Herbivore impacts to the moss layer determine tundra ecosystem response to grazing and warming

Abstract

Herbivory and climate are key environmental drivers, shaping ecosystems at high latitudes. Here, we focus on how these two drivers act in concert, influencing the high arctic tundra. We aim to investigate mechanisms through which herbivory by geese influences vegetation and soil processes in tundra ecosystems under ambient and warmed conditions. To achieve this, two grazing treatments, clipping plus faecal additions and moss removal, were implemented in conjunction with passive warming. Our key finding was that, in many cases, the tundra ecosystem response was determined by treatment impacts on the moss layer. Moss removal reduced the remaining moss layer depth by 30% and increased peak grass biomass by 27%. These impacts were probably due to observed higher soil temperatures and decomposition rates associated with moss removal. The positive impact of moss removal on grass biomass was even greater with warming, further supporting this conclusion. In contrast, moss removal reduced dwarf shrub biomass possibly resulting from increased exposure to desiccating winds. An intact moss layer buffered the soil to increased air temperature and as a result there was no response of vascular plant productivity to warming over the course of this study. In fact, moss removal impacts on soil temperature were nearly double those of warming, suggesting that the moss layer is a key component in controlling soil conditions. The moss layer also absorbed nutrients from faeces, promoting moss growth. We conclude that both herbivory and warming influence this high arctic ecosystem but that herbivory is the stronger driver of the two. Disturbance to the moss layer resulted in a shift towards a more grass-dominated system with less abundant mosses and shrubs, a trend that was further enhanced by warming. Thus herbivore impacts to the moss layer are key to understanding arctic ecosystem response to grazing and warming.

Long-Term Effects of Grazing and Global Warming on the Composition and Carrying Capacity of Graminoid Marshes for Moulting Geese in East Greenland

“…Jefferies et al 2006). The effect of below-ground foraging (so-called grubbing) by pink-footed geese on Svalbard has also caused dramatic changes in plant species composition and the establishment of open depressions void of vegetation (Gornall et al 2009;Speed et al 2009). A likely explanation for the more subtle effects observed in Jameson Land is that the moulting geese exclusively graze the aboveground vegetation instead of below-ground material whereby moss and roots are pulled out of the vegetation carpet, and a larger component of the plant community other than the forage is disturbed in the process.…”

Balancing positive and negative plant interactions: how mosses structure vascular plant communities

“…In fact, the combination of high temperature and low nutrient supply reduced biomass, possibly because root respiration and turnover increase with temperature (Forbes et al 1997). Thus, should a moss layer be disturbed, for example by the activity of herbivores, Salix will only respond to the resultant increase in soil temperature as far as any associated increase in nutrient availability will allow (Gornall et al 2009). …”

The effects of a hydrogen-rich ground cover on cosmogenic thermal neutrons: Implications for exposure dating

“…; DM values are provided in the references; DM values assumed/reconstructed for Plug et al 2007): Savannah vegetation in semi-arid to arid regions have a relatively low biomass with shrubs at ~0.12 g/cm 2 and grass at ~0.06 g/cm 2 (Patagonia; (Flombaum and Sala, 2007), dry tropical evergreen forests ~1 g/cm 2 (trees only, India; (Mani and Parthasarthy, 2007), neotropical forests 1.5-3 g/cm 2 (Central America; (Drake et al, 2003), Acadian boreal forest ~2 g/cm 2 (trees and branches; Plug et al 2007); temperate forests 3.3±0.8 g/cm 2 (Central Europe; (Szwagrzyk and Gazda, 2007); and temperate rain forest ~9.5 g/cm 2 (trees and branches; Plug et al 2007). Estimates for moss cover range between ~0.01 g/cm 2 (Binkley and Graham, 1981) and 0.02-3 g/cm 2 (Gordall et al, 2009;Rieley et al, 1979;Tamm, 1953;Telfer, 1972;Traczyk et al, 1973;Weeman and Timmer, 1967). For mosses it is important to note that they can retain significant amounts of extracellular water; nonsaturated extracellular water contents of moss can range widely: e.g.…”