Timberline along the Khangchendzonga National Park in eastern Himalayas Credit: Badola et al

In eastern Himalayas’ Khangchendzonga National Park (KNP), eye-catching Wight’s rhododendron shrubs and graceful firs, frame the upper limits of the sub-alpine forests. Beyond this boundary of closed canopy forests, called timberline, the climate is too harsh for trees to grow.

But as the climate warms, globally, studies have documented an elevational upwards expansion of the forest line in many mountain ranges, such as the Polar Urals (Russia), the central Swiss Alps, western Himalayas (India), the Rocky Mountains (Southern Alberta) and central Himalaya (Nepal).

A recent assessment has said that Sikkim, India’s first organic farming state, is bucking the trend so far. This particular forest timberline is unlikely to budge in the near future. However, the edges around the forest limits may become greener due to densification as more tree individuals spring up.

“The upward advancement of timberlines, associated with climate warming, is unlikely to occur in the near future in the Sikkim Himalayas given the present stress on conservation and on the green and organic farming policies under the state chief minister Pawan kumar Chamling,” said Hemant K Badola, advisor-biodiversity, Chief Minister’s Office in Sikkim and one of the lead authors of the study.

Worldwide, the alpine timberline in high mountains, which characteristically distinguishes the upper limit of closed canopy forests and represents a major zone between the forested and non-forested vegetation, is one of the most sensitive ecotones (transition zone) and an indicator of a warming climate.

In the remote Ural mountains, comparisons with historical photographs indicate that the timberline has increased by 40 to 80 metres in altitude during the last century, very likely due to climatic changes.

Study area Khangchendzonga National Park

“Timberlines advance upwards as a rise in temperature enhances productivity of plants and due to increase in reproduction, the number of trees goes up and hence the timberlines shift. One may think that this is good, but upslope forest expansion also has consequences,” Badola explained.

This upward forest expansion is expected to shrink the extent of the alpine ecosystems and possibly result in species loss and ecosystem degradation.

“As the forest expands, the alpine ecosystem becomes crowded and may lead to competition between species and their populations. For example, forests invading alpine ecosystems may result in loss of foraging habitat for species including musk deer, blue sheep, pheasants, etc,” Badola said.

Local greenery, organic biome mitigates warming

Sikkim has been able to arrest the upslope forest expansion because local greenery and organic biome has influenced its microclimate in a way that warming effects are mitigated, Badola explained.

For example, the state’s Green Mission to maintain green cover and cleanliness and its organic mission, have been feted globally for ramping up conservation efforts.

At the ongoing climate change summit (COP 24) in Poland, the state also shared its innovative initiatives of springshed management to reduce climate change-induced risks and vulnerabilities from glacial lake outburst flood.

A report presented by Indian researchers at the summit revealed that Sikkim has the lowest vulnerability to climate change among 12 Himalayan states.

The report adds that although Sikkim has three major drivers of vulnerability – low area under forests per 1,000 households, low percentage area covered by insurance and low percentage of farmers taking loans, it has the highest per capita income and the lowest area under open forests, which relatively lowers vulnerability of the state when compared to other states in the Indian Himalayan region.

Study authors Badola, Aseesh Pandey, Sandhya Rai and S.P. Singh warn that growing climate warming, if not controlled, in the long-term, would be detrimental to high altitude ecosystems.

Sikkim government’s data shows how climate change is already making a mark. Extreme climate events have become more frequent. Rainfall patterns have become erratic, monsoons are usually late and in general torrential rainfall has replaced the monsoon drizzle.

This has increased the surface runoff and dry period during winters, resulting in a higher incidence of forest fires and drying up of springs.

SP Singh of the Uttarakhand-based Central Himalayan Environment Association noted that despite its importance, timberline shift is not discussed in the context of biodiversity conservation and climate change in India.

“Himalayan timberlines are among the highest in the world. They have been much less studied than their European counterparts due to remoteness of these mountains. In this study, we have provided a baseline for Sikkim for future comparisons. Timberline shifts should be incorporated in policies,” Singh said.

In eastern Himalayas’ Khangchendzonga National Park (KNP), eye-catching Wight’s Rhododendron shrubs and graceful firs, frame the upper limits of the sub-alpine forests. Beyond this boundary of closed canopy forests, called timberline, the climate is too harsh for trees to grow

Studying timberlines to understand climate impact on mountains

Positives aside, the timberline study also drew attention to a potential problem: the presence of a large number of seedlings and saplings in the timberline ecotone suggesting that in near future the forest edge will become denser and woody trees may invade spaces where herbs flourish.

“This is also supported by occurrence of the higher weightage of juveniles and younger trees, especially Sikkim fir (Abies densa), Sorbus microphylla and Rhododendron lanatum,” the study said.

“Climate warming will likely enhance plants’ growth, productivity and regeneration, which in turn will accelerate densification of tree saplings in the area,” Badola said, adding such densification may then lead to interplant and interspecies competition.

“Advancement of woody trees in alpine meadows and their encroachment would impact medicinal plants growing in these meadows,” Singh noted.

For the present study, the researchers worked their way on foot along a 20km timberline stretch along the Dzongri landscape of KNP, at a height of 3787–3989 metres above sea level (m asl), surveying woody trees at nine sites within that area.

The park represents a protected core area ensconced within the massive Khangchendzonga Biosphere Reserve (KBR), in the eastern Himalayas. Khangchendzonga Biosphere Reserve was included in the UNESCO’s World Network of Biosphere Reserves in July 2018.

The nine study sites are located at the confluence of two snow fed rivers Prekchu and Rathongchu originating from the Mount Khangchendzonga range and constitute the Rangit basin. The Rangit is a tributary of Teesta, the biggest river of Sikkim.

“The Himalayan timberlines are less explored for climate change-driven vegetation changes than those of European timberlines. The eastern Himalayas, on the other hand, which generally maintains greater moisture with richer biological and cultural diversity than the other Himalayan parts, is less explored, comparatively,” the authors said.

The timberline vegetation was classified into five forest types/communities and mainly dominated by Sikkim fir (Abies densa), followed by a small flowering tree (Sorbus microphylla), Rhododendron species such as R. lanatum, and R. wightii in the tree layer. Silky rose (Rosa sericea), followed by Ribes glaciale and Himalayan juniper (Juniperus recurva), are conspicuous in the shrub layer.

Applied statistical assessments indicated that environmental factors such as presence of humus (organic matter), elevation and slope played an important role in shaping the vegetation composition as well as timberline boundaries of the landscape.

In other findings, the analysis points to the presence of a remnant treeline.

“Presence of dead standing trees of A. densa above the current timberline indicates that timberline was higher. It could be remnant treeline. Their higher position in the past can also be related with the prehistoric anthropogenic disturbance, as the yak herders were using this area before governmental ban on grazing,” the study said.

Badola and co-authors called for getting a holistic picture of timberlines to understand how a warming climate impacts mountain ecosystems.

“The study demonstrates a methodology for monitoring the status of timberlines and that can act as a blueprint for further assessment of timberlines across the Himalayas and elsewhere in high mountains, for getting a holistic picture of timberlines, as one of the most sensitive ecotones, which may offer vital clues on the impacts of warming climate,” the study concluded.

The rhododendrons of Sikkim Himalayas

(The article first appeared in Mongabay-India)

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