The neighbourhood of green

by Aditi Mishra

“We make our friends; we make our enemies; but God makes our next door neighbour.” – Gilbert K. Chesterton

“You can be a good neighbour only if you have good neighbours.”

– Howard E. Koch

Neighbours don’t share homes, but they do share the entire environment nearby. And with that they share its public goods and nuisances.

It is a delicate balance of maintaining camaraderie while maintaining distance. And as mere mortals know, loving thy neighbour can be hard.

But humans are not the only one suffering.

Undisclosed to us, trees in forests always had their own neighbourhoods. And like our neighbourhoods, plant neighbourhoods also buzz with interaction.

Sounds crazy right? But truth is sometimes indeed stranger than fiction.

The neighbourhood of leaves

For more than a decade now, scientists knew that some tree species coexist better as neighbours in the forest than others. Trees impact the survival and growth of other trees in their immediate neighbourhood. Immediate neighbourhood for a woody tree species could be any area within a radius of fifteen meters. To help us visualize, fifteen meters is approximately the length of a standard city bus.

Woody neighbours compete and justifiably so, but the interaction between them can also be much beyond a straightforward competition for survival. Plants engage with their neighbours in myriad ways – some positive, some negative and sometimes with neutral but vital indifference.

Neighbours might challenge each other for water, sunlight, pollinators and nutrients from the soil. But the same neighbours also help each other by bringing up water to the surface soil, enriching beneficial soil microbes and defending each other from herbivores hungry to devour them.

Sometimes maintaining a community does not even demand effort. Mere presence is enough.

Being facilitative – the art of being positive while being passive

Scientists have repeatedly observed that certain neighbouring trees can facilitate the survival and growth of other plants under their benign canopy. The shade of such facilitative trees produces microhabitats helping other plant species to germinate and thrive.

Microhabitats are small patches with conditions allowing survival in otherwise harsh and discouraging environments. And these microhabitats can arise in multiple ways. The presence of shady facilitative trees in extremely sunny environments is one of them.

These shady facilitative trees are aptly named nurse trees. The nurse tree modifies the environment around them by their mere presence and opens up previously unviable spaces for other species to germinate and grow in.

Shading others is only one example of facilitative interaction. There are several other facilitative interactions between plants. Increased pollinator visits, immobilization of toxic heavy metals in heavy metal-tolerant plant and simply acting as physical barriers to plant pathogens are few examples of them.

In all of these cases plants impact their neighbours passively yet positively.

What makes passive positive?

Facilitative interactions occur without a doubt in plant communities. However, we don’t understand what encourages such interactions to emerge.

One school of thought is that neutral facilitative interactions emerge when the positive and negative impacts of engaging with other species balance each other out. Hence it is thought that facilitative interactions are more likely to emerge in extreme environments. Extreme environment could tip the scales to strengthen the value of positive interactions relative to negative interaction.

It is a theory that neatly explains the facilitation observed in harsh environments of deserts, alpine tundra and salt marshes. Experiments conducted by researchers give this theory more foothold. Researchers observed that occurrence of facilitative interactions among alpine plants increased as they examined plants at higher altitude.

Another school of thought proposes that phylogenetic distance, that is, relatedness between two organism determines if they will facilitate each other. In this case relatedness is all about when the two organisms shared a common ancestor. In this theory the force of competition drives and modulates facilitation.

To illustrate this theory, consider a highly competitive environment. Competition drives organisms to specialize on specific resources to survive. Much like Nestle specializes on KitKat and Cadbury on Cadbury Silk to survive in the competitive chocolate market. Both of these companies manufacture chocolate and compete for the chocolate guzzlers in the market. What ensures that they don’t wipe each other out of existence – well the fact that KitKat is slightly different than Cadbury Silk.

Similarly, competition in the wild sometimes compels organisms to specialise to survive. This allows organisms to play to their strengths and to extract the resource of their choice really efficiently.

No coming back to the phylogenetic theory, since the ability to exploit a resource is usually genetic, species closely related to each other are more likely to have similar strengths and weaknesses. Hence closely related species, when spatially close together, are more likely to exhaust a similar kind of resource.

They would then more likely crowd themselves and step on each other’s toes. Closely related individuals have similar needs hence they have no choice to compete rather than cooperate.

Familiarity does breed contempt.

Over time special proximity forces closely related species with similar interests to spread away from each other. After all it is quite hard if you and your neighbour are always competing for the same thing.

When plant species are distantly related to each other, the species are more likely to have different needs. Differences in needs leads to exploitation of different resources and can potentially allow these neighbours to play to each other’s advantage without competing.

Hence it could actually be beneficial to have a neutral distantly related neighbour rather competitive kin. As is seen for the columnar cacti of Mexico. Studies find that more distantly related neighbouring columnar cacti thrive better.

But what happens when siblings are nothing alike and strangers turn out to be functional doppelgangers? The above theory assumes that closely related species would behave similarly.

Unfortunately, the real world is not so organised, similar individuals behave differently in different environments while diverse individuals could exhibit similar behaviours in the same environment.

Additionally, convergent evolution, that is, unrelated organisms developing same traits independent of each other, is also fairly common. Hence genetically dissimilar species could still be similar in their resource choices, making each other their competitors. Genetic closeness is a good proxy for similarity but not similarity itself. After all, individuals are more than their genetic barcode

What could predict a cooperative neighbourhood?

While it is hard to say what leads to facilitative interactions between neighbours, one thing is certain that facilitation is built upon the traits of the participating neighbours. After all non-competitiveness is also a trait.

Hence a true predictor of a community’s dynamics is relying both on the environment and participant’s genetics – quirks that genes encode, and environments express.