Tracking seasons through changing tree behaviour [Commentary]

Over the past two decades, citizen science has emerged as a powerful means to democratise the production and accessibility of scientific knowledge, enabling non-professionals to initiate, contribute, or participate in scientific research, making problem-solving mainstream. Citizen science is especially valuable in documenting biodiversity — people can collaborate and collect, collate, and verify the diversity of life forms across large spatial scales and even over the long-term. It is now an essential tool for understanding enduring ecological phenomena, such as shifts in species distributions and changes in population dynamics over time.

Take, for instance, iNaturalist. Anyone with a smart phone and an eye for plants, insects, fungi, birds, and larger animals can contribute photos to the portal, and get a verified identification from a pool of experts who volunteer their time for this purpose. This location and species data can then be accessed by diverse end-users. The data has also been utilised in understanding multiple ecological processes — from first records of invasive species, to the timing of flowering plants and animals.

Long-term, large-scale data on the occurrence of birds via millions of observers on eBird have helped identify migration paths of long-ranging species. The same database has also been used to understand the population dynamics of birds in the State of India’s Birds. It comes as no surprise then that citizen science is emerging as a vital tool for understanding biodiversity response to climate change.

Seasonality of living beings and climate

Living beings respond to changing seasons through altered seasonal behaviours. Invertebrates lay eggs, molt, and become adults in specific seasons; birds’ nesting is seasonal; and most trees put out leaves, flowers, and fruits during specific seasons. As the climate changes, and the warm season becomes longer across the globe, trees are responding by (usually) advancing the emergence of leaves and flowers. Trees can thus act as reliable indicators of changing weather patterns.

The changing seasonality of trees may have cascading effects on the life-cycles of other organisms up the food chain. For instance, in northern temperate latitudes, an advancing spring season has also resulted in pedunculate oaks advancing their time of leafing post-winter. Herbivorous winter moth larvae that feed on these emergent leaves, have also adapted to this change by emerging earlier from eggs. However, vertebrates such as passerine birds that often rely on moth larvae as a food source while nesting, have not advanced their seasonal behaviour; it is likely that they either face a food shortage or will need to shift to a different food source for hatchlings and fledglings.

The Japanese Sakura tree is another culturally iconic species which has been celebrated and observed both informally and formally for more than a 1,000 years now (it is perhaps the oldest citizen science programme in the world)! Like many other temperate trees, this species is also responding to a warming climate by advancing its flowering time. But a 1,200-year chronology of peak flowering dates in Kyoto, Japan, helps us determine the time when this advancement began: the early 1900s, concurrent with the increased concentration of greenhouse gases in the atmosphere, following the industrial revolution. Similar advancing flowering patterns are also being inferred at citizen science projects like the National Phenology Network for some plant species in the USA. And what would become of the pollinators that rely on the emergence of flowers to complete their life cycles? One can only speculate: A mere 10-day advancement in flowering dates may mean life or death for short-lived invertebrates that rely on this floral resource.

Tropical trees respond to seasons

But what of trees in the tropics? Tropical trees are diverse and have as many ecological strategies to survive and thrive in these environments. In some cases, the same species may show different seasonal strategies depending on the local climate and environmental conditions. It is a Herculean task to firstly understand how a tropical tree species responds to the local climate given all the variation, and then secondly to understand what climate change is doing to that variable seasonality. The only way to disentangle the two is to collect tree seasonality information at large scales and over the long term for as many tree species as possible. Mainstream scientific research is able to achieve very high quality, high resolution seasonality information for trees at monitoring stations. But in countries like India, often such stations are unsustainable over the temporal scales at which the weather and consequent tree seasonality change.

Citizen scientists have come to the rescue here, too. Volunteer observers for SeasonWatch monitor and record the seasonality of more than 170 tree species from across India, following a simple, standardised observation protocol. Since its inception in 2010, SeasonWatch with most of its volunteers being school children, has accumulated more than 850,000 observations on more than 1.5 lakh trees, especially from the state of Kerala. This data gets stored in a database, gets screened via an algorithm for potentially erroneous observations on seasonality, and then gets analysed and disseminated back to the public. SeasonWatch’s data products include the annual seasonality of all monitored species across India, and the seasonality of three tree species in relation to the environment in Kerala (where enough sample size allows us to understand these correlations).

Over the past 10 years, culturally beloved trees like mango, jackfruit, and the Indian Laburnum have been showing interesting seasonality patterns in Kerala. Mango and jackfruit track temperature and precipitation patterns — flowering during warm, dry periods and fruiting when it is wetter. Rapid urbanisation seems to be exacerbating the effect of temperature on flowering. Imagine a future climate where floods and droughts are more frequent, or when the usually predictable monsoon season becomes more unpredictable: What would happen to our most favourite summer fruits? SeasonWatch is now exploring these scenarios using machine learning techniques. The Indian Laburnum seems to be showing a small shift in the timing of peak flowering in the year. Usually coinciding with the Malayalee festival of Vishu (which has consistently fallen on April 14 or 15 for at least the past 50 years), peak flowering may be occurring up to a week earlier than usual. The next question to explore, is then in the context of invertebrate pollinators who depend on the grand golden inflorescences of this species. These seasonal patterns are brand new insights with potential application in agriculture, and building resilience to climate change.

In addition to invaluable scientific data, the programme offers children the precious opportunity to befriend and interact with trees in their surroundings, reviving a sense of connection and appreciation for nature, something that our current schooling systems often overlook or disallow. This motivated us to create the Climate Change Educators Handbook for teachers who want to bring nature into the classroom and talk about this global phenomenon through curiosity and hands-on experience, instead of fear and anxiety. We believe that this community of young people will develop a scientific temper; use experimentation, observation, and evidence to understand their environment; and feel pride in becoming a part of the solution.

Geetha Ramaswami is a scientist and the team lead at SeasonWatch by Nature Conservation Foundation, India

Banner image: Flowering of Dillenia pentagyna trees in Sanjay Gandhi National Park, Maharashtra. Trees act as great indicators of climate change, as many are responding to warming by flowering sooner than normal. Representative image by Dr. Raju Kasambe via Wikimedia Commons (CC BY-SA 4.0).