Watching Trees to Understand Climate Change

The neem tree growing outside my house is a shape-shifter. Every year, without fail, in the cool dry winter its leaves become tired and yellow-brown, and sometimes fall off completely. Just as the weather turns a bit warmer, heralding the summer, the neem miraculously becomes green again. Then the shy white flowers emerge, hiding in the foliage, difficult for the human eye to spot except from the hum of tireless pollinating insects and the scent of honey. Two months later, yellow-green fruits invite a menagerie of fruit-eating birds and animals to a summer feast, couriering seeds away in their bellies, which would grow into shape-shifting neem trees elsewhere. At other times mosses, mushrooms, spiders, geckoes, squirrels, bugs, and the occasional snake lead their own secret lives in the shaded sanctuary of the neem.

The study of the seasonality of trees can tell a lot about the environment – even provide evidence for climate change. In temperate regions of the world, plants shed all their leaves in the extremely cold winters and can start growing again only after the warmer spring season arrives. The arrival of the growing season is outwardly visible in the form of new leaves. Global-warming induced climate change is now affecting the seasons – it is getting warmer earlier and cooler later in the year. Over the past few decades, trees have been responding by putting out new leaves earlier and dropping leaves later than usual in the year. In the tropical latitudes where we live, information on tree seasonality is difficult to come by, and therefore the effects of climate change on these patterns is as yet unknown.

How can one detect such patterns, though? Through lots of research and data collected about a factor of interest. Here’s an illustration – think about your height; it is most likely determined by genetics, but nutrition may also be a factor. The heights of all the other humans you know are also affected by their own peculiar individual conditions. As a population of humans, we can therefore ascertain an average height with an estimate of the variability around it. If one detects an increase in this average human height over time (say 150 years), one can speculate about the overall reasons (say, better childhood nutrition) for these changes occuring at the level of human populations.

It is exactly the same for quantifying seasonality in trees! Understanding seasonal patterns in trees requires information on the onset date of change, frequency of the change, and quantum of change over many years. Once this information is acquired, long-term averages and the variability in these patterns can be ascertained, and the causes for shifts from known patterns can then be explored. To detect the effect of factors like climate change, one must first find evidence of environmental change (such as an increase in temperature), and then spot a corresponding departure from the expected long-term tree seasonality. Researchers are often logistically constrained in collecting information of this kind, at large enough scales (of space or time), to be able to reliably estimate the seasonality in living organisms. This is where non-professional scientists, trained in collecting the desired information, can contribute through citizen science projects to fill in gaps.

SeasonWatch is a citizen science project aimed at understanding the seasonality of trees, and the effects of climate change on this
Photo Credit: Geetha Ramaswami

One such citizen science project documenting country-wide seasonality of common Indian trees is SeasonWatch. Since 2010, the project has collected > 4,00,000 observations on > 90,000 trees belonging to 134 common species of India. This long-term data has helped deduce the patterns of emergence of leaves, flowers and fruits of the most-observed species like Mango, Jackfruit, and Indian Laburnum. This information will now serve as a baseline to compare any future changes in these species. In the Indian Laburnum, SeasonWatch data showed a mild advancement in peak flowering dates compared to culturally known peak flowering dates. Whether this is a result of climate change is a conclusion that can be arrived at only by observing more trees for more time. For now, I will make a note of the arrival of flower buds on my neem tree, contributing to an ever-increasing repository of information, one tree at a time.

SeasonWatch is a citizen science project aimed at understanding the seasonality of trees, and the effects of climate change on this seasonality, across India. Students from more than 1200 schools and more than 1100 interested individuals contribute weekly information on tree species that are commonly found across the country. Anyone can become a citizen scientist with SeasonWatch by registering on the site as a contributor and registering as many trees as one wants for observation. There is also a SeasonWatch Android app on Google Play, to help with making observations on-the-go.

Geetha Ramaswami is a Programme Manager at SeasonWatch, based at Nature Conservation Foundation. She is interested in all things plants, and finds their quiet lives intriguing and inspiring at the same time. She especially enjoys studying plants that have gone rogue – invasive plants – and how they interact with animals.

This series is an initiative by the Nature Conservation Foundation (NCF), under their programme ‘Nature Communications’ to encourage nature content in all Indian languages. To know more about birds and nature, join The Flock.