When a fire sweeps through a forest, or a lumber company strips an area of all of its trees, the greenery will eventually grow back. Or so many forestry researchers thought. But a new study in the tropics suggests that these second-growth forests can look very different from what they replaced—a finding that may cause biologists to wonder what biodiversity will be restored and forestry experts to reconsider how much they should or can intervene in the regrowth.

“There’s a high degree of random effects” in what comes back, says Jefferson Hall, a forest ecologist at the Smithsonian Tropical Research Institute in Panama who was not involved with the work. “It’s a very important study.

When a forest is burned or cut down and farmed temporarily, that land tends to undergo a series of changes. Some pioneer plants will quickly take hold, gradually changing the landscape—how much the ground is shaded and the soil composition—such that a new set of plants will thrive there. This in turn creates yet another set of conditions that eventually allows for the return of the forest. For a long time, ecologists have thought this process, called succession, followed a fairly preordained course such that the same trees ultimately dominated the landscape once again. But they have been limited by imperfect evidence. Not likely to get funding for a 200-year-long study, plant ecologists have examined succession by studying regrowth in plots where original forests that had been cut down at different times, an approach called chronosequencing.

Robin Chazdon, an ecologist at the University of Connecticut, Storrs, wasn’t sure such work captures a true picture, however. So with her postdoc Natalia Norden and colleagues, she amassed data from several long-term studies of regrowing forests in Mexico, Costa Rica, Brazil, and Nicaragua. These studies had actually tracked in real time the changes in the kinds of trees present (species diversity), the density of trees, and how well the trees were growing (by measuring each tree’s diameter and from that forest’s growth over time). Norden then teamed up with a hydrologist to build a mathematical model that explored how these characteristics affected each other to set the trajectory of regrowth.

The model revealed that chance plays a big role in determining that trajectory, the researchers report online today in the Proceedings of the National Academy of Sciences. In Brazil, for example, among several plots studied as the land recovered from pasture, there was a great deal of variation in the density of trees and number of species present even at just 10 years of regrowth. The size of the cleared land, the plants that were present before farming or grazing began, the amount of time before the land was abandoned, and, perhaps more importantly, what seeds happen to sprout first all shape what happens next in the early stages of secondary forest growth, Norden suggests.

The new study “represents our best understanding of how tropical secondary forests change over time,” says Jennifer Powers, an ecosystems ecologist at the University of Minnesota, Twin Cities. And it suggests that when it comes to tropical forests, “generalizations are not warranted.”

Indeed, the work indicates it won’t be easy to predict how secondary forests growing now will turn out. “You cannot assume the forest that you know is 5 years old will look like the forest that you know is 20 years old,” agrees Stefan Schnitzer, an ecologist at Marquette University in Milwaukee, Wisconsin, who was not involved with the work. “It will be a big bummer for a lot of people studying chronosequences.”

Whether deliberately replanting trees is needed to help steer a land back to its previous state is also an open question, the study suggests. “What the paper says is, don’t count on the forest that you replant being the forest next door,” Schnitzer says. “You can still reforest but you still don’t know what you will get.”