Multiple dimensions of taxonomic tree diversity reveal complementary effects of environmental filtering and land-use legacy in an urban Atlantic Forest

In the crowded, concrete world of cities, patches of forest often survive as quiet remnants of a wilder past. These are not the ancient, untouched woods of deep wilderness, but secondary forests—lands that were once cleared for farming or grazing and have since been left to heal on their own. For decades, ecologists have known that these recovering forests are vital for holding onto the planet's biodiversity, yet a lingering question remains: what actually shapes the life within them? Is it the physical stage upon which the forest grows, such as the soil beneath the roots and the slope of the land, or is it the memory of how humans treated the land long ago? To answer this, scientists look at diversity not just as a simple count of how many species exist, but as a complex picture of who is there, how many of them there are, and whether the forest is dominated by a few common trees or shared equally among many. Understanding these layers is crucial because it tells us whether a forest is truly healthy or merely a collection of survivors, and it guides how we might help nature recover in our own backyards.

In the heart of Juiz de Fora, Brazil, a team of researchers turned their attention to a unique slice of the Atlantic Forest, a region known for its incredible variety of life but also for being heavily fragmented by human activity. They focused on a specific botanical garden that holds a rare natural experiment: three distinct patches of regenerating forest that all began as shaded coffee plantations. Decades ago, these areas were managed in different ways. One patch was left to recover naturally after the coffee was abandoned roughly eighty years ago. Another was managed with a specific technique to encourage the growth of the juçara palm, a native fruit tree, without cutting down other trees. The third patch underwent more intense management, involving selective logging and the clearing of undergrowth, before being abandoned about twenty years ago. Because these three communities share the same climate, the same soil type, and the same starting point as coffee farms, they offered a perfect setting to see how the past and the present environment interact to build a forest.

The researchers walked through these forests with a clear goal: to measure the trees not just by how many species they could find, but by how those species were distributed. They counted every tree with a trunk wider than five centimeters across three hectares of land, identifying each one down to the species level. They also dug into the earth to analyze the soil, measuring everything from the amount of sand and clay to the nutrients available for plants. They collected fallen leaves from the forest floor to understand the quality of the litter, and they recorded the exact elevation of every plot to account for the subtle changes in the landscape. By using a method that treats a simple count of species differently from a measure of how evenly those species are shared, the team could see the forest through multiple lenses.

What they found was a story of two different forces pulling the forest in different directions. The number of different tree species present in each patch—the richness—was largely dictated by the physical environment. The trees responded to the soil and the slope. Areas with higher elevations, soils rich in certain nutrients, and a specific balance of sand and clay tended to support a greater variety of rare species. The history of how the land was managed had very little to do with how many different species showed up. In this sense, the environment acts as a filter, deciding which seeds can successfully take root and grow based on the conditions they face.

However, the story changed completely when the researchers looked at how the trees were distributed. The balance between common and rare species, and the dominance of certain trees over others, was strongly shaped by the land's history. The forest that had been enriched with juçara palms decades earlier was now dominated by those palms. This historical intervention had left a lasting mark, creating a community where a few species held the majority of the space, reducing the evenness of the forest. Similarly, the area that had been intensively managed and logged showed a different structure, with fewer species overall and a distinct pattern of dominance. The past management practices had effectively rewritten the rules of competition, determining which trees would become the giants of the canopy and which would remain in the shadows, regardless of the soil conditions.

The study revealed that these two forces—environmental filtering and land-use legacy—operate on different aspects of the forest. The physical world decides who gets to enter the party, while the history of human management decides who gets to be the host. The forest that had been left alone for eighty years developed a more balanced community, with a wider variety of species sharing the space more equally. In contrast, the forests with more recent or specific human interventions showed signs of being dominated by a few species, a pattern that persisted long after the humans had left. This suggests that simply counting the number of species in a recovering forest can be misleading. A forest might look diverse because it has many different types of trees, but if a few of those types are overwhelming the rest, the ecosystem may be less resilient than it appears.

These findings offer a clearer view of how urban forests recover. They show that while nature is good at bringing back a variety of species when the soil and climate allow it, the scars of past human actions can linger for decades, shaping the structure of the community in ways that are not immediately obvious. For cities trying to restore their green spaces, this means that looking only at the number of species is not enough. To truly understand the health of a forest, one must also look at how those species are arranged and whether the balance of power among them has been altered by history. The path to a resilient urban forest requires paying attention not just to the ground beneath the trees, but to the stories written in the land long before the first sapling of the new forest was planted.