Environmental evolution of cover deposits on chemically distinctive bare ground, New Zealand: review and temporal contexts.

Chemically distinctive features such as high salinity, low nutrient contents, and extremes of pH can limit the development of a vegetation cover on soil-free bare ground. This study synthesises our previous work on the geoecology of numerous chemically distinctive bare sites in the South Island and adds a temporal component within the last century. The rates of development of surficial cover on decadal time scales have been traced from available historical aerial photography of varying but adequate quality. Rates are highly variable but range between zero and 2 metres per year laterally, with some sites showing localised expansion of bare ground at times. Cover evolution involves the development of a cm-scale proto-soil layer that is at least partially biologically mediated. Geochemical thresholds in surficial substrates control the biological components of encroaching cover. Dynamic surface environments, especially steep and actively eroding slopes, help to maintain bare ground and resist cover development. Some chemically distinctive bare ground, and evolving adjacent proto-soil areas, host specialist tolerant native plant communities and shrinkage of these areas threaten this biodiversity. In contrast, some bare ground sites have elevated arsenic and/or highly acidic pH, and the development of proto-soil and vegetation cover is environmentally positive from a human perspective. [ABSTRACT FROM AUTHOR]