Spatial Modelling of Long-Term Urban Vegetation Greening and Browning in Ibadan, Nigeria Using Sentinel-1 and Sentinel-2 Time-Series Data

In the bustling heart of tropical cities, the story of nature is often written in shades of green and brown. When we look at a city from above, the plants that grow there are not just scenery; they are vital indicators of how the environment is changing. Scientists have long used satellite images to track these changes, looking for signs that vegetation is either thriving, known as "greening," or struggling, known as "browning." This process is rarely uniform. In some spots, trees might be growing back, while just a few streets away, concrete might be replacing the last patches of grass. Understanding exactly where these shifts happen and, more importantly, why they happen is crucial for city planners trying to balance rapid growth with the health of the urban ecosystem.

A researcher recently turned their attention to Ibadan, a sprawling metropolis in southwestern Nigeria, to decode these complex patterns. Ibadan is a city in transition, with a population projected to reach nearly five million people by 2030. As more people move in, the land changes, and the vegetation responds. To get a clear picture of what has been happening over the last decade, the researcher did not rely on a single type of data. Instead, they combined two powerful tools from space: optical cameras that see light like the human eye, and radar sensors that can see through clouds and darkness to measure the physical structure of the ground. By stitching together ten years of observations from 2015 to 2025, they created a detailed map of how the city's green cover has changed, moving beyond simple snapshots to understand the long-term trends of growth and decline.

The researcher built a sophisticated computer model to predict these changes, feeding it a vast amount of information about the city's surface. They looked at the average colors of the land, how much those colors varied over time, and the specific rates at which vegetation was increasing or decreasing. They also included data on moisture levels and signs of disturbance, such as areas that had been burned or cleared. The goal was to see if these factors could explain why some parts of the city were getting greener while others were turning brown. The model performed with remarkable accuracy, successfully explaining more than 84 percent of the variations in vegetation trends across the city. This high level of success meant the researcher could trust the model to reveal the true drivers behind the changes they were seeing.

When the researcher examined the results, a clear and surprising pattern emerged. The most powerful factor driving vegetation change was not the physical structure of the buildings or the radar signals from the ground, but rather the moisture available to the plants. Specifically, the model showed that areas where moisture was declining were the same places where vegetation was browning. Conversely, the second most important factor was the recovery from disturbances. When an area had been cleared or damaged, the data showed that if the land began to recover, the vegetation would start to green up again. Interestingly, the radar data, which measures the physical texture of the ground, contributed almost nothing to the prediction. In this specific urban landscape, the optical data regarding water and vegetation health told the whole story, while the structural radar signals were largely silent.

The map of Ibadan revealed a distinct divide between the city center and its outskirts. The greening trend was most extensive in the peri-urban areas, the zones on the edge of the city where development is still spreading. In the Ido local government area, for instance, nearly 78 percent of the land showed signs of greening. Other edge districts like Lagelu and Akinyele also saw significant growth in vegetation. In contrast, the older, densely built-up core of the city told a different story. Here, the urban expansion had already consumed much of the open space, and the vegetation was struggling. In the Ibadan South-West district, for example, browning was the dominant trend, affecting more than 37 percent of the land. The city center was characterized by a mix of stability and decline, with very little new growth compared to the edges.

This pattern suggests that the city is not changing in a simple, linear way. Instead, it is a patchwork of different realities. In the core, the pressure of construction and the lack of space have led to a loss of green cover, while the edges are still seeing vegetation take hold, perhaps in new gardens, farms, or young tree plantings. The researcher noted that this "greening" in the suburbs does not always mean a return to a forest; it can simply mean that grass or small shrubs are growing where there was once bare soil. However, the trend is clear: the heart of the city is losing its green cover, while the periphery is gaining it. This spatial difference highlights a critical challenge for the city's future. The areas that need help the most are the densely populated urban cores, where the loss of vegetation is most acute.

The study concludes that to protect the future of Ibadan, city planners must look beyond the edges and focus their efforts on the center. The findings suggest that simply planting trees is not enough; the strategy must be targeted. The researcher recommends prioritizing the underserved, densely developed urban core areas for new green spaces and tree planting initiatives. They point to a specific framework known as the 3-30-300 principle, which aims to ensure that every resident can see at least three trees from their home, that every neighborhood has at least 30 percent tree canopy cover, and that every citizen has access to a public park of at least 300 square meters within a short walk. By applying these principles to the areas where browning is most severe, the city can begin to reverse the trend of vegetation loss. The data provides a clear roadmap: the moisture of the land and the history of disturbance are the keys to understanding the city's green future, and the time to act is now, before the concrete spreads any further.