The ecological impact of volcanic ash (tephra) deposition

Biogeochemical Cycles



Explosive volcanic eruptions can distribute ash and other rocky fragments (known collectively as tephra) on a continental scale. Tephra deposits constitute a significant natural hazard: as well as threatening human health, tephra smothers vegetation, alters hydrology and slope stability, and interrupts biogeochemical flows. Tephra deposits can remain unconsolidated – and potentially detrimental to ecosystems – for years. Understanding the response of ecosystems to tephra deposition is crucial for hazard mitigation, and long-term ecological restoration. The responses of aboveground (plant) communities to tephra deposition have been studied in some detail. In contrast, the effects on tephra deposition on the belowground communities (soil organisms) that underpin terrestrial ecosystem function are poorly understood. Furthermore, the factors that promote the stabilisation of tephra deposits have not been investigated before. This project addresses these knowledge gaps with an experimental study in Washington State, USA, centred on two key questions.

Q1: How do tephra deposits impact soil biology and biochemistry?
The immediate ecological impact of thick (> 0.5 m) tephra deposits is catastrophic. However, most tephra deposits are thin (< 0.1 m) and their impacts are less predictable. Even thin tephra deposits change the physical properties of land surface, altering surface albedo and hydrology, with knock-on effects for soil temperature and moisture levels. Tephra deposition may impact soil chemistry directly. For example, soils underlying tephra deposits are likely to receive a pulse of carbon (C) and nutrients from the decay of buried vegetation, and a deposit just a few cm thick will isolate the soil from aboveground sources of organic matter. Gas diffusion into the soil could be interrupted, with impacts for soil redox chemistry (e.g., onset of anaerobic conditions and increased production of acidic fermentation products, sulfides and methane). Tephra also has indirect impacts on soil chemistry, as stressful surface conditions will likely reduce net primary productivity (NPP) and belowground C inputs via root exudates.

Soil microbial communities (SMCs, primarily microscopic bacteria and fungi) are sensitive to changes in the soil environment. Thus, it is likely that tephra deposition will change the structure and function of these communities; e.g., the relative proportion of bacteria and fungi and overall microbial diversity. Changes in SMCs are likely to be accompanied by shifts in quantity and quality of soil C and soil respiration, as organic C is consumed by soil organisms, but not replaced by fresh inputs. It is important to understand these changes, because SMCs play a critical role in decomposition and the cycling of C and nutrients in terrestrial ecosystems; they also influence the resilience of these ecosystems to future disturbance.

Q2: What are the biotic and abiotic factors that influence the stabilisation of tephra deposits?
Fresh tephra deposits are often mobilised by the wind and slope processes, endangering human wellbeing and burying terrain unaffected by original deposit. Some tephra deposits can remain mobile for years, whilst others stabilise rapidly and are incorporated into soils without causing further disruption. The stabilisation of tephra deposits is understudied but is probably influenced by a combination of biotic and abiotic factors, e.g. climate, vegetation cover and NPP and tephra characteristics. It is likely that a) fine tephra will be more mobile than coarse material; b) thick deposits will be more mobile than thin layers (because they are less readily fixed by vegetation) and that areas with high NPP will be more favourable to stabilisation, due to the sheltering impact of vegetation and rapid burial by litter. Greater understanding of these factors would enhance efforts to predict (and mitigate) the impact of future tephra-producing eruptions.

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Image Captions

Cascades forest.jpg: coniferous forest in the Cascade Mountains, close to Mount St Helens. Image: N Cutler
Devastated zone.jpg: the areas around Mount St Helens was devastated during the 1980 eruption. Image: N Cutler
Eastern Washington State.jpg: the tephra from the 1980 eruption left a visible layer >600 km from the volcano. Image: N. Cutler
Mt St Helens.jpg: Mount St Helens is one of five active volcanoes in Washington State. Image: N. Cutler
Sagebrush scrub.jpg: vegetation in E. Washington State. Image: N. Cutler
Tephra layer.jpg: a shallow pit showing the MSH1980 tephra layer (off-white). Image: N. Cutler


The project will be based on a field experiment – designed by the candidate – to directly observe tephra impacts and stabilisation. The experiment will involve the application of tephra to vegetated plots in Washington State, USA, a volcanically active, ecologically varied and agriculturally productive region that is vulnerable to tephra disturbance. In collaboration with US researchers, the candidate will identify sites for the experiment and obtain tephra originating from the 1980 eruption of Mount St Helens. The precise impacts of tephra deposition are likely to vary according to a) the peculiarities of the site (climate, vegetation cover, etc.) and b) the characteristics of the tephra deposit (grain size and thickness, primarily). Thus, the experiment will encompass two contrasting habitats: closed coniferous forest in the humid west of the State, and sagebrush stepped in the arid east. In each area, the candidate will establish replicate experimental plots with varying tephra depth and grain size.

The plots will be monitored for the duration of the project. Soil temperature, moisture and wind speed will be monitored continuously using data loggers. The candidate will also make two field trips to collect soil samples (at the initiation and completion of the project; samples from the midpoint of the experiment will be collected by US collaborators) and measure soil respiration. The soil samples will be analysed in the UK. The analyses will determine a) SMC structure using molecular (DNA) analysis and b) soil properties, with a focus on soil C (quality and quantity) and nutrient status. At the same time the soil samples are collected, the candidate will conduct high resolution scans of tephra surface profile, estimate tephra loss and compaction (in terms of mass per unit area) and assess the experimental tephra deposits for evidence of biocrust-forming organisms (primarily microbes such as green algae and cyanobacteria, mosses and lichens).

Project Timeline

Year 1

Preparatory research & liaison with US collaborators; experimental design; field trip 1 (experimental set-up and collection of soil samples).

Year 2

Molecular and chemical analysis of soil samples for year 1 & 2 of experiment (includes training); training in statistical methods.

Year 3

Continued analysis of soil samples; field trip 2 (in situ data acquisition and collection of samples); conference presentation of preliminary results; draft manuscript preparation.

Year 3.5

Completion of thesis and manuscript(s) for publication.

& Skills

During the project, the candidate will receive the following training:

  • Use of molecular (DNA) methods to characterise soil microbial communities
  • Lab-based soil analysis
  • Statistical techniques, specifically statistical modelling of biological communities and bioinformatics
References & further reading

Ayris, P.M., and Delmelle, P. (2012) The immediate environmental effects of tephra emission: Bull Volcanol, 74:1905-1936.
Cutler, N.A., et al. (2020) How does tephra deposit thickness change over time? A calibration exercise based on the 1980 Mount St Helens tephra deposit. J Volcanol Geothermal Res, 399: 106883
Dugmore, A.J., et al. (2020) The interpretative value of transformed tephra sequences. J Quat Sci, 35:23-38.
Hotes, S., et al. (2004) Effects of tephra deposition on mire vegetation: a field experiment in Hokkaido, Japan: J Ecol, 92:624-634.
Panebianco, J.E., et al. (2017) Dynamics of volcanic ash remobilisation by wind through the Patagonian steppe after the eruption of Cordon Caulle, 2011, Scientific Reports, 7.

Further Information

Dr Nick Cutler, School of Geography, Politics & Sociology, Newcastle University. E-mail:

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