Last December, as I strolled along a boulevard lined with temperate deciduous Ginkgo trees (Ginkgo biloba) in Hiyoshi, Japan, I was astounded by the beauty of the foliage. The vibrant yellow leaves transformed into a mesmerising display as the setting sun bathed them in its golden rays. Such a picturesque sight naturally called for photography! As I posed to capture the moment, a wave of doubt washed over me. To begin with, I wondered if I had ignored the nuanced cues of "autumn" in the tropical city of Mangaluru, Karnataka, India, for three decades!?
Readers familiar with South India may raise questions about my doubt, stating that the region traditionally acknowledges three seasons: summer, monsoon, and winter. I know! But, we "have" winter in South India even though many coastal cities, including Mangaluru, do not have a pronounced winter. So why not "have" autumn, I wondered? I confess! I was confused by the traditional characteristics associated with "autumn."
What characteristics am I talking about!? Autumn is characterised by shorter photoperiods (shorter days and longer nights), a gradual decline in temperature marked by cooler mornings and evenings, crisp air, decreased humidity, and the transformation of leaves on deciduous trees into hues of yellow, orange, or red before shedding*. Tropical climates like that in Mangaluru do not exhibit a significant temperature change from the rainy season to winter. However, other autumnal characteristics, including the shedding of leaves by deciduous trees, remain apparent. Hence, I assumed the subtle transition period after monsoon to be an autumn-like phase.
Evergreen trees/ forests dominate Southern and Western India, creating a landscape where synchronised leaf shedding, a characteristic observed in deciduous trees/ forests of Central and Northern India, may not be as prevalent. However, deciduous, and semi-deciduous species still thrive in South Indian landscapes. The Teak tree (Tectona Grandis), Gulmohar (Delonix regia), and Rain tree (Albizia saman) are deciduous trees commonly found in South India. If you pay close attention, the leaves of these tree species transform in colour and are shed shortly after the rainy season.
After conducting a comprehensive literature review, I came to the realisation that I had mistaken the leaf shedding in "tropical deciduous trees" as a response similar to shedding in "temperate deciduous trees." Let me delve into the science behind the changing colours and shedding leaves to state why only temperate climates have "autumn" and not tropical climates. To comprehend this, we first need to understand what "autumnal leaf senescence" is from a botanical perspective. To enhance understanding, I have structured the information in a question-and-answer format.
What is an autumnal leaf senescence?
"Leaf senescence" is the "last days" of the leaf. Cells in the leaves may die accidentally or upon the completion of their life. For temperate deciduous trees, autumn marks the end of the life cycle for its leaves. During the months of significantly decreasing temperature and photoperiod (length of the day), the trees prepare for senescence by triggering a choreographed gene regulation! The regulated genes* lead to a chain of events. From the slow dismantling of the photosynthetic machinery to the weakening detoxifying system, the temperate deciduous leaves are slowly changing colours.
The "photoreceptors" or light sensors in the leaves detect the changes in red light levels within the visible light spectrum due to reducing photoperiod. As the red light activates chlorophyll, limited red light causes limited chlorophyll activation. Simultaneously, with the photosynthetic machinery shutting down, the chlorophyll* content in the "senescing" leaves decline. Chlorophyll, which would otherwise mask the other leaf pigment (carotenoid*) with its green hue, relinquishes as its amount drops. Hence, the unmasked carotenoid casts a vibrant autumnal palette of yellow or orange before the leaves fall.
The weakening detoxifying system accumulates toxic free radicals*. In response, some temperate deciduous species produce anthocyanins, red pigments, recorded to possess anti-oxidant properties. Anthocyanins radiate in the form of red leaves while neutralising the free radicals within. However, the weakening detoxifying system prompts a domino effect that produces excess free radicals beyond any significant neutralisation, thus damaging and killing cell structures. Before the leaves "fall" to free-radical-induced necrosis*, the valuable nutrients synthesised during photosynthesis are "shifted" to other parts of the tree.
Do temperate deciduous trees require autumnal senescence?
There is more to autumnal senescence than mere leaf shedding. Autumnal senescence is the epitome of adaptation! During the adverse conditions of low temperatures (sub-zero to 15 °C), potential water scarcity, and reduced sunlight, the temperate deciduous trees undergo a phase of growth dormancy due to "discontinued" growth hormone* production and photosynthesis. Thus, autumnal leaf senescence and dormant growth phase help the trees conserve energy and avoid severe damage/ death by the cold.
Is climate change affecting the adaptive mechanisms of temperate deciduous trees?
The onset of the "dormant growth" phase is characterised by the setting of the bud, i.e., underdeveloped shoots, at the tip of the tree's stem. Shielded from adverse external conditions by protective scales, these "buds" require a certain cold period before they burst open during the spring.
As of late, with the changing climate, the cold period has significantly shortened. Prolonged warm spells are pushing autumnal leaf senescence further into the calendar year. Erratic weather patterns and early warm springs may deprive temperate deciduous trees of adequate time for a seamless transition, thereby impacting the health and survival of these trees.
How is the leaf shedding in Tropical deciduous trees different?
While tropical and temperate deciduous trees experience a leafless period, the triggers for this phenomenon are distinct. In contrast to temperate deciduous trees, tropical deciduous trees undergo leaflessness primarily in response to "water stress" rather than temperature. The onset of the dry season is marked by prolonged rainless days following the monsoon. Despite the mild temperatures during the early months of the post-monsoon period, the senescence in tropical deciduous trees is a response to diminishing water availability in the soil.
How does shedding leaves help the tropical deciduous trees?
To prevent water loss through transpiration* and subsequent death, these trees have evolved to adapt to the dry period by shedding their leaves. The absence of leaves eliminates transpiration, preventing the roots from absorbing water from the water-deficient soil. Instead, the already-absorbed water is stored. The ensuing months constitute a growth-dormancy phase until a bountiful water supply, such as during the monsoon, is restored.
Can leaflessness in tropical deciduous trees be avoided?
You may wonder if the water stress-induced leaf shedding in tropical deciduous trees can be prevented by addressing the soil-water deficiency. Well, I thought of the same! However, a review of the existing literature indicated that though water stress can be reduced, shedding could still occur (to a certain extent, if not entirely) due to a reduced photoperiod.
To conclude, do we have autumn in India?
By now, the concept of "autumn" has been thoroughly scrutinised and well understood. It is apparent that autumn is characterised by a substantial change in seasonal temperature, leading deciduous trees in the region to shed their leaves. However, South India (Except for a few districts) does not experience a significant transition in temperature between seasons. Consequently, temperature, the primary driving factor for autumnal senescence, is NOT the cause behind the observed leaf shedding in Mangaluru. Instead, the notable phenomenon of leaflessness in tropical deciduous trees such as the Teak tree, Gulmohar, and Rain tree in South India is attributed to water stress. No subtle autumn cues there!
The Indian Meteorological Department (IMD) officially designates the period following the monsoon as "post-monsoon," and does not acknowledge autumn as a distinct season. I find this term to oversimplify India's diverse climate and regional disparities by encapsulating them under a single term!
Let me pin this food for your thought. Unlike South India, Northern regions of the country experience noticeable changes in seasonal temperatures. Winters here bring about significantly lower temperatures, transforming the deciduous trees into a vibrant display of autumnal hues from September to November. It raises the question: why doesn't the IMD recognize this period as "autumn"? Considering the deciduous trees undergoing autumnal senescence, isn't this, in essence, the true manifestation of autumn?
Before delving further, I acknowledge a slight deviation from the main topic as I attempt to make a pertinent point. It's worth noting that "autumn" tourism in North India could play a pivotal role in boosting India's tourism sector. The picturesque landscapes painted with the rich colours of the fall could attract domestic and international tourists, contributing to growth in the tourism sector and India’s economy.
*Scientific terms for you
Abscission = Shedding of leaves.
Genetic regulation = Genes are activated (upregulated) or deactivated (downregulated).
Photosynthesis = Conversion of light energy to chemical energy.
Chlorophyll = The green pigment responsible for photosynthesis.
Carotenoid = The yellow or orange pigment in leaves.
Xanthophyll = A carotenoid responsible for yellow pigment.
Necrosis = Cell death.
Growth hormones in plants = Auxins, Gibberellins, and Cytokinins.
Stress phytohormones = Abscisic Acid (ABA) and ethylene.
Transpiration = The water movement through the plants driven by evaporation from parts such as the leaves.
References
Climate. (n.d.). KnowIndia. Retrieved January 23, 2024, from https://knowindia.india.gov.in/profile/climate.php
Fadón, E., Fernández, E., Behn, H., & Luedeling, E. (2020). A conceptual framework for winter dormancy in deciduous trees. Agronomy, 10(2), 241. https://doi.org/10.3390/agronomy10020241
Lev‐Yadun, S., & Gould, K. S. (2007). What Do Red and Yellow Autumn Leaves Signal? The Botanical Review, 73(4), 279–289. https://doi.org/10.1663/0006-8101(2007)73
Rosenthal, S. I., & Camm, E. L. (1997). Photosynthetic decline and pigment loss during autumn foliar senescence in western larch (Larix occidentalis). Tree Physiology, 17(12), 767–775. https://doi.org/10.1093/treephys/17.12.767
Smart, C. M. (1994). Gene expression during leaf senescence. New Phytologist, 126(3), 419–448. https://doi.org/10.1111/j.1469-8137.1994.tb04243.x
Comentarios