Crassulacean plant succession over eight years on an unirrigated green roof in Beijing

Abstract

The selection of plants on green roofs should be tailored to the local climate. However, little is known about the long-term performance of plants in a Dwa climate (in the Köppen climate classification). This study follows the performance of 10 Crassulacean species (four species of Phedimus and six of Sedum) grown at four substrate depths (5, 10, 15, and 20 cm) over eight years on an unirrigated roof in Beijing, China. We found that the climate in Beijing, particularly the specific distribution of precipitation, had a substantial influence on plant growth and the community dynamics on extensive green roofs, and the dry and cold winters were the main factors that affected the plant survival on unirrigated green roofs. In such a climate, species with rootstocks or thickened fibrous roots performed better than species with entirely fibrous roots. Although deeper levels of substrate promoted better plant survival and performance, the species richness at all four depths continued to decrease. By the end of this research, only Phedimus aizoon and Phedimus floriferus ‘Weihenstephaner Gold’ still survived, and Phedimus aizoon was the only species that was alive at all of the four substrate depths. This research emphasized the importance of long-term research. The results of this study also revealed that the substrate moisture content was a more important factor than the substrate depth on the survival of plants over winter. To increase the rate of survival of plants on extensive green roofs over winter in Beijing, we advise the provision of supplementary irrigation before winter in years when the plants have experienced a dry autumn.