Morphological and genetic variability of Vitellaria paradoxa C.F. Gaertn populations across a climate gradient in Benin (West Africa)

In the vast, sun-drenched landscapes of West Africa, a single tree species holds a place of profound importance for both the land and the people who live there. The shea tree, known scientifically as Vitellaria paradoxa, is a long-lived giant that can survive for more than two centuries, providing food, oil, and income for millions. For conservationists and ecologists, understanding how such a widespread species survives in different environments is a central puzzle. They are particularly interested in how a tree's physical appearance—its leaf size, its height, the weight of its wood—relates to its genetic makeup. This relationship is crucial because it reveals whether a tree is simply reacting to its immediate surroundings or if it has inherited specific traits that help it thrive in a particular climate. As the climate changes and human activity alters the land, knowing whether these trees can adapt or if their genetic diversity is shrinking becomes a matter of urgent importance for the future of the region's forests and the communities that depend on them.

A team of researchers from Benin and Switzerland recently set out to solve this puzzle for the shea tree across its entire range within the Republic of Benin. They traveled from the dry, arid north to the humid, forested south, covering the full spectrum of climates where the tree grows. Their goal was to take a comprehensive look at 188 individual trees, measuring their physical characteristics and analyzing their DNA to see how the two are connected. By combining detailed field measurements with modern genetic testing, they aimed to understand if the trees in the north are fundamentally different from those in the south, or if they are all part of one large, interconnected family that simply looks different because of where they live.

The researchers began by selecting mature trees from three distinct climate zones: the dry Sudanian zone in the north, the transitional Sudano-Guinean zone in the middle, and the humid Guinean zone in the south. For each tree, they recorded its height and trunk width, and they carefully collected leaves to measure their length, width, and the length of the stem that attaches them to the branch. They also weighed the dried leaves and calculated the total weight of the tree's above-ground parts, a measure known as biomass. To understand the genetic side of the story, they took leaf samples from the same trees and sent them to a laboratory in Kenya. There, scientists used a high-tech method called DArTseq to scan the trees' DNA for thousands of tiny variations, known as single-nucleotide polymorphisms, which act like unique genetic fingerprints. This allowed them to compare the genetic code of every tree in the study with incredible precision.

The results painted a picture of a species that is physically flexible but genetically surprisingly uniform. The researchers found that the trees changed their appearance as they moved across the climate gradient. Trees in the humid south tended to have longer leaves and longer leaf stems compared to those in the dry north. The weight of the leaves also shifted, being slightly heavier in the middle zone before dropping off in the south. However, the amount of wood a tree produced told a different story. The trees in the dry northern zone were the most massive, with an average above-ground weight of 709.27 kilograms per tree, while those in the middle and southern zones were significantly lighter, weighing 355.42 kilograms and 253.99 kilograms respectively. This suggests that the dry conditions of the north might actually encourage the trees to grow larger, perhaps as a way to store water or compete for light in an open savanna.

When the researchers looked at the DNA, they discovered that despite these physical differences, the genetic landscape of the shea tree is remarkably flat. They identified three distinct genetic groups across the country, but the differences between these groups were extremely small. In fact, more than 98 percent of the genetic variation existed within the local populations themselves, rather than between the different regions. This means that a tree in the north is genetically very similar to a tree in the south. The study also found an excess of genetic mixing, with more variation within the trees than would be expected if they were breeding only with their closest neighbors. This indicates that pollen and seeds are traveling long distances, carried by insects, animals, and humans, keeping the gene pool well-mixed across the entire country.

The most striking finding was the disconnect between what the trees looked like and what their genes said. While the trees in different zones had distinct physical traits, these traits did not align neatly with the three genetic groups the researchers found. A tree with the genetic makeup of the "northern" group could be found in the south, and vice versa. This suggests that the physical differences observed are largely due to phenotypic plasticity—the ability of a single genetic blueprint to produce different physical forms depending on the environment. The trees are not necessarily evolving into different species for each climate; rather, they are adapting their growth and shape to the local heat, rain, and soil conditions.

These findings have significant implications for how the shea tree should be protected. Because the genetic diversity is spread out across the entire landscape rather than concentrated in specific pockets, conservation efforts cannot focus on just a few isolated groves. Instead, the researchers suggest that protecting the species requires maintaining the connectivity of the landscape so that pollen and seeds can continue to travel freely. The study highlights that the trees' ability to change their physical form is a key survival strategy, but it also warns that this flexibility has limits. As climate change accelerates and land use shifts, preserving the natural regeneration of these trees in their current locations is essential. The study concludes that the best way to safeguard the future of the shea tree is to protect the diverse populations across all climate zones, ensuring that the species retains its capacity to adapt to a changing world.