Climate change disrupting seasonal rhythm of forests in Nilgiri Biosphere Reserve, study finds

The Nilgiris district's water woes due to deficient rains this year have drawn attention to erratic weather patterns. But beyond the visible crisis, researchers warn that warming temperatures are quietly destabilising native forests in the Nilgiri Biosphere Reserve (NBR), altering the very timing of their growth cycles.

A study published in the peer-reviewed journal Remote Sensing in 2024 mapped satellite imagery of the NBR's forests between 2000 and 2020. The researchers recorded an annual temperature rise of 0.01 degrees Celsius across the reserve between 1950 and 2018, alongside an average rainfall decline of 3.97 millimetres each year during the same period.

The study examined four forest types — moist deciduous, dry deciduous, semi-evergreen, and wet evergreen forests. By tracking the 'Start of Season' (when vegetation greening begins), 'End of Season' (when active greening ceases), and 'Length of Season' (the duration between the two), the team sought to understand how these shifts affect forest ecology.

During the first phase of the study (2001–2010), the Start of Season was delayed across all four forest types, leading to shorter growing seasons for dry deciduous, semi-evergreen, and wet evergreen forests. However, moist deciduous forests experienced a lengthening of season by 4.37 days per year during this period.

The pattern reversed dramatically in the second phase (2011–2020), when the Start of Season advanced across all forest types, resulting in longer growing seasons overall.

Bodi Surya Pratap Chandra Kishore, one of the paper's authors and Technical Officer at the School of Planning and Architecture in Bhopal, said the findings reveal that the seasonal rhythm of NBR forests has changed over the last two decades. “In some periods, vegetation started its growing season later; in another period, it started earlier. The end of the growing season also changed differently in different forest types,” he explained.

Kishore emphasised that climate change is not simply making forests greener or less green, but is fundamentally altering the timing of their biological activity. “Monitoring these changes can help forest managers identify vulnerable ecosystems and plan conservation and restoration strategies earlier,” he added.

The study also noted that temperature alone does not dictate seasonal shifts. Kishore pointed to interactions with rainfall, soil moisture, drought, and photoperiod — the amount of light organisms receive. “Warmer conditions can promote earlier vegetation development when sufficient moisture is available, but warming accompanied by water stress can have the opposite effect,” he said, predicting greater variability in seasons ahead.

Godwin Vasanth Bosco, a restoration ecologist and founder of Upstream Ecology in The Nilgiris, observed that the clearest signs of climate change are visible in the cloud forest sections of the sholas — the high-altitude tropical montane forests unique to the Western Ghats. These fragile ecosystems, already under pressure from land-use changes, are now facing the additional stress of a shifting climate.

The findings underscore the need for sustained ecological monitoring and adaptive conservation strategies. As climate models project more extreme weather events, the ability of forest managers to anticipate phenological changes will be critical in preserving the biodiversity of the Nilgiri Biosphere Reserve, a UNESCO-designated site.