Growth rings show limited evidence for ungulates’ potential to suppress shrubs across the Arctic

Abstract

Global warming has pronounced effects on tundra vegetation, and rising mean temperatures increase plant growth potential across the Arctic biome. Herbivores may counteract the warming impacts by reducing plant growth, but the strength of this effect may depend on prevailing regional climatic conditions. To study how ungulates interact with temperature to influence growth of tundra shrubs across the Arctic tundra biome, we assembled dendroecological data from 20 sites, comprising 1,153 individual shrubs and 22,363 annual growth rings. Evidence for ungulates suppressing shrub radial growth was only observed at intermediate summer temperatures (6.5-9°C), and even at these temperatures the effect was not strong. Multiple factors, including forage preferences and landscape use by the ungulates, and favourable climatic conditions enabling effective compensatory growth of shrubs, may weaken the effects of ungulates on shrubs, possibly explaining the weakness of observed ungulate effects. Earlier local studies have shown that ungulates may counteract the impacts of warming on tundra shrub growth, but we demonstrate that ungulates’ potential to suppress shrub radial growth is not always evident, and may be limited to certain climatic conditions.

Large herbivore diversity slows sea ice–associated decline in arctic tundra diversity

“…3). Across the arctic tundra biome, local richness of large herbivores typically ranges from 0 to 2 species ( 19 ), so the magnitude of the increase in herbivore diversity over the course of this experiment is comparable to extant biome-wide variation in large herbivore diversity (but not density, which is comparatively low at this site) ( 24 , 31 ).…”

Alpine shrubs have benefited more than trees from 20th century warming at a treeline ecotone site in the French Pyrenees

“…Besides rising temperature, we also hypothesize that the growth rate increase over the last century may have been triggered by rising CO 2 concentrations and changing competition level. We tend to exclude any anthropogenic influence through land abandonment, as none of the species studied is grazed by livestock, which probably influences the establishment of woody plants (Améztegui et al, 2010;Galop et al, 2011), but not the radial growth of already established individuals (Vuorinen et al, 2022). Yet, uncertainties remain about possible changes in the level of competition between plants in a context where shrubs have been favored over herbaceous species in the past.…”

Shrubification along Pipeline Corridors in Permafrost Regions

“…Herbivories were anticipated to slow, stop, or even reverse shrub expansion resulting from climate warming, e.g., [105,106]. Field exclusion experiments showed that releases from both small and large herbivories may change the plant species composition of tundra communities [104,107].…”

“…For instance, a 20-year exclosure experiment on tundra vegetation in Scandinavia and Alaska indicated that mammalian herbivories helped in reducing the leaf area index (LAI), NDVI, and abundance of vascular plants, and large herbivories aided in lowering plant statures, while small herbivories assisted in boosting plant species diversity [104]. A 3-year exclosure experiment documented that the phytomass of deciduous shrubs, forbs, and silica-poor grasses increased by 40-50% in the absence of ungulate (e.g., reindeer) and small rodent herbivores (e.g., voles and lemmings) [106]. The impacts of herbivory on tundra shrub may depend on experiment periods, landscapes, characteristics of herbivore species and herbivory density in the plant community [106,108].…”

“…A 3-year exclosure experiment documented that the phytomass of deciduous shrubs, forbs, and silica-poor grasses increased by 40-50% in the absence of ungulate (e.g., reindeer) and small rodent herbivores (e.g., voles and lemmings) [106]. The impacts of herbivory on tundra shrub may depend on experiment periods, landscapes, characteristics of herbivore species and herbivory density in the plant community [106,108].…”