Leaves of several tropical forest and agroforestry species in the Western Ghats are already reaching temperatures that could push them toward irreversible heat damage, scientists have found.
Researchers monitored 13 agroforestry species and four native forest species over a 4.5-month period in 2023 at Hosagadde village near Sirsi in Karnataka — an area that frequently experiences high air temperatures, sometimes above 40 degrees Celsius.
They found that many plants were exposed to leaf temperatures surpassing critical physiological thresholds, raising concerns over how tropical species may respond to rising global temperatures.
Scientists used a parameter known as T₅₀ — the temperature at which the photosynthetic efficiency of a leaf declines by half. This threshold, widely applied in global plant studies, marks a point of no return for leaves, often leading to irreversible cellular damage and eventual death.
The authors of the paper noted that while global warming is projected to raise average global temperatures by up to 2°C — or potentially even higher — it remains unclear how such extreme heat will affect plants, particularly commercially important crops.
“Tropical plants may be particularly vulnerable to future climates with increased mean temperatures and more frequent exposure to extreme temperatures,” the authors noted in the journal Global Change Biology. “They may possibly already be living close to their upper limits of temperature tolerance.”
Because leaves are the primary site of photosynthesis and water loss through transpiration, their thermal responses provide important insights into how trees and crops cope with environmental stress, said Ron Sunny, assistant professor of biology at St. Joseph’s College (Autonomous), Devagiri, Calicut, in an earlier conversation with Down To Earth.
“All the vital functions such as photosynthesis happens in the leaf. It is also the leaf that senses the temperature, light availability, etc,” Sunny said.
Leaves are the site where the plant regulates temperature through water loss, making it a centre of immense physiological activity.
Among commercially valuable agroforestry species, the scientists examined plants such as cardamom, cashew, sapota (chikoo), cinnamon, citrus, clove, cocoa, coffee, lemon, pepper, rambutan, syzygium and vanilla.
While cocoa, cinnamon, coffee, lemon and rambutan were found to experience leaf temperatures that could cause damage, the duration of exposure was often less than 10 minutes. According to the researchers, such brief spikes are “unlikely to lead to irreversible damage and leaf death”. Laboratory assessments of T₅₀ typically involve subjecting leaves to extreme temperatures for at least 30 minutes to assess heat tolerance.
Among native forest species, the researchers tested kindal (terminalia paniculata), ceylon box wood (psydrax dicoccos), ironwood (memecylon umbellatum) and rose sandalwood (olea dioica). Kindal—the only deciduous species tested, with all others being evergreen — and ironwood were found to occasionally experience episodes of extreme leaf temperatures.
The study highlighted that during peak sun exposure, leaf temperatures exceeded surrounding air temperatures by 10°C-12°C in most species, due to heat absorption by sun-exposed foliage. This led to visible damage in some plants. In several agroforestry species, researchers observed blackened leaf areas, which could indicate thermal injury.
However, lead author Akhil Javad cautioned against attributing such symptoms solely to heat. “There may be other stressors involved — such as pathogens or water deficits — that could produce similar signs or exacerbate heat stress,” he said.
The researchers emphasised that understanding the upper thermal limits of leaves is essential, especially for the commercially valuable species like cardamom, cashew, chickoo, clove, citrus, pepper, syzygium and vanilla, which form the backbone of agroforestry economies in the Western Ghats.
Most earlier studies have focused on thermotolerance and thermal safety margins, rather than measuring how frequently plants exceed those thresholds in real-world conditions. Thermal safety margins are defined as the difference between a leaf's temperature and its T₅₀ threshold. Smaller margins suggest higher vulnerability to heat stress.
While the study found that present-day exposure durations remain mostly short, even under simulated global warming scenarios of 2°C and 4°C, it raised red flags about the increasingly narrow thermal buffers available to many tropical species.