In arctic tundra ecosystems mosses dominant the vegetation in terms of productivity and diversity.  Despite this, mosses are often overlooked in studies of tundra ecology.  However, evidence from this thesis suggests that mosses maybe integral to the functioning of these systems.  Mosses insulate soil keeping it cooler than air temperature, an effect more apparent under deeper moss.  The effects of the moss layer on soil characteristics alter conditions for microbial populations resulting in higher nitrogen availability in soil under shallow moss.  This thesis shows that the role of mosses in determining vascular plant success may dictate many higher plant interactions.  There are both positive and negative effects of the moss layer on vascular plant growth.  The relationship between positive and negative impacts of the moss layer on vascular plants is species specific, meaning that moss cover may be a key determinate of vascular plant community structure.  Climatic warming and herbivory are important drivers of vegetation change in the Arctic.  This thesis shows that grazing by reindeer and grubbing by geese is detrimental to moss cover.  As mosses insulate the soil, a reduction in depth or integrity increases soil temperatures and enhances microbial activity and thus nitrogen availability.  This in conjunction with addition of nutrients from faeces enhances vascular plant productivity to the further detriment of mosses.  Warming increases soil temperature and accelerates decomposition, but has little affect on either biomass of moss or vascular plants.  Moss grubbing has a greater negative effect on mosses in a warmed environment.  This thesis concludes that mosses are integral to the current functioning of tundra heaths.