Shrubification in a changing High Arctic

Overview

Observational studies have shown a widespread greening of the Arctic, caused by ‘shrubification’, an increase in the coverage of shrubs (1,2). This process is vital for limiting the release of the vast store of organic carbon (c. 1,672 gigatonnes (3)) in Arctic permafrost soils to the atmosphere as CO2, yet the mechanisms explaining the process remain obscure (4), and the rate at which shrubification has occurred in the High Arctic, at the climatic extremes of the phenomenon, is poorly documented. This project uses remote sensing to measure the rate of shrubification in the High Arctic, and, using DNA-based methods, investigates the role in the process of ectomycorrhizal fungi, symbionts associated with shrub roots that help plants to acquire nutrients from soil.

The Arctic has warmed at more than twice the rate of the rest of the planet over the past 50 years (5). Summer 2020 saw temperature records broken across the region, with highs of 38 °C in Siberia and 22 °C on Svalbard, in June and July, respectively (6,7). Precipitation patterns are also changing, with rainfall now being more frequent in summer, and rain even falling during winter (5,8,9). These changes to the Arctic’s climate are having widespread impacts on its fauna and flora, with one of the most apparent effects being the rapid spread of shrubs such as Salix, Bistorta and Betula in the Low Arctic (1,2,4). However, in the climatically more extreme High Arctic, little is known of the rate of shrub expansion over recent decades (1,2).

The precise mechanisms underlying shrubification are obscure (4). However, a consistent feature of the shrubs that are spreading across the Arctic is that they form symbioses with soil fungi termed ectomycorrhizas (ECMs) (10). These symbioses, which are absent from plant species not spreading in the region, help plants to acquire nutrients, notably nitrogen (N), from soil, leading to enhanced growth (11). The project here presents a postgraduate student with an excellent opportunity to use DNA-based methods to determine if ECMs have a significant role in shrubification, and to use remote sensing to measure the rate of High Arctic shrub expansion.

Methodology

Remote sensing: through a collaboration with Dr Shridhar Jawak at the Svalbard Integrated Arctic Earth Observing System, an expert in the remote sensing of polar vegetation, the student will analyse changes to plant cover in recent decades on the Brøgger Peninsula on Svalbard in the High Arctic (78° 55′ N, 11° 44′ E). The primary source of satellite imagery will be two 10 × 10 km very high resolution (31 cm per pixel) WorldView-2 (WV-2) panchromatic visible and near infrared (VNIR) images of the peninsula and its adjacent islands, taken as near as possible to August 2023 (Objective 1, Table 1). The student will compare these images with additional high-resolution (e.g., Quickbird or IKONOS) cloud-free archive images from 2000 and 2011. The bands available in WV-2 imagery will enable the calculation of a wide range of vegetation indices (VIs). Statistical models (based on linear regressions and generalised linear models) will be used to calculate which VIs have most explanatory power with regard to the expansion of deciduous shrubs and spatial vegetation dynamics (12).

Analyses of pre-existing plots: through a collaboration with Dr Clare Robinson, fractional vegetation cover in pre-existing plots established close to Ny-Ålesund on the Brøgger Peninsula in summers 1991 and 2000 (13,14) will be compared with that in summer 2023 using a digital adaptation of the field-based point frame technique (15). Either BAS staff or Norwegian Polar Institute (NPI) staff based permanently at Ny-Ålesund, with whom we have an ongoing collaboration, will take images of the plots. These analyses will determine if vegetation cover has changed over the last 23–32 years (Objective 2, Table 1), over which time mean annual air temperature on Svalbard has risen by 3–5 °C (5,8,9), and whether plant species symbiotic with ECMs (viz., Salix polaris and Bistorta vivipara) account for any increase in cover.

ECM analyses: in 2024, Salix polaris and Bistorta vivipara will be sampled by either BAS or NPI staff from each plot of an open top chamber warming experiment established at Kongsfjordneset on the Brøgger Peninsula in autumn 2014 (Fig. 1a; see also https://www.youtube.com/watch?v=lAQUWjAO5ZI). The experiment consists of 48 plots, 24 of which are warmed with open top chambers and 24 of which are bi-annually irrigated, simulating summertime rainfall. Analyses of images of plots indicate that OTCs increase the cover of both S. polaris and B. vivipara (Fig. 1b–d) (16). Leaves of both species will be analysed for N concentrations and natural abundance of 15N, which is depleted in ECM plants in the High Arctic owing to isotopic fractionation of organic N during uptake by fungi (17). Root tips colonised by ECM fungi (Fig. 1a, inset) will be counted, excised and surface sterilised, and, following protocols developed by our collaborator Dr Filipa Cox (18), DNA will be extracted from 576 individual tips using kits. Internal transcribed spacer (ITS) regions of fungal ribosomal DNA will be amplified using the ITS1F/ITS4 primer set and amplicons bidirectionally sequenced at a commercial facility. The sequences will be compared with those deposited in UNITE (https://unite.ut.ee/), a publicly-accessible database of fungal ITS sequences. Using generalised linear models and regression, the student will determine treatment effects on the abundance and identity of ECM fungi, and will compare the frequency of ECM taxa with leaf N concentrations and δ15N values, with the specific aim of determining if fungal symbionts that are efficient at soil N capture are more frequent on the roots of warmed and irrigated plants (Objective 3, Table 1).

Collectively, the analyses described above will break new ground in understanding the rate of shrubification in the High Arctic, and the role of ECMs in this process. We anticipate several peer-reviewed scientific papers from the project, written and led by the student, which will significantly advance current knowledge of shrubification.

Project Timeline

Year 1

Student to train in remote sensing techniques, conduct literature review and to address Objective 1 (Table 1).

Year 2

Student to train in molecular biological methods and to address Objectives 1–3 (Table 1).

Year 3

Student to address Objective 3, to write thesis and papers, and to engage in outreach (Table 1).

Year 3.5

Student to write thesis and papers, and to engage in outreach (Table 1).

Training
& Skills

The postgraduate student will receive training in remote sensing and molecular biological methods (please see Training Component).

References & further reading

1Epstein et al. (2011) doi:10.1088/1748-9326/7/1/015506

2Myers-Smith et al. (2020) doi:10.1038/s41558-019-0688-1

3Tarnocai et al. (2009) doi:10.1029/2008GB003327

4Mekkonen et al. (2018) doi:10.1029/2017JG004319

5IPCC (2021) Cambridge University Press, in press

6www.livescience.com/hottest-arctic-circle-temperature-ever-siberia.html

7svalbardposten.no/nyheter/nesten-21-grader/19.12748

8Bintanja & Andry (2017) doi:10.1038/NCLIMATE3240

9Hanssen-Bauer et al. (2019) doi:10.13140/RG.2.2.10183.75687

10Väre et al. (1992) doi:10.1007/BF00203256

11Smith & Read (2008) doi:10.1016/B978-0-12-370526-6.X5001-6

12Guay et al. (2014) doi:10.1111/gcb.12647

13Robinson et al. (1998) doi:10.1890/0012-9658(1998)079[0856:PCRTSE]2.0.CO;2

14Madan et al. (2007) doi:10.1007/s00300-006-0213-7

15Molau & Mølgaard (1996). ITEX Manual, 2nd ed., Copenhagen

16Newsham et al., unpublished data

17Michelsen et al. (1998) doi:10.1007/s004420050535

18Cox et al. (2010) doi: 10.1111/j.1461-0248.2010.01494.x

Further Information

Kevin Newsham (kne@bas.ac.uk)

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