Why So Many Tree Species Can Coexist In Species-Rich Forests

Eddie Gonzales Jr. – MessageToEagle.com – 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.

The spatial distribution of tree species in species-rich tropical forests is highly complex. The figure shows the spatial position, species identity (colour) and size of individual trees in a 500 m × 1000 m study plot on Barro Colorado Island (Panama), which was also analysed in the study. Photo: UFZ

An international team of scientists led by the Helmholtz Centre for Environmental Research (UFZ) discovered unexpected patterns in tree species distribution.

The Forest Global Earth Observatory (ForestGEO) network of the Smithsonian Tropical Research Institute features over 75 permanent forest dynamics plots in 29 countries. It offers excellent forest inventories to study ecosystem dynamics and understand the processes driving forest structure and function.

On these 20-to-50-hectare plots, every tree thicker than a pencil is identified, measured, and mapped every five years—often totaling over 200, 000 trees.

UFZ researchers Dr. Thorsten Wiegand and Prof. Dr. Andreas Huth studied 21 forest megaplots from tropical, subtropical, and temperate zones. Their team used ForestGEO data to analyze tree species distribution and the processes responsible for their spatial patterns.

Image credit: Pexels – Pixabay 

“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 Thorsten 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 centimetres as found in the forests.

“The closer the forest plot to the equator, the less likely rare trees had a nearby tree of the same species,” says Andreas Huth.

In temperate forests, only slight differences exist between common and rare species, leading to systematic changes in spatial patterns from the tropics to temperate latitudes. This raises two questions: What are the consequences for tree species coexistence, and what processes cause these changes?

To find answers to these questions, the researchers used information on the dispersal mechanisms of the different species.

“Roughly 70 to 80 percent of tree species in the tropics are dispersed by animals, but much less in temperate forests,” says Thorsten 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 neighbourhood 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 Thorsten Wiegand.

To understand the consequences of spatial patterns for species coexistence, UFZ researchers used spatial simulations and a new mathematical theory.

“We wanted to know under what circumstances tree species could coexist,” says Andreas Huth. Stable coexistence requires that rare species increase in abundance. Using mathematical models, UFZ researchers developed a formula describing population growth at low abundances, incorporating a risk factor from various influences.

They found that more common species with many same-species neighbors have lower risk factors and higher coexistence chances. In temperate forests, species generally have low risk factors. Although tropical forest risk factors are greater, other aspects like animal seed dispersal compensate for this. “Overall, tropical and temperate forests exhibit optimal but contrasting spatial structures promoting coexistence,” concludes Thorsten Wiegand.

Now, researchers aim to develop a general theory for understanding the spatial dynamics and stability of species-rich forests, supported by an Advanced ERC Grant from last year.

“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. Around 2.5 million euros will be available to them for this work over the next five years.

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Written by Eddie Gonzales  Jr. – MessageToEagle.com Staff Writer