Every now and then, biologist Deepak Barua takes a break from his usual assignments at the Indian Institute of Science Education and Research (IISER), Pune, and plans a trip to the 500 km away Sirsi forest. Located in the heart of the Western Ghats, the forest near Sirsi town of northern Karnataka is known for its tree species diversity. This is one of the latest forests that Barua has been monitoring, along with other scientists, to investigate the threat that climate change poses to the tropical tree species of India.
Studies suggest that heatwaves and frequent and intense droughts result in increased tree mortality and forest die-back across the world. There is also evidence that extreme temperatures threaten the survival of sensitive species, resulting in local extinctions, range shifts and altered vegetation composition. A 2010 study, published in Forest Ecology and Management, documented 88 events of forest mortality, driven by water or heat stress since 1970. A May 2024 study, published in the Proceedings of the National Academy of Sciences shows that about 37 per cent of the Amazon forest in Brazil is unable to recover due to frequent droughts, suggesting the possibility of mass die-offs in near future.
However, not much is known about how tropical forests in India are going to respond to extreme temperatures and frequent and severe droughts.
Mahesh Sankaran, professor of ecology and evolution at the National Centre for Biological Sciences (NCBS), Bengaluru, says events of forest diebacks have not been documented in India. But given future climate predictions, identifying which tree species are likely to be susceptible, or alternately resistant, to droughts and extreme temperatures can help us make more informed decisions about the future management of our forests. For example, he adds, in restoration efforts, it would make more sense to select tree species that can withstand droughts and extreme temperatures. Sankaran is part of the research that Barua has been spearheading for the past 10 years, along with scientists from NCBS, the Kerala Forest Research Institute, Thrissur, St Joseph’s College Devagiri, Calicut, and Leeds University, UK. The group has so far monitored 200 tree species across the Western Ghats and has quantified their temperature and drought tolerance, beyond which survival of the species may become difficult. The species also include mangroves, montane rainforests, grass, grasses, bamboos, palms, herbaceous plants and grasslands.
To assess climate vulnerability, the group monitors two organs of a tree: leaf and stem. While the leaf reveals temperature tolerance of the species, the stem indicates its tolerance to drought or water loss.
“Leaves are the primary organ of a tree that interact with the environment, temperature and light. It is also the organ responsible for many vital functions, such as photosynthesis that helps produce glucose and oxygen, and transpiration that helps maintain the plant’s internal temperature,” explains Ron Sunny, assistant professor of biology at St Joseph’s College Devagiri. Besides, adds Barua, using leaves serves a practical purpose. “Carrying out experiments by taking a leaf from an individual and studying it in controlled conditions such as a laboratory can bring us closer to representing impacts of an entire tree,” he adds.
In fact, in 2014, when Barua started monitoring the trees to assess their vulnerability to climate change, along with another scientist from IISER Aniruddha Sastry, the study primarily relied on the leaves. They would collect leaf samples of 41 tree species, commonly found in the urban parks of Pune, both during the hot-dry (pre-monsoon) and the cool-wet (monsoon) seasons and bring those to the laboratory to analyse temperature tolerance. Using a globally recognised method, Barua and Sastry would rehydrate the leaves overnight and then expose those to a range of temperatures, say from 25°C to 55°C, and measure the leaves’ efficiency to harvest light for photosynthesis. The temperature at which the leaf’s efficiency drops to 50 per centdenoted as T50—is a critical point as beyond this, the leaves undergo irreversible damage, experiencing cell mortality, tissue necrosis and ultimately death. T50 also helps scientists estimate thermal safety margins of the species—indicator of the species’ vulnerability to warming—by assessing how close the trees’ temperature tolerance is to the maximum temperatures it experiences in the region.
Initial findings of the study, published in Scientific Reports in 2017, suggest that most tropical forest tree species in India “may be highly vulnerable to future warming”. Thermal safety margins of the 41 species in Pune are “precariously low” at 3.5°C to 8°C, which implies that the trees are likely to be severely affected by increased temperatures. When compared with evergreen species like banyan, eucalyptus and mango, the study found that deciduous and fast-growing species with low leaf mass per area, such as teak, Indian almond and frangipani, are likely to be more negatively affected by global warming.
During the subsequent years, the group has expanded their research to monitor the branches as well. Extreme heat is not the only risk in a warming world. Trees are also vulnerable to droughts and water loss. Sunny says this vulnerability of a plant can be measured by assessing the risk of embolism or damage caused to the plant’s vascular tissues due to disrupted water supply. On a pleasant day, Sunny says, the roots “pull” water from the soil and transport it to the leaves through a vascular tissue pipeline in the branches known as xylem. However, during drought, dry soil “pulls” water threads away from roots. This tug of war, if intensifies, can snap the water threads and lead to embolism or formation of air bub-bles in the xylem that ultimately kills the tree.
To determine tree species’ tolerance to drought, the scientists bring branches to the field station and measure the water potential in the branch (the amount of water available in the xylem) and estimate the ability of the branch to conduct water. As the branch dries with time, both water potential and conductance decrease. The point at which the water potential that results in a 50 per cent decline in the branch’s ability to conduct water is taken as the hydraulic vulnerability threshold, denoted as P50.
While the findings of the study are yet to be published, the team is constantly at work to further fine-tune the methodology. For instance, says Sunny, hydraulic thresholds are calculated with the assumption that all leaves, irrespective of the species, lose water at the same rate. But this is not the case, and hence the rate at which the leaves lose water may change the threshold risk in plants. The rate may also change depending on the species, geographic location, soil moisture, surrounding temperatures, and time and rate of environmental exposure. “It is not just the threshold, but also how quickly you reach the threshold that counts. Long exposures to heat will put a tree in a dilemma of whether to continue transpiring to cool leaves or close its stomata to conserve water but stop photosynthesis,” Sunny explains. If trees continually transpire to cool down, they risk dehydration.
“Moreover, a species may not even experience xylem embolism if it has a thick wax coat on the leaf that minimises water loss. This is akin to a person putting on Vaseline balm to lock in moisture on a dry day,” he adds. The scientists are now incorporating these parameters to their research methodology for a robust prediction.
At the same time, they are deploying new tools for real-time leaf temperature data. Leaf temperatures can often be higher than that of the air. Yet in 2017, due to lack of technology, Barua and Sastry based their calculation on ambient temperature. The group now uses a light-weight sensor that remains clipped to the leaves and transmit the data wirelessly to the scientists’ computers on a real time basis. Data shows that at Sirsi, leaf temperatures can be up to 10°C or higher than air temperatures for some species, says Barua. This suggests that the thermal safety margins of these species are extremely low. Simply put, future warming will pose a major challenge for trees in this region.
While there is an urgent need to accurately monitor trees across the country to assess their climate vulnerability, Barua says, the sensors are limited to Sirsi for now, due to resource limitations. Maintaining and analysing data from them involves a lot of time, effort and money, he adds.
This was first published in the 16-30 September, 2024 print edition of Down To Earth