Tropical rainforests, grasslands and
deserts, temperate and boreal forests, scrub and tundra. The view of this incredible
diversity of biomes has been, in this series so far, more or less confined to lowland regions.
But what happens when you take these biomes, and climb upwards, into thinner, colder air?
Plants must adapt to the colder conditions and in rising altitude, the biomes
shift in an echo of increasing latitude. Set against dramatic mountains and plateaux, plant
life leaves its mark creating living landscapes which are arguably the most stunning on
our planet, in the highlands of Earth. If you look at a map of the
elevation of Earth’s topography, you’ll find a good proportion of the land
surface is significantly above sea-level. And so in a series on biomes, it would be grave
omission to not spend some time looking at how ecosystems have evolved to cope with altitude
across these diverse regions of our planet. Due to various effects relating to heat transfer,
the troposphere, which is the lower portion of the earth’s atmosphere, cools as altitude
rises. In dry air this effect results in about 10°C of cooling per km (5.4°F/1000ft). As the
troposphere extends up to about 12km (39,000ft) above sea level, and since the highest point
of Mount Everest is some 3km below this point, this cooling effect applies to
all land surfaces on the planet. How this temperature drop affects plant life
that sits upon the higher elevations of earth is determined by the initial
temperature at sea level, and this of course, is determined by latitude,
with the hottest areas in the tropics and in subtropical deserts, and the coldest sea
level temperatures being at the poles. But regardless of the starting latitude, the
result of increasing altitude is usually to push the biome poleward in effective latitude.
For example, a temperate forest at sea level will shift toward taiga in nearby mountains,
and again to alpine tundra and eventually to a glacial icecap if the mountains are
high enough. All at the same latitude. Let’s return to our Holdridge Lifezones
chart. At last we can look at the right hand side of the diagram, the altitudinal belts
that closely mimic the latitudinal regions. So we find a general relationship between increasing
altitude and latitude in terms of effective biome. A second concept to notice is that the type of
altitude biome we will get will be determined by rainfall. As we increase in altitude, and the
temperature cools, for the same amount of rain, there will be less evapotranspiration,
allowing a higher biomass to accumulate. This explains why you often find forests upon the
slopes of hills and mountains in drier regions, while the plains below are grassland or scrub.
The chart doesn’t describe what type of forests we’ll find, however, and I’ll discuss that detail
a bit later. As always, when it comes to biomes, simple changes in atmospheric effects and climate
can lead to complex changes in ecosystems. In order to break this down, let’s take a journey
from the poles of our planet to the equator, and examine the effects of altitude in each
zone of latitude. Starting at the poles we have, at sea-level, the ice biome. The north
and south poles are covered in ice, but while the ice at the north pole, sitting
on the surface of the Arctic Ocean, is close to sea level, the ice across most of Antarctica
is at several kilometres in altitude, due to the accumulation of snow over thousands of years into
the great ice sheets that cover this continent. And this gives us our answer as to what happens
to the ice biome with increasing altitude. No surprises – it’s just more ice. Well,
actually, a bit less, as there is less snowfall in these much colder areas than can be found at
sea-level. But this doesn’t affect the overall picture of a lifeless world in these areas.
Moving into sea-level tundra areas, and these are most widely found in the arctic. If we
increase altitude here we quickly reach freezing temperatures where no plant life is possible,
and accumulations of snow produce glaciation as portions of the ice biome. The most marked example
of this is in Greenland, which lies at a similar latitude to arctic tundra, evidenced by the
fact that the coasts are all in that biome. But the world’s largest island is mostly
covered in ice, due to the accumulation of millenia of snow upon its ice-sheet. This
is likely a hangover of a past ice age, with temperatures upon the top of the sheet
remaining below freezing and preventing melting. As we move into the sea-level boreal forest,
or taiga, we find that this vast dark green ocean of conifers is peppered with higher
altitude sections that are turned to tundra. Here is the first time we see the tree-line, a
horizontal line that can be seen on mountains where trees give way to low-lying vegetation
such as grasses, dwarf shrubs, heather, sedges, moss and lichen. Trees cannot grow if summer
temperatures do not rise above 10°C, and so this tree-line is where we find this subdivision
of average summer temperature. Rising beyond the alpine tundra, we again reach glaciers and the
ice biome if the mountains are high enough. Continuing toward the equator, we reach
the temperate and subtropical zones where things become a bit more complex, because
at these latitudes we can find temperate forest, grassland, scrub or desert, depending upon
prevailing levels of rainfall and other factors. Our Holdridge Lifezones Chart navigates us through
this complexity. Simplest is the case of temperate forests. As we increase in altitude here,
broadleaf forests of beech, oak and maple give way to hardier broadleafs such as birch and aspen,
but more commonly conifers such as spruce, fir, pine and larch. Eventually, only conifers remain
in a form of alpine taiga, which eventually gives way to alpine tundra and ice if high enough. The
treeline here is often lower in altitude than the 10°C summer temperature average earlier mentioned,
however, as high-winds at these higher elevations often prevent the development of trees. An example
of this are the heather-covered uplands of Great Britain that were deforested millennia ago,
and have never recovered despite being unused for agriculture, due to constant high winds that
blow across these regions for most of the year. Trees on the border of alpine tree line zones
tend to have twisted trunks and stunted growth when exposed to constant freezing winds and
this type of landscape is called krummholz. In the case of grassland and scrub at
sea-level, as earlier mention, slopes of hills and mountains in these regions tend to
be tree-covered as evapotranspiration falls with reducing temperature. These “montane” forests are
composed of mixed broadleaf and conifer species such as ponderosa pine, juniper and evergreen oak.
Above the treeline, these forests yield once again to a form of alpine tundra, although this is
usually very sparse due to the comparatively low precipitation. And if high enough,
glaciers will be found on mountain tops. The largest of the temperate latitude alpine
tundra areas are the vast and barren steppes of the Tibetan plateau, with this region having
an average altitude of 4,500m (14,800ft). Sea-level deserts offer the only departure
from the general pattern of alpine slopes having forests. Semi-desert, like grassland and
scrub, gives way to forest at higher elevations, but particularly dry deserts, such
as those of the Atacama in Chile, are so devoid of moisture, that no forests
occur, or even tundra for that matter, with just a thin dusting of snow at the highest
peaks of these areas which are accumulations of occasional precipitation that has remained
frozen for centuries or even millennia. But it’s upon our arrival in the tropics where
the most interesting effects of altitude upon biomes occur. The fundamental reason for this
is the lack of seasonal temperature changes. In temperate regions, changes in seasonal
temperature yield frosts in the mountains as well as at sea-level, and so we tend to find
similar types of trees and shrubs in both. But in the tropics, one can rise in altitude
into a temperate climate that, unlike climates of the temperate latitudes has little seasonal
temperature change, and importantly, no frost, which dramatically affects the type of plants
that can grow here. Such is the uniqueness of this climate zone, that I devoted a whole video
to it in my Secrets of World Climate series, and briefly spoke of the unique plant
life that can be found in such regions. In the tropics, we find, at sea-level, tropical
rainforest, tropical seasonal (or “dry”) forest, or savannah, depending upon the
amount and annual distribution of rainfall. At higher elevations in all
of these cases, we’ll find some kind of specialised forest composed of various
broadleaf tree species as well as palms. In the case of savannah, less evapotranspiration
due to cooler temperatures increases the covering of trees, while the tropical forests morph
in terms of the type of species present. What species occur depends upon the continent, but
like their sea-level companions, fall into three broad groups of Neotropical in Central and South
America, African and Malesian in South-East Asia. Tropical Rainforest often gives way to Cloud
Forests such as those found in Ecuador and Costa Rica, which, in addition to regular
rainfall, receive water in the form of fog drip. The broadleaf trees in these forests are usually
not as tall as their sea-level counterparts, but often have thicker and more gnarled trunks.
They are also referred to as Mossy Forests owing to the moss that covers practically every surface
owing to the saturated water passing through the forest in the form of fog. In nearby Colombia,
we find the world’s tallest palms – the Wax Palms of Cocora that rise up to 60m in height.
In fact, this country, due to its large areas of subtropical highland climate, is believed
to have more species of palms than any other. Above the treeline, however, things become even
more unusual as we enter a subtropical grassland, moorland or heathland, depending upon
region. For in these tropical regions, the altitude where the temperature drops
below 10°C that prevents tree growth, is very high. This results in two effects
– the first being much thinner air than in comparable alpine tundra, the second being
very high day-night temperature variation. And, as mentioned, there are no seasons in
regard to temperature. So plants found here are uniquely adapted and frankly don’t look
like plants found anywhere else in the world. An example of these almost alien-like landscapes
can be found on the slopes of Mount Kenya in East Africa. A more “normal” form is the Puna
Grassland that is found in the most extensive of these subtropical highland areas which occur
on the vast raised Andean Altiplano of Peru and Bolivia as well as adjoining highland
areas in Ecuador, Argentina and Chile. In conclusion of this episode and of all
the land biomes covered in this series, it is perhaps fitting to look at Africa’s mightiest,
and most famous peak, the giant Mount Kilimanjaro. This dormant volcano rises to almost 6km
(actually 5,895 metres, 19,341 ft) above sea level and almost 5km (4,900 metres,16,100 ft)
above its base on the East African plateau. In those five vertical kilometres, we go from
savannah grassland (which is now largely farmed), through tropical then temperate montane forests,
moorland like that just described on Mount Kenya, and then an almost desert-like tundra zone,
before reaching the glaciers on its summit. Very few places in the world
allow someone to experience almost the full gamut of our planet’s
biomes in a short trip. Kilimanjaro, then, really is like our planet in miniature -
diverse, fascinating, awe-inspiring and majestic. And that’s the highlands. I hope you enjoyed this
overview of the biomes of the high places of our world. If you did please hit the like button,
and let me know your thoughts in the comments. I’d especially love to hear from you if you live
up in one of these regions. Don’t forget to check out the website. I have provided a link in
the description to this particular video with the full transcript and high-resolution maps
available. If you haven’t already done so, then please hit the subscribe button so you don’t miss
future episodes. Thanks again for watching, and I’ll see you in the next video, where we at last
leave behind the land and head out onto the seas.