Vegetation Disturbance Triggers Greenhouse-Gas Emitting Feedback Loop in Permafrost

by Lindsay McCord

Vegetation changes have the ability to rapidly destabilize permafrost soil, illustrating vulnerability of these ecosystems to disruptions. Study sites that removed shrub vegetation experienced both increased thaw depth of permafrost as well as soil subsidence, lowering the permafrost table by 31 cm in comparison to control sites. This created localized wetlands of water-saturated depressions, which become hotspots for additional thawing as well as increased methane emissions.

The removal of shrubs triggered a positive feedback loop. Permafrost soils can contain up to 80% ice by volume and as the permafrost melts, the soil surface sinks. The subsided soil collects more snow, which insulates the soil and keeps it warmer in winter, allowing it to melt faster in the warmer months. These depressions pool water which absorbs solar energy better than soil. The solar energy warms the water and facilitates further melting of permafrost. The feedback loop between shrub disturbance, subsidence, and permafrost melting is expected to most strongly affect particularly ice-rich regions, about 20% of permafrost regions.

Methane emissions, a powerful greenhouse gas, was measured at both disturbed and control sites. In sites where shrub cover had been removed, measurements indicated a net release of methane, while control sites recorded a net uptake. Subsidence and permafrost melting probably facilitated decomposition of now-thawed organic matter, producing methane gas.

These findings indicate that small disturbances in vegetation cover can trigger strong positive feedback loops that cause permafrost collapse. These ecosystems are vulnerable to human activities that may perturb vegetation, causing permafrost collapse that transforms frozen soil that is currently uptaking methane gas into wetlands that emit this strong greenhouse gas. This localized feedback cycle has the potential to exacerbate climate change which already poses a threat to permafrost.

Nauta, A. L., Heijmans, M. M., Blok, D., Limpens, J., Elberling, B., Gallagher, A., … & Berendse, F. (2015). Permafrost collapse after shrub removal shifts tundra ecosystem to a methane source. Nature Climate Change, 5(1), 67-70.

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