An introduction to the tropics and tropical forests

This is the start of a series of blog posts about the tropics and tropical forests, this is my favourite ecosystem. It is green and lush and is known for its incredible biodiversity. Let’s start with some quick facts to put the great biodiversity in perspective; You can find 473 tree species per hectare in an Ecuadorian forest, 365 vascular plant species in 0.1 hectares in Ecuador, 19 individuals of one tree species (Luehea seemannii) harboured 955 species of beetles, 15km2 La Selva Reserve in Costa Rica has 4000 species of moths. 

As this is the first in the series we are going to stick to the basics and look what the tropics are, what shapes the tropics and the type of ecosystems you can find. Further in the series, we will look at the origin and evolution, the reason for the high biodiversity, the differences between the regions and the human activity.

The tropics

The therm tropics is sometimes used in a general sense for a tropical climate to mean warm to hot and moist year-round, often with the sense of lush vegetation. But ‘the tropics’ is actually the full region found between the tropics of cancer (23.5°N) and the tropics of Capricorn (23.5°N). Which makes 36% of the earth’ surface. Or 20% of the ocean’s surface and 42% of the land surface. Although I wish this could all largely be covered with tropical forests, it actually is only 6 to 7% of the earth’s surface.

The tropics are distinguished from the other climatic and biotic regions of Earth, which are the middle latitudes and the polar regions on either side of the equatorial zone.

What shapes the tropics?

The tropics is shaped by 4 main factors: (1) shape and tilt of the earth, (2) climate, (3) geography and (4) anthropogenic activities.

Due to the shape and tilt of the earth, the sun is directly overhead the equator which makes the tropics hotter than temperate regions. Resulting in a mean temperature being above 18°C and the absence of a cold season.

The climate of the tropics

Not all of the tropics is green and lush and filled with impenetrable forests. It ranges from areas with up to 4 meters of precipitation to savannah that receives only a small amount of rain per month and precipitation free seasons. From lush forests to never-ending grasslands, and everything in between.

The tropics can be divided into 3 regions when you follow the Köppen climate classification. The first letter indicates the region, in this case, it is A. The second letter indicates the seasonal precipitation type.  This means that the tropics are divided in according to the amount of precipitation they receive.  Summers are defined as the 6-month period that is warmer either from April–September and/or October–March. While winter is the 6-month period that is cooler. Later in the blog I will tell more about the type of ecosystems that are found in the climate regions.

  • Af is the tropical rainforest climate, it receives precipitation in every month and at least 60 mm. But this is the minimum, and most places receive double or triple the amount. This climate has no natural seasons in terms of temperature or moisture changes.
  • Am is the tropical monsoon climate, it experiences seasons regarding the amount of precipitation. There are months where the precipitation is less than 60mm.
    • The driest month nearly always occurs around the winter
  • Aw/s is the savanna climate, the dry season can become severe and often drought conditions prevail during the year.
    • Aw is a tropical savanna climate with dry-winter characteristics
    • As is tropical savanna climate with dry-summer characteristics

Geography 

One thing that can affect weather is the topography of an area. This refers to the arrangement of natural and manmade features of an area. It can include mountains, rivers, or cities. Topographical features like mountains affect the weather mostly in the way that they direct air currents. For example, the air is forced to rise over the mountains. Moist air will cool as it rises, and then the clouds release the water, causing precipitation like rain or snow. This is why one side of a mountain range – the side nearest the ocean – often gets more rain. This way areas that would be tropical forest when looking at the location might actually be a savanna in reality or vice versa.

An area’s latitude on the surface of the Earth (location in terms of north and south) also affects the weather. Because it changes the intensity of the sun’s light that the area receives, as mentioned before.

Anthropogenic activity

The impact of anthropogenic activities is mostly negative, removing rainforest where it would naturally occur. I will not go into details in this now, as later in the series there is will be a blog dedicated to human activity in the tropical forests.

The Intertropical convergence zone (ITCV)

Wondering how all that rain gets formed over the tropical forest? That has to do with the intertropical convergence zone (ITCV). Air at ground level gets heated, heated air expands which makes it rise. When the air rises it starts to cool down again. Which causes the clouds to be able to hold less water. The water that can no longer be hold will fall as rain. The air that rises in the first places gets constant replacement by air from north and south. So, the ITCV is the zone where the winds meet and precipitations results, but this is not a continuous feature. 

The ITCV moves north during the northern summer and migrates south during the autumn. This means it produces seasonal precipitation following the seasonal migration of the sun. So the equatorial region has two rainy seasons, while the area north and south of the equator has only one rainy season. 

Ecosystems of the tropics

So far, we have seen that the tropics are shaped by different things. As these are not constant and change during time, the ecosystems are not the same over the complete region. We can divide the tropics into three major climates, and the ecosystems can be divided into four major groups (1) Tropical rainforest, (2) Seasonal tropical forest, (3) Dry forests and (4) Tropical grasslands, shrubland and savannas.  

Similar to the climate, the ecosystems, are mostly determined by the amount of precipitation. Where the amount of precipitation decreases from tropical rainforest to the tropical grasslands, shrubland and savannas. As we focus on the tropical rainforest here, I will only focus on the tropical rainforest and the seasonal tropical forest.

Tropical rainforest

Tropical rainforest ecosystems have the highest biodiversity and often combined with high species endemism.  At least half of all the plants and animals we know live in these forests. They are tall, dense and evergreen forest. We also call them ‘mega thermal’ forest, which means frost-free. The monthly temperature is between 26°C and 27°C, average humidity of 80% and the annual precipitation is usually more than 2000mm.  There is no single typical rainforest, even here there is a great variety. For example, lowland rainforest, montane rainforest, freshwater swamp forest, heath forest, peat swamp forest and mangroves.

The rainforests are divided into different layers where the vegetation is organized in a vertical pattern from the soil to the canopy.  Where each layer harbours unique communities of plants and animals, adapted for each layer. Even though the first three layers (forest floor, understory and canopy) are found in all types of forest. The fourth layer, the emergent layer, is unique to the tropical rainforest. It contains a small number of very large trees, some of these species can grow up to 60 meters high!

Seasonal tropical forest

Seasonal tropical forest ecosystems generally receive high total precipitation but also have a distinct dry season. It typically contains a range of tree species from which only some drop part of their leaves during the dry season. There are different layers in the forest, but these may be less pronounced in these mixed forests. Mixed forests are often characterized by numerous lianas, as they have a growth advantage during the dry season. These forests are found close to the tropical rainforest and the visual and climate difference are minimal in some places. The temperatures can become slightly higher and the humidity lower.