The structure of a tropical forest is extraordinarily complex , comprising multiple levels, from the deepest roots that take root in bedrock, through a large number of different soil horizons, and already on the surface, various levels of tree canopies, to the atmosphere, above the highest canopies, where forest birds fly and grains of pollen and spores float.
Its complexity is also reflected in the relationships established between different living beings, forming dense food webs that intertwine and interconnect with each other.
Refuges of terrestrial biodiversity
Tropical forest ecosystems not only have many levels, but are ecologically extraordinarily heterogeneous . Unlike a temperate forest, dominated by one or a few species of trees and a relatively small floral procession, in the tropical forest there are multiple tree species, with different characteristics, that coexist and compete constantly.
Tropical forests are estimated to be home to up to 96% of the planet’s tree species . Added to this enormous wealth is a large number of shrubs, herbaceous plants, epiphytic plants —that grow on other plants— and a wide variety of animal species. In its soil, microorganisms proliferate and diversify in a way that is difficult to observe outside the borders of a tropical forest.
The myth of oxygen production
There is a belief that a significant proportion of the oxygen we breathe comes from tropical forests. And that, therefore, its disappearance would drastically decrease the amount of oxygen available on the planet.
It is true that the vegetation of a tropical forest produces tons of oxygen every day, but they are enormously complex ecosystems, with many strata, and many of the living beings that live in them breathe. In a mature tropical forest, most of the oxygen produced by photosynthesis is used by respiration , and the balance is quite low.
Only when the forest grows and expands does it retain more carbon than the oxygen it releases and contribute to terrestrial oxygen levels. But, since for every molecule of carbon dioxide that a plant obtains from the environment, it releases only one molecule of oxygen, even if that were to happen massively, the current concentration of carbon dioxide would give the atmosphere a minimum amount of oxygen, compared to what is already there. Most of the oxygen we breathe does not come from plants, but from photosynthetic bacteria that lived billions of years ago.
The tropical forest as a carbon store
Something similar to what happens with oxygen, happens with carbon dioxide. Tropical forests have relatively low carbon use efficiency.
It is estimated that of all the carbon that a tropical forest obtains, about 70% is released again through respiration , and approximately 30% is retained to form part of the biological structures of the forest. It really is a low ratio, and in fact, other communities have a much higher efficiency. However, the factor to take into account here is not so much its efficiency in sequestering carbon, but rather how long the tropical forest is capable of retaining that captured carbon .
As the forest grows and matures, the soil becomes a carbon store. When a plant dies, it is buried under successive layers of leaf litter and other animal remains, before it completely decomposes, thickening the soil and retaining the carbon that those living organisms had. It is estimated that tropical forests currently store up to 25% of the world’s carbon .
This carbon storage capacity in the soil is greater the greater the taxonomic diversity of the forest, that is, when it presents a greater number of species and less related to each other, in evolutionary terms.
The consequences of tropical forest loss
The loss of biodiversity of tropical forests implies the loss of the capacity of that carbon, and losing the forest implies releasing all that carbon into the atmosphere . Actions such as logging, even if there is a repopulation afterwards, cause a homogenization of the tropical forest, making one or a few species dominate over the rest, reducing biodiversity. This phenomenon becomes extreme if the cleared land is used for agricultural or forestry cultivation. Other human impacts that reduce tropical forest biodiversity are pollution or the introduction of invasive species.
For this reason, the conservation of the biodiversity of tropical forests is essential , especially in a scenario of global change such as the one we are currently experiencing. In addition, many tropical forests depend directly on other ecosystems or processes to function properly, as is the case with the Saharan haze and the Amazon rainforest. A change in these types of dynamics, perhaps caused by climate change, can be fatal for the forest, and in a scenario of global change, we must also pay attention to these relationships to avoid a domino effect.
That tropical forests expand would be a good sign, a mitigating event of global change , by retaining greater amounts of carbon. On the contrary, the disappearance of these ecosystems implies a very high risk: turning the tropical forest from a carbon store into a carbon source, which would contribute to climate chaos.
References:
Cavanaugh, K. C. et al. 2014. Carbon storage in tropical forests correlates with taxonomic diversity and functional dominance on a global scale. Global Ecology and Biogeography, 23(5), 563-573. DOI: 10.1111/geb.12143
Chambers, J. Q. et al. 2004. Respiration from a Tropical Forest Ecosystem: Partitioning of Sources and Low Carbon Use Efficiency. Ecological Applications, 14(sp4), 72-88. DOI: 10.1890/01-6012
Poorter, L. et al. 2015. Diversity enhances carbon storage in tropical forests. Global Ecology and Biogeography, 24(11), 1314-1328. DOI: 10.1111/geb.12364
Putz, F. E. et al. 2001. Tropical Forest Management and Conservation of Biodiversity: An Overview. Conservation Biology, 15(1), 7-20.
