A long-term study has yielded four decades of data on palm trees (Astrocaryum mexicanum) and tropical evergreen forests, providing much more than just demographic information about the understory species.

A variety of human activities on the edges of tropical forests can have severe, cascading consequences — one of the most important being the conversion of forests to cropland and grazing land, which reduces habitat and contributes to forest fragmentation.

The researchers found that edge effects, primarily peripheral deforestation and hunting, are something of a one-two punch that caused 3.3 times as many Astrocaryum mexicanum palm trees to thrive in the forest reserve’s understory, which could jeopardize biodiversity.

A study started in the early 1970s to research population demography of tropical trees confirms that even deforestation on the edges of a rainforest can have deleterious effects across the ecosystem.

José Sarukhán of the Universidad Nacional Autónoma de México wanted to compare different evolutionary strategies between different species of trees, from canopy trees to those that make up a forest’s understory, such as Astrocaryum mexicanum, a palm tree that is the subject of a new study Sarukhán and a team of researchers will publish in the Proceedings of the National Academy of Sciences (PNAS) this week.

Sarukhán’s long-term study has now yielded four decades of data on Astrocaryum mexicanum and tropical evergreen forests, providing much more than just demographic information about the understory species.

“It gave us insight on how the density of palms influences the recruitment of the dominant arboreal species,” Sarukhán told Mongabay, “a history of the natural disturbances of the forest through gap-formation due to the fall of old trees (especially after heavy storms), which is responsible for keeping a high species diversity in the forests, and the role of seed and seedling eaters in a forest that has been losing its top predators.”

The study was carried out in the Los Tuxtlas Biological Station in the state of Veracruz in southeast Mexico, which belongs to the Universidad Nacional Autónoma de México (UNAM), helping Sarukhán and team to keep their study sites free of non-natural disturbances.

But a variety of human activities on the edges of tropical forests can have severe, cascading consequences, Sarukhán explained. One of the most important is the conversion of forests to cropland and grazing land, which reduces habitat and contributes to forest fragmentation, directly diminishing the viability of populations of top carnivores. That in turn upsets the balance between these carnivorous species and their prey, which in most cases are herbivores that depend on fruit seeds and seedlings of both canopy and understory species.

“When this effect last for long periods there is a substantial effect on the composition of the forest due to the over abundance of plant species that, prior to those effects, were not so abundant and which in turn starts severely changing the [forest’s] composition and structure,” Sarukhán said.

Increased fragmentation also exposes more of the forest to edge effects such as strong winds from hurricanes. When heavy rains soak the soil of the forest, older, larger trees fall more frequently in those heavy winds, producing gaps of close to one hectare, according to Sarukhán.

Under normal circumstances, large trees falling helps promote species diversity in tropical evergreen forests, since it allows more sunlight into the understory, allowing species there to thrive. But when its occurs with increasing frequency in a fragmented forest, this can also affect the composition and structure of the forest, Sarukhán said.

At the same time, people living in and around the Los Tuxtlas forest reserve hunt for bushmeat, leading to defaunation, which Sarukhán calls “a central culprit in the causes that break the equilibrium of well preserved tropical forests.”

The researchers found that these edge effects, hunting and peripheral deforestation, are something of a one-two punch that caused 3.3 times as many Astrocaryum mexicanum to thrive in the forest reserve’s understory.

Increased light availability in the understory due to more frequent treefalls allowed for greater seed production and growth of young palms. Meanwhile, populations of large herbivores such as tapirs, peccaries, and deer were decimated by hunters, freeing the palm’s seeds, seedlings and saplings from the pressures brought to bear by herbivorous species that help keep the palm’s population in check.

“The resulting extraordinary increase (3.3-times greater/40 years) in the palm’s population density obliterated space and monopolized resources otherwise available for other trees in the understory, eventually decreasing understory species diversity and affecting forest biomass dynamics,” Sarukhán and team wrote in a statement.

“Rippling, dramatic effects can be expected, as the understory tree community represents the species-rich regenerating forest of the future. This work highlights that human disturbances, even if they occur predominantly around rainforest reserves, jeopardize tropical biodiversity and compromise conservation efforts.”

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