Environmental perturbation, grazing pressure and soil wetness jointly drive mountain tundra toward divergent alternative states

Abstract

Summary1. Plant communities are structured by complex interactions between multiple factors, which veil our understanding of the effects of environmental changes on communities and ecosystems. Besides the relative role of biotic and abiotic factors as community-structuring processes, addressing how they jointly affect the ecological resilience and resistance of plant communities is crucial to understand better the long-term response of communities facing global changes. 2. Here, we used the results from a long-term (23 years) perturbation experiment set up in Fennoscandian mountain tundra to test these mechanisms. The experiment consisted of a transplantation of twenty blocks of Vaccinium myrtillus heath vegetation including upper soil layer from a lower elevation tundra heath habitat to a snowbed habitat 150 m higher in elevation where V. myrtillus lies at its upper limit. In the snowbed with contrasting levels of soil wetness, half of the transplanted blocks were protected from mammalian herbivores. 3. Our results revealed that in addition to the important role of environmental conditions as a structuring force, the joint effects of multiple drivers resulted in divergent patterns in both plant functional composition and species diversity among transplanted communities. Under environmental perturbation (i.e. transplantation to snowbed), the heath vegetation was altered by grazing pressure that reduced the cover of shrubs (especially V. myrtillus). In grazed dry snowbed, a species-rich community with high functional type evenness and diversity developed. Reversely, in dry exclosures, V. myrtillus gained high dominance associated with only few graminoids and forbs. In wet snowbed conditions, shrubs tended to decline both in grazed plots and exclosures whereas bryophytes attained high abundance. Grazing promoted species richness while soil waterlogging tended to promote among-plot heterogeneity (b-diversity) which was highest in wet exclosures. 4. Synthesis. Our long-term experiment reveals that environmental perturbation, grazing and soil wetness exhibit joint effects that induce divergent trajectories of tundra plant communities. We suggest that a strong environmental perturbation triggers mountain tundra heath community to move away from its equilibrium state. The outcome of this shift depends on the interplay between grazing pressure and soil wetness that drive tundra plant communities toward divergent alternative states.

Site fertility drives temporal turnover of vegetation at high latitudes

“…Most likely, also reindeer grazing influences the structure of main plant strategies over time. Grazing buffers against climate‐driven increase of tall shrubs and certain forbs (Olofsson et al, ; Pajunen, Virtanen, & Roininen, ; Saccone, Pyykkonen, Eskelinen, & Virtanen, ), which have generally high shares of competitive strategy, and maintains disturbance that is reflected in the relatively large proportion of R‐strategy within studied communities. Thus, the larger turnover on fertile sites may be related to secondary succession due to continuous disturbance (Pierce et al, ).…”

“…Despite their characterization as stress‐tolerators with slow growth (Grime, Rincon, & Wickerson, ), mosses with clonal growth and high spore‐dispersal capacity might be especially responsive (see also Becker Scarpitta, Bardat, Lalanne, & Vellend, ). In the time scales of two or more decades, even slow‐growing species can attain a dominant position (Saccone et al, ).…”

“…Most likely, also reindeer grazing influences the structure of main plant strategies over time. Grazing buffers against climatedriven increase of tall shrubs and certain forbs(Olofsson et al, 2009;Pajunen, Virtanen, & Roininen, 2008;Saccone, Pyykkonen, Eskelinen, & Virtanen, 2014), which have generally high shares of competitive strategy, and maintains disturbance that is reflected in the relatively large proportion of R-strategy within studiedF I G U R E 4 Posterior simulations of main strategy class (C, S, and R) proportions for fertile and infertile sites with 95% credible intervals. The observed means of each subregion during the original survey and resurvey are indicated as gray-scale lines F I G U R E 5 Simulated (a) vascular plant species diversity and (b) bryophyte species diversity for fertile and infertile sites with 95% credible intervals.…”

Human–animal agency in reindeer management: Sámi herders’ perspectives on vegetation dynamics under climate change

“…Therefore, a strong interaction exists between grazing pressure and abiotic effects on plant community transition during environmental change (Van der Wal , Saccone et al. ).…”

The importance of herbivore density and management as determinants of the distribution of rare plant species

“…The dependence of the effect of grazing upon elevation for alpine plant species richness has also been observed in the Italian Alps (Dainese et al, 2015) while Saccone et al (2014) highlighted how grazing in tundra environments may shift the system's trajectory following environmental perturbation. The mechanisms through which herbivory may affect plant diversity in such systems include colonisation and extinction effects (Speed et al, 2012).…”