Early vegetative development and ontogenetic phase transitions in Juglans neotropica Diels: a BBCH-scale approach

In the world of botany, understanding how a tree grows is often a matter of timing. For decades, scientists have relied on counting days or measuring height to track a plant's life, assuming that a seedling's age tells the whole story. However, many trees do not grow in a straight, predictable line. Instead, they move through distinct chapters of life, shifting from a slow, protected juvenile phase into a more robust adult stage. These shifts, known as ontogenetic phase transitions, are critical moments where a plant's shape, leaf structure, and growth habits change fundamentally. For conservationists trying to save rare species, knowing exactly where a tree stands in this journey is vital. If a tree is still in its juvenile phase, it may need different care than one that has matured. Without a clear map of these changes, efforts to restore forests or grow trees in nurseries can stumble, simply because the plants are being treated as if they are all the same age, when in reality, they are at very different points in their development.

This challenge is particularly acute for the South American walnut, Juglans neotropica, a species now listed as endangered. While this tree is valuable for its wood and its role in local ecosystems, scientists lacked a standardized way to describe its early life. Conservation teams in Colombia could not easily tell if a seedling was ready for the wild or if it was still too young, because there was no common language to describe its growth stages. To solve this, researchers observed the tree under both nursery and field conditions to create a new framework, similar to a universal code used for many crops, that tracks development based on what the plant actually looks like rather than how long it has been in the ground.

The researchers focused on three main chapters of the tree's early vegetative life: the moment the seed sprouts, the growth of its leaves, and the lengthening of its stem. They found that the seed germination process is notably slow and variable. After being planted, the seeds took anywhere from 30 to 140 days to emerge, with some cases stretching out to 180 days. When the seed finally broke through the soil, it did so in a specific way where the seed leaves remained underground, a trait that helps protect the young plant. Once the seedling appeared, the team watched closely as it moved from its first leaves into a more complex structure. They discovered that the tree does not just grow bigger leaves; it undergoes a series of distinct, organized changes. The shape of the leaf tips, the way the base of the leaf connects to the stem, the texture of the edges, and the overall form of the leaf blade all shifted in a clear, step-by-step pattern as the tree moved from one stage to the next.

These observations revealed that the early life of the South American walnut is not a smooth, continuous slide from baby to adult. Instead, the tree moves through separate, well-defined phases. The changes in leaf architecture were so consistent that they marked a clear boundary between the juvenile stage and the vegetative adult stage. This means that a tree's development is organized into specific chapters, and a plant can be identified by which chapter it is currently in, regardless of how many days have passed since it was sown. This distinction is crucial because it proves that chronological age is not a reliable indicator of a tree's maturity or needs.

By establishing this new scale, the researchers have provided a practical tool for anyone working with these trees. Nursery workers can now identify exactly which stage a seedling is in to provide the right care, and restoration projects can select trees that are truly ready for the field. This approach moves beyond simple counting of days and offers a way to monitor the health and progress of the species based on its physical form. It also sets a precedent for studying other tropical trees, suggesting that many species may follow similar patterns of distinct developmental phases. For the endangered South American walnut, this clarity offers a better chance for survival, ensuring that conservation efforts are guided by the actual biology of the tree rather than an estimate of its age.