Herbivore absence can shift dry heath tundra from carbon source to sink during peak growing season

Abstract

In arctic tundra, large and small mammalian herbivores have substantial impacts on the vegetation community and consequently can affect the magnitude of carbon cycling. However, herbivores are often absent from modern carbon cycle models, partly because relatively few field studies focus on herbivore impacts on carbon cycling. Our objectives were to quantify the impact of 21 years of large herbivore and large and small herbivore exclusion on carbon cycling during peak growing season in a dry heath tundra community. When herbivores were excluded, we observed a significantly greater leaf area index as well as greater vascular plant abundance. While we did not observe significant differences in deciduous dwarf shrub abundance across treatments, evergreen dwarf shrub abundance was greater where large and small herbivores were excluded. Both foliose and fruticose lichen abundance were higher in the large herbivore, but not the small and large herbivore exclosures. Net ecosystem exchange (NEE) likewise indicated the highest carbon uptake in the exclosure treatments and lowest uptake in the control (CT), suggesting that herbivory decreased the capacity of dry heath tundra to take up carbon. Moreover, our calculated NEE for average light and temperature conditions for July 2017, when our measurements were taken, indicated that the tundra was a carbon source in CT, but was a carbon sink in both exclosure treatments, indicating removal of grazing pressure can change the carbon balance of dry heath tundra. Collectively, these findings suggest that herbivore absence can lead to changes in plant community structure of dry heath tundra that in turn can increase its capacity to take up carbon.

Model responses to CO2 and warming are underestimated without explicit representation of Arctic small‐mammal grazing

“…Grazing animals can have large effects on ecosystems (Grellman 2002, McLaren and Jefferies 2004, Post and Pedersen 2008, Sj ögersten et al 2008, Rinnan et al 2009, Cahoon et al 2011, Kaarlejärvi et al 2015, Leffler et al 2019, Min et al 2021. Our simulations suggest that long-term exclusion of voles or maintenance of vole populations at high densities can result in large gains or losses of both plants and soil C (Fig.…”

“… 2019 , Min et al. 2021 ). Our simulations suggest that long‐term exclusion of voles or maintenance of vole populations at high densities can result in large gains or losses of both plants and soil C (Fig.…”

Small but mighty: Impacts of rodent‐herbivore structures on carbon and nutrient cycling in arctic tundra

“…This conclusion is supported by our observation that increases in N pool size under structures coincides with higher C‐acquiring enzyme activity, suggesting that microbes increase their effort to acquire C in response to reduced N‐limitation. Such changes in soil nutrient availability, paired with the coupling of C‐N‐P cycling in arctic tundra ecosystems (McLaren & Buckeridge, 2019), may impact local C sink‐source dynamics (Min et al, 2021) with consequences for other ecological processes.…”

Large and small herbivores have strong effects on tundra vegetation in Scandinavia and Alaska

“…This effect was only absent in A‐DH where field layer density was low in general and contained mainly prostrate dwarf shrubs that in general are less sensitive to grazing than more erect growth forms (Kaarlejärvi et al., 2017 ). The negative impact of herbivore presence on LAI and total plant hits in A‐DH has been associated with lower carbon uptake meaning that at least these dry heath tundra systems have the potential shift between being carbon sinks or sources depending on if herbivores are present or not (Min et al, 2021 ). In A‐MAT, S‐MH, and S‐DH, herbivory influenced all three measures related to plant density (LAI, NDVI, and total plant hits), but the strength of the effect differed among habitats and measurements.…”