Understanding the Patterns and Mechanisms of Alder Shrub Expansion in Arctic Tundra using Dendrochronology and Very-High-Resolution UAS Remote Sensing

Expedition Location:  Nome, Alaska

Expedition Dates:  July 8, 2023 – July 16, 2023

Field Team Members:  Wouter Hantson, Daryl Yang

Funding Support: The Robert and Judith Sturgis Family Foundation and NGEE Arctic

Background and Significance: The expansion of tall shrubs into northern high-latitude ecosystems is driving widespread changes in tundra ecosystems that may fundamentally modify land-atmosphere interactions and feedback to global climate change. However, the patterns of shrub recruitment and expansion, and the fundamental mechanism that drives this change remain largely unknown. This is because most historical observations (resolution: >30 m) that are used for tracking land surface changes cannot capture the fine-scale, individual establishment, spread, and growth of Arctic shrubs which are commonly less than 5 m in size. In recent years, the use of high-resolution unoccupied aerial system (UAS) remote sensing has revolutionized the way that ecologists describe and study natural ecosystems, by providing land surface observations at high-resolution resolution (<5 cm). This new technology provides an unprecedented opportunity to study shrub changes at the scale of individuals, by allowing researchers to map individual shrubs and relevant structural and life history traits. Through this project, we aim to investigate the potential of combining dendro- chronological and UASs to study the patterns and mechanisms of alder shrub expansion in a representative low-Arctic study site.

Photo 1: the Kougarok Hillslope site from the NGEE-Arctic project, with alder shrublands seen on the summit and alder savannahs on the lower slopes.


Fieldwork: In July 2023 (peak growing season), Wouter Hantson and Daryl Yang, PhD candidates at UMaine and Stony Brook University, completed a successful field campaign in Nome, Alaska. The objective of this field trip focused on collecting representative shrub samples needed for modeling alder shrub age across the Kougarok Hillslope (Foto 1), a low-Arctic tundra field site established by the Department of Energy’s Next-Generation Ecosystem Experiment – Arctic (NGEE-Arctic) project, through in-situ sampling of shrub stems and associated canopy and environmental characteristics (shrub height and diameter, tussock depth, soil moisture, active layer depth, and stem diameter – Foto 3). Through this campaign, a total of 52 alder shrubs were sampled, distributed across alder shrublands and alder savannahs, where we anticipate alder may differ in growth form and expansion mechanisms. For each selected shrub individual, the ‘main’ (largest) stem was sampled for age analysis which we will use to represent the age of the shrub (Foto 2). Before harvesting, we measured canopy height, crown diameter, soil moisture, and tussock layer depth. The coordinates of the selected individuals were recorded using dGPS which will allow us to accurately identify them from UAS imagery. The harvesting of the main stem was done carefully using an extendable hand saw. We labeled the stems using Brookhaven National Lab (BNL) barcodes. The DBH at the root collar was measured using a high-accuracy caliper. After that, we placed the stems into an oven in UAF Lab to avoid any molding before shipping them to BNL. Two nearby discs will be collected from each stem for age analysis, and processed separately for cross-comparison. For each sampled shrub, we also collected leaves to analyze for leaf mass/area, water content, and N & C content.


Photo 2: Example photo of alder shrub sampling. Stem at the root collar was harvested for further shrub ring and age analysis.


Photo 3: Alder before harvesting, measuring height, diameter, and marking for dGPS measurement.


Preliminary Results:  We sampled 52 Alder shrubs ranging from 31 – 335 cm in height and 10 – 600 cm in diameter. Figure 1 shows the variation in canopy height/crown diameter versus the diameter at the bottom of the main branch. Once stem age is analyzed, spatial models will be developed and applied to describe size/age distributions over the landscape.

Fig. 1: First analysis showing the relationship between DBH and Canopy height and DBH and Crown diameter for alder savannah and shrubland.


Acknowledgements:  We would like to thank all collaborators from NGEE-Arctic (Shawn Serbin, Jeremiah Anderson, and Verity Salmon)  for their help with fieldwork and the Mary’s Igloo Corporation for their guidance and for allowing us to conduct this research on their native lands.