Grazing and warming effects on shrub growth and plant species composition in subalpine dry tundra: An experimental approach

Abstract

Questions: Vegetation in the forest-tundra ecotone faces changes in both climate and land-use. While climate warming is an important driver of vegetation growth and composition, herbivory may have opposing effects. In the present study, we experimentally test how removal of sheep herbivory affects the vegetation in an alpine forest-tundra ecotone, and how responses are manifested at higher temperatures. Location: Dovre Mountains, Central Norway. Methods: Shrub growth (height and cover) and ground layer composition were analysed each third year over an 18-year period in a nested, three-factorial experiment (ambient temperature and herbivory; ambient temperature and no herbivory; increased temperature and no herbivory). Fencing and open-top-chambers were used as expedients. Treatment effects and interactions over time were analysed using linear mixed effects models and ordination.Results: Shrub height and cover increased over time due to reduced herbivory, but without additional warming effect. Lichen cover declined in all treatments over time, but more rapidly and earlier under warming treatment (significant after three years).Contrary to expectations, there was no statistically significant increase in woody species due to warming, although evergreen woody species displayed a trend shift after six years, comprising a sharp decline towards year twelve. Litter accumulated in all treatments, but at higher rates under warming (significant after nine years). Conclusions:Our results disclose removal of sheep herbivory as a prominent driver of shrub growth, with warming as a subordinate driver in the studied alpine vegetation. The warming-driven increased litter abundance may, however, be caused by the decrease of wind inside chambers and the subsequent absence of wind-driven removal of litter. This chamber effect and the displayed timing differences in vegetation responses call for the critical use of short-term experimental data in predictions of long-term consequences of environmental change.

The Response of Culturally Important Plants to Experimental Warming and Grazing in Pakistan Himalayas

“…This reinforces the idea that in some cases, warming may generate new opportunities for people living under cold conditions [64]. While previous studies suggested that overgrazing is one of the major drivers of rangeland degradation [18,65– 70]. Our results only partly supported this assumption, while grazing treatments did have an overall negative effect on plant biomass and percentage cover, the general trend was not significant due to the high variability in plant response among the sites.…”

Vegetation response to climate warming across the forest–tundra ecotone: species‐dependent upward movement

“…combined with an upward shift of field layer vegetation suggest that the link with the tree layer might become weaker in a warming climate (Camarero et al, , 2006; Batllori et al, , 2009). These contrasting results suggest that vegetation in the forest–tundra ecotone responds in layer‐specific manners to climate warming, possibly moderated by the presence of herbivores (Vuorinen et al, , 2017; Løkken et al, , 2019). In alpine and arctic tundra ecosystems, herbivory is a well‐known driver counteracting effects of climate warming on vegetation (Olofsson et al, , 2009; Speed et al, , 2012; Eskelinen et al, , 2017), that might inhibit the expansion of deciduous woody species, but with low effect on evergreen woody species (Ylänne et al, , 2015; Vowles et al, , 2017).…”

“…While gradient studies reflect the present environment, they tend to overestimate effects of climate change as a gradient reflects the average environment over a larger timespan and thus is an expression of site history rather than present climate (Elmendorf et al, , 2015). Experiments, on the other hand, yield detailed control over some selected environmental factors, but are prone to artefacts especially when used over longer timespans (De Boeck et al, , 2015; Ettinger et al, , 2019; Løkken et al, , 2019).…”

Lichens facilitate seedling recruitment in alpine heath

“…Recent studies report declines in lichen abundance in arctic‐alpine areas (Fraser, Lantz, Olthof, Kokelj, & Sims, ; Løkken, Hofgaard, Dalen, & Hytteborn, ; Maliniemi, Kapfer, Saccone, Skog, & Virtanen, ; Vanneste et al., ; Vuorinen et al., ), often driven by expansion of vascular plants, and in particular shrubs (Cornelissen et al., ; Fraser et al., ; Vanneste et al., ). Whether the negative effects of vascular plants on lichen communities are mediated by expansion of the already established plants within or close to lichen mats, or by colonizing plants from outside, is not known.…”

“…Recent studies report declines in lichen abundance in arcticalpine areas (Fraser, Lantz, Olthof, Kokelj, & Sims, 2014;Løkken, Hofgaard, Dalen, & Hytteborn, 2019;Maliniemi, Kapfer, Saccone, Skog, & Virtanen, 2018;Vanneste et al, 2017;Vuorinen et al, 2017), often driven by expansion of vascular plants, and in particular shrubs (Cornelissen et al, 2001;Fraser et al, 2014;Vanneste et al, 2017).…”