1. Introduction
Roof greening refers to planting on top of buildings or on elevated platform areas including roofs, terraces, and podiums, as well as on aerial platforms and overpasses that are not directly connected to natural soil on the ground [
1]. It is usually divided into three categories: extensive, semi-extensive, and intensive [
2,
3]. Extensive green roofs have a characteristic shallow substrate of <15 cm in depth, and are usually covered with herbaceous or low-shrub plants that are tolerant to environmental stresses, tolerant to mowing or pruning, demand little or no irrigation, and require little maintenance. Intensive green roofs integrate green function and leisure function as a whole. Their substrate is deep and they can support woody plants including trees and shrubs accompanied by herbaceous ground-level vegetation, most of which is ornamental [
4]. Semi-extensive green roofs have characteristics lying between the above two categories, and may offer a combination of environmental and aesthetic benefits [
5].
Green roofs can supplement the ground-level greening space and provide urban ecological functions, especially in high-density urban areas [
6]. The ecological functions of roof greening refer to the ability of the roof greening ecosystem to provide material products, ecological public welfare, environmental resources, and aesthetic value to enrich outdoor recreational opportunities and quality of life [
7]. The ecological functions provided by roof greening can be summarized through four types based on a four-point method for the classification of ecosystem services [
8]. Firstly, regulating functions include regulating ambient temperature and humidity [
9,
10,
11,
12], mitigating air pollution [
13], conserving water [
7,
14,
15], and abating noise pollution [
16]. Secondly, cultural functions include landscape services, recreational services, spiritual services, etc. Thirdly, supporting functions include nutrient cycles and providing habitats for wildlife [
17,
18]. Fourthly, supplying functions include medicinal, edible [
19], fodder, landscape, economic [
20], and nutritional [
21] value.
Plant functional traits, such as plant life form, plant height, life-span, and regeneration pathways, refer to a series of plant attributes closely related to planting, survival, growth, and reproduction [
22,
23]. Four kinds of functional trait classification systems are widely used at present, including: (1) the functional trait system that responds to disturbances on a global scale for herbaceous plants proposed by McIntyre et al. [
24]; (2) the morphological and life-history trait system for temperate zone woody plants proposed by Mabry et al. [
25]; (3) the handbook of standardization and simple measurement procedures for plant functional traits worldwide by Cornelissen et al. [
26]; and (4) the classification system of plant traits in the Mediterranean region proposed by Barboni et al. [
27]. Such systems focus on the relationships among plant functional traits, environment, and ecological contributions.
Plant functional traits reflect the distribution and utilization of light, temperature, nutrients, water, and other resources [
26,
28,
29,
30]. Different plants have different permutations of functional traits. They exhibit a variety of strategies to acquire, leverage, and provide resources, and to increase the diversity of the ecosystem and the stability of ecological systems [
31,
32]. Firstly, there are environmental regulating functions: plant growth form, leaf texture, leaf shape, and leaf size can influence ambient temperature and humidity, alleviate air pollution, conserve water, and reduce noise pollution [
7,
33,
34]. Secondly, there are landscape–cultural functions: foliage and ornamental plants cultivated in roof greening can create a unique roof landscape and endow the site with cultural aesthetic, recreational, and spiritual value. Thirdly, there are wildlife-supporting functions. Roof greening can provide forage, shelter, and habitats for wildlife [
18], and safeguard the normal operation of ecological processes such as energy flow and nutrient cycling. Fourthly, there are utilitarian supply functions: some plants can provide multiple products of value to humans, including medicinal, gardening, and economic benefits. The combination of different traits influences the interactions between plant community and ecological functions [
35,
36]. Green roof ecosystems with diverse plant functional traits can enhance ecological services [
37,
38,
39,
40].
Understanding the characteristics of plant functional traits facilitates the explanation and analysis of ecological functions [
7,
41,
42] and provides a reference for the planning and design of roof greening. The choice of species in roof greening should depend on commercial availability, aesthetic value, environmental adaptability, and root-depth standards [
43]. However, the choice and types of species that are more able to enhance ecological functions can be beset by various problems. Beijing was one of the first cities in China to develop roof greening. In the course of implementation, ecological functions implied from the functional traits of roof-greening plants were considered. With rapid urbanization and population growth, Beijing is facing various urban problems such as air pollution, heat islands, congested urban spaces, etc. In terms of per unit and per capita provision, ground-level green spaces are not sufficient to alleviate environmental problems and enhance the quality of urban life. On the other hand, the roof greening area of Beijing only occupies about 1% of the total roof area. The current research on roof greening mainly focuses on plant screening [
44], stress analysis, roof drainage [
45], roof load and substrate selection [
46], green infrastructure, and the spatial distribution pattern of roof greening and the corresponding factors [
47]. Other studies on roof greening in Beijing tackle the choice of plants and their resistance to stress [
48], the greening management mode [
49], policy formulation, and development, construction, and maintenance methods [
50]. Thus, Beijing has great potential for developing roof greening to improve its ecological environment. On a more practical level, whether the current roof greening plants provide adequate support for ecological services has not been adequately investigated. Furthermore, few studies touch on the assessment of all the four sets of ecological functions in the context of the relatively comprehensive ecosystem services that could be provided by roof-greening plants.
Based on field investigations, classification systems of plant functional traits [
24,
25,
26,
27], the Millennium Ecosystem Assessment (MA) ecosystem service function classification [
8], and other related studies [
40], this study probes plant functional traits and evaluates the ecological functions of roof greening plants in Beijing. From the findings, useful recommendations found for plant selection and ecological-function enhancement are provided to improve the planning and design of roof greening and maintain sustainable urban development, especially in rapidly urbanized areas.
3. Results
3.1. Distribution of Roof Greening in Beijing
All 201 roof greening sites and the surveyed 51 plots are displayed in
Figure 1. The site number and per cent of roof greening in each district are listed in
Table 4. It can be inferred from
Figure 1 and
Table 4 that most existing roof greening and surveyed plots are concentrated in four districts: Haidian, Chaoyang, Dongcheng, and Xicheng, which are more densely populated and have more intense urbanization than other districts. Xicheng and Chaoyang contribute over half of the total number and area of roof greening sites.
The surveyed plots of roof greening were classified into extensive roof greening, extensive platform greening, intensive roof greening, and intensive platform greening. The number of greening types in the field investigation and their ratios are shown in
Table 5. It can be inferred from
Table 5 that greening on roofs and extensive greening occupied more than 75% of the total plots. The plots were usually located in hospitals, schools, commercial districts, residential areas, government offices, etc. Among these, the commercial area has a higher availability of roof greening with plants that are more recreational and ornamental in nature. Most government office areas have extensive roof greening with low accessibility. In hospitals, both extensive and intensive roof greening exist. Intensive roof greening provides patients and their families with leisure and entertainment services, whereas extensive roof greening is almost impossible to reach in most cases. In residential areas, most public greening is on a platform within the community. The public platform greening always plants on a high platform for amusement or for parking, as seen in the intensive platform greening of Wangjing Garden and Jianwai SOHO (small office, home office). Private roof greening in residential areas has not been included in this field investigation. From long-distance visual observation its plant composition is rich and beautiful with high entertainment value, but it only serves the residential area owners.
3.2. Taxonomic Characteristics of Roof Greening Plants
According to the field investigation and related literature [
48], 207 plants used in Beijing roof greening are summarized (see
Supplementary Materials). They belong to 161 genera from 61 families (
Supplementary Materials). It can be inferred from the
Supplementary Materials that
Compositae has 31 species from 24 genera, occupying 15%;
Rosaceae has 21 species from 12 genera, occupying 10%;
Gramineae has 13 species from 13 genera, occupying 6%; and
Cupressaceae has nine species from five genera, occupying 4%.
Crassulaceae sp.,
Cucurbitaceae sp.,
Leguminosae sp.,
Oleaceae sp.,
Chenopodiaceae sp.,
Lamiaceae Martinov sp. and
Convolvulaceae sp. occupy 3% of the plants. Each of the other families account for less than 3% of the plants.
3.3. Functional Traits of Roof Greening Plants
According to the characteristics of plant functional traits listed in
Table 2, a large number of roof greening plants in Beijing are phanerophytes (76 species, 58% of the total, dominated by
Cupressaceae sp. and
Rosaceae sp.) and therophytes (61 species, 29% of the total, dominated by
Compositae sp.,
Cucurbitaceae sp. and
Gramineae sp.). Most are herbaceous plants (119 species, 58% of the total, dominated by
Compositae sp. and
Gramineae sp.), perennial species (144 species, 70% of the total, dominated by
Rosaceae sp.,
Compositae sp., and
Cupressaceae sp.), and those with a faster growing rate (201 species, 97% of the total). They often have cauline leaves and multiple stems and branches, such as
Ixeridium sonchifolium (Maxim.)
Shih of
Compositae and
Buddleja davidii Franch. of
Loganiaceae. Most plants have strong adaptation and are tolerant to the cold, heat, drought, and wind. They usually also have multiple supplying values, especially medicinal, gardening, and economic value, such with
Sedum lineare Thunb. of
Crassulaceae,
Ixeridium chinense (Thunb.)
Nakai of
Compositae, and
Agrimonia pilosa Ldb. of
Rosaceae.
The plant heights are dwarf (55 species, 26% of the total, dominated Compositae sp. and Crassulaceae sp.), medium (42 species, 20% of the total, dominated by Compositae sp.), and high (37 species, 18% of the total, dominated by Rosaceae sp. and Oleaceae sp.). Most plants have erect stems (162 species, 78% of the total), and no woody characteristics (115 species, 56% of the total, dominated by Compositae sp., Gramineae sp., and Cucurbitaceae sp.).
Most plants are deciduous (178 species, 86% of the total), and the senescence period is mainly in autumn (122 species, 59 of the total). Most leaves of the plants are small (94 species, 45% of the total). The texture of the leaf is mostly papery (111 species, 54% of the total) and not fleshy (197 species, 95% of the total). The leaf color is mostly green (194 species, 94% of the total).
The flower color of 30% of plants is yellow (62 species). The anthesis of most plants is in the summer (64 species, 31% of the total) or the summer–autumn transition season (62 species, 30% of the total). There are 12 species where the anthesis lasts 5 months or longer. The fruit period is always in autumn (99 species, 48% of the total) and approximately seven species have a fruit period of more than 5 months.
Most plants have shallow roots (188 species, 91% of the total, dominated by Compositae sp., Rosaceae sp., and Gramineae sp.), and have no underground organ to store nutrients (160 species, 77% of the total, dominated by Rosaceae sp., Compositae sp., and Gramineae sp.). A few plants have fleshy roots (Sedum sarmentosum Bunge of Crassulaceae), root tubers (Cirsium japonicum Fisch. ex DC. of Compositae), and rhizomes or tubers.
The propagating methods of the plants are diverse. Most plants have more than one propagation method. They usually select entomophily (141 species, 68% of the total, such as Compositae sp., Rosaceae sp., and Crassulaceae sp.) as the main method, because most of such plants have nectar and pollen (131 species, 63% of the total).
3.4. Ecological Functions by Roof Greening Plants
The relative values of ecological functions calculated by the different functional traits of the plants are listed in
Supplementary Material Table S1. Based on this, a comparison chart of four ecological functions is obtained (
Figure 2).
The regulating function values of the plants range from 6 to 12, with a mean of 8.52. Phyllostachys propinqua McClure of Gramineae, Rosa chinensis Jacq. of Rosaceae, Ligustrum japonicum Thunb. of Oleaceae, and Ulmus macrocarpa Hance of Ulmaceae have the highest value of 12. About nine species including Artemisia annua Linn., Artemisia selengensis Turcz. ex Bess., and Heteropappus altaicus (Willd.) Novopokr. of Composita have the lowest value of 6. Overall, 88 species have a value higher than the mean. Rosaceae sp. are the most frequent, with 15 species including Malus spectabilis (Ait.) Borkh., Prunus Cerasifera Ehrhar f. atropurpurea (Jacq.) Rehd., and Cotoneaster horizontalis Dcne. About 118 species have values below the average, and Compositae sp. are the most frequent, with 27 species including Taraxacum mongolicum Hand.-Mazz., Saussurea japonica (Thunb.) DC., and Bidens pilosa L. var. radiate Sch. Bip.
The cultural function values of the plants range from 4 to 13, with a mean of 7.82. Ligustrum japonicum Thunb. of Oleaceae and Hedera nepalensis var. sinensis (Tobl.) Rehd of Araliaceae have the highest value of 13. Triarrhena sacchariflora (Maxim.) Nakai of Gramineae, Amaranthus lividus L. of Amaranthaceae, and Cuscuta chinensis Lam. of Convolvulaceae have the lowest value of 4. A total of 68 species have higher than average culture function values, and Compositae sp. are the most frequent, with 25 species including Taraxacum mongolicum Hand.-Mazz. and Cirsium japonicum Fisch. ex DC. There are 138 species which have values below the average, and Gramineae sp. are the most frequent, with 12 species including Eragrostis pilosa (L.) Beauv. and Poa pratensis L.
The supporting function values of the plants range from 4 to 14, with a mean of 8.72. Ligustrum japonicum Thunb. of Oleaceae has the highest value of 14. Amaranthus lividus L. of Amaranthaceae has the lowest value of 4. In total, 123 species have higher than average values, and Rosaceae sp. are the most frequent, with 17 species including Rosa chinensis Jacq., Amygdalus persica L., and Cerasus yedoensis (Matsum.) Yu et Li. Overall, 83 species have values below the average, and Compositae sp. are the most frequent, with 15 species including Saussurea japonica (Thunb.) DC., Ixeris sonchifolia Hance, and Tagetes erecta L.
There are 144 species that have multi-use supplying functions, and Compositae sp. are the most frequent, with 21 species including Sonchus oleraceus L., Ixeridium chinense (Thunb.) Nakai, and Lagedium sibiricum (L.) Sojak. There are 59 species that have a single use, and Compositae sp. and Rosaceae sp. both occupy the majority, with nine species including Bidens pilosa L., Cirsium japonicum, and Gerbera anandria (L.) Sch.-Bip. of Compositae and Spiraea japonica L. f., Potentilla tanacetifolia Willd. ex Schlecht., Potentilla supine L. of Rosaceae. There are three species with no use: Heteropappus altaicus (Willd.) Novopokr. of Compositae, Cyperus nipponicus Franch. et Savat of Cyperaceae, and Axyris amaranthoides L. of Chenopodiaceae.
The total values of plant ecological functions range from 18 to 41 with two species above 35, Calystegia hederacea Wall.ex.Roxb. of Convolvulaceae and Ulmus macrocarpa Hance of Ulmaceae. Four species have comparatively low total ecological function values of less than 20, including Amaranthus lividus L. of Amaranthaceae, Saluia plebeia R. Br. of Labiatae, Gueldenstaedtia stenophylla Bunge of Leguminosae, and Carex heterostachya Bge. of Cyperaceae.
3.5. Plant Ecological Functions in Different Families
Table 6 shows the mean values of plant ecological functions in different families. It can be inferred from
Table 6 that about eight families have the highest mean regulating function values as compared to other families, at ≥11. These are
Buxaceae (11),
Ulmaceae (12),
Rhamnaceae (11)
, Aceraceae (11),
Moraceae (11),
Buxaceae (11),
Celastraceae (11),
Sapindaceae (11), and
Ginkgoaceae (11).
Solanaceae,
Apiaceae,
Oxalidaceae,
Calycanthaceae,
Labiatae,
Liliaceae, and
Amaranthaceae have the lowest mean regulating function value of 7.
Araliaceae and
Nyctaginaceae have the highest mean cultural function values of 13 and 11, respectively. In addition,
Gramineae and
Amaranthaceae have the lowest mean cultural function value of 5. Eight families have the highest mean supporting function values at ≥11. They are
Nyctaginaceae (15)
, Loganiaceae (13),
Araliaceae (12),
Lythraceae (11),
Bignoniaceae (11),
Ulmaceae (11),
Berberidaceae (11), and
Verbenaceae (11).
Gramineae (6),
Apiaceae (6), and
Amaranthaceae (5) have the lowest mean supporting function values at ≤6. Overall, 51 families have a mean supplying function value of 2, and 10 families have a mean supplying function of 1. There are 39 families that have a higher total function value than the mean value of 26.9, of which
Ulmaceae has the highest value of 37, followed by
Sapindaceae and
e with a value of 35, and
Berberidaceae and
Aceraceae with a value of 34.
Labiatae has the lowest total function value of 20, followed by
Amaranthaceae and
Saxifragaceae with a value of 21.
4. Conclusions and Discussion
By analyzing the functional traits of roof greening plants in Beijing, it was found that the four ecological functions differed significantly by plants. Firstly, Phyllostachys propinqua McClure of Gramineae, Rosa chinensis Jacq. of Rosaceae, Ligustrum japonicum Thunb. of Oleaceae, and Ulmus macrocarpa Hance of Ulmaceae have the highest regulating function value. Around nine species have the lowest values, including Artemisia annua Linn., Artemisia selengensis Turcz. ex Bess., and Heteropappus altaicus (Willd.) Novopokr. of Composita. Secondly, Ligustrum japonicum Thunb. of Oleaceae and Hedera nepalensis var. sinensis (Tobl.) Rehd of Araliaceae have the highest cultural function value, and Triarrhena sacchariflora (Maxim.) Nakai of Gramineae, Amaranthus lividus L. of Amaranthaceae, and Cuscuta chinensis Lam. of Convolvulaceae have the lowest value. Thirdly, Ligustrum japonicum Thunb. of Oleaceae has the highest supporting function value, and Amaranthus lividus L. of Amaranthaceae has the lowest. Fourthly, more than half of the plants have a wide range of supplying uses, whereas Heteropappus altaicus (Willd) Novopokr of Compositae, Cyperus nipponicus Franch. et Savat. of Cyperaceae, and Axyris amaranthoides of Chenopodiaceae have the lowest supplying function value. Fifthly, the overall mean ecological function values were obtained by summarizing the four ecological functions. Calystegia hederacea Wall.ex.Roxb. of Convolvulaceae and Ulmus macrocarpa Hance of Ulmaceae have the highest overall mean ecological function values. Amaranthus lividus L. of Amaranthaceae, Saluia plebeia R. Br. of Labiatae, Gueldenstaedtia stenophylla Bunge of Leguminosae, and Carex heterostachya Bge. of Cyperaceae have the lowest value. In addition, Rosaceae sp. have more species with higher levels of overall ecological function than the others, and Compositae sp. have more species with lower levels. Differences in the four ecological functions were also significant by families. Compared to other families, Araliaceae and Nyctaginaceae have a higher mean cultural and supporting function values. Ulmaceae has a highest mean overall function value of 37. Ulmaceae, Sapindaceae, Ginkgoaceae, Berberidaceae, and Aceraceae have higher mean regulating, cultural, supporting, and overall function values. Amaranthaceae, Umbelliferae, Lamiaceae, Saxifragaceae, Ericaceae, and Gramineae have lower mean regulating, cultural, supporting, and overall function values.
The key aim of this paper was to reveal the ecological function values of Beijing roof greening plants by analyzing their functional traits. Inferred from the analyses, the following problems exist in Beijing’s roof greening plants. Firstly, the composition structure of roof greening plants is unreasonable. Although 161 species of plants from 61 families are applied to roof greening, the number of plants from different families varies greatly. For example, the number of
Compositae sp. accounts for 44% of the total, and about 50 families make up less than 3% of the total as a whole. This is also a common problem in the current roof greening in China. For example, the survey on roof greening in Pu’er City, Yunnan Province, showed that the local form of roof greening is singular and the ecological benefits are not maximized [
66]. A survey on roof greening in Lanzhou, Gansu Province, showed that the local roof greening plants are not properly selected. The types are simple and seasonality is lacking [
67]. Another survey found that Chongqing has abundant plant resources, but insufficient plant species are used in roof greening [
68]. Secondly, the selection of plants does not consider the ecological functions. For example, many species of
Compositae and
Gramineae are used in roof greening, but their ecological function levels are low. The overall ecological function levels of
Berberidaceae and
Sapindaceae are high, but their species are deficient.
The future construction of roof greening in Beijing should focus on plants with high values in ecological function. The following are suggestions for improvement. Firstly, a possible course of action is increasing the numbers of plants that are strong at adaptation and have shallow roots, especially those tolerating wind, cold, drought, heat, and barren habitats, such as
Sedum lineare Thunb.,
Sedum sarmentosum Bunge.,
Malus spectabilis (Ait.) Borkh.,
Amygdalus persica L.
var. persica f. duplex Rehd., and
Hedera nepalensis var. sinensis (Tobl.) Rehd. Most
Crassulaceae sp. have good drought resistance through drought stress and other means [
69]. Plants with deep roots such as
Wisteria sinensis,
Ulmus pumila L., and
Morus alba L., are not suitable for planting on roofs. A second suggestion is enriching the ecological community and diversifying the species of herbaceous plants, shrubs, and lianas with high ecological functions.
Sedum lineare Thunb. is the most widely used vegetation roofing material. However, due to long-term single planting and lack of cultivation, problems such as baldness, degeneration, and death are prominent. Among the species that belong to
Crassulaceae sp., the drought-resistant perennial flowers are a large class of roof plants and should be studied deeply for future planting in roof greening [
44]. More plants with rich colors and long florescence can be cultivated, such as
Chaenomeles speciosa (Sweet) Nakai,
Cerasus yedoensis (Matsum.) Yu et Li, and
Rosa chinensis Jacq. Native plants with colorful leaves or evergreen native plants can be considered as well. For example,
Berberis thunbergii var.
atropurpurea Chenault has purple-red leaves, and
Cedrun deodara and
Ilex ficoidea Hemsl. var. parvifilia S. H. Fu var are evergreen. A third suggestion is carrying out rational ecological planning and management of roof greening to enhance ecological function levels [
7]. Ecological planning includes the connection between the composition structure and distribution characteristics of plants and the regional ecosystem. Ecological management includes advanced ecological engineering technology, an appropriate management system, and the training of professional managers.
Screening suitable species is necessary for successful greening on roofs, where there are often extreme environmental conditions such as high illumination intensity, long illumination time, high-temperature differences between day and night, low air humidity, high wind speed, and a thin soil layer. This study indicated that the functional traits of roof plants are closely related to the roof environment. Usually, roof plants characterized by small leaves, short and shallow roots, fast growth rate, and diverse breeding methods are tolerant to cold, heat, drought, and wind. Thus, characterizing plant functional traits is an efficient way to predict their functions or services for various purposes in roof greening, without considering geographic distribution, ecological niche, and taxonomic/phylogenetic relationships [
7]. Each type of ecological function has many detailed perspectives. One single plant functional trait may not be enough to reflect each ecological function. Thus, in order to get relatively accurate values of ecological functions, a set of related plant traits should be considered.
This study is helpful in green roof establishment for several reasons. Firstly, it has theoretical meaning because it assesses all the four ecological functions such as regulating, supporting, cultural, and supplying functions using a relatively comprehensive set of functional traits of plants on roof greening, and evaluates the differences of each function by plants and families. Secondly, it has practical meaning because it provides conclusions on the problems occurring in Beijing roof greening and provides useful information about screening suitable species and enhancing ecological functions. Because of the large amount of data and high technical requirements, more studies should be performed in the future to specify plant functional traits and to evaluate more detailed ecological functions such as regulating temperature and humidity, alleviating air pollution, conserving water, enhancing aesthetic, entertainment, and spiritual functions, and providing nutrient circulation, as well as medicinal, gardening, and economic value.