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The size and light requirements of plants might determine the future of a whole forest

If you take a look at a tropical rainforest, the species of plants you see can tell you the story about their life.

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The loss of biological diversity is a major problem in tropical rainforests around the world, and in Los Tuxtlas, Mexico, this is no exception. A group of tropical ecologists found that fragmentation causes a reduction in plant diversity but that the magnitude of the effects of deforestation on each plant depends on their size. These results would allow for the design of conservation, management and restoration plans of this highly transformed ecosystem.  


The understory of the rainforest in Los Tuxtlas, which is dominated by Chamaedora and Astrocaryum palms. These species need shady environments with low levels of sunlight which are highly affected when the canopy is removed. Photo courtesy of Juan Carlos López-Acosta.

Interested in understanding how deforestation affects the plant communities of the region, Juan Carlos López-Acosta of the Centre of Tropical Investigations from the Universidad Veracruzana, along with researchers from the Institute of Ecology (INECOL) and Stanford University, traveled to Los Tuxtlas, in the Mexican state of Veracruz. The group of ecologists measured the variation of plant richness and composition in forest fragments of different areas taking note of their size and light requirements.

They found that smaller fragments hold less species. However, when they considered the size of the plants in their analysis, this relationship changed. No matter the size of the fragment, the number of species of trees with 10 cm or more in diameter remained constant. That is, a fragment of barely 2 hectares could be home to the same number of species of adult trees than a patch of one thousand hectares.

These results are different from what other researchers have found in the Amazon and can be explained by when considering the behaviour of people living in Los Tuxtlas. Plant ecologist Juan Carlos López-Acosta and author of the paper published in the first edition of 2021 of Botanical Sciences explains:

“The locals are very aware of living fences and of leaving trees for shade.” He has noticed that isolated trees become nuclei of regeneration by creating microhabitats for local fauna.

López-Acosta says they have found that deforestation is selective and that locals only cut down species of small trees, leaving the tallest and oldest untouched. In the long term, this causes a change in species composition in the rainforest depending on the size of the trees selected for logging.

The team also found that species composition differed depending on fragment size: Light-demanding plants dominated in smaller ones.

This is because deforestation not only reduces species richness by cutting down trees, but also through light availability. Logging creates open spaces that allow sunlight to pass through the canopy favouring sun-loving plants but displacing those that grow better in the shade. Over time, this situation results in the disappearance of plants that prefer shade while light-demanding species take over.

As is well known, deforestation not only affects plants. The loss of trees creates a cascading effect. López-Acosta says that when the rainforest is fragmented, shade conditions, that both plants and their associated insects need, are lost.  

“An example of this is Astrocaryum mexicanum, a species of palm that needs shade and the fact that the canopy protection is lost affects the growing conditions of the plants. This also has consequences for its pollinators, which tend to disappear,” explains the ecologist.

The forest of Los Tuxtlas, the northernmost limit of the tropical rainforests in America, has experienced an intense transformation as a result of cattle raising, an activity that has been practised since the arrival of European ranchers during the Mexican colonial period in the sixteenth century.   

“The story about cattle raising in the region is the story of land transformation and appropriation,” says the researcher.

The rainforest of Los Tuxtlas is composed by a variety of human-made habitats. Living fences, grasslands, forest fragments of various sizes, abandoned agricultural fields all hold plant diversity; however, larger fragments, harbouring the largest diversity, are less common. Photo courtesy of Juan Carlos López-Acosta.

On the other hand, the area has been a place for research since the National Autonomous University of Mexico (UNAM) opened its first biological station there in 1967. “Los Tuxtlas is a place where I have become a biologist,” López-Acosta reflects.

And he’s not alone in the experience. The fact that the field station is in the region—the second most studied tropical forest in the Americas only behind Barro Colorado in Panama—has generated interest among national and international scientists.

Despite the grim outlook, in recent years the local community—even if unintentionally—has favoured the regeneration of their forests and of its wildlife.

“The synergy between humans and nature is what matters in the end; it’s fundamental for the conservation of this region,” he says.

RESEARCH PAPER:

Aguirre-Jaimes, A., López-Acosta, J. C., & Dirzo, R. (2020). Tropical rainforest fragmentation affects plant species richness, composition and abundance depending on plant-size class and life history. Botanical Sciences, 99(1), 92-103. https://doi.org/10.17129/botsci.2679


Rocío Ramírez Barrios is working towards her Biology degree at the National Autonomous University of Mexico (UNAM). She’s interested in vascular plant diversity and taxonomy. Rocío enjoys fieldwork and has visited most of the vegetation types in her country and is currently working on the first floristic study of the conifer forest in Huacalapa, in the Mexican state of Guerrero.  She’s also passionate about botanical photography. You can check out her images by following her on Instagram as rociorb_rmrz. 

English translation by Lorena Villanueva Almanza

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