This is what the bare earth looks like, the
bones of our planet revealed as the skin of vegetation is peeled back. Despite the lack of life, these landscapes
nonetheless inspire us with their stark beauty. A product of endless sunshine and droughts,
life yet still finds a way here, for plants to survive in these regions have gotten smart. One of the most extreme biomes on our world,
from the Sahara to the Sonora, the Gibson to the Gobi, these are the deserts of planet
Earth. Deserts are among the most open form of country
on earth, where vegetation is sparse or effectively absent. The cause of this is simple enough – it’s
just a lack of water. Plants, like all forms of life, need that
universal solvent, and when it’s sparse, they struggle, and in order to survive, they
must get creative. Deserts exhibit some of the most interesting
and strange forms of plant life to be found anywhere in the world. The strict definition of deserts in the biome
classification system we’re using in this series, known as LONS08, is any biome containing
bare earth, either in part between grasses, shrubs or trees, or completely bare. This distinguishes the deserts from other
desert like biomes, such as scrub, since the latter will always have a complete covering
of grass at ground level between the shrubs. But while shrubs and grasses, and occasionally
even trees, are also often present in deserts, they are separated by sections of bare earth. Deserts occur over extensive parts of the
continental surface of the earth, present on every continent, and are in fact the largest
biome in terms of land area. They are basically subdivided into four forms,
hot and cold, arid and semi-arid. Arid deserts have practically no vegetation,
while semi-arid regions contain a mix of plants separated by bare earth as described a moment
ago. Hot deserts occur in the subtropics and are
characterised by mild winters and very hot summers, while cold deserts occur in the temperate
latitudes and although they still have relatively hot summers, experience winters that can be
brutally cold. These elements, hot and cold, arid and semi-arid
are mixed up in various ways to produce biomes that are unique within the greater sphere
of deserts. The correlation between hot and cold deserts
and their corresponding Koppen climate zones is very clear. Hot desert areas occur in regions defined
by the Koppen Hot Arid (BWh) and Hot Semi-Arid (BSh) zones, while cold deserts are found
in the Koppen Cool Arid (BWk) and Cool Semi-Arid (BSk) zones. The climate phenomena and causes of these
four basic types are discussed at length in the two episodes of my Secrets of World Climate
series that relate to deserts, which you’ll find linked top right. If we consult our Holdridge Lifezones chart,
we can find the deserts at the extreme left of the diagram, where rainfall is at its lowest. Note that the deserts span every latitude
of earth, from the tropics to the poles. Although parts of Antarctica are the driest
on Earth, these regions are classified within the Ice biome since the perpetually cold temperatures
on that continent are considered to be the defining characteristic over any localised
absences of precipitation. The aridity index – the ratio of evapotranspiration
to available water - is at its highest anywhere on earth in the hot deserts of the subtropics,
where searing temperatures combine with a near-absence of rain to make any plant growing
extremely difficult. The deserts are the only biome where, mostly
or completely devoid of vegetation, the bones of our planet are laid bare. These "bones" come in many forms. Astronomically, earth is regarded as a rocky
planet, and beneath any vegetation, or oceans, its crust is some form of rock, usually oxides
of silicon mixed with other minerals. These other minerals, and the way in which
these rocks were formed beneath the earth, give them different colours. So where you see deserts, these exposed rocks
reveal their many colours, from the white of sodium and calcium salts, to the red of
iron, the black of basalt, and the beige of sandstone, the colour most commonly associated
with deserts. Lacking cloud cover, deserts have high ranges
in temperature from night to day. In a process called weathering, these temperature
changes stress the rock where it is exposed, and eventually break it up, depositing it
in heaps of shattered rock below, known as scree. Additionally, with little root vegetation
to hold onto it, desert winds, laden with sand, further wear at the rocks to create
more sand, and in places, this sand gathers in enormous dunes. Dunes spread out over a wide area are referred
to as ergs, and some of these ergs can stretch for thousands of square kilometres, such as
those found in the Taklamakan desert of Central Asia. Salt is a common feature of this biome. Rivers running into deserts very often end
there, due to evaporation from the intense sun, and as they dry out, they deposit any
salt dissolved within. In geological times past, ancient seas once
connected to the oceans were cut off, and, within the parched desert climate, dried out
to form salt lakes. So very often the soil is salty, and as a
consequence many desert plants have adapted by becoming highly salt tolerant and such
plants are called halophytes, after the Greek word “halas” for salt. Plants that can withstand extensive droughts,
on the other hand, are called xerophytes, after the Greek word “xeros” for dry. The mechanisms of how such plants deal with
drought is numerous, but all centre around retaining as much moisture as possible from
the infrequent rains that might come their way. Moisture in plants is lost through leaves,
and in particular through stomata, the breathing holes in leaves that exchange CO2 for oxygen. Since sunlight is readily available in deserts,
small leaves are standard for desert plants, since these leaves provide enough for photosynthesis,
but are kept small to reduce moisture loss. Others keep their stomata closed during droughts,
preventing growth, but also greatly reducing moisture loss. Another adaptation is storing as much water
as possible within the plant itself, and this is the defining characteristic of the class
of plants known as Succulents, which have thick fleshy stems and leaves. The most well-known sub-class of the succulents
is, of course, the cactus family. Despite being only found only in the Americas,
cacti are regarded as the iconic desert plant, and could be mistaken for naturally occurring
throughout the world, when they are not. Cacti’s excellent ability to store water
is well-known – in the case of the Saguaro cactus, one of the largest, a fully grown
specimen can soak up to 200 gallons of water after rainfall. In these brief interludes between the long
droughts, when rain comes to the desert, the plants therein frantically attempt to reproduce,
and so a desert will bloom in a riot of wildflower colour after a rainstorm, transforming the
landscape overnight. So where in the world do we find the deserts? Starting in North America, in the Western
half of the United States we have the extensive Great Basin cold desert of Nevada and Utah,
while further south, and at lower altitudes, we have the hot deserts of the Mojave, Sonoran
and Chihuahuan deserts extending from Southern California, through Arizona and New Mexico,
down into most of north and western Mexico. In South America, we have two principle desert
regions. Practically the entire long coastline of Peru
is extremely arid desert, second only to the Atacama of Chile, the driest of all deserts. Further south and on the opposite side of
the Andes, we have the hot semi-arid deserts of North-western Argentina that continue south
down to the tip of that continent as the cold desert of Patagonia. Skipping across the South Atlantic and we
reach Namibia and Western South Africa, where the arid Namib desert of the coast extends
inland to the more semi-arid landscapes of the Kalahari bushland. Heading north, and in Europe, we can find
some localised semi-arid regions in Eastern and Southern Spain, while the Canary Islands
to the south west are mostly desert at the lower elevations. East of the Canaries and we arrive at the
world’s largest, and most famous desert, the Sahara. Most of this hot desert is so dry that it
supports almost no vegetation, with semi arid conditions occurring on the northern and southern
fringes. This desert continues east into the Horn of
Africa, and the Arabian and Syrian deserts of the Middle East, which have alternately
arid and semi arid conditions. This region is further connected to the Iranian
plateau where cool deserts are found, and east again into the Baluchistan desert bordering
Pakistan and ending in the Thar desert of North-west India. Connected to the North of this region are
the cool deserts of Central Asia, with the Karakum of Turkmenistan being among the most
arid. The semi arid conditions north of here encompass
most of Kazakhstan and where these merge into the Russian steppes defines the actual border
of these two large countries. East over the Altai mountains, and we find
the Taklamakan desert of Western China, one of the most forbidding of all deserts, such
is its aridity and freezing winter temperatures. This desert is connected to the north east
to the most famous of cold deserts, the Gobi of Mongolia and northern China. Lastly, in Australia, the driest of all continents,
we have semi-arid conditions making up most of this country. Extending from the coasts of Western Australia,
through the “red centre” to the fringes of the savannahs that run down the eastern
side of the continent. In contrast to the isolated regional development
of species in the Shrublands that we discussed in the last episode, deserts have much more
extensive contiguous ranges, allowing individual species to spread over wide areas. Several plant families have colonised most
of the globe including that of the Daisy family and the Frankenia family of flowering shrubs. The Artemisia family are found across all
northern hemisphere deserts, including sagebrush and wormwood, while the Chenopodium and Atriplex
families are both referred to, rather confusingly, as saltbushes, and are found in every continent. If we look at regional variants, and firstly
in South America, west of the Andes along the Peruvian coast and down to the Atacama
desert of Chile, rainfall is so minimal that practically nothing grows. East of the Andes in Patagonia, however, we
have extensive semi-desert populated by low-lying shrubs such as Nassauvia and Chuquiraga, along
with Brachyclados daisies and Burkartia. The largest contiguous desert region connects
the Sahara of Africa to the Middle-East, Central Asia and India, and so species here tend to
be spread across this region, always on the fringes of the extreme desert interiors where
very little grows. Wormwood (Artemisia), Tamarisk and Nitre bushes
are the most common shrubs, while Bean caper, Atriplex and Feathergrass make up other common
species. Date Palms are perhaps the most significant,
being one of the few sources of food for humans in the desert areas of the Middle East, as
they have been for thousands of years. In the Namib and Kalahari deserts of South-West
Africa, we find many aloes including the Quiver tree (Aloidendron dichotomum) and Milk Bush
(Euphorbia virosa) and the uniquely strange Weltwischia. In Australia, in addition to Spinifex grasses
that were mentioned in the Shrublands, we find the Bush tomato, the Cabbage palm (Livistona
mariae), and the Desert Oak (Allocasuarina decaisneana). Lastly, in North America, we find perhaps
the most iconic of all desert plants. While Sagebrush (Artemisia) is found all across
the Great Basin of the American cool desert, further south we find the Saguaro Cactus ranging
from Arizona down into the Sonoran desert of Mexico. And then there’s the Blue Agave, now grown
commercially for its juice that powers that most famous of all Mexican drinks, Tequila. In parts of Califonia we find the Joshua Tree,
a type of Yucca, while Creosote Bushes are extensively ranged across the hot deserts
in this region. Clonal colonies of Creosote Bushes are among
the oldest living organisms known, with one colony, deriving from a single plant thought
to be over 11,000 years old. Talking of age, and the high altitude desert
of California is home to the oldest trees in the world. The Bristlecone Pine is incredibly hardy,
and one individual tree, known as Methuselah, is believed to be almost 5,000 years old. Just think about that for a second – it
was already a thousand years old when the Pharoahs of Egypt rose to prominence, three
millennia old when Ancient Rome was at its peak, and four and a half thousand when Europeans
first travelled to the Americas. When it comes to age, we are truly humbled
by the trees of this world. Due to the limited supply of vegetation, deserts
do not support a large amount of wildlife, and that which is present is mostly in the
form of snakes, scorpions and the like. Something to bear in mind if you’re camping
out in the desert, and have some unwelcome guests snuggling up in your warm sleeping
bag at night, or making a home of your boots the following morning. Despite the suppression of plant life by the
arid conditions of deserts, this biome has a special place in the minds of so many of
us, possessing a beauty that is both stark and so different to the rest of the much greener
biomes. Deserts have inspired artists and writers
for centuries, from the Arabian Nights that shaped western story literature, to the innumerable
books, TV series and movies of the Western genre, and as such, the deserts are the only
biome to have become the central feature of a whole class of popular culture. And that is the deserts. I hope you enjoyed this journey into the parched
lands of our planet. If you did, please like and share this video,
and let me know your thoughts in the comments. Don’t forget to subscribe, so you don’t
miss future episodes. Thanks again for watching, and for the next
episode, it’s time to giddy-up, as we venture into the horse latitudes of the grasslands.