Kalanchoe pinnata (Crassulaceae) invasion restructures plant community regeneration through phylogenetically related assemblages of species

In the natural world, ecosystems are not static backdrops but dynamic communities where plants constantly compete for light, water, and nutrients. When a new plant species arrives from another continent and begins to spread rapidly, it is called an invasive species. These invaders often outcompete native plants, altering the landscape and sometimes causing the local biodiversity to collapse. Scientists study these invasions to understand not just which plants are winning or losing, but how the entire community changes. A key concept in this field is that plants are related to one another by evolutionary history, much like members of a family tree. When an invader changes the environment, it might not affect every plant randomly; instead, it might specifically favor or harm certain branches of this family tree, leaving a distinct pattern in the surviving community. Understanding these patterns is crucial for conservation, especially in fragile environments like tropical dry forests, where the loss of native species can be irreversible.

In the central highlands of Veracruz, Mexico, a specific invasive plant known as Kalanchoe pinnata has become widespread. Native to Madagascar and southern Africa, this succulent was introduced to the Americas for its medicinal properties and as an ornamental garden plant. It thrives in the region's seasonally dry forests, which grow on poor, volcanic soils. Researchers set out to determine exactly how this green invader is reshaping the forest. They focused their study on the Actopan River basin, an area characterized by shallow soils and a climate with a distinct dry season. To get a clear picture of the invasion's impact, the team established a paired design across twenty different plots. Ten of these plots were heavily covered with Kalanchoe pinnata, while the other ten, located nearby, remained free of the plant. In each plot, the scientists counted and identified every plant, sorting them into three distinct groups based on their size: large adult trees and shrubs, smaller saplings, and tiny seedlings just beginning to grow. They also collected soil samples to check if the invader was changing the chemical makeup of the ground.

The results revealed a story of two very different realities within the same forest. The large, established adult trees and shrubs showed remarkable resilience. Whether they stood in a plot overrun by Kalanchoe pinnata or in a clean plot, the number of species, their variety, and their overall abundance remained largely the same. The adults, with their deep roots and tall canopies, seemed able to withstand the pressure of the invader. However, the story changed completely when the researchers looked at the forest floor. In the plots where Kalanchoe pinnata was present, the regeneration layer—the seedlings and saplings trying to grow into the next generation of forest—was in trouble. The number of different plant species in this layer dropped by nearly ten percent compared to the uninvaded sites. Furthermore, the few species that did survive in the invaded plots became much more dominant, crowding out the others. The data showed that the invasion was not just reducing the number of plants; it was actively replacing a diverse mix of native seedlings with a much smaller, less varied group of survivors.

This shift in the forest's future was not random. The researchers found that the plants which managed to increase in number within the invaded plots belonged to specific evolutionary families. These survivors were clustered together on the plant family tree, including groups such as the Malpighiales, Fabales, and various grasses and sedges. In contrast, the plants that disappeared or declined were scattered randomly across the evolutionary tree. This suggests that the invader is not simply wiping out plants at random; it is creating conditions that favor specific types of plants with particular traits, while excluding others. The soil analysis provided clues as to why this might be happening. The ground beneath the Kalanchoe pinnata contained significantly more organic matter and carbon than the soil in the uninvaded plots. This increase likely comes from the thick layer of leaves the invader sheds, which alters the nutrient cycle and potentially creates a soil environment that favors fast-growing, disturbance-tolerant species over slower-growing native ones.

The implications of these findings are significant for the long-term health of the tropical dry forest. While the current canopy of adult trees appears safe for now, the forest is failing to regenerate. If seedlings and saplings cannot establish themselves because they are being outcompeted or chemically suppressed by the invader, the forest will eventually lose its adult trees as they age and die, with no new generation to take their place. The invader is effectively rewriting the future composition of the forest, favoring a few specific lineages of plants while pushing out the diverse native community that has evolved there over thousands of years. This study highlights that the threat of Kalanchoe pinnata is not just about the plant itself, but about how it fundamentally restructures the community, ensuring that the forest of tomorrow will look very different from the forest of today. Immediate management is necessary to protect these unique ecosystems before the regeneration layer is lost entirely.