Natural History Of The Sonoran Desert - University Of Arizona

Final Exam Carbon Cycle Web Sites . Vehicle C emissions and wind credits: http://www.participate.net/terrapass Carbon quiz: ml Thurs May 11h 11-1 Koffler 204 100 multiple choice questions – Cumulative – 10-15% will be on Natural History of the Sonoran Desert You already have a study guide (outlines and key concepts) Review session: Wed the 10th 11 AM Koffer 204 Natural History of the Sonoran Desert Natural History of the Sonoran Desert 1. 2. 3. 4. What is a desert? Desert regions and desert habitats Adaptation in deserts (generally) Plants and animals of the Sonoran Desert Connections between the detail of the Sonoran Desert and general concepts you have been learning What is a desert? Communities are a continuum Deserts: – dry places with virtually no plant life – Saguaro National Park that’s teaming with plant life Deserts are extremely hot, others not What is a desert? Tucson Basin ‘moist’ ends of desert We don’t need to worry too much about where the exact cut-off point is on the continuum (except that we live in an ‘iffy’ desert) 1

What is a desert? A dry place with drought adapted plants and animals and an open canopy Often defined by annual precipitation: Old World Ecologists: 125 mm New World Ecologists: 250 mm (Tucson has 300 mm! San Diego has the same precip as Tucson) What is a desert? Ratio higher than 3 as semi-arid Tucson – 300 mm precip/ 1,524 mm PET The air can evaporate 5x more water than actually falls in precip – so even if we had 1,524 mm of precip it could all be evaporated! For comparison: Yuma PET/P 30 Sahara PET/P 600 What is a desert? Rainfall alone doesn’t capture aridity Also – how easily water is lost by plants Potential Evapotranspiration (PET): how much water evaporates and is transpired from plant leaves when it is not limiting An Index of aridity – evaporative strength of air The ratio of PET:Precip is what climatologists use to define a desert Where is a desert? Primary determinants of where deserts are: A. Sunlight falls perpendicular to the axis of the spherical planet B. 3 Laws of air and water: 1. Hot air rises, cool air sinks 2. Rising air expands and cools, sinking air compresses and heats up 3. Warm air can hold more water than cool air Where is a desert? Where is a desert? Warmest in tropics – near vertical sunlight Hot air rises from the tropics, cools, and drops rain in storms Creates convection cells - Hadley Cells Rising air spreads towards the poles Eventually cools and then sinks, flowing back toward the surface Air sinks around 30 degrees N and S 2

Where is a desert? Cool air sinks Æ Can it hold more or less water? No rain AND absorbs moisture Where is a desert? Rain shadows exacerbate the dryness of the desert Southwest There are 4 North American Deserts Great Basin Desert – Summer growing season (but isnt always rain!) Small leaved shrubs. Poor in trees, succulents and annuals. Dominants: big sagebrush (Artemisia) Where is a desert? Thus deserts are found mostly at 30 degrees N and S latitude There are 4 North American Deserts Great Basin Desert – northern most, highest elevation. Very cold winters (in the Old World – would be called a steppe) There are 4 North American Deserts Mojave Desert – winter rain. Hard freezes Many species of low shrubs & winter annual plants (only seen in wet years) Few succulents and trees Joshua tree is a tree yucca 3

There are 4 North American Deserts Chihuahuan Desert – high elevation and arctic air causes winter freezes. Many species of low shrubs, leaf succulents, and small cacti. Few trees. Summer rainfall – summer annuals There are 6 Subdivisions of the Sonoran Desert 1. 2. 3. 4. 5. 6. Lower Colorado River Valley Arizona Upland Plains of Sonora Central Gulf Coast Vizcaino Magdalena Plains Subdivisions of the Sonoran Desert Lower Colorado River Valley Valleys: creosote bush and white bursage (Very drought tolerant perennials) Trees grow along larger washes There are 4 North American Deserts Sonoran Desert – Biseasonal rainfall. Northern 2/3 gets winter storms, southern 2/3 gets summer thunderstorms The only NA desert with mild winters (only a few hard freezes) Allows tropical plants and animals to edge up here Dominants: legume trees and columnar cacti. (Also many shrubs and annuals) Subdivisions of the Sonoran Desert Lower Colorado River Valley Hottest driest part of the desert Temperature: just like Death Valley in the Mojave – Over 120 F (49 C), soil temperatures to 180 F (82 C) Rainfall: low as 76 mm (3 in) but doesn’t rain every year Subdivisions of the Sonoran Desert Lower Colorado River Valley Mountains – have more shrubs and cacti Few columnar cacti (they are restricted to valley floors) 50% or more of the plants are annuals, mostly winter annuals, and abundant only in wet years 4

Subdivisions of the Sonoran Desert Arizona Upland Subdivisions of the Sonoran Desert Arizona Upland Highest, coldest part of the Sonoran Desert More trees and succulents Trees on rocky slopes and drainageways Saguaros on slopes above valley floors Palo Verde is a common tree Hard frosts exclude some species found in other parts of Sonoran Desert Two equal rainy seasons Many plants and animals – 630 plant species in the Tucson Mountains alone Subdivisions of the Sonoran Desert Vizcaino Subdivisions of the Sonoran Desert Vizcaino Pacific side of Baja California Less than 125 mm rain, but humid breezes and fog Many stem succulents: Boojum (Fouquiera columnaris), Baja elephant tree (Pachycormus discolor), strangler figs Subdivisions of the Sonoran Desert Vizcaino Desert Adaptation Concepts Also Blue Palms (Brahea armata) The main stressors are aridity and heat: Aridity? water is needed for physiological functions (e.g. photosynthesis, transport) Heat? 5

Desert Adaptation Concepts Heat – Van’t Hoff’s Rule for every temperature rise of 10o C, rate of biochemical reactions double (up to a point) This is why it is bad to get too hot or too cold – Too cold body functions slow due to Van’t Hoff’s Rule. – Too hot proteins break down and body functions slow Desert Adaptation Concepts Options? 1. EXPIRE 2. EVADE: when the going gets tough avoid – Store Water. Example: cactus – Spatial – migrate, select comfortable microhabitat – Temporal – nocturnal, CAM, torpor Desert Adaptation Concepts Options? 1. EXPIRE 2. EVADE 3. ENDURE: when the going gets tough, tough it out. Several adaptations: Desert Adaptation Concepts Options? 1. EXPIRE: when the going gets tough die – Have annual lifecycle and be a tough egg/seed during harsh times e.g. annual wildflowers Torpor: a resting state lowering of body temp below activity temp Reduces water needs – less evaporation, less respiration, less excretion Also reduces energy needs – – Slower heat loss – Less tissue demand (Van’t Hoff’s Rule) Endure: Temperature Stress 1. Reduce Heat Input Ex. Use orientation/posture, shading from spines/hairs, small surface area What’s going on with the toes? – Temperature Stress – Water Stress 6

Endure: Water Stress Endure: Temperature Stress 1. Reduce Heat Input 2. Dissipate Heat Ex. Small size, long extremities, evaporative cool 3. Tolerate Hyperthermia (High Body Temp) 1. Store Water Animals – in fatty deposits Plants - roots, stems, and/or leaves of plants (plants that do this are called succulents) Ex. C4 photosynthesis, Harris’ Antelope Squirrel (up to 105 F!) Endure: Water Stress 2. Conserve Water Minimize loss through skin e.g. waxes, small surface area Minimize excretory loss concentrate urine, dry feces Endure: Water Stress 3. Tolerate Dehydration Many plants (prickly pear) and animals (desert toads) can tolerate great losses of water without dying Minimize respiratory loss C4/CAM photosynthesis, drought deciduous leaves, torpor Notable Plants and Animals Ambrosia – major cause of hay fever Different Species are very common throughout the Sonoran desert Critical nurse plants e.g. Humans: 12% loss Prickly pear: 80% loss Notable Plants and Animals Ambrosia Drought deciduous shrubs Avoid drought by loosing leaves in hot, dry season Usually have 2 leaf cohorts per year – monsoon and cool winter Downside: it takes several weeks to re-deploy their leaves (the photosynthetic machinery) 7

Notable Plants and Animals Ambrosias vary in leaf size and drought tolerance Larger leaves have more photosynthesis and can evaporatively cool (need plenty of water to keep stomata open) Boundary layer of stagnant air at the surface and can overheat when stomata are closed Notable Plants and Animals Ambrosia ambrosiodes - Canyon Ragweed A large broadleaf shrub Confined to washes and canyon bottoms – Where it can get enough water to evaporatively cool Notable Plants and Animals Ambrosia dumosa - White Bursage Small shrub with smallest leaves Dominant in driest areas of the Lower Colorado River Leave size matches aridity of habitat Notable Plants and Animals Smaller leaves, divided leaves - smaller boundary layer and loose heat more effectively by convection Don’t overheat Which type of leaf do you think is the most drought tolerant (large vs. small)? Notable Plants and Animals Ambrosia deltoidea – Triangleleaf bursage Shrub with smaller leaves Same range as canyon ragweed, but is on plains Dominant plant in Arizona Upland (with the Palo Verde and Saguaro) Notable Plants and Animals Defining life forms of the Sonoran Desert: Columnar Cacti and Legume trees Columnar cacti are found in arid tropical habitats throughout Mexico, Central and South America 41 Species in Mexico 8

Notable Plants and Animals There’s 4 big ones in the Sonoran Desert: Cardon Senita Organ Pipe Sagauro Final Exam Thurs May 11h 11-1 Koffler 204 100 multiple choice questions – Cumulative – 10-15% will be on Natural History of the Sonoran Desert You already have a study guide (outlines and key concepts) Review session: – Michod Tues 9th 1PM location TBA – P-Z and Hunter Wed the 10th 11 AM BSE 100 ExI ExII ExIII Total 94 96 96 286 96 94 96 286 92 96 98 286 98 92 96 286 94 96 94 284 100 90 92 282 92 94 96 282 98 92 90 280 92 94 92 278 98 94 86 278 Exam III – Top score 98 Natural History of the Sonoran Desert Mean 75 Mean top points so far: 282 out of 300 (roughly 94) So (roughly): A: 253-282 B: 225-252 C: 197-225 D: 155-197 E: 154 Notable Plants and Animals Saguaro The only columnar cactus coldhardy enough to grow in the Arizona Upland Largest cactus in the USA Notable Plants and Animals CAM succulents (like Saguaro) have shallow but extensive root system 4 in deep & extend out as far as the plant is tall 9

Notable Plants and Animals Notable Plants and Animals CAM metabolism ‘idles’ during drought It never really goes dormant Can resume full growth 24-48 hrs after rain Close their stomata – Recycle CO2 to photosynthesis Recycle O2 to respiration Notable Plants and Animals Only under nurse plants do they get protection from temperature, drought and predators Grow very very slowly Notable Plants and Animals The Sonoran Desert is dominated by columnar cacti and legume trees Legume trees: Palo Verde, Mesquite, Ironwood Uses shallow roots and idling photosynthesis to take advantage of frequent small rains Notable Plants and Animals Takes several wetter than average years Only occurs several times a century in the AZ Upland there is episodic recruitment Notable Plants and Animals Palo Verde - stem photosynthetic trees 72% of growth is from stem photosynthesis More chlorophyll in stems than leaves More photosynthesis with less water loss (higher WUE) 10