One of the key components in the planning of sustainable cities, according to current research, is plants and planted spaces. Whitford et al., 2001 (abstract), in developing indicators for the ecological performance of urban areas in the UK noted that “the greatest influence on ecological performance was the percentage of green space, particularly of trees.” The fields of landscape ecology and urban ecology have emerged from research to become the primary advocates of ecological design in cities. While a considerable body of research has been gathered, problems of integration and common standards remain to be resolved. Leitao and Ahern (2002), for example, noted that there are “literally hundreds of metrics developed” and that
these metrics are “frequently strongly correlated, and may be confounded” (p. 74). They suggested a reduced list of 7–9 metrics categorised under two aspects of landscape planning—composition and configuration. According to Pauliet and Duhme (2000),