HABITAT TYPE DETERMINES HERBIVORY CONTROLS OVER CO₂FLUXES IN A WARMER ARCTIC

Abstract

High-latitude ecosystems store large amounts of carbon (C); however, the C storage of these ecosystems is under threat from both climate warming and increased levels of herbivory. In this study we examined the combined role of herbivores and climate warming as drivers of CO2 fluxes in two typical high-latitude habitats (mesic heath and wet meadow). We hypothesized that both herbivory and climate warming would reduce the C sink strength of Arctic tundra through their combined effects on plant biomass and gross ecosystem photosynthesis and on decomposition rates and the abiotic environment. To test this hypothesis we employed experimental warming (via International Tundra Experiment [ITEX] chambers) and grazing (via captive Barnacle Geese) in a three-year factorial field experiment. Ecosystem CO2 fluxes (net ecosystem exchange of CO2, ecosystem respiration, and gross ecosystem photosynthesis) were measured in all treatments at varying intensity over the three growing seasons to capture the impact of the treatments on a range of temporal scales (diurnal, seasonal, and interannual). Grazing and warming treatments had markedly different effects on CO2 fluxes in the two tundra habitats. Grazing caused a strong reduction in CO2 assimilation in the wet meadow, while warming reduced CO2 efflux from the mesic heath. Treatment effects on net ecosystem exchange largely derived from the modification of gross ecosystem photosynthesis rather than ecosystem respiration. In this study we have demonstrated that on the habitat scale, grazing by geese is a strong driver of net ecosystem exchange of CO2, with the potential to reduce the CO2 sink strength of Arctic ecosystems. Our results highlight that the large reduction in plant biomass due to goose grazing in the Arctic noted in several studies can alter the C balance of wet tundra ecosystems. We conclude that herbivory will modulate direct climate warming responses of Arctic tundra with implications for the ecosystem C balance; however, the magnitude and direction of the response will be habitat-specific.

Spatial patterns of goose grubbing suggest elevated grubbing in dry habitats linked to early snowmelt

“…This so-called grubbing activity is particularly destructive for vegetation. It creates holes in the vegetation and may create vegetation-free craters at high grubbing intensities van der Wal et al 2007;Sjogersten et al 2008;Speed et al 2009;Speed, Cooper et al 2010;. Furthermore, grubbing exposes the organic layer to wind and flooding, and thereby increases the impact of erosion processes (van der Wal et al 2007).…”

Density-dependent switches in diet: a likely mechanism for negative feedbacks on goose population increase?

“…In that case, foraging conditions may quickly deteriorate above a threshold of grazing pressure. Deteriorating foraging conditions were most pronounced in the mesic tundra, likely as a result of the lower plant productivity Sjögersten et al 2008). Here, after the first year of grazing geese in general could not meet their daily energy requirements at any level of grazing pressure.…”

“…The design of the first grazing experiment was a randomised block design in which the effects of grazing (two levels) and increased surface temperature were studied. Temperature was increased by means of open top chambers, which increased soil surface temperature by 1-2°C (for further details see Sjögersten et al 2008). Two levels of grazing were applied on plots of 2 m 9 2 m; in 'low level' grazing, a pair of geese was present for 1 h on a plot and in 'high level' grazing, it was present for 5 h. Each block containing all treatments was replicated five times in the two habitat types; wet-moss tundra and mesic tundra.…”

Thermo-erosion gullies boost the transition from wet to mesic tundra vegetation

“…The presence of Carex aquatilis, Eriophorum scheuchzeri and Dupontia fisheri characterizes well the typical vegeta- tion of Arctic wetlands (Jorgenson et al, 2013;Sandvik and Odland, 2014;Lara et al, 2015) whilst that of Arctagrostis latifolia, Luzula and Salix spp. are common features of Arctic mesic environments (Audet et al, 2007;Sjögersten et al, 2008). Disturbed polygons were the most diverse habitats given that they offered a middle-range state between wet and mesic conditions where hydrophilic species were still present while mesic environment ones had successfully established.…”