Vegetation shift from deciduous to evergreen dwarf shrubs in response to selective herbivory offsets carbon losses: evidence from 19 years of warming and simulated herbivory in the subarctic tundra

Abstract

Selective herbivory of palatable plant species provides a competitive advantage for unpalatable plant species, which often have slow growth rates and produce slowly decomposable litter. We hypothesized that through a shift in the vegetation community from palatable, deciduous dwarf shrubs to unpalatable, evergreen dwarf shrubs, selective herbivory may counteract the increased shrub abundance that is otherwise found in tundra ecosystems, in turn interacting with the responses of ecosystem carbon (C) stocks and CO2 balance to climatic warming. We tested this hypothesis in a 19-year field experiment with factorial treatments of warming and simulated herbivory on the dominant deciduous dwarf shrub Vaccinium myrtillus. Warming was associated with a significantly increased vegetation abundance, with the strongest effect on deciduous dwarf shrubs, resulting in greater rates of both gross ecosystem production (GEP) and ecosystem respiration (ER) as well as increased C stocks. Simulated herbivory increased the abundance of evergreen dwarf shrubs, most importantly Empetrum nigrum ssp. hermaphroditum, which led to a recent shift in the dominant vegetation from deciduous to evergreen dwarf shrubs. Simulated herbivory caused no effect on GEP and ER or the total ecosystem C stocks, indicating that the vegetation shift counteracted the herbivore-induced C loss from the system. A larger proportion of the total ecosystem C stock was found aboveground, rather than belowground, in plots treated with simulated herbivory. We conclude that by providing a competitive advantage to unpalatable plant species with slow growth rates and long life spans, selective herbivory may promote aboveground C stocks in a warming tundra ecosystem and, through this mechanism, counteract C losses that result from plant biomass consumption.

Circumpolar arctic tundra biomass and productivity dynamics in response to projected climate change and herbivory

“…Other field studies found negative impacts of herbivory on the growth and morphology of an Arctic willow ( Salix glauca ) (Den Herder et al ., ) and other deciduous shrubs (e.g., Ylänne et al . , Olofsson et al ., ). Our results confirm that herbivores moderate the expansion of fast‐growing deciduous shrubs at local scales based on new and published data synthesized across the arctic ecosystems (Christie et al ., ).…”

Expansion of deciduous tall shrubs but not evergreen dwarf shrubs inhibited by reindeer in Scandes mountain range

“…), it is conceivable that, under moderate grazing pressure, less palatable evergreens could be given a competitive advantage despite their semi‐prostrate growth form (Christie et al . ; Ylänne, Stark & Tolvanen ). In our study, however, the advancement of evergreen dwarf shrubs in both ambient plots and exclosures suggests that this was not a result of altered competitive interactions between shrub species due to a release from grazing.…”

“…There could be several possible reasons for the observed expansion of evergreens. Though tall deciduous shrubs, such as B. nana, have been shown to be among the most responsive tundra species to increasing temperatures and have often been found to increase at the expense of evergreen shrubs (Chapin et al 1995;Bret-Harte et al 2001), it is conceivable that, under moderate grazing pressure, less palatable evergreens could be given a competitive advantage despite their semi-prostrate growth form (Christie et al 2015;Yl€ anne, Stark & Tolvanen 2015). In our study, however, the advancement of evergreen dwarf shrubs in both ambient plots and exclosures suggests that this was not a result of altered competitive interactions between shrub species due to a release from grazing.…”

Delayed herbivory by migratory geese increases summer‐long CO2 uptake in coastal western Alaska

“…Because NEE and GPP are closely tied to leaf area (Shaver et al, 2013), our results are broadly consistent with these studies suggesting early grazing diminishes overall production in the system. The largest effect of grazers on C uptake is removal of photosynthetic tissue although the impact of grazing on plant functional group composition can also be substantial (Cahoon et al, 2012;Metcalfe & Olofsson, 2015;Post & Pedersen, 2008;Ylänne, Stark, & Tolvanen, 2015). High-intensity grazing by geese also suppressed NEE and GPP compared to lower intensity or grazing removal from a wet tundra system (Sjögersten et al, 2008).…”