Cold-season soil respiration in response to grazing and warming in High-Arctic Svalbard (original) (raw)
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Distinguishing Rapid and Slow C Cycling Feedbacks to Grazing in Sub-arctic Tundra
Large grazers are known to affect ecosystem functioning even to the degree where ecosystems transition to another vegetation state. Alongside the vegetation change, several features of ecosystem functioning, such as ecosystem carbon sink capacity and soil carbon mineralisation rates, may be altered. It has remained largely uninvestigated how the grazing effects on carbon cycling processes depend on the duration of grazing. Here, we hypothesised that grazing affects ecosystem carbon sink through plant-driven processes (for example, photosynthesis) on shorter timescales , whereas on longer timescales changes in soil-driven processes (for example, microbial activity) become more important contributing to a decreased carbon sink capacity. To test this hypothesis, we investigated key processes behind ecosystem carbon cycling in an area that recently had become dominated by graminoids due to a high reindeer grazing intensity and compared these to the processes in an area of decades old grazing-induced graminoid dominance and in an area of shrub dominance with little grazer influence. In contrast to our hypothesis, areas of both old and recent grassification showed a similar carbon sink capacity. Yet the individual fluxes varied depending on the time passed since the vegetation shift: ecosystem respiration and mid-season photosynthesis were higher under old than recent grassification. In contrast, the extracellular enzyme activities for carbon and phosphorus acquisition were similar regardless of the time elapsed since grazer-induced vegetation change. These results provide novel understanding on how ecosystem processes develop over time in response to changes in the intensity of herbivory. Moreover, they indicate that both autotrophic and heterotrophic processes are controlled through multiple drivers that likely change depending on the duration of herbivory.
Geese are directing the plant and microbial communities of their arctic forage habitat
2014
The presented study aims to add more field evidence of goose grazing impact on the structure of Arctic ecosystems, which is necessary to better understand the effect of rising goose numbers on complex ecosystem processes. The conducted research made use of long-term exclosures on Svalbard to study the influence of Barnacle Goose Branta leucopsis grazing on vascular plants, the moss layer and abiotic soil conditions. Molecular fingerprinting using PCRDGGE was used to get also a first idea of the possible goose grazing effect on microbial communities. Barnacle Goose grazing was found to significantly influence on the vegetation composition and to reduce species number, vegetation biomass and depth of the moss layer. Our results suggest also the effect to trickle down to the decomposer food web influencing the microbial community structure. Those differences are probably leading to changes in important ecosystem processes such as soil nutrient dynamics. The presented study adds thus to...
Influence of goose grazing on plant availability of nutrients
2014
Nutrient availability is a primary limiting factor of biotic functioning in Arctic environments. We hypothesized that geese, whose numbers have increased dramatically, impact on the plant availability of nutrients. The moss layer was thought to play a key role herein. To test for these hypotheses we measured plant availability of macro- and micronutrients over the winter and growing season and moss depth in a goose exclosure experiment. Our results show that important nutrients were significantly influenced by goose grazing. For some elements this could be partially explained by the grazing impact on the moss layer. During the winter season nutrient availability was remarkably high and was influenced by geese, urging the need for more ecological research during this period.
Polar Science, 2021
Geese can profoundly affect arctic ecosystems directly (e.g., by grazing vegetation) and indirectly (e.g. by changing nutrient cycling resulting from faces inputs and by reducing plant litter). In the Arctic, behavior and abundance of geese have changed due to climate and land-use change. While the short-term effects of increased goose populations on tundra ecosystems are known, there is a knowledge gap for long-term consequences of goose population changes on nutrient cycling and plant communities, especially in the High-Arctic. Here, we compared wetland sites where geese have been almost absent for at least 50 years (Pond Inlet), and nearby sites where geese are abundant or sites where they have been excluded experimentally by cages and where the ground has been experimentally fertilized for over 16 years (Bylot Island). Long-term goose disappearance increased inorganic nutrients in wetlands through increased plant litter decomposition and changed community composition, likely by altering competitive relationships between three dominant vascular plant species that are different in terms of nutrient acquisition and use. In experimentally fertilized sites, inorganic nutrients were similarly increased, but fertilization had contrast effects on plant community composition compared with herbivory, released plant species from the nutrient limitation and converted wetland habitats to a dryer state. Overall, our results suggest that the direct effects of goose herbivory on vegetation are more profound than their indirect effects through an alternation of nutrient cycling even in nutrient-limited wetlands of the Arctic. These findings emphasize the need to assess long-term direct impacts of herbivores on vegetation.
Consequences of grazer‐induced vegetation transitions on ecosystem carbon storage in the tundra
Functional Ecology, 2018
1. 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. 2. 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. 14 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. 3. 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. 4. We show that the heavily grazed, graminoid-dominated, areas stored less C aboveground than the lightly grazed, shrub-dominated, areas. Yet, the belowground consequences of grazinginduced 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. 5. Our results indicate that despite the negative impacts of grazers on aboveground C storage, their impact on belowground 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.
Spring feeding by pink-footed geese reduces carbon stocks and sink strength in tundra ecosystems
Global Change Biology, 2007
Tundra ecosystems are widely recognized as precious areas and globally important carbon (C) sinks, yet our understanding of potential threats to these habitats and their large soil C store is limited. Land-use changes and conservation measures in temperate regions have led to a dramatic expansion of arctic-breeding geese, making them important herbivores of high-latitude systems. In field experiments conducted in high-Arctic Spitsbergen, Svalbard, we demonstrate that a brief period of early season belowground foraging by pink-footed geese is sufficient to strongly reduce C sink strength and soil C stocks of arctic tundra. Mechanisms are suggested whereby vegetation disruption due to repeated use of grubbed areas opens the soil organic layer to erosion and will thus lead to progressive C loss. Our study shows, for the first time, that increases in goose abundance through land-use change and conservation measures in temperate climes can dramatically affect the C balance of arctic tundra.
Consequences of warming on tundra carbon balance determined by reindeer grazing history
Nature Climate Change, 2014
Arctic tundra currently stores half of the global soil carbon (C) stock 1 . Climate warming in the Arctic may lead to accelerated CO 2 release through enhanced decomposition and turn Arctic ecosystems from a net C sink into a net C source, if warming enhances decomposition more than plant photosynthesis 2 . A large portion of the circumpolar Arctic is grazed by reindeer/caribou, and grazing causes important vegetation shifts in the long-term. Using a unique experimental set-up, where areas experiencing more than 50 years of either light (LG) or heavy (HG) grazing were warmed and/or fertilized, we show that under ambient conditions areas under LG were a 70% stronger C sink than HG areas. Although warming decreased the C sink by 38% under LG, it had no e ect under HG. Grazing history will thus be an important determinant in the response of ecosystem C balance to climate warming, which at present is not taken into account in climate change models.
Foraging geese, vegetation loss and soil degradation in an Arctic salt marsh
Applied Vegetation Science, 2002
The North American mid-continent population of Lesser snow geese (Chen caerulescens caerulescens L.) has increased by ca. 7% per year, largely as a result of geese feeding on agricultural crops in winter and on migration. We describe the long-term effects of increasing numbers of geese at an arctic breeding ground (La Pérouse Bay, Manitoba) on intertidal salt-marsh vegetation. Between 1985 and 1999 goose grubbing caused considerable loss of graminoid vegetation along transects in intertidal marshes. Loss of vegetation led to bare sediment with a plant cover of less than 2%. Changes in vegetation could not be described by simple linear, geometric or exponential functions; most losses occurred between 1988 and 1990 and losses were staggered in time between individual transects, some of which had all vegetation removed.