Beyond the large mother tree: a multi-approach modeling framework reveals the predominance role of site conditions on Parkia biglobosa seedling growth, with a maternal effect limited to germination

In the vast, sun-drenched landscapes of West Africa, a tree known as the néré, or African locust bean, stands as a pillar of life for local communities. Its fruit is fermented into a prized condiment, its wood is used for construction, and its presence defines the agroforestry parklands that sustain millions. Yet, despite its importance, the natural regeneration of this species is faltering. Fewer young trees are taking root, threatening a future where these vital resources might vanish. For decades, a prevailing belief among foresters and conservationists held that the size of the parent tree was the key to the future of the forest. The logic seemed sound: a massive, ancient tree should produce the heaviest, most robust seeds, which in turn should grow into the strongest, most vigorous saplings. It was a simple, intuitive chain of cause and effect that guided how people selected seeds for planting.

However, nature often defies simple intuition. A new study conducted across the diverse climates of Côte d'Ivoire has challenged this long-held assumption, offering a more nuanced picture of how these trees begin their lives. The research, led by Beda Innocent Adji, set out to test whether the sheer size of the mother tree truly dictates the success of its offspring, or if other factors play a more dominant role. By tracking thousands of seeds and seedlings across two very different environments—one dry and northern, the other humid and central—they applied a rigorous set of statistical tools to untangle the complex web of influences. What they found was a clear distinction between the moment a seed wakes up and the years that follow. While the size of the parent tree does influence the very first step of life, it is the environment where the seed is planted that ultimately decides whether the tree will thrive or struggle.

The study began with a massive collection effort. Researchers gathered seeds from 120 different parent trees, carefully grouping them into six categories based on the diameter of their trunks, ranging from small, slender trees to giants with massive girths. These seeds were then sown in two distinct locations: Korhogo in the north, characterized by a drier climate and specific soil conditions, and Daloa in the west-central region, which is wetter and sits on different soil types. The goal was to see if the offspring of the largest trees would consistently outperform the offspring of smaller trees, regardless of where they were planted. The researchers monitored these seedlings at regular intervals, measuring their height, the thickness of their stems, and their overall weight over a period of two years.

The results were striking and immediate. The location where the seed was planted proved to be the single most powerful factor determining the seedling's fate. The difference between the two sites was profound. In the drier northern site of Korhogo, the germination rate reached 73.3 percent, whereas in the humid Daloa site, it was lower at 62.8 percent. More importantly, the growth of the young trees was overwhelmingly dictated by the soil and climate they encountered. At every stage of measurement—whether at four months, six months, or two years—the seedlings in Daloa grew significantly taller and thicker than those in Korhogo, simply because the conditions there were more favorable for this species. When the researchers looked specifically at the influence of the mother tree's size, the expected pattern vanished. There was no direct link between the diameter of the parent tree and the height or weight of its children. A seed from a giant tree did not grow into a bigger seedling than a seed from a smaller tree if they were planted in the same spot.

This does not mean the mother tree is entirely irrelevant, however. The study uncovered a subtle, indirect effect that operates only at the very beginning of life. Through a detailed statistical analysis, the researchers confirmed that larger parent trees do indeed produce heavier seeds. These heavier seeds, in turn, have a higher chance of successfully germinating. It is a small but real advantage: a bigger tree gives its offspring a better start by packing more energy into the seed, which helps the sprout break through the soil. Yet, this advantage stops there. Once the seedling has emerged, the extra weight of the seed no longer matters. The growth trajectory of the young tree is entirely taken over by the conditions of the ground it stands in. The initial boost from a heavy seed cannot compensate for poor soil or a harsh climate, nor can a light seed prevent a tree from thriving in a perfect environment.

The researchers also explored how these young trees grow over time, finding that their growth follows a rapid, accelerating curve rather than a steady, straight line. They discovered that the most accurate way to estimate the weight of these young trees without cutting them down was to measure both their height and their stem thickness together, rather than relying on thickness alone. This finding provides a practical tool for nursery managers who need to track the health of their stock. Furthermore, the study revealed that the sensitivity of these seedlings to their environment changes as they age. The difference in performance between the two sites was not constant; it peaked when the seedlings were about one year old, suggesting a critical window where the young trees are most vulnerable to local conditions, before settling into a more stable growth pattern as they mature.

The implications of these findings are significant for anyone involved in restoring forests or managing agroforestry systems in West Africa. For years, the strategy for conservation seed orchards has often focused on selecting the largest, most impressive trees to harvest seeds from, assuming that size equates to quality. This study suggests a shift in that approach. While selecting larger trees might slightly improve the germination rate, it will not guarantee the growth of stronger trees. The most critical decision for a restoration project is not which tree to harvest from, but where to plant the seeds. Choosing the right site, with the right soil and climate, is far more important than the pedigree of the seed. The study concludes that conservation efforts should prioritize the diversity of planting locations and the suitability of the environment over the selection of the largest individual trees. By focusing on the ground rather than the canopy, managers can ensure that the néré trees of the future have the best possible chance to grow tall and strong, securing the resources and landscapes they support for generations to come.