Winter is giving way to Spring in Delhi.
Days are getting hotter, and longer; Sparrows are getting noisier as they compete for mates and nesting sites, and the lapwings have started doing aerobatics to keep stray dogs off their nesting grounds...and some trees are busy shedding leaves.
"Fall" is a season commonly associated with Autumn (Sep-Nov). It is easy to understand the logic behind trees shedding their leaves to prepare for a few months of cold winter. There is a need to conserve energy, to minimize exposure to the elements, especially when energy from sunlight is hard to come by.
Why then do so many trees retain their leaves right through winter, only to shed them in spring, just when the days are getting brighter?
In NCR Delhi, many avenues are littlered with yellow leaves time of the year. Neem (Azadirachta indica), Kadamba (Neolamarckia cadamba), and even the Rangoon Creeper (Quisqualis indica / "Madhmalti") go almost completely barren even while they are putting out fresh shoots and leaves. Why is this so?
Turns out that botanists and ecologists are still grappling with this puzzle. They call this phenomenon "characteristic and counterintuitive ‘spring-flushing’ of monsoon forest trees". Latest available research covering Costa Rica, India and Thailand seems to suggest that the trees do this because "it optimizes use of large subsoil water reserves for photosynthetic activity during seasonal drought and thus extends the relatively short, wet growing season."
This does not make sense. Neem trees thrive in dry, drought prone areas and it seems unlikely that Rangoon Creepers are capable of sending their roots deep down to the 'large subsoil water reserves'.
So this makes 'spring flushing' yet another of nature's puzzles, and the answers continue to be in the realm of speculation.
LINKS & REFERENCES
*(2006): Leaf flushing during the dry season: the paradox of Asian monsoon, Global Ecology and Biogeography - http://ctfs.si.edu/Public/pdfs/ToDelete/Elliott_etal_2006_GlobalEcolBiogeogr.pdf
* Spring Flushing - https://link.springer.com/article/10.1007/s00468-002-0185-3
- Establishment of new foliage shortly before the wet growing season is likely to optimize photosynthetic gain in tropical forests with a relatively short growing season.
- Deciduous - https://en.wikipedia.org/wiki/Deciduous