Consequences of grazer‐induced vegetation transitions on ecosystem carbon storage in the tundra

Abstract

Large herbivores can control plant community composition and, under certain conditions, even induce vegetation shifts to alternative ecosystem states. As different plant assemblages maintain contrasting carbon (C) cycling patterns, herbivores have the potential to alter C sequestration at regional scales. Their influence is of particular interest in the Arctic tundra, where a large share of the world's soil C reservoir is stored. We assessed the influence of grazing mammals on tundra vegetation and C stocks by resampling two sites located along pasture rotation fences in northern Norway. These fences have separated lightly grazed areas from heavily grazed areas (in close proximity to the fences) and moderately grazed areas (further away from the fences) for the past 50 years. Fourteen years earlier, the lightly and moderately grazed areas were dominated by dwarf shrubs, whereas heavy grazing had promoted the establishment of graminoid‐dominated vegetation. Since then, both reindeer densities and temperatures have increased, and more time has passed for transient dynamics to be expressed. We expected that the vegetation and C stocks would have changed under all grazing intensities, but not necessarily in the same way. At the site where relative reindeer numbers and trampling intensity had increased the most, graminoid‐dominated vegetation was now also found in the moderately grazed area. At the other site, the dominant vegetation types under all grazing intensities were the same as 14 years earlier. We show that the heavily grazed, graminoid‐dominated areas stored less C above‐ground than the lightly grazed, shrub‐dominated areas. Yet, the below‐ground consequences of grazing‐induced grassification varied between the sites: Grazing did not alter organic soil C stocks at the site where both evergreen and deciduous shrubs were abundant in the lightly grazed area, whereas heavy grazing increased organic soil C stocks at the site where the deciduous shrub Betula nana was dominant. Our results indicate that, despite the negative impacts of grazers on above‐ground C storage, their impact on below‐ground C may even be positive. We suggest that the site‐specific responses of organic soil C stocks to grazing could be explained by the differences in vegetation under light grazing. This would imply that the replacement of deciduous shrubs by graminoids, as a consequence of grazing could be beneficial for C sequestration in tundra soils. A http://onlinelibrary.wiley.com/doi/10.1111/1365-2435.13029/suppinfo is available for this article.

Removal of grazers alters the response of tundra soil carbon to warming and enhanced nitrogen availability

“…), previous reports from the same study site (Ylänne et al. ) and other subarctic areas (Köster et al. , ).…”

“…At the site, a continuous soil organic layer of approximately 4.25 cm is found above the mineral soil layer (Ylänne et al. ). The area is bisected by a pasture rotation fence, been in place at least since 1966, which creates a legal border between the reindeer summer ranges and their migration range.…”

Contrasting vegetation states do not diverge in soil organic matter storage: evidence from historical sites in tundra

“…It has been suggested that whether a transition to another vegetation state in response to grazing increases or decreases soil C sequestration could depend on the type of vegetation that is replaced through grazing (Ylänne et al. ). Heath‐ and shrub‐dominated tundra with mixed B. nana and E. hermaphroditum or Salix willows store carbon in the woody biomass and produce slowly decomposable litter, but also maintain high rates of C cycling via labile C inputs and ectomycorrhizal symbionts that efficiently degrade soil organic matter (De Deyn et al.…”

“…, Ylänne et al. ). Sites with historical vegetation transitions could provide a tool for predicting the impacts of ongoing grazer‐induced vegetation transitions on SOM stock because the vegetation transitions at these sites occurred several centuries ago and sufficient time has passed for the slow process of SOM accumulation or loss to be recognizable.…”

Long‐term heavy reindeer grazing promotes plant phosphorus limitation in arctic tundra

“…The area experiences a suboceanic climate with an annual precipitation of 935 mm and an annual mean temperature of −0.6°C (gridded data, 2006–2015, Norwegian Water Resources and Energy Directorate, http://www.senorge.no). The mean annual precipitation and temperature in 2013 and 2014 (the years in which this study was performed) were fairly normal for the last decade, but since mean annual temperatures have increased over the last 50 years at a rate of almost 1° per decade, these years were warmer than the long‐term averages (Ylänne, Olofsson, Oksanen, & Stark, ). The study site is divided by a reindeer fence, established in the 1960s to help keep the reindeer within their legal summer ranges.…”