Reindeer trampling promotes vegetation changes in tundra heathlands: Results from a simulation experiment

Abstract

Question:Herbivores exert strong influences on vegetation through activities such as trampling, defoliation, and fertilization. The combined effect of these activities on plant performance may cause dramatic vegetation shifts. Because herbivore pressures and the relative importance of their different activities are not equally distributed across the landscape, it is important to understand their isolated effect. One example of an herbivore-induced vegetation shift is the reindeer-driven transition from a subarctic tundra vegetation dominated by dwarf shrubs into a more productive, graminoid-dominated state. Here, we asked how each of the grazing activities by reindeer separately and combined shape vegetation composition.Location: Nordreisa, Norway. Methods:We used a field experiment over six summers to study the separate and interacting effects of reindeer trampling, defoliation, addition of faeces and removal of moss on tundra heath vegetation, and to identify which of these factors were most important in driving the plant community towards a graminoid-dominated state. Results:The combination of all treatments resulted in the strongest changes in vegetation, but trampling was the single most important factor altering the vegetation composition by reducing the abundance of both evergreen and deciduous dwarf shrubs. In contrast to what was expected, none of our treatments, separate or combined, resulted in an increased abundance of graminoids in 5 years, although such rapid vegetation changes have been observed in the field in similar environmental conditions. Conclusions:Trampling is the key process by which reindeer influence the abundance of functional groups, but only many processes combined result in strong changes in community composition. Moreover, additional factors not included in this experiment, such as urine, may be important in causing a state shift to a graminoid-dominated community.

Stomping in silence: Conceptualizing trampling effects on soils in polar tundra

“…Trampling may drive shifts in plant functional or species composition towards species with higher resilience and resistance to mechanical damage (Cole, 1995; Forbes et al., 2001; Jónsdóttir, 1991; Lezama & Paruelo, 2016). These vegetation shifts may arise via direct trampling damages altering plant fitness and survival and could further intensify due to ungulate defoliation and excretion (Egelkraut et al., 2020). Alternatively, vegetation shifts may arise via trampling‐induced changes in soil resource availability and plant–soil feedbacks (Egelkraut et al., 2018) and these changes likely emerge and evolve as the different trampling effect pathways act alone and in interaction over time (Figure 1A–C).…”

“…While transient trampling effects of these ungulates may be subtle in comparison to past Pleistocene megafauna, their ability to locally drive graminoid‐moss state transitions accompanied by shifts in ecosystem and soil processes and albedo has been reported, especially for Old World reindeer (Forbes & Kumpula, 2009; Olofsson et al., 2004; van der Wal, 2006). Indeed, much of the complex vegetation and ecosystem responses to herbivores in tundra may be driven by trampling either alone or in concert with defoliation and fertilization that alone usually do not recreate the observed changes in vegetation or ecosystem processes caused by ungulate presence (Egelkraut et al., 2020; Falk et al., 2014, 2015; Olofsson, 2009). In tundra, climate warming is rapidly shifting species pools, altering biogeochemical cycles (Abbott et al., 2016; IPCC, 2014; Meredith et al, 2019) and changing the populations and behaviour of ungulates (Cuyler et al., 2020; Mallory & Boyce, 2017; Mysterud, 2013; Uboni et al., 2016).…”

Idiosyncratic responses to simulated herbivory by root fungal symbionts in a subarctic meadow

“…Trampling is a disturbance associated with grazing that is rarely studied (Heggenes, Odland, and Bjerketvedt 2018). Trampling may damage roots (Olofsson 2009;Xu et al 2012) and influence vegetation composition more strongly than intensive defoliation or fertilization (Egelkraut et al 2020). In the present work, trampling reduced Saussurea nutrient acquisition, suggesting a belowground-mediated negative effect on the plant.…”

“…Trampling was simulated by dropping a pole (12 kg and 80 mm in diameter) from a height of 40 cm on the vegetation using an intensity of 100 pole drops per square meter. The pressure from the pole is similar to that from a reindeer hoof and the frequency of the simulated trampling was set to mimic the trampling observed by trampling indicators in natural settings (Egelkraut et al 2020). The trampling treatment was applied after defoliation and fertilization (between 7 and 12 July).…”

Vegetation science during hectic times

“…We also mention two forerunners. Dagmar Egelkraut with colleagues aimed at separating different grazing effects on tundra vegetation in northern Norway (Egelkraut et al, 2020). Reindeer influence plant communities through defoliation, fertilization and trampling.…”