One might think that a tree produces a particularly large number of offspring in its immediate vicinity through its seeds. But in tropical forest ecosystems, the opposite is true, one study documents, where trees grow surprisingly far from other specimens of their own kind. This has to do with the negative effects on growth that occur when trees of the same species are in close proximity. However, the inhibiting effect has a beneficial effect on the development of biodiversity in tropical forest ecosystems, the researchers explain.
Their biodiversity is legendary: The tropical rainforests of the world are hotspots for animal and plant biodiversity. As far as the trees are concerned, there are hundreds of different species per square kilometer in some locations. A better understanding of the factors that lead to this small-scale coexistence can benefit the protection of threatened forest ecosystems in the tropics. Scientists in a study area in Panama have been dedicating themselves to this research topic for many years. It is a forest research area the size of 100 soccer fields, from which detailed information on the tree species and their spatial distribution on the area is available.
Patterns of tree distribution on the trail
As part of the current study, the team led by Michael Kalyuzhny from the University of Texas at Austin has now further explored a phenomenon that has already emerged from earlier study results: Species-rich tree populations are more productive than less diverse or even monocultures. Neighborhood effects are considered to be the cause: A single tree thrives better if it is surrounded by different tree species with different resource requirements and susceptibility to pathogens. In order to uncover patterns in the tropical forest area that could reflect this principle, the scientists recorded the spatial distribution of tree specimens from 41 species and analyzed them using computer models.
“Because of the abundance of available data on this particular forest area, we knew the exact location of each tree and also how far its respective seeds might travel. This allowed us to first investigate what the forest would look like if the trees established themselves where the seeds are most likely to land,” says Kalyuzhny. The subsequent comparison of the models with the real distributions showed: "The real forest does not match the predictions at all - instead, the trees are much further apart," reports the scientist. On average, the trees in the studied rainforest in Panama even grow three times farther from other adult specimens of their own species than the adage “the apple doesn't fall far from the tree” would suggest.
factor of plant biodiversity
The team also used the models to explore what might be driving the apparent suppression of young trees in the neighborhood of their own species. It became clear that each tree species is much more negatively influenced by members of its own species than by others. A particularly important aspect seems to be that they suffer from species-specific enemies, the scientists explain: pathogens such as fungi or herbivorous insects can jump over more easily in close proximity and kill the more sensitive young plants.
Overall, however, this self-incompatibility of the trees leads to more biodiversity, the scientists explain. Because the effect prevents one tree species from dominating the forest and habitats are formed for others: in the vicinity of a specimen of a certain species, other tree species can grow, which in turn ensure that the surrounding area does not become overgrown. According to the researchers, their study thus contributes to a further understanding of the development processes in the globally important and severely threatened tropical forest ecosystems. "Trees are the engineers that provide resources for the entire ecosystem, and since most of the world's species live in the tropics, we need to better understand what sustains the biodiversity of planet Earth," concludes Kalyuzhny.
Source: Communication from the University of Texas at Austin, professional article: Science, doi: 10.1126/science.adg7021