‘Crown shyness’: Social distancing phenomenon observed in trees

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July 26, 2020 12:01 AM

Crown shyness, a phenomenon observed in some tree species wherein the crowns of trees do not touch each other, helps trees stay healthy

Some trees that display crown shyness include black mangrove, eucalyptus, camphor and Japanese larch.Some trees that display crown shyness include black mangrove, eucalyptus, camphor and Japanese larch.

Social distancing is being observed by millions of people around the world today to slow the spread of coronavirus. It’s not, however, a uniquely human intervention. Plants also follow the concept. Take, for instance, crown shyness, a phenomenon observed in some tree species, wherein the crowns of trees do not touch each other, forming gaps, which help trees share resources and stay healthy. Some trees that display crown shyness include black mangrove, eucalyptus, camphor and Japanese larch.

To understand the process, it’s important to first know about the growth and morphology of trees in the forest ecosystem, which is largely governed by plant genotype, soil and environment (especially precipitation and temperature). Trees in rainforests with dense stocking tend to have close canopies where the crowns seem to be touching each other, whereas in dry deciduous forests with low rainfall, trees are found scattered with large gaps between canopies, says Jyoti K Sharma, professor and head, Center for Environmental Sciences & Engineering, Shiv Nadar University, Greater Noida.

“When viewed standing under the trees in rainforests, the channel-like gaps running in a zig-zag fashion between canopies of adjacent trees become very conspicuous. This is a form of social distancing, or crown shyness, in which the crowns of fully-stocked trees do not touch each other. In the case of plant pests and diseases (such as coffee rust, eucalyptus blight disease), it is well known that where the plant density is very high, with no social distancing, the occurrence and spread of disease is very rapid. Social distancing (plant density per unit area) is one of the important pest management strategies,” says Sharma, who was the former director of the Kerala Forest Research Institute.

Crown shyness has been a topic of scientific debate among scientists since the 1920s, with them proposing multiple possible reasons behind it. Some say it maximises resource collection and minimises harmful competition, while others hypothesise that it’s from abrasion of leaves due to wind. “Some hypotheses suggest that the intertwining of canopy branches leads to ‘reciprocal pruning’ of adjacent trees. As a result of abrasions and collisions, there will be an induced crown shyness response. In dense forests with closed canopies, there will be intense competition between plants for light. Possibly, the gaps in the canopy resulting from crown shyness allow trees to increase their exposure to light and optimise the process of photosynthesis. There are many hypotheses suggesting that crown shyness might possibly inhibit tree-to-tree spread of leaf-eating insect larvae or spread of diseases. But there is no scientific evidence available for these hypotheses,” shares Sharma.

Gurugram-based rewilder Vijay Dhasmana works on ecologically restoring degraded landscapes and has curated the Aravalli Biodiversity Park on a mining site. Dhasmana feels social distancing has an entirely different meaning for plants. “The semi-arid forests of Aravalli have some trees that live in close proximity, some are sparse, some are confined to specific habitats, while others are generalists. Dhau trees live in close proximity and are found on hilltops or steep slopes most of the times if the forest is not degraded. However, many plants, which are nepotists, do not let other kinds of plants grow near them such as Vilayati kikar, an invasive alien species brought by the British from central America. It is chemically armed with alkaloids that it secretes in its root zone to discourage other plants to germinate or establish. It is so successful that Vilayati kikar forests are the least diverse in the NCR region. There are some other unusual species too…. there is a patch of wild moringa, of about eight trees, in Jaisalmer, that may be 150 km from the nearest moringa patch,” says Dhasmana.

There are other resilient species as well. Grass may look delicate and prone to being bulldozed by large trees, but it has strategies to prevent it from being consumed by forests. It might burn down, but its rhizome remains potent and with the first rain it comes alive again, dominating the landscape.

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