Peak season carbon exchange shifts from a sink to a source following 50+ years of herbivore exclusion in an Arctic tundra ecosystem

Abstract

Summary 1.To date, the majority of our knowledge regarding the impacts of herbivory on arctic ecosystem function has been restricted to short-term (<5 years) exclusion or manipulation experiments. Our understanding of long-term responses of sustained herbivory and/or herbivore exclusion on arctic tundra ecosystem function is severely limited. 2. Recent evidence suggests lemming population outbreaks, which have historically been common in tundra regions, have become less frequent and in some cases disappeared. Here, we evaluate how 50+ years of lemming absence have impacted ecosystem carbon and energy exchange in an Arctic tundra region near Barrow, Alaska, using some of the oldest herbivore exclosures in the Arctic. 3. We show that sustained lemming absence dramatically altered peak growing season ecosystem function (i.e. thaw depth, water-table, NDVI, albedo and land atmosphere CO 2 and CH 4 exchange) in all land cover types (i.e. wet, moist and dry) investigated but was most profound in wet tundra, where the long-term absence of lemmings altered the largest carbon sink during peak growing season into a source to the atmosphere. Landscape-level analysis suggests the long-term absence of lemmings may increase peak growing season albedo by +6% and decrease net ecosystem exchange, CH 4 flux and the combined radiative forcing potential by À96, À79 and À104%, respectively. 4. Identified long-term functional responses to herbivory differed from many short-term exclusion studies, as we found higher rates of carbon uptake where herbivores are present, associated with a shift in plant community composition (i.e. graminoid to moss), while short-term studies typically report a reduction in carbon uptake where herbivores are present, associated with defoliation events that decrease above-ground biomass/photosynthesis. 5. Synthesis. This work highlights the long-term impact of herbivory on ecosystem function, which suggests (i) the measured response of arctic tundra to herbivory may be dependent on the duration of experimental observation, and (ii) if negative trends in lemming populations persist in arctic tundra regions, the historically strong carbon sink capacity may be severely reduced.

Delayed herbivory by migratory geese increases summer‐long CO2 uptake in coastal western Alaska

“…Worldwide, grazers have a substantial influence on NPP and ecosystem-level CO 2 exchange (Augustine & McNaughton, 2006;Knapp et al, 1999;Lara, Johnson, Andresen, Hollister, & Tweedie, 2017;Welker, Fahnestock, Bilbrough, et al, 2004), but here we suggest that influence is modified by timing of grazing. Worldwide, grazers have a substantial influence on NPP and ecosystem-level CO 2 exchange (Augustine & McNaughton, 2006;Knapp et al, 1999;Lara, Johnson, Andresen, Hollister, & Tweedie, 2017;Welker, Fahnestock, Bilbrough, et al, 2004), but here we suggest that influence is modified by timing of grazing.…”

“…Our experimental manipulation suggests that the influence of climate change on the timing of migratory goose grazing in the Y-K Delta of western Alaska may have a larger impact on ecosystem CO 2 exchange than direct changes in response to a locally warming climate. Worldwide, grazers have a substantial influence on NPP and ecosystem-level CO 2 exchange (Augustine & McNaughton, 2006;Knapp et al, 1999;Lara, Johnson, Andresen, Hollister, & Tweedie, 2017;Welker, Fahnestock, Bilbrough, et al, 2004), but here we suggest that influence is modified by timing of grazing. Moreover, the timing of peak grazing is linked to timing of arrival to the breeding range (Eichholz & Sedinger, 1998;Lindberg, Sedinger, & Flint, 1997), which is determined by climate and weather conditions in their southern winter range and in staging sites along the migration route (Bauer, Gienapp, & Madsen, 2008;Clausen & Clausen, 2013).…”

Nutrient Release From Permafrost Thaw Enhances CH4 Emissions From Arctic Tundra Wetlands

“…Across the pan‐Arctic, tundra plant communities are shifting (Elmendorf et al, ) and have implications for biophysical (Chapin et al, ; Euskirchen et al, ) and biogeochemical feedback to the climate system (Euskirchen et al, ; Lara, Nitze, Grosse, & Mcguire, ). Because of the array of interacting climate (Elmendorf et al, ), environmental (Lin et al, ; Robinson et al, ), and biotic (Lara et al, ) processes that can influence long‐term patterns of vegetation change across heterogeneous tundra landscapes, evaluating the underlying controls on ecosystem structure and function has been difficult. Using an environmentally controlled experiment, we provided direct evidence to show that nutrients released from permafrost thaw can increase macrophyte biomass (Figure ) and the magnitude of CH 4 emission in arctic tundra wetlands (Table ).…”

Phenological mismatch in coastal western Alaska may increase summer season greenhouse gas uptake

“…However, the effects of grazing may counteract an advanced growing season because grazing decreases biomass (Sjögersten et al 2008) and reduces net CO 2 uptake (Sjörgersten et al 2012, Cahoon et al 2012), despite the potential for increased CO 2 emission through soil respiration in response to less shading and warmer soil temperatures (Risch et al 2013, Welker et al 2004. In contrast, some grazing exclosure studies in high latitude systems suggest the opposite response with grazing exclusion resulting in greater CO 2 emission, particularly where there is a change in species composition in response to the removal of grazing (Falk et al 2015, Metcalfe and Olofsson 2015, Lara et al 2017. While the mechanisms are complex, it is clear that presence or absence of grazing is an important driver of CO 2 flux through effects on soil and vegetation properties, and changes in timing of grazing is also likely an important control on local CO 2 flux.…”