Sayana Sabu, a class X student from Kerala, was intrigued by a tree in her backyard that she had been observing for three years. This was a Kanikonna tree (Cassia fistula, a.k.a. Indian laburnum), expected to be in full bloom by the festival of Vishu on April 14/15 every year. However, this tree was flowering on Vishu as well as at all other times of the year! This was strange. In the past, one knew it was Vishu when the countryside was painted bright yellow by Kanikonna trees in full bloom. What was happening?
In tropical peninsular India, changes in air temperature through the year are small compared to those happening in the temperate regions of the world. As a result, the textbook seasons – summer, autumn, winter and spring – don’t apply. However, most trees are seasonal anyway, flowering and fruiting at specific times of the year. They sense the passage of time using a combination of changes in temperature, day length and moisture, and react by dropping old leaves, flushing new ones, putting out flower buds and/or ripening their fruits.
The harbingers of the tropical Indian summer are the bright flowers of trees that have lost all their leaves during the drier, cooler months of December, January and February. The flame of the forest tree, the Indian coral tree and the red silk cotton tree change from bare and stark into their orange-red floral regalia and further to sombre green within weeks. Later, the arrival of fruits like mango and jamun (Syzygium cumini) tell us that it is summer. The greening of the ground, crowded with grasses, ephemeral flowering herbs and tree seedlings, indicate that the monsoon has arrived in earnest.
In Kerala, the Kanikonna flowers are used during Vishu celebrations and are an inseparable part of the local culture. The tree itself is unmistakable when in full bloom, golden-yellow flowers held in droopy chandeliers on (sometimes) completely leafless trees, visible from far away. But the residents of Kerala have been noticing that, over the last few years, the Kanikonna has been blooming at odd times during the year. This could be a cause for worry; are trees receiving mixed signals from their environment? Is climate change to blame, working via an erratic monsoon and increasing temperature?
To understanding the typical pattern of Kanikonna flowering, we need to look at lots of trees, and for a long enough period, to find when in the year each tree blooms fully and whether all trees bloom together. This is a daunting task and requires lots of eyes, lots of trees and lots of patience!
As it happens, a group of dedicated teachers and school students, like Sayana Sabu, have been doing just this as part of a citizen science project called SeasonWatch. What can their efforts contribute to the Kanikonna question?
The easiest way to check flowering patterns is to count the number of trees in groups of 100 trees each that have flowers at any given point of time (say, every week). This percentage gives the magnitude of trees blooming at the state level. If we were to plot this percentage on a graph, where the horizontal axis runs along weeks from 2015 to 2018, and the vertical axis shows the percentage of trees with at least a few flowers, we get something that looks like a cardiogram. But instead of showing the beating of a heart, this cardiogram shows the pulse of the trees:
The red lines in the graph indicate Vishu week in that year. You may notice that at least 10% of trees always have a few flowers throughout the year, but that 80-100% of trees have some flowers around the time of Vishu.
This is how the percentage of trees with any flowers changes through the year. If we were instead to plot the percentage of trees in full bloom – trees in which all or most branches have flowers – we get another, more revealing tree cardiogram:
You might notice that up to 80% of all trees are in full bloom just before Vishu week. In 2017, 75% of trees had already fully bloomed before Vishu. What do these graphs tell us? It seems that the Kanikonna’s maximum flowering happens shortly before the Vishu festival begins in Kerala, rather than coinciding exactly with it. In 2018, peak full bloom happened earlier than all previous years; further, the trees came back into full bloom after Vishu week as well.
So school teachers and children in Kerala have observed two things: Kanikonna comes into full bloom just before Vishu and some fraction of the trees are in flower throughout the year. Are these patterns ‘aberrant’? Some caveats are needed here. Of the lakhs of Kanikonna trees in Kerala, only about 60 are represented in these graphs. Do these 60 trees reflect the larger pattern?
Additionally, the information represented above represents only the last few years. Whether flowering patterns have been shifting can only be answered with several more years of observation.
Even by observing a few trees, intriguing seasonal signals are evident. To properly understand how the natural world is changing, more trees (of all species) will need to be monitored, and over a longer period of time. Then, we might be able to say if climate change has indeed been playing a hand, whether those who celebrate Vishu will have to make do without their iconic flowers 10 years from now, etc. This is a call to all interested citizens: let’s join hands and work together towards a better understanding of the unprecedented changes unfolding around us.
Geetha Ramaswami heads the SeasonWatch citizen science project and, together with Suhel Quader, is with the Nature Conservation Foundation. SeasonWatch is run as a collaboration between the NCF, the National Centre for Biological Sciences and Wipro Foundation. The regional partner in Kerala is Mathrubhumi-SEED.