The Desert What makes a desert? U.S. deserts Life in the desert What makes a desert? U.S. deserts Life in the desert

What is a desert? ◊All deserts are dry. Not all deserts are hot. ◊Some definitions say deserts are arid (dry) regions that get less than 25 cm (ten inches) of rain a year. The problem with this is that there are places that get less than 10” of rain a year, but aren’t deserts. ◊ A better definition is one that relates the amount of rain to how fast water can evaporate (or transpire from plants) in that place. This is the PET/P ratio (or Potential EvapoTranspiration to Precipitation ratio). ◊Simply, a desert has a PET/P ratio of 3 or higher. ◊All deserts are dry. Not all deserts are hot. ◊Some definitions say deserts are arid (dry) regions that get less than 25 cm (ten inches) of rain a year. The problem with this is that there are places that get less than 10” of rain a year, but aren’t deserts. ◊ A better definition is one that relates the amount of rain to how fast water can evaporate (or transpire from plants) in that place. This is the PET/P ratio (or Potential EvapoTranspiration to Precipitation ratio). ◊Simply, a desert has a PET/P ratio of 3 or higher. This is what most people think a desert looks like. In fact, even only a small part of the Sahara is sand dunes.

PET/P ratios ◊To measure this, the rate of water evaporating from a pan is measured (pan evaporation). Studies with plants show that actual transpiration loss is about 60% of the pan evaporation rate. ◊Studies show that in Tucson, for instance, pan evaporation is 100” a year (so PET = 60”), and we get at most 12” of rain a year. From this, PET/P = 60/12 = 5 ◊Since this is higher than 3, Tucson is officially a desert. ◊Yuma has a PET/P ratio of 30. ◊The Sahara Desert has a PET/P ratio of 600! ◊To measure this, the rate of water evaporating from a pan is measured (pan evaporation). Studies with plants show that actual transpiration loss is about 60% of the pan evaporation rate. ◊Studies show that in Tucson, for instance, pan evaporation is 100” a year (so PET = 60”), and we get at most 12” of rain a year. From this, PET/P = 60/12 = 5 ◊Since this is higher than 3, Tucson is officially a desert. ◊Yuma has a PET/P ratio of 30. ◊The Sahara Desert has a PET/P ratio of 600!

Another definition ◊A desert is a place where water is severely limiting to life most of the time. ◊Without the word “severely” this could also apply to semi-arid habitats such as chaparral or grasslands. ◊A desert is a place where water is severely limiting to life most of the time. ◊Without the word “severely” this could also apply to semi-arid habitats such as chaparral or grasslands.

Why so dry? ◊Several situations can result in deserts: Rainshadow effect Global wind patterns Ocean currents Distance from water ◊Several situations can result in deserts: Rainshadow effect Global wind patterns Ocean currents Distance from water

Rainshadow ◊As warm moist air rises, it cools, and rain precipitates out. ◊This means most of the moisture falls on one side of mountains. ◊As warm moist air rises, it cools, and rain precipitates out. ◊This means most of the moisture falls on one side of mountains.

Wind patterns ◊Sunlight is stronger and delivers more energy near the equator, and much less as you move toward the poles. ◊This causes a zone of warm rising air near the equator, so (like with the rain shadow effect) it rains a lot near the equator. As the air cools and drops (around 20-30° N and 20-30° S) it is very dry. ◊Because of this, there are many deserts near these latitudes. ◊Sunlight is stronger and delivers more energy near the equator, and much less as you move toward the poles. ◊This causes a zone of warm rising air near the equator, so (like with the rain shadow effect) it rains a lot near the equator. As the air cools and drops (around 20-30° N and 20-30° S) it is very dry. ◊Because of this, there are many deserts near these latitudes. Wind pattern animation

Ocean Currents ◊Warm or cold ocean currents also affect local climates. ◊Cold water holds less moisture, and there is less evaporation from cold water. ◊For instance, the Galapagos Islands are on the equator, and we would expect them to be warm and humid. The cold Humboldt current, though, robs them of much moisture and causes them to be a desert. ◊Warm or cold ocean currents also affect local climates. ◊Cold water holds less moisture, and there is less evaporation from cold water. ◊For instance, the Galapagos Islands are on the equator, and we would expect them to be warm and humid. The cold Humboldt current, though, robs them of much moisture and causes them to be a desert.

◊Distance from water is also important. The Gobi Desert in Asia is dry because it is a very long way from most bodies of water. ◊Together, these factors explain the distribution of most major deserts. ◊Distance from water is also important. The Gobi Desert in Asia is dry because it is a very long way from most bodies of water. ◊Together, these factors explain the distribution of most major deserts. Gobi Desert

U.S. Deserts ◊Four deserts: ◊Great Basin Desert ◊Mojave Desert ◊Sonoran Desert ◊Chihuahuan Desert ◊Note: By some measures, the Sonoran and Chihuahuan deserts do overlap in Southeastern Arizona, around Bisbee. ◊Four deserts: ◊Great Basin Desert ◊Mojave Desert ◊Sonoran Desert ◊Chihuahuan Desert ◊Note: By some measures, the Sonoran and Chihuahuan deserts do overlap in Southeastern Arizona, around Bisbee.

Great Basin Desert Nevada, Utah, Oregon, Idaho High elevation and farthest north, the Great basin is in the rain shadow from mountains along Pacific coast. Cold desert - average temperature 50°F. Severe hard freezes in winter. Not all deserts are hot! Most of the precipitation is snow. Typical (indicator) plants: big sagebrush (Artemisia tridentata). Many small-leaf shrubs, few trees, few succulents. Nevada, Utah, Oregon, Idaho High elevation and farthest north, the Great basin is in the rain shadow from mountains along Pacific coast. Cold desert - average temperature 50°F. Severe hard freezes in winter. Not all deserts are hot! Most of the precipitation is snow. Typical (indicator) plants: big sagebrush (Artemisia tridentata). Many small-leaf shrubs, few trees, few succulents.

Great Basin Desert

Mojave Desert Southern California and Western Arizona. In the rain shadow from mountains along Pacific coast. Hard freezes in winter but not as severe as in Great Basin. Most precipitation is in winter. Indicator plants: Joshua tree (Yucca brevifolia), low shrubs, a few succulents and trees. Southern California and Western Arizona. In the rain shadow from mountains along Pacific coast. Hard freezes in winter but not as severe as in Great Basin. Most precipitation is in winter. Indicator plants: Joshua tree (Yucca brevifolia), low shrubs, a few succulents and trees.

Mojave Desert Michael Frank 2006 Joshua Tree National Monument

Sonoran Desert Our desert Arizona, Baja and Sonora in Mexico In Pacific rain shadow and at 20-30° latitude. Two rainy seasons, winter and summer, with about half the rainfall during each. Freezes happen but are rare and usually don’t last long. Indicator species - Saguaro (Carnegiea gigantea), creosote (Larrea tridentata), mesquite (Prosopsis spp). Our desert Arizona, Baja and Sonora in Mexico In Pacific rain shadow and at 20-30° latitude. Two rainy seasons, winter and summer, with about half the rainfall during each. Freezes happen but are rare and usually don’t last long. Indicator species - Saguaro (Carnegiea gigantea), creosote (Larrea tridentata), mesquite (Prosopsis spp). Michael Frank 2005

Southeastern Arizona, Southern New Mexico, North/Central Mexico High elevation, not protected from arctic air masses so hard winter freezes. Summer rain. Low shrubs (creosote, sage), leaf succulents (Yucca spp), small cacti, few trees. Southeastern Arizona, Southern New Mexico, North/Central Mexico High elevation, not protected from arctic air masses so hard winter freezes. Summer rain. Low shrubs (creosote, sage), leaf succulents (Yucca spp), small cacti, few trees. Chihuahuan Desert

Living in a desert For both plants and animals, the lack of water is the biggest factor in determining what plants and animals live there. Most have adaptations that let them take advantage of what water there is, and survive when there is no water. These can take two forms: Drought avoidance Drought tolerance For both plants and animals, the lack of water is the biggest factor in determining what plants and animals live there. Most have adaptations that let them take advantage of what water there is, and survive when there is no water. These can take two forms: Drought avoidance Drought tolerance

Drought avoidance - annuals Annuals (those that just live one season) escape harsh conditions by not existing. They only sprout from seed when conditions are right. Air and soil temperature, and available water have to be right. Then they grow and flower very quickly. If rain continues, they may continue to grow and flower until things dry out. Since we have two rainy seasons here, we often get two growing seasons for annuals. Strategy for annuals: grow fast, set seeds while the conditions are right. Seeds may go several years before the conditions are right for sprouting. Over half of the plant species in the Sonoran desert are annuals. Annuals (those that just live one season) escape harsh conditions by not existing. They only sprout from seed when conditions are right. Air and soil temperature, and available water have to be right. Then they grow and flower very quickly. If rain continues, they may continue to grow and flower until things dry out. Since we have two rainy seasons here, we often get two growing seasons for annuals. Strategy for annuals: grow fast, set seeds while the conditions are right. Seeds may go several years before the conditions are right for sprouting. Over half of the plant species in the Sonoran desert are annuals.

Desert annuals Desert Marigold Mexican Poppy Lupin

Perennials are plants that live more than one year Many desert plants avoid drought by going dormant in hot dry times. They slow or shut down their metabolic processes (including photosynthesis) They may lose their leaves, or even drop some limbs. Dropping leaves cuts down on water loss from transpiration. Many look dead until the next rains. Palo verde (Parkinsonia spp) trees have green bark that allows them to continue a low level of photosynthesis even when they have lost their leaves. Strategy for perennials - most growth during wet seasons. Perennials are plants that live more than one year Many desert plants avoid drought by going dormant in hot dry times. They slow or shut down their metabolic processes (including photosynthesis) They may lose their leaves, or even drop some limbs. Dropping leaves cuts down on water loss from transpiration. Many look dead until the next rains. Palo verde (Parkinsonia spp) trees have green bark that allows them to continue a low level of photosynthesis even when they have lost their leaves. Strategy for perennials - most growth during wet seasons. Drought avoidance - perennials

◊A Blue Palo Verde tree that has dropped limbs in order to conserve water. ◊(Tumamoc Hill, Tucson, April 2006) ◊A Blue Palo Verde tree that has dropped limbs in order to conserve water. ◊(Tumamoc Hill, Tucson, April 2006) Michael Frank 2006

Drought tolerance in plants Many plants have adaptations that allow them to survive and even thrive in the dry times. Succulence Succulence: The ability to store water. Cacti and other plants store water in tissues in stems (saguaro), leaves (aloe), or roots (many). Root structure Root structure: Some plants (mesquite, cottonwood) have deep roots that reach to the water table. Others have a wide network of very shallow roots just under the surface that catch all the rain in a wide area. Leaf structure Leaf structure: Many desert plants have small leaves or compound leaves (many leaflets connected together) to reduce surface area. This reduces water loss. Others shade leaf surfaces by color or hairs or a waxy surface. Some even fold their leaves up during the hot part of the day. Many plants have adaptations that allow them to survive and even thrive in the dry times. Succulence Succulence: The ability to store water. Cacti and other plants store water in tissues in stems (saguaro), leaves (aloe), or roots (many). Root structure Root structure: Some plants (mesquite, cottonwood) have deep roots that reach to the water table. Others have a wide network of very shallow roots just under the surface that catch all the rain in a wide area. Leaf structure Leaf structure: Many desert plants have small leaves or compound leaves (many leaflets connected together) to reduce surface area. This reduces water loss. Others shade leaf surfaces by color or hairs or a waxy surface. Some even fold their leaves up during the hot part of the day.

◊Compound leaves on a screwbean mesquite help reduce surface area and so reduce water loss. Michael Frank 2006

Leaves on cacti? ◊On young new pads, you can see little rubbery spikes. These are remnants of what were once leaves. Now they are coverings for the spines as they grow. Spines are also modified leaves.

Metabolic adaptations Some plants in hot or dry climates have adapted by changing how they do photosynthesis. CAM plants can keep stomates (structures on the leaf surface where transpiration happens) closed during the day, and only open them during the cooler night. This saves 90% of their water, but is less efficient, so CAM plants grow slowly. Cacti are CAM plants. Some desert grasses use the C 4 version. A few species, including some agave can switch back and forth between C 4 and CAM. Some plants in hot or dry climates have adapted by changing how they do photosynthesis. CAM plants can keep stomates (structures on the leaf surface where transpiration happens) closed during the day, and only open them during the cooler night. This saves 90% of their water, but is less efficient, so CAM plants grow slowly. Cacti are CAM plants. Some desert grasses use the C 4 version. A few species, including some agave can switch back and forth between C 4 and CAM.

One more survival adaptation Since many desert plants store water, this makes them a good source of water for animals. Many plants have adapted to this by developing spines or thorns as a defense Some (such as Catclaw Acacia - A. greggii - and other Acacia species) don’t grow thorns until an animal has browsed on them. The damage due to browsing triggers the growth of thorns in the new plant growth. Since many desert plants store water, this makes them a good source of water for animals. Many plants have adapted to this by developing spines or thorns as a defense Some (such as Catclaw Acacia - A. greggii - and other Acacia species) don’t grow thorns until an animal has browsed on them. The damage due to browsing triggers the growth of thorns in the new plant growth. Michael Frank 2006 Many also produce defensive chemicals that make them taste bad or are poisonous.

Desert adaptations - animals ◊Some animals avoid heat by being active only at night, or at dusk and dawn. They stay in cool shady places during the day. ◊Others burrow under ground to escape heat. The Round-tailed Ground Squirrel hibernates during the hot dry season (= estivation). Desert toads stay dormant underground until the rains start, then they emerge, breed, and replenish themselves for the next season. ◊Some animals avoid heat by being active only at night, or at dusk and dawn. They stay in cool shady places during the day. ◊Others burrow under ground to escape heat. The Round-tailed Ground Squirrel hibernates during the hot dry season (= estivation). Desert toads stay dormant underground until the rains start, then they emerge, breed, and replenish themselves for the next season.

Dissipating heat ◊Some animals shed the heat absorbed from their surroundings. The huge ears of jackrabbits act as radiators to give off excess heat. ◊Many desert animals are paler in color than their non-desert relatives. This helps them keep from absorbing as much heat. It also helps them hide from predators by blending in with their environment. ◊Some animals shed the heat absorbed from their surroundings. The huge ears of jackrabbits act as radiators to give off excess heat. ◊Many desert animals are paler in color than their non-desert relatives. This helps them keep from absorbing as much heat. It also helps them hide from predators by blending in with their environment.

Acquiring and retaining water ◊Desert animals often have to get their water from what they eat. They may get it from eating succulent plants, or from other animals. Most will drink water when it is available. ◊They also may have elaborate physiological adaptations that help them retain water. Reptiles and birds, for instance, excrete wastes as uric acid, which is a white solid (so little or no water is lost). Mammals excrete urea which is a water-soluble compound. This causes them to loose quite a bit of water, so they need access to a good supply of fresh water. ◊Desert animals often have to get their water from what they eat. They may get it from eating succulent plants, or from other animals. Most will drink water when it is available. ◊They also may have elaborate physiological adaptations that help them retain water. Reptiles and birds, for instance, excrete wastes as uric acid, which is a white solid (so little or no water is lost). Mammals excrete urea which is a water-soluble compound. This causes them to loose quite a bit of water, so they need access to a good supply of fresh water.

Biome comparison