Different vegetation types used for the extensive green roofs have characteristic physiological and morphological traits (e.g., C 3, C 4, or CAM photosynthesis, deciduous or evergreen). Several Sedum species are recognized as “inducible CAM” type plants. These differences in the physiological and morphological traits have a considerable effect on the carbon sequestration in the green roofs. The objective of the present study was to quantify the carbon sequestration in several green roof plants during the first year after the construction of the green roofs and to clarify the relevance of the physiological and morphological traits to each plant’s ability to sequester carbon in its body using the growth analysis method. We used Zoysia matrella , Ophiopogon japonicus , and Sedum mexicanum species for the study wherein, S. mexicanum was assigned to the wet, dry, and non-irrigation treatments, and Z. matrella and O. japonicus only received the wet treatment. During the first year after the construction, carbon sequestration in the plants and the substrate of S. mexicanum was in the range of 276 to 364 g-C/m 2/year, which was similar to that of O. japonicus and the finding of a previous study. In contrast, Z. matrella exhibited the highest carbon sequestration (670 g-C/m 2/year), which is also expressed as the relative plant C-sequestration rate per whole-plant C-content (RGR c), because Z. matrella is a C 4 plant and exhibits the highest net assimilation rate (NAR c) of all species. Significant differences were not observed in RGR c , NAR c , and RMF (root mass fraction) in S. mexicanum between the wet and dry treatments. These results suggest that in countries with high rainfall, a high frequency of irrigation has an insignificant effect on the physiological and morphological characteristics, and carbon sequestration in the Sedum green roofs.
Green roofs are considered an effective technology for solving urban environmental problems. Some of their benefits include mitigation of the urban heat- island effect [
Green roofs are classified as either extensive or intensive. The extensive type is characterized by a shallow substrate (<20 cm deep) and the requirement of low maintenance. In contrast, the substrate depth of the intensive green roofs is greater than 20 cm and can support the growth of woody plants. However, an intensive green roof imposes architectural constraints because of its weight; it requires careful maintenance and is costly. Owing to these reasons, the extensive green roof is used almost exclusively in Japan.
The most common types of vegetation used for the extensive green roofs in Japan are the warm-season turf grasses (e.g. Zoysia species) and Sedum species, as they possess physiological and morphological traits that render them suitable for this purpose. For example, warm-season turf grasses are defined as C4 plants whose aboveground parts die in winter. Sedum species are regarded as Crassulacean acid metabolism (CAM) plants whose stomata close during the day and the gas exchange occurs at night [
The aim of the present study was to quantify the carbon sequestration potential of several green roof plants during the first year after the construction of green roofs and to elucidate the relevance of the physiological and morphological traits to each plant’s ability to sequester carbon in its body. Each plant’s ability to sequester carbon in its body was assessed by growth analysis, a widely used analytical model for characterizing plant growth. This model is based on the association between the relative growth rate of the plant (RGR) and the physiological (net assimilation rate―NAR) and morphological (leaf area ratio―LAR) traits [
The growth analysis provides a more informative comparison of a plant’s relative performance because it can decrease the influence of the initial size and mass of the experimental plants [
This study was conducted at the Center for Environment, Health and Field Sciences, Chiba University, Japan. The ground cover herbaceous green roof plants such as Zoysia matrella (L.) Merr. “Kourai-shiba”, Ophiopogon japonicus (Thunb.) Ker Gawl. “Tama-Ryu”’, and Sedum mexicanum Briton were used for the experiment. The three species are common green roof plants in Japan. Z. matrella is a warm-season turf grass and a C4 plant. O. japonicus is an evergreen perennial. S. mexicanum is one of the most popular Sedum species used for the extensive green roofs in Japan.
All the species were propagated as cuttings in plug flats (128 cells per tray) filled with the seedling propagation soil (Metro Mix; Sun Gro Horticulture, USA). After approximately one month, the plugs were planted in 0.2 L polyethylene pots (44 cm2) filled to a depth of 5 cm with commercial artificial soil for green roofs (114 mg/kgNO3-N, 323 mg/kgNH4-N, 159 mg/kgP2O5, 32 mg/kgK2O, 41 mg/kgCaO, and 2 mg/kgMgO), and grown in the greenhouse for two months. The composition of the artificial soil was 75% perlite, 22% bark and peat, and 3% zeolite. Thereafter, they were placed on the rooftop and watered once every two days for three weeks. During this period (September 29, 2014, to October 19, 2014), the minimum temperature was 10.5˚C and the maximum temperature was 29.1˚C in Funabashi, Chiba, Japan.
Our experiment was carried out for one year. At the start of the experiment on October 20, 2014, fifteen pots of all species were sampled over a period of approximately ten days. Green roofs of turf grasses or other perennials are generally installed with irrigation systems to prevent drought stress. In contrast, Sedum green roofs are less often installed because Sedum uses the CAM photosynthetic pathway. In our experiment, S. mexicanum was randomly assigned to wet, dry, and non-irrigation treatments after the first sampling. Plants of the wet treatment group were watered once a week from January to March, once every two days from April to June, every day from July to September, and once every two days from October to December. The plants of the dry treatment group were not watered from January to March, and watered once every two weeks from April to June, once a week from July to September, and once every two weeks from October to December. The plants of the non-irrigation treatment group were not watered at any time of the year during the experiment. The wet treatment alone was applied to Z. matrella and O. japonicas in accordance with the general cultivation practices.
Samplings were carried out every second month (on the 20th of October and December 2014, and the 20th of February, April, June, August, and October 2015) in order to assess the seasonal plant growth subjected to the wet and dry treatments. The plants of the non-irrigation treatment were only harvested at the end of the experiment (October 20, 2015). All treatments additionally received approximately 0.1 g/pot (20 g/m2) of controlled-release fertilizer (8N:8P:8K) on June 13 and August 13, 2015, in accordance with the general cultivation practices of the extensive green roofs.
The aboveground biomass, leaf area, and greenness of leaves of all the plants were studied. The aboveground parts were separated into leaves and other non-leaf parts. Leaves were scanned (LP-A500; Epson, Japan) and the image analysis software ImageJ [
To investigate the morphological traits and the carbon sequestration potential of the plants during the first year after construction, we measured the biomass allocation and the carbon content of the plants and the substrate. At the start and end of the experiment, all the fifteen plants were separated into leaves, stems, roots, flowers, and substrate. They were dried at 70˚C for 72 h. The biomass allocation was analyzed using five indices: the leaf mass fraction (LMF; leaf mass per whole-plant mass), stem mass fraction (SMF; stem mass per whole- plant mass), root mass fraction (RMF; root mass per whole-plant mass), flower mass fraction (FMF; flower mass per whole-plant mass), and specific leaf area (SLA; leaf area per leaf mass). The carbon and nitrogen concentrations were measured using an organic elemental analyzer (2400 SeriesⅡ CHNS/O System; PerkinElmer, USA). The carbon content was quantified by multiplying the carbon concentration and the dry weight. Carbon sequestration was calculated by subtracting the plant and substrate carbon content in October 2014 from the values in October 2015.
We employed the fundamental growth analysis equations, which was the classical approach [
where Wc1 and Wc2 are the plant carbon contents, and LA1 and LA2 are the total leaf areas of the plants at times T1 and T2, respectively. In this experiment, T1 and T2 correspond to October 20, 2014, and October 20, 2015, respectively.
Data were analyzed using IBM SPSS Statistics version 22.0 (IBM Japan, Japan). ANOVA was used to assess the effect of the treatments and the plant species. Differences in the mean values were assessed with Student's t-test or multiple comparisons (Tukey-b with homoscedasticity assumed, Dinnett-T3 with homoscedasticity not assumed).
The values for ambient air temperature and precipitation in Funabashi, Chiba, Japan, are shown in
The maximum monthly total precipitation is September was 385 mm, while that in May and October 2015 was approximately 50 mm (
The time course of the aboveground biomass is shown in
The time course of LAI was similar to the aboveground biomass except for S. mexicanum in the dry treatment (
S. mexicanum in the wet treatment showed a significantly higher aboveground biomass and LAI on August 20, 2015, than that in the dry treatment. However, at the end of the experiment, there were no significant differences between the treatments.
The G/R ratio of O. japonicus was significantly higher than that in the other species during the entire experimental period, except at the fifth sampling (
All the three measurements of S. mexicanum in the non-irrigation treatment were lower than those of the wet and dry treatments at the end of the experiment (non-irrigation treatment: above ground biomass = 1.71 ± 0.09 g, LAI = 4.65 ± 0.22, G/R = 0.91 ± 0.00).
The morphological traits of all the species under each treatment on October 20, 2015, are presented in
Species and treatment | Oct-14 | Dec-14 | Feb-15 | Apr-15 | Jun-15 | Aug-15 | Oct-15 | ||||||||||||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Z. matrella | Wet | 1.15 | ± | 0.00 | bz | 0.94 | ± | 0.00 | by | 0.96 | ± | 0.00 | by | 1.03 | ± | 0.00 | by | 1.14 | ± | 0.01 | cy | 1.10 | ± | 0.01 | cy | 1.02 | ± | 0.01 | by |
O. japonicus | Wet | 1.21 | ± | 0.00 | c | 1.16 | ± | 0.00 | c | 1.06 | ± | 0.01 | c | 1.06 | ± | 0.01 | c | 1.11 | ± | 0.00 | b | 1.19 | ± | 0.00 | d | 1.18 | ± | 0.00 | c |
S. mexicanum | Wet | 1.04 | ± | 0.01 | a | 0.91 | ± | 0.00 | a | 0.92 | ± | 0.00 | a | 1.02 | ± | 0.00 | ab | 1.08 | ± | 0.01 | a | 1.04 | ± | 0.01 | b | 1.03 | ± | 0.01 | b |
Dry | 0.91 | ± | 0.00 | a | 0.92 | ± | 0.00 | a | 1.01 | ± | 0.00 | a | 1.11 | ± | 0.01 | b | 0.95 | ± | 0.01 | a | 0.93 | ± | 0.00 | a |
Data represented by Mean ± SE (n = 15). ZSignificant differences among the means are indicated by different letters based on Dunnett-T3 at P < 0.05. ySignificant differences among the means are indicated by different letters based on Tukey-b test at P < 0.05.
Species and treatment | LMF | SMF | RMF | SLA (m2/g) | |||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Z. matrella | Wet | 0.11 | ± | 0.00 | az | 0.70 | ± | 0.01 | dy | 0.19 | ± | 0.01 | ay | 0.016 | ± | 0.000 | n.s.y |
O. japonicus | Wet | 0.22 | ± | 0.01 | b | 0.17 | ± | 0.01 | a | 0.61 | ± | 0.02 | d | 0.017 | ± | 0.001 | |
S. mexicanum | Wet | 0.48 | ± | 0.02 | c | 0.28 | ± | 0.01 | c | 0.23 | ± | 0.02 | ab | 0.016 | ± | 0.000 | |
Dry | 0.48 | ± | 0.01 | c | 0.24 | ± | 0.01 | b | 0.28 | ± | 0.01 | bc | 0.016 | ± | 0.000 | ||
Non | 0.46 | ± | 0.01 | c | 0.21 | ± | 0.01 | b | 0.33 | ± | 0.02 | c | 0.017 | ± | 0.001 |
Data represented by Mean ± SE (n = 15). ZSignificant differences among the means are indicated by different letters based on Dunnett-T3 at P < 0.05. ySignificant differences among the means are indicated by different letters based on Tukey-b test at P < 0.05.
any treatment, at the end of the experiment. The SMF of Z. matrella was significantly higher than that of O. japonicus and S. mexicanum, which indicates that Z. matrella allocated significantly more biomass to the stems than did the other species. Likewise, the LMF and RMF values clearly demonstrate that S. mexicanum allocated significantly more biomass to the leaves, and O. japonicus allocated significantly more biomass to the roots.
S. mexicanum did not exhibit significant differences in the LMF and SLA values among the treatments (
The plant and substrate nitrogen concentrations at the end of the experiment (October, 20 2015) are shown in
The dry weight, carbon concentration, and carbon content of the plants on October 20, 2014, and 2015 are shown in
Species and treatment | Plant nitrogen concentration (%) | Substrate nitrogen concentration (%) | |||||||
---|---|---|---|---|---|---|---|---|---|
Z. matrella | Wet | 0.77 | ± | 0.05 | bz | 0.29 | ± | 0.01 | n.s.y |
O. japonicus | Wet | 1.05 | ± | 0.02 | c | 0.27 | ± | 0.01 | |
S. mexicanum | Wet | 0.65 | ± | 0.02 | a | 0.28 | ± | 0.01 | |
Dry | 0.65 | ± | 0.02 | a | 0.29 | ± | 0.01 | ||
Non | 0.66 | ± | 0.01 | a | 0.28 | ± | 0.02 |
Data represented by Mean ± SE (n = 15). ZSignificant differences among the means are indicated by different letters based on Tukey-b test at P < 0.05. ySignificant differences among the means are indicated by different letters based on Dunnett-T3 at P < 0.05.
Species and treatment | Plant dry weight (g/pot) | Plant carbon concentration (%) | Plant carbon content (g-C/pot) | ||||||||||||||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Oct-14 | Oct-15 | Oct-14 | Oct-15 | Oct-14 | Oct-15 | ||||||||||||||||||||||
Z. matrella | Wet | 0.41 | ± | 0.02 | 5.80 | ± | 0.33 | * | - | 41.9 | ± | 0.7 | 43.2 | ± | 0.3 | cz | 0.17 | ± | 0.01 | 2.50 | ± | 0.14 | * | - | |||
O. japonicus | Wet | 0.51 | ± | 0.03 | 3.36 | ± | 0.11 | * | - | 41.3 | ± | 0.4 | 40.9 | ± | 0.3 | b | 0.21 | ± | 0.01 | 1.37 | ± | 0.05 | * | - | |||
S. mexicanum | Wet | 0.47 | ± | 0.03 | 3.40 | ± | 0.18 | * | Bz | 33.6 | ± | 0.8 | 36.6 | ± | 0.3 | * | a | 0.16 | ± | 0.01 | 1.24 | ± | 0.07 | * | Bz | ||
Dry | 3.37 | ± | 0.12 | * | B | 37.0 | ± | 0.4 | * | a | 1.25 | ± | 0.04 | * | B | ||||||||||||
Non | 2.70 | ± | 0.11 | * | A | 36.8 | ± | 0.4 | * | a | 0.99 | ± | 0.04 | * | A |
Data represented by Mean ± SE (n = 15). *Represents significant differences between the results of October 2014 and October 2015 (Student’s t-test: *P < 0.05). zSignificant differences among the means are indicated by different letters based on Tukey-b test at P < 0.05.
The highest carbon concentration of the three species was observed in Z. matrella (
The carbon content of the plants among all the species and treatments was significantly increased during the experimental period (
The substrate dry weight, carbon concentration, and carbon content on October 20, 2014, and 2015 are shown in
Z. matrella and S. mexicanum exhibited a significant increase in the substrate carbon concentration and content during the experimental period (
The total carbon content and the annual carbon sequestration are shown in
During the first year after construction of the green roofs, the carbon sequestration potential of Z. matrella was the highest among all the species (
Species and treatment | Substrate dry weight (g/pot) | Substrate carbon concentration (%) | Substrate carbon content (g-C/pot) | ||||||||||||||||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Oct-14 | Oct-15 | Oct-14 | Oct-15 | Oct-14 | Oct-15 | ||||||||||||||||||||||
Z. matrella | Wet | 31.8 | ± | 0.2 | 33.2 | ± | 0.4 | * | - | 5.6 | ± | 0.1 | 7.2 | ± | 0.5 | * | n.s.z | 1.77 | ± | 0.03 | 2.39 | ± | 0.12 | * | - | ||
O. japonicus | Wet | 29.1 | ± | 0.3 | 28.2 | ± | 0.5 | - | 5.4 | ± | 0.2 | 6.0 | ± | 0.4 | 1.59 | ± | 0.04 | 1.69 | ± | 0.06 | - | ||||||
S. mexicanum | Wet | 30.5 | ± | 0.3 | 29.7 | ± | 0.6 | n.s.z | 5.0 | ± | 0.2 | 6.5 | ± | 0.3 | * | 1.54 | ± | 0.03 | 1.94 | ± | 0.07 | * | n.s.y | ||||
Dry | 29.8 | ± | 0.6 | 6.9 | ± | 0.3 | * | 2.06 | ± | 0.04 | * | ||||||||||||||||
Non | 30.4 | ± | 0.3 | 6.2 | ± | 0.4 | * | 1.92 | ± | 0.09 | * |
Data represented by Mean ± SE (n = 15). *Represents significant differences between the results of October 2014 and October 2015 (Student’s t-test: *P < 0.05). zSignificant differences among the means are indicated by different letters based on Tukey-b test at P < 0.05. ySignificant differences among the means are indicated by different letters based on Dunnett-T3 at P < 0.05.
Species and treatment | Total carbon content (g-C/pot) | Carbon sequestration | ||||||||
---|---|---|---|---|---|---|---|---|---|---|
Oct-14 | Oct-15 | (g-C/pot/year) | ||||||||
Z. matrella | Wet | 1.95 | ± | 0.03 | 4.89 | ± | 0.15 | * | - | 2.95 |
O. japonicus | Wet | 1.80 | ± | 0.05 | 3.04 | ± | 0.09 | * | - | 1.24 |
S. mexicanum | Wet | 1.70 | ± | 0.02 | 3.18 | ± | 0.07 | * | ABz | 1.48 |
Dry | 3.30 | ± | 0.09 | * | B | 1.60 | ||||
Non | 2.92 | ± | 0.10 | * | A | 1.21 |
Data represented by Mean ± SE (n = 15). *Represents significant differences between the results of October 2014 and October 2015 (Student’s t-test: *P < 0.05). zSignificant differences among the means are indicated by different letters based on Tukey-b test at P < 0.05.
The growth analysis applied to the plant carbon contents is presented in
There were no significant differences in the NARc and LARc values of S. mexicanum among the treatments. However, the non-irrigation treatment resulted in lower RGRc values than by the other treatments.
During the first year after the construction of the green roofs, carbon sequestration of the three species is represented as grams of carbon per m2 per year (
S. mexicanum receiving the wet treatment exhibited significantly higher above- ground biomass and LAI than it did under the dry treatment in summer (August 20, 2015), whereas at the end of the experiment, there were no significant differences (
Species and treatment | RGRc | NARc | LARc | ||||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|---|
(day-1) | (g-C/m2/day) | (m2/g-C) | |||||||||||
Z. matrella | Wet | 0.0074 | ± | 0.0001 | cz | 1.38 | ± | 0.06 | cy | 0.0094 | ± | 0.0005 | az |
O. japonicus | Wet | 0.0052 | ± | 0.0001 | a | 0.54 | ± | 0.04 | b | 0.0155 | ± | 0.0006 | b |
S. mexicanum | Wet | 0.0056 | ± | 0.0000 | b | 0.26 | ± | 0.01 | a | 0.0281 | ± | 0.0010 | c |
Dry | 0.0057 | ± | 0.0001 | b | 0.27 | ± | 0.01 | a | 0.0277 | ± | 0.0009 | c | |
Non | 0.0050 | ± | 0.0001 | a | 0.24 | ± | 0.01 | a | 0.0280 | ± | 0.0011 | c |
Data represented by Mean ± SE (n = 15). ZSignificant differences among the means are indicated by different letters based on Tukey-b test at P < 0.05. ySignificant differences among the means are indicated by different letters based on Dunnett-T3 at P < 0.05.
significantly higher than that under the dry treatment (
However, the substrate carbon contents of Z. matrella and S. mexicanum in all treatments significantly increased during the experimental period, while their G/R values were below 1.0 in winter (
A significant correlation (r = 0.97; P < 0.01) was observed between RGRc and carbon sequestration in the plant body during the experimental period for all species and treatments (
A previous study carried out under a controlled environmental system demonstrated that S. mexicanum in a wet and an increased nutrient condition exhibited the C3 photosynthetic pathway and its LARc was greater than triple that of Z. matrella. Further, these physiological and morphological responses of S. mexicanum have led to a higher RGRc than that of Z. matrella and their drought treatments [
Therefore, it is clear that in countries with high rainfall, a high frequency of irrigation has little effect on the physiological and morphological characteristics and carbon sequestration in the Sedum green roofs. Therefore, it is necessary to consider the maintenance practices, for example, fertilizer management, which will enable effectiveness of the Sedum green roofs.
In this study, although we focused on carbon sequestration during the first year after the construction of the green roofs, the carbon balance would reach equilibrium where the decomposition of organic matter equaled the sequestration. However, our method of applying the growth analysis to the assessment of the plant carbon content will contribute to the understanding of the relevance of carbon sequestration to the growth of the green roof plants.
Our results demonstrated the carbon sequestration capacity of several green roof plants during the first year after construction, which suggests that the warm- season turf grasses are effective choices because of their high NARc and RGRc.
In addition, the growth analysis of the plant carbon content enabled us to understand the relevance of the physiological and morphological traits of plants to the ability to sequester carbon in their body. This would support the selection of plants with higher capacities for carbon sequestration, and aid the employment of maintenance practices that increase NARc and LARc. We believe that our results would contribute to the development of more appropriate designs and maintenance for each green roof.
Kuronuma, T. and Watanabe, H. (2017) Relevance of Carbon Sequestration to the Physiological and Mor- phological Traits of Several Green Roof Plants during the First Year after Construction. American Journal of Plant Sciences, 8, 14-27. http://dx.doi.org/10.4236/ajps.2017.81002