ECR feature: Alpine treelines with Shalik R. Sigdel

Shalik is a postdoc at the Institute of Tibetan Plateau Research. He an ecologist working in the Himalayas to understand how climate change might cause shifts in alpine treelines. Shalik shares his recent work on the combined influence of climate and intraspecific interactions on these treelines.

Shalik collecting treeline data in the Manang valley, central Nepal. (Photo credit: Samresh Rai, 2 November 2015)

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Institute. Institute of Tibetan Plateau Research, Chinese Academy of Sciences, Beijing China

Academic life stage. Postdoc.

Major research themes. My research focuses on understanding alpine treeline responses to changing climate at multiple spatial scales using dendroecological tools, particularly in the Himalayas. I have a keen interest in the use of spatio-temporal data for understanding how treeline responds to changing climate.

Current study system. Currently, I am working on the treeline ecotone in the central Himalayas. The Himalayas host one of the longest natural treeline ranges, extending from Pakistan to southeast China crossing different climatic zones (westerly- and monsoon-dominated areas). As this area is experiencing a faster warming rate than other areas, the alpine treeline is considered as a potential indicator to track influence of climate warming on alpine ecosystem. In response to climate warming, the treeline is expected to move upslope. However, our early study showed a non-linear relationship between rates of treeline shift and warming climate, raising some questions about the mechanisms affecting the treeline shift.

A natural Himalayan birch (Betula utilis) treeline located at ablout 4100 m. a.s.l. in the Langtang valley, central Nepal. (Photo credit: Shalik Ram Sigdel, 17 April 2014)

Recent publication in JBI. Sigdel, S. R., Liang, E., Wang, Y., Dawadi, B., Camarero, J. J. (2020). Tree-to-tree interactions slow down Himalayan treeline shifts as inferred from tree spatial patterns. Journal of Biogeography, 47(8):1816–1826.

Major motivation for this paper. The major motive of this research was to investigate the role of intra-species interactions (spatial patterns) on responsiveness of treelines to climate warming. Mountainous regions across the globe (including the Himalayas) have been experiencing increased recruitment as a result of climatic warming. However, in the Himalayas, the rate of treeline shift has been slower, relative to other parts of the world. Heterogeneous response of Himalayan treeline to a warming climate indicates that rates of treeline shifts may be affected by local-scale interactions (particularly intra-species interactions). However, it is unknown whether Himalayan treeline dynamics show lagged or weak responses to climate warming due to treeline densification and clustering intensity of trees (tree-to-tree-interactions) or not. We expected to discern if tree-to-tree-interactions regulate the pace of treeline shifts, and to determine if those interactions are more important drivers of changes at treelines than climate. We hope this study will help to better understand how local interactions drive large scale pattern formation at treeline and their response to changing climate.

Key methodologies. We established a network of treeline plots across the central Himalayas, encompassing a wide longitudinal gradient characterized by increasing precipitation eastwards. The location of each individual tree within the plot was measured, and their ages were estimated by applying a dendrochronological approach. We used the locations and age of the trees to calculate the changes in tree density, treeline elevation changes, distance between neighbouring trees, and the spatial patterns (clustering intensity) for 50-year age-classes (1-50, 51-100, 101-150 years) of two main tree species at the treeline (Himalayan birch and Himalayan fir). The relationships between these parameters were used to understand the driving mechanism of Himalayan treeline shift.

Taking tree-ring sample from Himalayan fir (Abies spectabilis) to get the germination date and age of the trees (Photo credit: Samresh Rai, 31 October 2015)

Major challenges. Our treeline sites were located in remote mountain region of the Himalayas and can be reached after 3-4 days continuous trekking. The Himalayan mountains are characterized by a complex regional climate system. Hostile climatic conditions and the remoteness of treeline sites are big challenges to surveying a large-scale network of treeline plots, which took several months. Even though we had only a very short time window suitable for conducting field surveys, we were able to collect data along the east-west precipitation gradient (more than 800 km) across the central Himalayas after 3 years continuous effort.

Major results. Based on the network of treeline plots across an east-west precipitation gradient in the central Himalayas, our study revealed that treeline shift rates were not only limited by climate but also affected by intra-species relationships. Higher clustering of young trees increased with increasing moisture stress from the eastern to western sites but treeline shifts rates were higher at wet eastern treelines where clustering intensity is lower than at dry western sites. The higher distance between neighboring trees, the faster the shifting rate, and vice-versa. This is an important empirical advance in the study of driving mechanism of alpine treeline shift, showing how climatic and non-climatic factors interact at the local scale to drive treeline patterns. Furthermore, it explains the spatial differences in treeline shifts from the perspective of intraspecific relationships, and quantifies the role of biological factors on treeline shifts. It also underscores the lag effect of treeline shifts in response to climate warming.  

Next steps? In this study, we considered the treeline sites dominated by single species (either Betula utilis or Abies spectabilis). But we found some treelines with both species together. Currently, we are working on such mixed species treelines to determine if species specific spatial patterns shape treeline structure and to examine if those patterns drive the shift rate of particular species.

If you could study any organism on Earth, what wold it be? I am fascinated by alpine grassland ecosystems, which are highly sensitive to global change. Alpine plants are the most interesting organisms to me and I would love to study the impact of climate change on their establishment, survival, reproduction and distribution, particularly in the central Himalayas (which is a global biodiversity hotspot). As one of the most affected terrestrial ecosystems, alpine plant communities in the Himalayas are ideal systems to monitor the impacts of climate warming. Additionally, I feel relaxed while working in the beautiful Himalayan mountains, and enjoy the pristine environment.

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