Experimental herbivore exclusion, shrub introduction, and carbon sequestration in alpine plant communities

Abstract

BackgroundShrub cover in arctic and alpine ecosystems has increased in recent decades, and is predicted to further increase with climate change. Changes in shrub abundance may alter ecosystem carbon (C) sequestration and storage, with potential positive feedback on global C cycling. Small and large herbivores may reduce shrub expansion and thereby counteract the positive feedback on C cycling, but herbivore pressures have also changed in the alpine-arctic tundra; the increased shrub cover together with changes in herbivore pressure is leading to unpredictable changes in carbon sequestration and storage. In this study we investigate the importance of herbivory and shrub introduction for carbon sequestration in the short term. We measured standing biomass and daytime mid-growing season carbon fluxes in plots in a full factorial design where we excluded small and large mammalian herbivores and introduced Salix by planting Salix transplants. We used three study sites: one Empetrum-dominated heath, one herb- and cryptogam-dominated meadow, and one Salix-dominated shrub community in the low-alpine zone of the Dovre Mountains, Central Norway.ResultsAfter 2 years, significant treatment effects were recorded in the heath community, but not in the meadow and shrub communities. In the heath community cessation of herbivory increased standing biomass due to increased biomass of dwarf shrubs. Cessation of herbivory also reduced biomass of bryophytes and ecosystem respiration (ER). Except for an increase in biomass of deciduous shrubs caused by the Salix introduction, the only effect of Salix introduction was an increase in biomass of graminoids in the heath.ConclusionsOur short-term study demonstrated that herbivore exclusion had small but still significant effects on heath vegetation, whereas such effects were not apparent in the herb-and cryptogam-dominated meadow and the Salix-dominated shrub community. Following the treatments over more years is needed to estimate the long-term effects on community structure and the consequences for C sequestration in the three plant communities. Such data are important for predicting the impact of shrub expansion on C budgets from alpine regions.Electronic supplementary materialThe online version of this article (10.1186/s12898-018-0185-9) contains supplementary material, which is available to authorized users.

Drivers of C cycling in three arctic-alpine plant communities

“…The sampling design of this study was part of a larger experiment with four different treatments in each of eight blocks for each community (Sørensen et al 2018; Figure S1). In the present study, six replicate blocks from each community were randomly selected for measurements.…”

Herbivores reduce seedling recruitment in alpine plant communities

“…One prominent example of expected changes in species interactions is the negative effects of ongoing shrub encroachment on many understorey plants (Pajunen et al 2011, Mod and Luoto 2016, Wallace and Baltzer 2019). Increased shrub cover (‘shrubification') increases vegetation productivity and changes the nutrient and carbon dynamics of tundra heath and meadow vegetation (Sørensen et al 2018a). Beyond these plant–plant interactions, it is also increasingly clear that patterns of vegetation change depend on interactions among trophic levels, notably herbivory (Olofsson et al 2009, Kaarlejärvi et al 2017).…”

Functional group contributions to carbon fluxes in arctic-alpine ecosystems

“…Seasonal data from arctic ecosystems suggests that winter fluxes may comprise 30-50 percent of total annual belowground respiration (Grogan and Chapin 1999) and may be strongly affected by warming (Mikan, Schimel, and Doyle 2002). While GP and aboveground respiration are seasonal and strongly affected by variation in air temperature, belowground respiration continues yearround, where temperatures are buffered by vegetation and snow and are higher in winter in the shrub than heath or meadow communities (Sørensen et al 2018a). Differences in root density and soil microbial communities among the three communities and as affected by functional group composition may also contribute to source-sink relations outside of the growing season in these regions.…”

“…Measurements were made in conjunction with a long-term experiment on the effects of grazing and shrub expansion on alpine community composition and C balance (Sørensen et al 2018a(Sørensen et al , 2018b. The Dovre Mountains in Norway are a high plateau with moderate, rolling relief.…”

“…We selected study sites on south-facing slopes with representative examples of heath-, herbaceous-, and Salix-dominated communities, designated heath, meadow, and shrub ( Figure S1). For further details about the study site and community composition, as well as C pools in the soil and vegetation, see Sørensen et al (2018aSørensen et al ( , 2018b. We analyzed six plots within each community.…”