Rhythms of tropical trees

In nature, change is constant. Most ecological processes – from species’ reproductive activity to carbon cycling by entire ecosystems – show rhythmic cycles across seasons and years. However, the nature of these rhythms is itself changing. As temperatures rise, rainfall patterns shift, and droughts and extreme rainfall events become more frequent and destructive in a changing climate, and human pressures on land- and water-scapes intensify, ecosystems are experiencing very different conditions from those under which they evolved. Studying how ecological cycles are responding to these novel and changing conditions can provide a bellwether of emerging conservation and restoration challenges. For slow-growing and long-lived organisms such as trees, this can take multiple years to decades of meticulous long-term ecological monitoring.

Our long-term monitoring and research efforts in the Western Ghats focus on forest ecosystem dynamics and tree phenological cycles. The phenology project has tracked the flowering, fruiting, and leafing cycles of over 170 rainforest rainforest tree species in the Southern and Central Western Ghats.

In 2017, we began monitoring 1584 trees of 172 species monthly in 7 different existing forest trails across the Anamalai Tiger Reserve and Valparai Plateau in the Anamalai Hills. From 2021 January, onwards our phenology monitoring was rationalised to focus on species with at least 10 individuals on six trails. We additionally included wild fig (Ficus) trees and a set of 10 globally threatened tree species, which is the focus of a parallel study. We now monitor the trees fortnightly for a more close-grained study of the phenological rhythms and cyclical patterns. In 2022, we also began monitoring 640 trees of 40 species along six trails in rainforests of the Central Western Ghats near Sakleshpur.

In the southern Western Ghats, initial results from our phenology monitoring during 2017-2020 shows that flowering in the tree community peaked during March (the dry hot season) with 50% species bearing open flowers, and declined during July-September (the wet season). Peak fruiting coincided with the southwest monsoon (June-August), with 35% species fruiting during the peak rainfall months. The emergence of young leaves peaked in March (the dry hot season) with 89% of species flushing leaves, and declined during August to January (the post-monsoon and winter seasons). Across the years, timing of flowering and leaf flushing were relatively consistent, while fruiting was more variable.

The phenology research is beginning to generate valuable insights into the response of Western Ghats rainforests to climate change, along with a more immediate practical application in helping streamline the seed collection calendar for our rainforest restoration efforts.