Invasion by Ulex europaeus L. and fire history: patterns in vegetation composition, diversity, and structure in Colombian páramos.

High in the Andes, above the tree line but below the permanent snow, lie the páramos. These are tropical high-mountain ecosystems, vast landscapes of grasses, shrubs, and unique rosette plants that have evolved over millennia in a world where fire was a rare visitor. Unlike the savannas or forests of lower elevations, which are accustomed to burning, these delicate communities did not develop under frequent fire regimes. For a long time, they remained relatively untouched by flames, recovering slowly when fires did occur. However, the modern world is changing this balance. Human activity and shifting climates have brought wildfires to these heights with increasing frequency. At the same time, aggressive plants from other parts of the world are moving in. When fire and invasive species meet, they can create a dangerous cycle that reshapes the entire landscape, potentially turning a diverse, resilient ecosystem into a simplified, fire-prone one.

Researchers set out to understand how this double threat is playing out in the páramos of Colombia's Eastern Cordillera. They focused on a specific invader: a spiny shrub from Europe known as Ulex europaeus, or gorse. This plant is notorious for its ability to spread quickly and for being highly flammable, often fueling intense wildfires. The team wanted to see how the presence of this shrub, combined with the time that had passed since the last fire, was altering the plants that call these mountains home. They traveled to six different locations, ranging from the páramo itself to the transition zones where the forest meets the high grasslands. Some of these sites had been burned recently, others decades ago, and some had not burned in living memory. Crucially, some sites were overrun by the spiny gorse, while others remained free of it.

To get a clear picture of what was happening, the scientists walked through these landscapes with a specific plan. They laid out long, narrow strips of land, measuring 25 meters by 2 meters, and counted every plant they found within them. They also placed small square plots on the ground to measure the tiny herbs and grasses that grow close to the soil. By doing this across all six sites, they could compare the height of the plants, how many different species were present, and how evenly those species shared the space. They recorded over 130 different types of plants, from the towering rosette plants unique to the Andes to the small herbs that carpet the ground.

The results painted a stark picture of what happens when the spiny shrub takes hold. In the areas where the gorse had invaded, the plant community looked very different. The shrub grew so thick and dense that it dominated the landscape, reaching a level of importance where it accounted for nearly a third of the entire plant cover in some spots. This dominance came at a cost. The invaded sites had far fewer different species sharing the space, and the plants that remained were not spread out evenly; instead, the gorse crowded out its neighbors. The vertical structure of the vegetation, which usually has layers of different heights, became simplified. The complex mix of tall shrubs, small bushes, and ground cover was replaced by a uniform wall of spiny stems. In contrast, the sites without the invader, even those that had burned years ago, showed a much richer variety of life and a more complex, layered structure.

The researchers also looked at how the plants changed over time after a fire. They found that the time since the last fire mattered, but not in a simple, straight line. In the places that had not been invaded, the plant communities were shifting and changing as years passed, with different species taking the lead at different stages. However, in the invaded sites, the story was different. At one site called Escobal, where the gorse was already present before a fire in 2023, the shrub returned with incredible speed. Just three years after the fire, it was already the dominant plant again. This suggests a troubling feedback loop: the fire clears the way for the gorse, and the gorse, with its dry, flammable leaves, makes the next fire more likely and more intense.

One of the most surprising findings was about how the plants were replacing one another. The scientists expected that after a fire and invasion, the landscape would simply lose species, becoming poorer and poorer. Instead, they found that the main change was a swapping of species. Different groups of plants were replacing each other, rather than just disappearing. This turnover was happening everywhere, whether the site was invaded or not, and whether it had burned recently or long ago. It showed that the ecosystem was still dynamic, but the path it was taking was being steered heavily by the presence of the invasive shrub.

The study also highlighted a few native plants that seemed to hold their ground. Certain members of the heather family, which include some of the iconic rosette plants of the Andes, were found persisting even in the invaded areas. These plants likely have tough traits, like the ability to regrow from their roots or bases, that allow them to survive the fire and the competition from the gorse. However, their survival was not enough to stop the overall simplification of the landscape. The invaded sites remained less diverse and more uniform than the healthy ones.

The authors caution that their study design, which compared different locations rather than manipulating them in a controlled experiment, means they cannot prove exactly how the fire and the shrub interacted in every detail. The sites varied in soil, slope, and climate, which adds complexity to the picture. Yet, the patterns were strong enough to raise a serious alarm. The case of Escobal, where the shrub returned to dominance in just three years after a fire, serves as a warning. It suggests that once this invasive plant establishes itself in a fire-prone area, it can lock the ecosystem into a new, degraded state that is difficult to reverse.

The path forward, the researchers suggest, is not to wait for the damage to be done. Because the shrub thrives on fire and spreads quickly from seeds stored in the soil, the most effective strategy is early action. Finding and removing the young plants before they take over and before the next fire occurs is critical. If managers can deplete the seed bank and stop the shrub from becoming dominant, the native páramo might still have a chance to recover its natural complexity. Without such intervention, these high-mountain ecosystems risk shifting permanently into a simpler, fire-driven world dominated by a single, aggressive species.