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The reason why so many tree species can coexist in species-rich forests has long been a subject of debate in ecology. This question is key to understanding the mechanisms governing the dynamics and stability of forests. An international team of scientists led by the Helmholtz Centre for Environmental Research (UFZ) has now discovered unexpected patterns in the spatial distribution of tree species, as in the journal Nature.

Their results suggest that tree species in tropical and temperate forests manifest contrasting coexistence strategies as a result of differences in the patterns of tree clustering and the abundances of tree species.

The data sets are very large: with more than 75 permanent forest dynamics plots in 29 countries worldwide, the Forest Global Earth Observatory network (ForestGEO) of the Smithsonian Tropical Research Institute (STRI) provides excellent forest inventories for investigating the dynamics of forest ecosystems and better understanding the processes that drive the structure and function of forests.

On these 20-to-50-hectare plots, every single tree with a diameter not much larger than a pencil has been identified, measured and mapped every five years, often totaling more than 200 000 trees.

The two UFZ researchers, Dr. Thorsten Wiegand and Prof. Dr. Andreas Huth took a closer look at 21 of these forest megaplots, which cover a gradient from the tropical to the subtropical and temperate zones. Their international team then used the ForestGEO data to analyze the distribution of tree species in the forests and which processes are responsible for their spatial patterns.

"The search for simple principles underlying the spatial structure and dynamics of plant communities is a long-standing challenge in theoretical ecology," says first author Wiegand, describing their research question.

For their analyses, the research team examined all individual trees with a diameter at breast height of at least 10 centimeters as found in the forests. "The closer the forest plot was located to the equator, the less likely it was that trees of rare species had a tree of the same species nearby," says Huth.

In temperate forests, in contrast, they found only slight differences between common and rare species. This results in unexpected and systematic changes in the spatial patterns from the tropics over the subtropics to the temperate latitudes. This intriguing finding immediately raised two questions: What consequences do these changes have for the coexistence of tree species and which processes cause them?

To find answers to these questions, the researchers used information on the dispersal mechanisms of the different species. "Roughly 70 to 80% of tree species in the tropics are dispersed by animals, but much less in temperate forests," says Wiegand.

Another important factor is mycorrhizal fungi. This network of fungi forms a symbiotic relationship with the fine roots of the trees to benefit both organisms: The fungi supply the trees with nutrients and water, receiving glucose in return.

"In temperate forests, mycorrhiza usually protect the roots of young trees in the neighborhood of large conspecifics from pathogens or insect pests," explains UFZ researcher Dr. Samuel M. Fischer, who was also involved in the study. In tropical forests, on the other hand, this is mostly not the case.

"That's why seeds in the tropics have to ensure that they are dispersed away from their parent trees, a job mostly done by animals," he says.

The conclusion: "In tropical forests, mechanisms such as seed dispersal by animals lead to the observed patterns, while in temperate forests, the patterns are shaped by mycorrhizal fungi," says Wiegand.

In order to better understand the consequences of the observed spatial patterns for species coexistence, the UFZ researchers used spatially explicit simulations and a novel mathematical theory. "We wanted to know under what circumstances tree species would be able to coexist," says Huth.

Stable coexistence generally requires that species that have become rare can increase in abundance again. Based on mathematical models of forest dynamics, the UFZ researchers have developed a novel formula to describe the population growth rate at low abundances. A key element of their formula is a risk factor that combines several influencing factors.

The result: the more common the species currently is and the more neighbors of the same species it has, the smaller the risk factor and the higher the probability that the species can coexist. Species in temperate forests generally have a low risk factor. However, the risk factors are often greater in tropical forests, but the formula includes additional factors that compensate for this disadvantage, such as the specific spatial patterns generated by animal seed dispersal.

"Overall, it turned out that species in tropical and temperate forests exhibit optimal—but contrasting—spatial structures that each promote coexistence," concludes Wiegand.

This newly discovered spatial mechanism now provides the starting point for further research. Wiegand and Huth want to develop a more general theory for understanding the spatial dynamics and stability of species-rich forests as part of their research.

"We want to substantially expand our methods and analyses, such as by taking into account the size of the trees, the immigration of species and more detailed species characteristics, as well as by using remote sensing data," he says.