake: rain or snow. It is absorbed, more or less, by the soil, and the plants suck it up with their roots. Sometimes this is easy, sometimes it ain t. It depends on the soil. In general, the soils that can hang on to water are very fine grained like clay but they hang on so tight that plants can t suck it out quickly. That s water uptake affected by

4 Rain, Snow (water in) Soils (holds water) Competition with other plants (lose water to others, who might take it first e.g., Saguaros have roots like spokes on a wheel. Mesquite go deep.) What about water use? Photosynthesis (water + light + CO2 = sugar + O2). Where does the carbon dioxide come from? The air. Must enter through pores called stomates. But you can t open the door to let carbon dioxide in without letting water vapor out. Transpiration is the loss of water vapor through evaporation. So water is not only part of the chemical reaction for photosynthesis, but water is also lost because of transpiration. How much? 200 to 1,000 lbs ( kg) of water are transpired for each pound of solid material added to the plant. So, Water use is by. Photosynthesis Transpiration Anything else? Theft by animal. Many a barrel cactus is knocked over bighorn. Rodents gnaw on saguaros. The plant isn't using the water beneficially, but it's certainly losing it. So, let s assume something, something that s mostly true. Plants can t make it rain more; plants can t get up and move to a new place; they must grow and reproduce with available resources. What they can do is lower water use. How to use water more efficiently? Three different ways of carbon fixation in Photosynthesis C3 most common, not arid adapted (95% of earth s plant biomass) C4 high sunlight, drought adapted common in monocots (40%), like grasses. Changes the location of carbon fixation from the mesophyll to the bundle sheath cells. But not as efficient a producer of (GMO rice?) CAM allows the plant to shut stomates during the day. Takes in CO2 during the night, stores it as organic acid. Less transpiration. Common in cacti, agave, yucca, aloe, bromeliads. Any other ideas? How to cope with aridity? Ways to cope with aridity:

5 Use it NOW. Ephemeral (annuals), can be luxurious. Live fast, die young but leave seeds behind. Succulent -includes the cactus and euphorbs (stem-succulents); and agaves, yuccas, dudlyeas..(leaf succulent) They suck up and store water when available. They re canteens. What about the shrubs? Fairy duster. Jojoba bush. Whitethorn. Wolfberry. I call these the sippers because even though they can drink deeply like a succulent, they can squeak by when water isn't available by growing slowly, or not at all. They can be: Drought-deciduous: lose your leaves and go dormant. Or hold onto your leaves and still go dormant creosote, jojoba. Recount: four ways I just wrote up here. Ephemeral. Stem succulent. Leaf succulent. Sipper (drought deciduous, etc). There are plenty of in-between guys. E.g., about the ajo lily? Bulb under ground. Not an annual. But waits. Kind of a giant seed. So, let s get out and look at some of these plants. They live right outside, in the Joseph Krutch garden. Go for tour. HAND OUT MAPS. Everyone should list the plants we see, and try to characterize it s water use strategy. For example: Agave: leaf succulent Cactus: stem succulent Wolfberry: sipper Chenopod: use-it-or-lose it an annual SET THEM Loose. Have them find species, and try to figure out their water strategies. E.g., Drought deciduous wolfberry (FOR CONVERGENCE, COLLECT EUPHORB AND CACTUS FLOWERS. Bring back to class, dissect. The two flowers are not much alike, and it's no wonder, because they are not closely related. ) Return to class. Emphasize that if our tour was broader, the more we would see, like coral beans and limber bush. They have big leaves, but they wait for the summer rains.

6 Can t see everything, every adaptation: desert plants have hidden adaptations, like CAM photosynthesis. Allows them to open their stomates at night. So: how did these plants end up like this? Remember, they all came from the sea, then moss, etc. Via evolution, i.e., natural selection, i.e., adaptation: Variation in offspring, differential survival and reproduction. Adapation is powerful. Evidence is everywhere. In people, just check out a barrel-chested Bolivian. In general, the more powerful the selective force, the greater the push to adapt. To move fast in the water, you must be hydrodynamic. Aquatic mammals: manatee, and dolphin which one really moves out? So you can tell, then, from just looking at an organism, something about where it lives. Like a cactus. There are about 1600 species of cactus, native to the Americas. (that s in the family). Most are stem succulents, with spines. They often have Plicated stems = accordion pleats. Water storage. There are also stomates in the valleys the drier the cactus, the better protected, because the accordion pleats close as the plant shrinks. Are there cactus elsewhere besides the Americas? Not native. But there are cactus-like plants that have resulted. This sort of adaptation is called Convergent Evolution = when unrelated organisms evolve similar adaptations to similar environmental or selective pressures.