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A new angle in the theory that explains the variety of tree species in the rain forest

"Until now, the theory explained the existence of a variety of species of trees in the rain forest only through the existence of the natural enemies of each species, mainly insects and fungi. The current study puts emphasis for the first time on the conditions that will allow these enemies to thrive," explains the lead researcher

Rainforest. Illustration:
Rainforest. Illustration:

A new study conducted at the university - Oranim campus and published in the prestigious journal NATURE He found that in high soil moisture conditions, the survival chances of plant species that are in high density decrease and as a result, in the following year, a higher variety of species will be found in the same area. "Until now, the theory explained the existence of a variety of species of trees in the rain forest only through the existence of the natural enemies of each species, mainly insects and fungi. The current study puts emphasis for the first time on the conditions that will allow these enemies to thrive," said Dr. Edwin Lavreha-Trakhos, one of the authors of the study.

Tropical rainforests are one of the most complex and important ecosystems for sustaining life on earth, mainly as the "green lungs" of the earth. A key component of the rainforests is the large species diversity of tree species. A large variety of tree species is important for the proper functioning of the ecosystem, because a large variety can cope with the vulnerability of one species or another. "You can think of it like a factory where there is only one person who knows how to do a certain task in the chain of operations, compared to a workplace where there are different employees who know how to do several tasks. In the first case, as soon as that person gets sick, all activity stops. In the second case, there will be someone who will back him up. The same is true in the rainforest. If there is a dominant species that can carry out photosynthesis in harsh conditions and it is harmed for any reason, the ability to photosynthesize will decrease. However, if there are other species that also know how to photosynthesize in difficult conditions, the forest will continue to produce oxygen," said Dr. Lavriha-Trakhos.

In the current study, Dr. Labriha-Trakhos from the Department of Biology and Environment at the Oranim University-Campus and researchers from the Smithsonian Tropical Research Institute in Panama based the 50-year-old theory of Janzen and Connell, which is one of the most prominent theories in the study of species diversity in forests. "One of the things that keeps a high variety of plants in these rainforests are the natural enemies of the species. The idea behind the theory is that the natural enemies of the plants: insects or fungi are unique to each species or at least, mainly harm one species. As soon as one species becomes dominant, and therefore its density increases, the ability of its natural enemies to harm it increases, especially in the young seedlings, which allows other species to establish themselves. Thus, in fact, a natural cycle is created that prevents a certain species of tree from maintaining dominance over time, which helps maintain the diversity of species in the forest," said Dr. Lavriha-Trakhos.

 At the basis of the new model that Dr. Labreha-Trakhos asked to develop, is the concept that if the theories of natural enemies are correct, the conditions that allow their development will also be of great importance. And since these are insects and fungi, a rainy season in which the soil will be saturated with water will increase the amount of insects and fungi - and make the mechanism "work better". Whereas a fallow year, in which the soil will be drier, will lead to the opposite result. "In fact, our model adds a new angle to the existing theory. "In order to maintain and preserve a variety of species in the tropical forest, it is important to have conditions that will allow the natural enemies of the trees to thrive," said the researcher.

However, in order to prove this, the researchers needed a lot of data, which fortunately exists in the rain forests. Dr. Joe Wright (Joe Wright), the research partner, managed since the 90s 800 plots measuring 1 square meter, and in each plot he knew on an annual basis which seedlings there were, which ones died and which ones survived. Another database used by the researchers included a count and description of each and every tree, the trunk of which is wider than 1.3 cm and 500 meters high on an area of ​​80 dunams in the forest, starting in the 90s. By crossing these two reservoirs it was possible to know for each year, which seedling survived, which did not - and which mature trees were around it, that is, whether there was a dominance of the same species around it or not. Another third database on which the researchers were based, included information from the last hundred years of soil moisture and precipitation amounts. With the help of this database, the researchers identified the hottest and wettest year in the century. These two extreme values ​​have been measured since the XNUMXs.  

Thanks to all these databases, the researchers in the current study were able to build a model that examines the effect of soil moisture on plant survival, as a function of the variety of species identical to it found in its immediate environment.

The results of the study show that in years of high humidity in the soil, species that are in high density, their chances of survival are low and in the following year they will find a greater variety and variety of plants in the area. It was also found in the study that the variety of plants is greater after a year in which the moisture in the soil was high compared to a dry year. That is, in a year when there was a lot of precipitation, the insects and fungi thrived, and could "do their job" in diluting individuals of dominant species. To prove that it is indeed the proliferation of the insects and fungi, and not that rainy years are "good" for all kinds of trees and are the ones that cause the increase in diversity - the researchers examined the places where there was not a high density of the same species and showed that in these places the rain or drought did not significantly change the survival of the trees . That is, only the combination of density and dominance of one species and a rainy year and moist soil that increased the natural enemies contributed to the fact that in the following year - the diversity of species will increase.

"The model we built proves that dryness and humidity indirectly affect the variety of species and compared to normal years there is a 15% decrease in the variety of species in the driest years and a 15% increase in the variety of species in the wettest years. Beyond the important addition to understanding the mechanism that maintains species diversity, the connection we found to soil moisture links us to climate change and global warming. Therefore, the more dry years we have, the more there is a fear of damage not only to the variety of species in the rainforest, but also to its contribution to humanity," said Dr. Lavriha-Trakhos.

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