Fig 1.
Examples of browning events arising from climatic, biotic and physical disturbance events.
(A) Empetrum nigrum shoot mortality following an extreme winter warming event in northern Norway. (B) Almost 100% mortality from frost-drought of the dominant dwarf shrub Calluna vulgaris, Norway. (C) Dead Cassiope tetragona following an icing event, Svalbard, High Arctic. (D) Droughted Sphagnum from a combined heat-wave drought, Utqiaġvik, Alaska. (E) Spread of an Epirrita autumnata caterpillar outbreak at treeline from birch woodland onto tundra. Grey vegetation in foreground is defoliated Betula nana (dwarf birch) shrubs, grey trees mid-picture are defoliated tree birch (B. pubescens spp. czerepanovii) that is the source of the outbreak (sub-arctic Sweden). (F) Dead shoots of E. nigrum infected with the snow mould Arwidssonia empetri. (G) Dead E. nigrum following a caterpillar outbreak of E. autumnata and Operophtera brumata. (H) Browning from lemming grazing. (I) Aftermath of fire in the Yukon-Kuskokwim delta, Alaska, showing a dead Sphagnum mound and significant removal of the peaty organic layer. Abundant resprouting of Eriophorum vaginatum (cotton grass) clearly visible in mid-ground. (J) Resprouting of E. vaginatum a year after fire. This heathland was fire-prone due to a previous frost-drought event that left dead, dry, flammable vegetation. (K) A large retrogressive thaw slump (a megaslump >20 ha), Peel Plateau, NW Canada. (L) Active layer detachments near Eureka, Ellesmere Island. Photos: (A, C, G) Rachael Treharne (B, J) Gareth Phoenix (D) Donatella Zona (E) Thomas Parker (F, H) Johan Olofsson (I) Chris Linder (K) Julian Murton (L) Antoni Lewkowicz.
Table 1.
Characteristics of browning events. Durations and reported ranges of % damage from real events, not field simulations. ‘nd’ indicates no data. Superscript letters refer to notes as follows: (a) based on recovery of biomass or cover; (b) note, total area of impact often not reported in surveys of amount of damage, (c) not quantified but based on similarity of impact compared to extreme winter warming; (d) combined drought and heatwave; (e) at small scales scorching can result in little biomass removal, though fire typically burns >70% vegetation cover; (f) a thermal erosion feature often continues to expand, so duration only refers to a single point; (g) area includes thermokarst lake and wetland development; (h) beaver dams listed separately since the browning mechanism is dam creation, not herbivory of biomass; (i) decades to centuries for complete biomass recovery. Nordic Arctic region here refers to Norway (including Svalbard for icing events), Sweden and Finland north of the Arctic Circle.
Fig 2.
Browning event impacts on ecosystem carbon sequestration in the first years immediately following the event.
Change in Net Ecosystem Exchange (NEE): +1 means a doubling of the pre-event (undisturbed) C uptake flux. 0 is no change with sink strength declining to −1 where there is no net uptake, x−5 means a shift to C fluxes opposite in sign (i.e., losses) but up to five-fold greater magnitude than the original C uptake flux. (1) Browning events arising from physical disturbance can result in NEE much greater in magnitude and opposite in sign than of the pre-event healthy vegetation (i.e., a shift from a C sink to a much larger C source). However, these disturbance events where there is less browning (2) can also cause more modest shifts resulting in reduced C sink size rather than conversion to a source (e.g., active layer detachments where vegetation remains partially intact or where ecosystem respiration is reduced due to lack of vegetation and reduced soil organic matter content). This more moderate change in NEE is also typical for biotic and climatic events (3) that typically result in lower C sink size rather than conversion to a source. The limitation on NEE impacts of browning from climatic and biotic drivers partially arises because photosynthesis from resilient plant species compensates for loss of sensitive species. Unique among events (4) herbivore outbreaks can also result in greater C sequestration from a rapidly recovering plant community being able to take advantage of nutrient inputs from frass, potentially doubling the C uptake compared to the original undisturbed vegetation. RTS = retrogressive thaw slump; ALDS = active layer detachment slide; Herbiv = herbivore outbreak; HwD = heatwave-drought; EWW = extreme winter warming.
Fig 3.
Overview of recovery rates of browning events.
Recovery is in years since event. % browning is % loss of live biomass. HwD is heatwave-drought, EWW is extreme winter warming, F-D is frost drought. (1) Climatic events and herbivore outbreaks all have similarly fast recovery with the majority of live biomass recovered within 4 years. (2) Much longer recovery times are associated with physical disturbance events (abrupt permafrost thaw and fire), and while significant initial recovery after fire can occur on similar timescales to climatic events from re-sprouting plants, (3) full recovery can take decades. (4) Abrupt permafrost thaw has the longest recovery time due to recovery potential being severely reduced where plant biomass is completely removed above and below ground. Events may also lead to greening in the long term. % browning and recovery rates vary within a single event type, so for each event type the graph represents a ‘typical’ trajectory based on data and text descriptions in the cited papers.