Tropical Evergreen and Seasonal Forests - Biomes Episode 1 - Lake Harding Association

Tropical Evergreen and Seasonal Forests – Biomes Episode 1

Tropical Evergreen and Seasonal Forests – Biomes Episode 1

By Micah Moen 13 Comments October 11, 2019

It is fitting that we begin our journey across
the biomes of Earth with the one that we perhaps hold the most dear in our hearts. The greatest biodiversity of any area of earth
with over half of all plant and animal species. The lungs of the world – absorbing more
CO2 and producing more oxygen than any other. A product of constant heat and abundant rain,
it’s no wonder that plants grow here more vigorously than in any other biome. But it is also the biome that is under the
greatest threat. Known as jungle, selva or rainforest, these
regions are in fact two distinct biomes – the evergreen and seasonal forests of the tropics. Encircling the equatorial regions of our planet,
conditions of constant heat, and plentiful rainfall produce the richest biomes of all,
since such conditions are the optimum for plant growth. The biomes of the tropical evergreen and seasonal
forests have the same basic components of tall broadleaved, hardwood trees dominating
the biomass. But beyond this, their characters diverge
out from that essential character. The presence of evergreen or seasonal forest
is determined mainly by the pattern of rainfall throughout the year. When there is a minimal to no dry season or
where extensive flooding occurs after a wet season to help the trees through the dry,
the trees retain their leaves and continue growing throughout the year. When there is a noticeable dry season and
no flooding the trees shed their leaves to retain internal moisture, and cease growing. This is why they are known scientifically
as tropical evergreen and seasonal forest. In popular understanding, we think of these
as the rainforest, but it should be noted, as we can see in Holdridge’s Lifezones chart,
that rainforests can also exist in more temperate latitudes where cooler temperatures prevail. This same chart also shows that as overall
rainfall is reduced in the tropical band, rainforest gives way to progressively drier
forests, and then to trees interspersed with grasses and shrubs, that we can no longer
truly call forest, but is its own concept – that of the tropical savannah. We’ll pick that one up in the next episode. For now, then, let’s address the forests
of the tropical regions, and consider their common elements. If we look at a cross-section of a typical
tropical forest, we will find four distinct layers. Starting at the ground, and we have the forest
floor, which is littered with fallen vegetation from upper layers. In the hot and wet conditions, and in the
presence of huge numbers of scavenger insects, millipedes and the like, this litter is reduced
rapidly. Combined with often heavy rainfall, this soil
is further leached of nutrients, so despite the biodiversity going on above ground, the
soil is remarkably poor. Due to the extensive leaf canopy above, only
about 2% of sunlight reaches the ground in rainforest, and so undergrowth is generally
quite sparse here, with selected low-lying shrubs with extra-large leaves battling to
receive as much light as they can in the dim conditions. As we move out into seasonal forest, the canopy
thins out, permitting more light to reach the ground, and with consequently more undergrowth. This trend continues until proper savannah
conditions exist – a mix of trees interspersed at the ground level with thickets of shrubs
and an abundance of grass. Above ground but under the canopy, we have
the understorey, where still only 5% of light can penetrate the canopy in rainforest. Here are found shrubs as well as seedlings
of the main tree species which will fully develop if a gap opens in the canopy due to
the fall of another tree. At around 20-40m in height we arrive at the
canopy itself – an almost continuous layer of leafed branches of the main tree species
that have grown to this height in competition with other trees for sunlight. This layer will absorb over 90% of the light
falling on the forest. It has by far the greatest biodiversity of
all the layers, and it is estimated that it is home to about half of all plant species
on earth, since the trees themselves are usually covered in other plants, such as air-rooted
orchids, and ground-rooted liana, or vines, that usually feed upon the host tree. Considered parasitic, these liana often connect
trees together to act as highways for wildlife to move from tree to tree – from ants and
lizards to rodents, sloths and monkeys. This layer, comprising most of the life of
the rainforest, makes this biome unique among all on earth, as being the only one that is
so distinctly far above ground. The last layer, where very tall trees extend
beyond the main canopy, growing up to 70 metres or more, is known as the emergent layer. Such trees, while basking in the fullest sunshine,
have to be resilient to the fierce tropical sun, as well as resisting strong winds which
are commonly found at this height. The tropical forests are believed to be the
oldest of all earth’s biomes still in existence, being as much as 100 million years old. Compare this to the temperate forests at about
60 million years, and temperate grasslands at only 25 million years in origin. So, where in the world do we find the tropical
forests? As clued by their name, they occur only in
the tropics, between 23 degrees north and 23 degrees south, where high temperature exist
year round. In general there is a close relationship to
the evergreen forests and the Tropical Rainforest (Af) and Tropical Monsoon (Am) Koppen climate
zones, that have rain year round or for most of the year. Seasonal Forest is generally found on the
periphery of these Koppen zones and most commonly within the Koppen Tropical Savannah (Aw) zone,
which has longer or more severe dry seasons. There are exceptions to this relationship,
some of which can be explained by extensive flooding from the wet season sustaining growth
and leaf retention throughout the dry season. In other cases, higher altitudes, such as
in SE Brazil, Colombia and Ecuador push the climate zone into a Subtropical Highland type
of warm instead of hot temperatures year round, which is nonetheless still favourable to trees
maintaining their leaves throughout the year. So, region by region, and starting in the
Americas, tropical forests dominate Central America, with southern and western Mexico
having seasonal forest, and as we travel further south, these become evergreen, all the way
to the Pacific coasts of Colombia and Ecuador. Most of the Caribbean islands have either
evergreen or seasonal forests on them, wherever they are exposed to moist rain-bearing winds. South America has the most extensive tropical
forests in the world, with the Amazon basin being the most famous, covering an area equivalent
to the continent of Europe. Most of this is within Brazil, but it extends
into the adjoining countries of the Guyanas, Venezuela, Colombia, Ecuador, Peru and Bolivia. The vast majority of this is evergreen, with
seasonal forest on the periphery as it morphs into the Savannahs of Central Brazil, Venezuela
and Bolivia. South Eastern Brazil has extensive seasonal
forest, which becomes evergreen along much of the coast owing to moist trade winds year
round. Moving onto Africa, and we have the home of
the second largest contiguous rainforest – that of the Congo basin. The entire coast of West Africa from Nigeria
to Senegal has evergreen forest which, like the Congo, has an edge of seasonal forest
before it transforms gradually into the extensive savannahs that dominate the continent. Evergreen and seasonal forests also exist
in parts of Ethiopia where a subtropical highland climate exists, and the western and northern
coasts of Madagascar, where favourable trade winds deliver plenty of rain year round. Onto Asia, now, and the Indian Subcontinent,
which would have the largest area of tropical seasonal forest in the world if so much of
it had not been cut down for agricultural purposes to feed the world’s second largest
population. This natural vegetation type dominates here
due to the dramatic shifts in wet and dry seasons brought by the Indian Monsoon, the
world’s most extreme. Evergreen forests exist on the western coast
of India where the monsoon rain is so high that the ground stays wet for most of the
year. Flooding in the lower Ganges basin throughout
the dry season also keeps most of Bangladesh in the natural evergreen type, although again,
most of these forests are gone due to extensive agriculture to feed its massive population. The island of Sri Lanka is home mostly to
evergreen forests due to its exposure to moist trade winds all year round. The South East Asian nations of Myanmar, Thailand,
Laos, Cambodia and Vietnam are home to a mix of seasonal and evergreen forests depending
upon the drainage and retention of water as all these countries experience a significant
dry season. The southern coasts of China all the way to
Taiwan also have a mix of tropical forest types. As we move into the SE Asian archipelagos
of the Philippines, Malaysia and Indonesia, however, the evergreen forest becomes dominant,
and this, collectively, makes up the third largest rainforest in the world, with much
of it still in its natural state, particularly on the world’s second and third largest
islands – New Guinea and Borneo. As we move into Oceania, and we have a narrow
band of rainforest existing along the northern and north-western coasts of Australia, where
exposure to moist trade winds leads to plenty of rain allowing favourable conditions for
evergreen and seasonal growth of tropical forest. Lastly, we come to the Pacific, and all islands
that lie within the tropical zone have evergreen forests, from the Solomon Islands to Fiji,
Micronesia and certain coasts of the Hawaiian Islands that are exposed to moist trade winds
for much of the year. As is well known, the tropical rainforests
are the most biodiverse habitats on Earth, and this biodiversity runs across all kingdoms
of plants and animals. As an example in one area of Malaysia of less
than a quarter of a square kilometre, 375 different tree species with significant trunk
diameters were recorded. Most plants, including almost all of the trees
are in the angiosperm group, namely, those plants that produce flowers. Among these are the more famous hardwoods
such as mahogany, teak, ebony and rosewood. Hundreds of species within the palm family
are also common. Coniferous trees, such as pine etc., on the
other hand, are relatively rare in these parts. When we move into the seasonal or “dry”
forest, where trees lose their leaves in the dry season, biodiversity is significantly
less, and in contrast to the multi-species canopies of the rainforest, often the forests
can be dominated by a single species over wide areas, such as the teak forests of Myanmar. Tree species distribution varies geographically,
and is roughly divided into three subkingdoms. The Neotropical, African and Malesian. In Central and South America, mahogany, cedar,
myrtle, laurel, palm, acacia, rosewood and Brazil nut are common. In Africa we find mahogany again, along with
ebony, limba, wenge, agba, iroko and sapele. In SE Asia, in addition to the well known
teak, a significant proportion of trees belong to the dipterocarpus family, while others
include durian, sandalwood and ironwood. In closing, we come, sadly, to the issue of
the destruction of this biome – one practically all of us are aware of, especially in a year
of unprecedented burning of forests in Brazil and Bolivia. At one time logging of prized tropical hardwoods
was considered the greater threat. But, like so many other biomes worldwide,
it is farming that is now the greatest threat to forests of the tropics. In Bolivia and Brazil, illegally burning the
land to then claim it as “dead” land that can be used for cattle ranching is being carried
out to circumvent the usual rules that prevent such claims. While in South East Asia, habitat destruction
is now at a scale never been seen, much of it being due to the demand for palm oil. I have seen with my own eyes, from the air,
vast swathes of the forests of Indonesia and Malaysia turned into the hexagonal monoculture
of palm oil plantations. Many of you might think, no it’s not possible
that the rainforests could disappear – they’re too big, and just too precious. Well, if you need any precedent for this,
look at the forests that once covered England, or India, or the prairies that once covered
a dozen states of the USA. You can’t, because they’re not there anymore. Destruction of natural habitats is very much
in our history, stretching back thousands of years. And unless we learn how to accommodate the
needs of a growing population throughout the tropics, then we may lose the most precious
biome of all. I hope you enjoyed this tour of the forests
of the tropics. If you did, please like and share this video,
share your thoughts in the comments and ensure you’re subscribed to my channel, so you
can get notified when new releases occur. Thanks again for watching, and I’ll see
you in the next of the biomes, the Savannah.

13 Comments found





Cool Blue Tunes

huizaches trespelaques


Uneti Tree



Dante Garafalo

Great video! Can you make a video about Florida’s climate, specifically south Florida and its biomes? Thank you!


Bruno Posavec

I love your videos so much! They're so well made and relaxing, it's like I'm watching a super big-budget documentary. Looking forward to seeing more videos! πŸ™‚



Ahhh, Finally!

I've been eagerly waiting for this


Nostalgic memories

ahh dude i forgot about your channel, just today reinvented your videos :)) such a quality content, and still dont have followers numbers you deserve



Could you do a video on Tropical Coniferous Forest? I'm not familiar with it and it's strange to me



I loved the end. It was definetly a very nice way to wrap up


Keyboard Warrior

Isnt the ocean the lungs


Richard Torres

Excellent work as always πŸ‘πŸ‘πŸ‘, I loved it, I'm dying to see the following biomes. These forests being so important in terms of biodiversity and balance for the planet, our mission must be apart from admiring, protecting and caring for them. Thank you for highlighting the importance and beauty of these jewels.


Lazy Travelers

very informative post


C. E. Torchia

Nice music. Human greed is destroying many ecosystems.


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