Abdul Wahab Khan
Panaji : A study spanning over two decades has uncovered a paradox in the Western Ghats – forest productivity is declining even as the forest cover expands, with Goa, Gujarat and Maharashtra showing concerning trends in the northern region of this critical biodiversity hotspot.
The research, conducted by Sanjay Kumar, Jyothika Karkala, Joyal Arya and lead scientist Dr Rajiv Kumar Chaturvedi of BITS Pilani, Goa, analysed 22 years of satellite-derived Net Primary Productivity (NPP) data from 2001 to 2023.
The objective was to fill a longstanding scientific gap by understanding how climate factors, vegetation health and land-use changes influence terrestrial productivity across the Western Ghats.
Net Primary Productivity (NPP) represents the rate at which plants convert solar energy into organic compounds through photosynthesis, minus the energy they use for their own respiration. The NPP serves as a pivotal marker for assessing ecological security and measuring vegetation dynamics, indicating how much carbon forests and other ecosystems can absorb from the atmosphere. This makes it crucial for understanding both, historical and future carbon cycles and climate change mitigation.
The findings paint a complex picture of ecological stress across the region. The research found that NPP distribution in southern states like Kerala, Tamil Nadu, and Karnataka – predominantly covered with wet evergreen forests – showed significantly higher values compared to Goa, Maharashtra and Gujarat, which feature moist deciduous and semi-evergreen forests.
The study highlights fluctuations over the 22-year period.
The NPP across the Ghats peaked in 2011 and again in 2022, yet suffered sharp declines during the periods 2003-2004, 2009-2010 and most severely from 2017 to 2019. The year 2019 stands out as the lowest point, with productivity dropping to 0.73 kilograms of carbon per square metre per year, a fall attributed to persistent low rainfall since 2015, rising maximum temperatures and soaring land surface temperatures that reached 41°C. These conditions “caused heat stress, increased respiration rate and created feasible conditions for NPP to drop”, according to the study.
During the first five years of this century, average NPP in forests stood at 1.23 kilograms of carbon per square metre annually, but this decreased to 1.11 in the final five years of the study period, marking a decline despite forest area expansion by 9,432 square kilometres, or 1.93%. The total mean NPP came to 1.17 kilograms of carbon per square metre annually, with the maximum recorded in 2002 and minimum in 2019.
The researchers suggest this counterintuitive finding could be explained by forest expansion into less fertile regions or the establishment of younger forests that are naturally less productive than mature forest ecosystems.
A key insight concerns the surprising dominance of land-use change. The ecological detector analysis notes that “LULC (land use and land cover) has more impact than all other variables,” indicating that shifts in croplands, forest edges and built-up areas exert strong control over the region’s productivity. This finding challenges the conventional expectation that increased human activity invariably lowers NPP.
In certain zones, especially along the Ghats’ periphery, expanding croplands contributed to a rise in productivity, even as forests themselves showed an overall decline.
The researchers noted regional variations in productivity trends. Significant NPP increases were spread mainly in the northern boundary from Vansda through the Chandoli forest, covering Gujarat and Maharashtra, while highly downward trends were observed in lower elevation tropical-monsoon and tropical-savanna zones in Karnataka and Kerala.
