Alternative stable states and human-environment interactions in forest mosaics and ecotones

Henderson, Kirsten
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University of Guelph

Alternative stable states result in drastic vegetation shifts, without much advanced warning. Although common in theory, they are often more di cult to observe in ecological systems and the mechanisms responsible for shifts remain largely unknown. The fragility of bistable systems when faced with large external forces, such as human in uence or climate, provides a plausible explanation for the elusiveness of alternative stable states. The work presented here aims to provide quanti able mechanisms for threshold responses in forest mosaic and ecotone systems. First, a model of tree-grass recruitment is parameterized with empirical data to gain an understanding of mechanisms driving vegetative shifts. The following chapter uses climate moisture index to model vegetation structure and stability in a boreal/temperate forest/non-forest system. In addition to climate, anthropogenic land-use change is considered to be one of the greatest stresses on natural ecosystems. Landowner behaviour, from questionnaire data, is incorporated into a human-environment coupled model to determine the in uence of conservation and economic values on landscape dynamics. In both human and environment driven tree-grass systems, recruitment is a limiting factor and as such has a fundamental role in characterizing vegetative state shifts. After reviewing multiple tree-grass dynamical models and calibrating the model with empirical data, we nd that soil moisture content acts as a proxy for re and precipitation regimes, providing a simple, yet descriptive, recruitment function. The ndings from the boreal/temperate climate model support an alternative framework for conceptualizing alternative stable states and suggest that future climate regimes may eclipse bistability. Likewise, human in uences disrupt the natural bistable characteristic of forest-grassland mosaics. Generally, strong human in uence precludes bistability. However, long-term conservation behaviour reintroduces bistability, driven by rarity-based conservation rather than natural processes. Ecosystem interactions are intricate and the presence of alternative stable states often adds to the complexity. This thesis highlights the fragility of bistable systems, particularly under strong environmental and human forces. Therefore, models that include empirical data and mechanistic processes, which enhance our understanding of biological processes, are key to determining what triggers shifts in states and assessing the vulnerability of a system to change.

tree-grass system, mosaic, ecotone, alternative stable states, threshold response, human-environment coupling, modelling