Introduction

It is said that humans and animals are the only living beings that are able to communicate, saying that they can send and perceive signals from others. Plants on the other hand are quiet. They don’t have a mouth to talk, no eyes to see nor ears to hear. It seems that they are isolated and only live for themselves. But this is not true as research has shown that plants just use different channels to communicate with each other. Usually this happens below-ground via their roots which are part of a symbiosis with mycorrhiza fungus that act as connection between the plants and transport nutrients, water and information from one to another. Above-ground communication takes place via chemical substances released by the plants and transported by the wind to their companions. However, communication is not only about talking, but also interpreting “body language”: Researchers were able to prove that plants can sense the presence and behaviour of other plants in their environment and act accordingly. And as in humans, also plants have friends or species they like and others they dislike. So, one must decode their body language and see how they behave next to another species in order to figure out the relation between them. This is very useful when implementing new (agro-) forestry parcels to achieve a productive and harmonic plant network. It especially is of mayor importance, if wood or fruit tree species are involved as their growing pattern crucially determines the latter wood quality and productivity. This systematization aims to describe the (symbiotic or competitive) relationship between tree species found at Mollesnejta, a research centre for Andean agroforestry.

It is said that humans and animals are the only living beings that are able to communicate, saying that they can send and perceive signals from others. Plants on the other hand are quiet. They don’t have a mouth to talk, no eyes to see nor ears to hear. It seems that they are isolated and only live for themselves. But this is not true as research has shown that plants just use different channels to communicate with each other. Usually this happens below-ground via their roots which are part of a symbiosis with mycorrhiza fungus that act as connection between the plants and transport nutrients, water and information from one to another. Above-ground communication takes place via chemical substances released by the plants and transported by the wind to their companions. However, communication is not only about talking, but also interpreting “body language”: Researchers were able to prove that plants can sense the presence and behaviour of other plants in their environment and act accordingly. And as in humans, also plants have friends or species they like and others they dislike. So, one must decode their body language and see how they behave next to another species in order to figure out the relation between them. This is very useful when implementing new (agro-) forestry parcels to achieve a productive and harmonic plant network. It especially is of mayor importance, if wood or fruit tree species are involved as their growing pattern crucially determines the latter wood quality and productivity. This systematization aims to describe the (symbiotic or competitive) relationship between tree species found at Mollesnejta, a research centre for Andean agroforestry.

Study site and methodology

Mollesnejta is situated on the hillside of the Tunari Cordilleras in the valley of Cochabamba, Bolivia. Its position is at 2.800m above sea level and the climate is semi-arid. One principle at Mollesnejta is about keeping the system as natural as possible which means that the planting method follows the natural succession of plants that includes high biodiversity and very dense planting. Because of the proximity of all species planted many interactions take place. Perceiving the below-ground interactions and body language is barely impossible, only having simple methods, but above-ground, simple observations can tell a lot about how trees get along with each other. There are around 90 tree species at Mollesnejta, mostly native to the area like Molle (Schinus molle), Jacarandá (Jacaranda mimosifolia), Tecoma (Tecoma ssp.), or Ch’acatea (Dodonaea viscosa) but also introduced species like Pines (Pinus ssp.), Walnut (Juglans regia), Apple trees (Malus domestica) and Eucalyptus (Eucalyptus ssp.). All species were recorded in an Excel sheet to form a table in which each behaviour of a species can be compared to another. An important factor is the height or age of the two interacting trees. Usually one is taller and has more influence on the smaller tree. Therefore, the vertical list of species in the table always represents the smaller tree in the observation while the horizontal list of tree species represents the taller one. When interacting, the following outcomes are possible:

1. The tree species like each other and the small tree grows straight through the crown of the taller one. They share the given space and respect the presence of each other. The relationship is symbiotic. (marked green in the table)

Example: Molle (Schnius molle) and Chirimolle (Zanthoxylum coco)

• The trees do not like each other and one species (usually the smaller one) has to find its way around the crown of the other species or tries to escape by growing in unusual directions. The relationship is competitive. (marked red in the table)

Example: Jacarandá (Jacaranda mimosifolia) does not like Paraíso (Melia azeradach)

• The trees actually might get along with each other but, it is not clear. The relationship can be symbiotic or competitive, depending on other factors. (marked orange in the table)

Example: Molle (Schnius molle) and Carawancara (Vasconcellea quercifolia)

The observations sometimes can be contradictory because not only other trees influence the growth of a species. As the institute is at a slope wind plays a major role. Sunlight, the availability of water and the tree-cut itself also determine the growing behaviour of a tree. Therefore, it is crucial to examine which of the mentioned factors is the driving force.

To better explain how the observations work, here is the example of Molle (Schnius molle) and Carawancara (Vasconcellea quercifolia):

In the picture above, you can see the small tree Carawancara that first grows straight but then bends over almost horizontally because it apparently does not like the Molle above it. One might say that there is still plenty of space above to grow some more, but if you look closely, there is a broad wound on the stem of the Molle that indicates that there used to be a branch that grew directly above the Carawancara causing the adverse growth pattern. However, it cannot be said that they do not like each other like described in 2. Carawancara is an endangered species that cannot be sown by hand, its germination only occurs naturally which makes conservation very difficult. It was observed that most Carawancara trees germinated right next to a Molle tree, sometimes less than 20cm away from the stem. This suggests that the below-ground environment of a Molle with its microorganisms favours the growth of the Carawancara seed. It seems like it needs the Molle to germinate and live. Subsequently, it can be concluded that the endangered species depends on the Molle, but its crown cannot compete/live in harmony with the crown of the Molle and gives way. Being a very rare species, the Carawancara has a higher value and its survival is crucial for species conservation. Therefore, the Molle needs to be pruned in order to reduce stress in the Carawancara and guarantee a natural growth pattern. Because of this complex relation, this tree combination belongs to the third group that gets along with each other under certain conditions.

Results and conclusions

As already mentioned, there are 90 tree species at Mollesnejta. So, in total 8100 different interactions and observations are possible, which requires much time. Especially because new trees are planted continuously in order to increase biodiversity. Further, there are some examples like oak (Quercus ssp.) or orange trees (Citrus sinensis) of which only few specimen grow on the property so that they physically cannot interact with all other species. Additionally, some newly planted species are still too small to assess their compatibility with others. Therefore, the table cannot be completed yet. However current results show that most trees have a symbiotic relationship. Even though, it cannot be said that one tree species is friends with all others, so no tree is generally compatible or not compatible with others. It also appeared that some species like Eucalyptus (Eucalyptus globulus) or Ch’acatea (Dodonaea viscosa) have very strong auras so that they are usually not affected by other taller trees, neither let smaller tree species grow through their crown. Further, it was observed that there seems to be more competition during the rainy season than during the dry season. This suggests that the trees might depend on each other more in the months without rain and try to work together in order to save energy instead of using it to compete with others. Eventually, it can be said that there is still a lot to observe and many interactions will only be visible in a few years. Therefore, the current main conclusion is that all tree species are individuals that have their own preferred consortium of trees in their environment which must be considered when implementing a new system.

Autor: Jakob