Urban forests of Indian Cities at greatest risk of Climate Change: Study

A global study has found that 86% of urban tree species in New Delhi and 70% of urban tree species in Bengaluru currently exceed their temperature tolerance. The research published in the journal Nature Climate Change has predicted that these proportions are likely to increase in the future and by 2050, all species in New Delhi (100%) are predicted to be at risk of increases in mean annual temperature, while 88% of the species in Bengaluru might be at risk.

The researchers used a database called the Global Urban Tree Inventory to assess the threat to 3,129 tree and shrub species currently grown in 164 cities globally. They used three metrics related to climate vulnerability (exposure, safety margin and risk) under a medium-emissions scenario called RCP6.0, as part of the study and found that climate change threatens the health and survival of urban trees across the world.

CFC India contacted the lead author of the study, Manuel Esperon-Rodriguez from Western Sydney University in Australia. He wrote in an email to CFC India, “For Indian cities, the proportion of species currently exceeding their temperature tolerance is very high and these proportions are predicted to increase in the future, by 2050. These findings highlight the need of taking immediate action in terms of the climate emergency to secure the survival and persistence of urban forests and the benefits they provide to Indian citizens.”

The Species at risk 

The global study included two Indian cities, Bengaluru and New Delhi, with a total of 97 species. The researchers found that for New Delhi, some of the most vulnerable species are Poplar (Populus deltoides), Mulberry (Morus alba), Peach (Prunus persica), Morpankhi (Platycladus orientalis), Trifoliate orange (Citrus trifoliata),  Olive (Olea europaea), and Mediterranean Cypress (Cupressus sempervirens). Whereas for Bengaluru, the most vulnerable species are Paper mulberry (Broussonetia papyrifera), Silver Oak (Grevillea robusta), Cockspur Coral Tree (Erythrina crista-galli), Blue Jacaranda (Jacaranda mimosifolia), and Australian Chestnut (Castanospermum austral).

Climate change has occurred too fast, cities unprepared to adjust 

The researchers found that despite lower exposure to future climate change in cities at low latitudes, a higher proportion of urban forest species were at risk in cities at low latitudes compared to high latitudes, particularly in the northern hemisphere which highlights a potential mismatch between species planted in low-latitude cities and baseline climatic conditions. 

Esperon-Rodriguez felt that just like in many other cities of the world, in the case of India as well, there is a potential mismatch between species planted in Indian cities and baseline climatic conditions. 

“Our results suggest that climate change has occurred too fast in the last 30 years, and cities have been unprepared to adjust the species selection to the changing climatic conditions,” Esperon-Rodriguez said. “Thus, we must consider the possibility that rapid climate change is causing time lags between the production of suitable species in nurseries and changing local climatic conditions.” 

The researcher also felt that presently, species selection during urban planning is largely based on past and current climate, without accounting for future climate change and on management considerations, prioritizing characteristics such as canopy size or aesthetics. 

The way forward for Indian cities

The study can be of great help in the way forward for Indian cities in terms of future planting programs, according to Esperon-Rodriguez. It can help the authorities to create climate-proof urban forests with climate-resilient species and species can be compared between Indian cities for replacements or substitution, Esperon-Rodriguez suggested. 

“By identifying vulnerable and resilient species, governments can make informed decisions about the future of their urban forests,” Esperon-Rodriguez said. “Species that are currently far exceeding their safety margin in Indian cities may be prioritized for monitoring and potential substitution with more resilient species in future planting programs, whereas species identified as low risk may represent a valuable resource for creating climate-proof urban forests.” 

“Our study reveals the need for wider research-based characterization of climatic tolerances of urban tree species. Are at-risk species particularly susceptible to heatwave and drought events? Are low-risk species suitable for planting in a range of urban environments?” Esperon-Rodriguez further added.  

According to Esperon-Rodriguez, the lead author of the study, the answer to these questions lies in redoubled efforts to monitor and record urban tree health to enable informed planning for climate risk mitigation at the local scale. 

Global Results

Esperon-Rodriguez and his colleagues found that 56% and 65% of species in 164 cities across 78 countries are currently exceeding temperature and precipitation conditions experienced in their geographic range, respectively. By 2050, 76% of these species will be at risk from rising average temperatures and 70% from decreasing rainfall, the researchers concluded.

Cities at low latitudes such as New Delhi and Singapore are at the greatest risk where all urban tree species are vulnerable to climate change, the study found. The study also found a tendency for the mean climate change risk to increase towards the equator in cities where economic resources to mitigate climate change are generally more limited. 

The study also found that climate risk for urban forests was higher in cities projected to undergo decreases in precipitation, increases in temperature and in countries with low ND-GAIN scores, for example, Pretoria in South Africa and New Delhi in India. (ND-GAIN is an index of a country’s vulnerability to climate change and its capacity to strengthen resilience.)

Other factors 

The researchers felt that their estimates are conservative as the study doesn’t take into account continued urban growth and the effects of urban land usage, which could warm cities even faster. It also doesn’t take into account greater weather extremes caused by climate change, the effects of pests and diseases, or human factors like vandalism, poor site conditions, and improper species selection. 

Esperon-Rodriguez also clarified that they don’t imply that species identified as at risk are likely to die. By at risk, they mean these species might be experiencing stressful climatic conditions that could affect their health and performance. 

The lead author of the study also said that the finding of a high number of species at risk indicates that there are potentially other factors helping them strive, which could include human management like irrigation or biological factors like local adaptation. 

“While microclimate and site effects as well as management actions are likely part of the explanation for how local trees can be uncoupled from their inferred climatic constraints, this may not be the whole story. It is well known that trees may exhibit plasticity in traits that govern survival, growth and environmental tolerance, particularly following their critical early establishment phase,” he concluded. 

Reference: Esperon-Rodriguez, M., Tjoelker, M.G., Lenoir, J. et al. Climate change increases global risk to urban forests. Nat. Clim. Chang. 12, 950–955 (2022). https://doi.org/10.1038/s41558-022-01465-8