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The installations of green roofs have been promoted worldwide, especially in European countries and United States. Extensive green roofs are often the target of scientific research since it costs less than intensive green roof. In addition, its weight adding to the building is less than intensive green roof; hence extensive green roofs are more common. Nevertheless, extensive green roofs face harsh climate, for instance high solar radiation, limited precipitation and shallow growing substrate; therefore it limits the choices of plants. These factors become obstacles in constructing extensive green roof; whereas comprehensive watering system can be installed on intensive green roofs. Therefore, water efficiency is not the major problem for intensive green roofs. Different types of green roof require different nature of plants; nonetheless, three common criteria of selecting plants using on extensive green roofs are their drought tolerance, albedo ability and native or nonnative. Typical plant species used on it is Sedum because of its feature of CAM which helps it to survive during harsh climate. Due to the harsh climate faced by the extensive green roofs, research usually focuses on the survival rates of plants. The survival rates of plants directly influence the esthetic of the green roofs; hence influence the acceptance of the general public. Other scopes of green roof studies are including the temperature reduction caused by green roofs, runoff quantity control as well as reduction of pollution. A research recently showed that broad leaves performed better than Sedum on rooftop, i.e., cooling efficiency. There are also arguments about whether nativespecies or non-native species should be introduced for green roofs. Native plants can provide homes and food for the native animals; however, one research claimed that non-native plants also provide same function for the native animals. Nevertheless, it is uncertain whether non-native species become invasive or not; thus using native plants is still the first priority.From soil formation on green roofs, it improves the urban biodiversity, for underground animals in the growth substrate, for the inserts in the canopy. Green roofs not only clean the air, but also the runoff. Plants on the rooftops can purify the air; plants and soil can purify the runoff as well as delay the storm peak. Green roofs act as a sink for nitrogen, lead and zinc from precipitation, but it also increases the concentration of phosphorus, which came from fertilizer used on green roofs. After reviewing research from mainly environmental perspectives, from the installation of green roofs, to its benefits to environment, with respect to urban area, it has been concluded that green roofs are good for rebuilding green areas in urban area; however, it should not be an excuse to destroy the outskirt green belts as green roofs cannot replace the role of natural habitat.
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