Deserts
Climate
Distribution and subtypes
Landscape-substrate-soils
Limiting factors
Adaptations to aridity
Adaptations to high temperatures
Definitions
• Boundary defined by absolute precipitation
(e.g. <4 inches or 100 mm mean annual
precipitation). Not an adequate definition.
• Boundary defined by intensity of moisture
deficit (e.g. ratio of available precipitation
to evaporative demand; see, for example,
the Budyko-Lettau dryness ratio)
Budyko-Lettau dryness ratio
D = R / (L x P)
where D
R
P
L
=
=
=
=
dryness ratio;
mean ann. net radiation;
mean ann. precipitation;
latent heat of vaporization of water
Original definition:
D > 2.3 = semi-desert; D ≥ 3.4 = desert.
UNESCO: D ≥ 10 = “extreme desert”
Dryness ratio
8
7
semiarid
(D≥2.3)
desert
(D≥10)
Unpredictability:
areas with >30% variability in
precipitation
Causes of regional aridity
• Persistent atmospheric subsidence associated with
the presence of sub-tropical anticyclones
suppresses convectional activity (e.g. Sahara,
Kalahari, Australia)
• Absence of humid airstreams (e.g. Gobi)
• Localized subsidence in rain-shadow areas (e.g.
Great Basin)
• Absence of cyclonic disturbances (e.g. Sonora)
• Inhibition of convectional activity by cold coastal
currents (e.g. Atacama, Namib)
“Coastal fog deserts”
E.g. Atacama desert
Namib desert
Temperature
Arid areas that are subject to subfreezing conditions (e.g. Gobi, Great
Basin) are “cold” deserts.
Areas where air temperatures seldom
or never fall below 0°C are “hot”
deserts (e.g. Sonora, Sahara)
“cold desert”
sagebrush steppe
(Idaho-eastern Oregon)
“Hot
desert”:
saguaro ocotillo
community,
Arizona
Sonoran cacti:
saguaro
(Carnegiea
gigantea)
range
controlled by
exposure to
freezing
temperatures
Sonoran plant ranges
Thermal microclimates
Desert landscape-substrate
elements
upland /
hillslope
pediment/bajada/reg
solutes
skeletal
soils
gravelly
soils
dunefield/
deflation erg
playa - sabkha
deposition
sandy silts
sands
Uplands of southern Baja
“a wilderness of thorn and rock”
Desert upland habitat:
skeletal soils
Uplands and bajada complex,
Death Valley, CA
Mesquite growing on gravelly
fan deposits, Arizona
Playa deposits
Mobile substrates:
Saharan dunes
Limiting factors
Five interlinked “stresses” for biological community:
REGIONAL
1. “permanent’ moisture deficit;
2. high surface temperatures during day; large diurnal
variation;
3. highly irregular and variable moisture supply;
LOCAL
4. mobile substrates; and
5. saline substrates
Limitation of primary productivity in
arid ecosystems (data from Tunisia)
Mean ann. rainfall (mm)
500
400
skeletal soils of uplands
300
200
gravelly soils
of pediments and
alluvial fans
100
0
0.01
0.1
1.0
Net primary production (kg ha-1 yr-1)
10
Forms of adaptation to
stresses in arid environments
Four strategies:
1. minimize heat intake or maximize heat
outflows;
2. maximize food reserves in times of plenty;
3. maximize water inflows; and
4. minimize water outflows
Evasion tactics to minimize
exposure to heat and drought
1. Organism dormant for substantial part of life-cycle:
e.g. ephemeral plants, some reptiles, most insects
persist through extended droughts as seeds, eggs,
or larvae (only the reproductive forms remain).
2. Nocturnal or crepuscular foraging (hottest parts of
day spent in burrows or shade).
[Is crassulacean acid metabolism an equivalent tactic
for succulent plants? CAM - stomates open at night;
CO2 absorbed, assimilated during day when stomates
closed to minimize water loss]
Desert ephemeral flora
• Large seedbanks (esp. in sites protected from wind,
e.g. around base of bushes; 100,000 seeds m-2);
• Long seed viability in dry soils;
• Rapid germination if rainfall sufficient (signalled by
leaching of inhibitors in seed coats [e.g. only rains
>25mm in Arizona produce germination] or
scarification of thick seed coat in flash floods)
• Short time [6-8 weeks] to seed-set;
• Some species heteroblastic [produce seeds with
varying germination requirements].
Sonoran desert in bloom
Refuging tactics:
chuckwalla lizard
How much cooler
is it at a depth
of 20 cm?
Refuging behaviour:
camels in shade
Refuging by desert aquatic species
e.g. pupfish in Death Valley
desert pupfish
winter
flow
live
summer
winter
salinity
water temp.
lay eggs
/die
hatch
Salt Creek, Death Valley
Reducing
heat load
• Low surfacearea/volume ratio;
• Reflective skin/bark
(colour changes in
lizards)
• Vertical shoot-body
architecture
Fouquieria/Idria columnaris
(the ‘cirio’ of central Baja)
Reducing heat
load:
a joshua tree
(Yucca
brevifolia)
in the Mojave
desert
Maximise food reserves in
times of plenty
• camel’s hump;
• berber sheep
(fat reserves in tail
vary from 2-10 kg);
• pack rats/gerbils
hoard seeds;
• succulents store
water.
Maximise water inflows
• extensive lateral (cacti) or vertical
(mesquite) roots;
• rapid root growth after rains
• beetles in Namib desert stand on hind legs
to catch fog droplets on raised abdomen;
• mice in Arizona often feed on low-protein
herbage with high water content
• camels can drink 100 L of water in 10 min!
• practice opportunistic migration to water
and food sources (desert locusts, nomadic
pastoralists)
Shrub/tree root
patterns, Arizona
Plant spacing
determined
by moisture
availability
and rooting
niche
40
Water conservation:
expandable storage organs and
palisade tissue in succulents
Tissue protection:
thorns and spines
Minimize water loss
1. Transpiration reduced in desert plants by
microphylly, deciduousness, sunken stomata, waxy or
pubescent leaves.
2. Water loss in desert fauna reduced by dry faeces,
low urine prodcution, low dilution of uric acid,
adaptive hyperthermia (camel’s body temperature
can vary by 6°C when animal is dehydrated).
3. Tolerate dessication: camel can withstand water
loss = 25% of body weight
Reducing water loss:
microphylly, deciduousness,
photosynthetic bark and shoots
Pachycormus
discolor
Reducing water loss: microphylly in ocotillo
(Fouquieria splendens)
Evidence of
climate
change:
lake levels in
the Great
Basin
Evidence
of climate
change
from
pack-rat
middens
QuickTime™ and a
TIFF (LZW) decompressor
are needed to see this picture.
Pink = desert; yellow=savanna grassland; brown=dry forest
NB Lake Mega-Chad (bigger than Caspian Sea at present,
and at least 40 m deep)
http://www.uni-mannheim.de/phygeo/8000BP.htm
Evidence for late
Holocene climate
change in the
Sahara
Lake sediments and pollen
Rock drawings, Tassili
Desertification in progress?
Rainfall in the Sahel zone of W Africa
Protracted drought
Cause: ?
Effect: desertification?
Desertification on savanna margins:
Rapp’s albedo hypothesis
Settlement
Nomadism
+
Grazing
Vegetation
-
-
+
Grazing
+
Vegetation
+
Albedo
Albedo
Convectional
rainfall
+
Convectional
rainfall
The evidence
for Rapp’s
model
The elements
of
anthropogenic
desertification