The edge paradox: An Icelandic case study investigating impacts of multiple and novel disturbances on forest ecosystem thresholds

Edges and ecotones have been thought to be sensitive locations for detecting ecosystem response to anthropogenic and natural disturbances. Previous studies have monitored fluctuations in species composition, microclimate, and animal populations at edges. Edges have been viewed both as barriers and invasible, both as outermost boundaries of community persistence and starting points for ecosystem expansion. Historically, the edge at the treeline zone has been studied to understand the upper limits of tree growth under extreme climatic conditions. The current paradigm is that plant responses to stress are amplified at edge zones, and that edges such as the treeline will be among the first to respond to climate change.; However, edge studies have not been designed to examine ecosystem level resistance and resilience characteristics. Nor have edge studies been used to tease apart the impact of one disturbance from another in landscapes where multiple disturbances are common. It is not clear how the function of the forest limit at the boreal-arctic ecotone changes when there is an overlay of human land use on climatic disturbances. Edge research has also not been able to show whether ecosystem edges can be used to predict the response of the more interior zones to disturbance.; An experiment was designed to assess the resistance characteristics of plant communities at the boreal-arctic forest limit, in landscapes with existing and long-term legacies of human land use (e.g., variable levels of sheep grazing). It was hypothesized that the existence of multiple disturbances and legacies of land use at the forest limit would decrease the resistance of the edge to disturbance, such that it would be less able to sustain its acquisition of limiting nutrients. It was also hypothesized that the accumulation of disturbance factors in the landscape could shift the location of functional thresholds through space, such that typical conservation and fluxes of limiting resources would shift on a landscape level.; Research plots were established at the Betula pubescens forest limit and at 50m and 100m in elevation below the forest limit at three mountain sites in Iceland. The sites were manipulated by the addition of sugar, with the aim of increasing microbial activity and immobilizing nitrogen in the soil. The following measurements were taken to track the response of the sites to the sugar disturbance: foliar carbon and nitrogen, relative foliar chlorophyll content, leaf area, and leaf weight. Air and soil temperature measurements were taken to incorporate the role of climate at the sites.; The results of this dissertation suggest that structural edges can have very different functional roles in the landscape, and that functional thresholds are not necessarily associated with the structural edge, but can shift throughout the landscape. Additionally, this study shows that predictable patterns typically associated with the treeline zone can be altered or disappear as legacies of disturbance accumulate in an ecosystem. This dissertation argues that both structural and functional thresholds hold vital clues for tracking the trajectory of change in a landscape. In the wake of global change, this kind of understanding is essential if we are to move toward the goal of sustainability.