Ecological benefits and species selection of tropical extensive green roofs

Abstract

The world’s urbanization has entered a rapid development stage. With more people moving into cities, more natural areas inside or surrounding cities have been transformed into buildings and impervious pavements. They have led to urban environmental degradation, such as the urban heat island effect, flooding, air pollution, urban biodiversity decline, and impacts on residents' daily life and health.

Extensive green roofs (EGR) provides an effective complement of urban green space (UGS) which can bring many economic, ecological and aesthetic benefits. EGR offers a feasible strategy to mitigate key urban environmental problems, and has become popular in many cities. However, EGR development in South China with hot-humid subtropical climate remains at the early stage and few studies have been attempted. Research on the ecological benefits in the local climate context and selecting suitable plant species provide a scientific basis to promote EGR application and inform optimal design and management.

Firstly, selecting plant species with high leaf area index and biomass, high plant diversity, vegetation coverage and soil thickness can improve the cooling effect of EGR for the rooftop environment. However, not all plant combinations with more species can provide better cooling effect, which is more dependent on vegetation cover. Plant coverage reaching 75% and above can significantly improve the cooling effect, and 5 cm extra soil layer can significantly reduce substrate temperature fluctuation, but the cooling effect is smaller than 60% vegetation coverage. Thus, for EGR, improvement in cooling effect can better be achieved by increasing vegetation coverage than soil thickness. This is a critical finding because EGR often have to be installed on existing buildings with limited loading capacity.

Through a two-year investigation, 94 volunteer vascular plant species belonging to 26 families and 76 genera, and 16 bird species belonging to 8 families and 14 genera, were assessed continually on an experimental EGR. Both local common ruderal plant species and common urban bird species successfully established and propagated on the EGR, which verified the potential contribution of EGR in urban biodiversity enrichment and conservation. With an increase in plant community diversity, evenness and coverage, avian community diversity and evenness also rose. Plant diversity and evenness had positive correlations with bird total abundance too, but plant coverage had a negative influence on bird total abundance. The high plant coverage has negative relation with Tree sparrow’s abundance, because it is a ground foraging species. Thus, different bird species had different specific habitat requirements.

Through a comprehensive evaluation of their esthetical value, adaptability, benefits and maintenance, 30 suitable species (including 9 perennial herbs, 4 sub-shrubs, 3 succulents and 14 climbers) were identified for use in EGR in humid-subtropical regions. Among them, 1 perennial herbs (Wedelia trilobata), 1 sub-shrubs (Cuphea hyssopifolia), and 2 climbers (Lonicera japonica and Parthenocissus dalzielii) had outstanding performance. The findings are condensed in practical hints to inform design and management of extensive green roofs in the humid-tropical region.