Research Paper
Quantifying carbon sequestration of various green roof and ornamental landscape systems

https://doi.org/10.1016/j.landurbplan.2013.11.015Get rights and content

Highlights

  • More complex plant communities increase the carbon per unit area of landscapes.

  • Landscapes contain more carbon per unit area in-ground than on green roofs.

  • Carbon per unit area only increased during the study in a few landscapes.

  • Deeper substrate may increase the carbon content of Sedum green roofs.

  • More complex plant communities decrease the carbon payback period of a green roof.

Abstract

Interest in reducing carbon emissions and carbon trading programs has been increasing. In order to calculate the carbon benefits of landscapes, we must first understand how they sequester and store carbon. Carbon dynamics have been extensively studied in natural and agricultural landscapes, and urban and community forests. Relatively little is known about carbon sequestration in ornamental landscapes. This study compared the carbon content of nine in ground and three green roof landscape systems of varying complexity to determine their carbon sequestration potential. Soil or substrate samples were analyzed prior to planting in 2009 and soil/substrate, below- and above-ground biomass were analyzed at the end of the 2010 and 2011 growing seasons. Landscape systems containing more woody plants, such shrubs (65.67, 78.75, and 62.91 kg m−2) and herbaceous perennials and grasses (68.75 and 67.70 kg m−2 for the in-ground and green roofs, respectively) had higher carbon content than other landscape systems. The native prairie mix (28.57 kg m−2) also had high carbon content, because of the high volume of plant biomass. The vegetable and herb garden and vegetable green roof contained a moderate amount of carbon (54.18 and 11.03 kg m−2, respectively). The Sedum and prairie green roofs contained less carbon than their counterpart in-ground landscape systems, suggesting that although green roofs do sequester a small amount of carbon, greater benefit can be achieved in ground level landscape systems. Ornamental landscapes have good potential for carbon sequestration but management practices can affect their net carbon sequestration and the permanence of the carbon sequestered.

Introduction

There are large differences in the ability of landscapes to sequester carbon. These differences are functions of species diversity (Tilman, Hill, & Lehman, 2006), ecosystem age (Matamala, Jastrow, Miller, & Garten, 2008), plant density (Fang et al., 2007, Matamala et al., 2008), species composition, climate (Matamala et al., 2008), and plant morphology (Fang et al., 2007, Rhoades et al., 2000). Tilman et al. (2006) found greater species diversity produced larger amounts of bioenergy and greater carbon sequestration. Roots in their plots containing 16 species sequestered 160% more carbon than those of monoculture plots. As landscapes age, litter, microbial and root biomass increase (Matamala et al., 2008) so does woody biomass (Fang et al., 2007), depending on the type of vegetation. Fang et al. (2007) observed a 10% increase in the proportion of carbon allocated to woody structures such as stems between years 5 and 9 of their study. They also found that management decisions such as the spacing of trees and the cutting cycle influenced carbon sequestration. Management practices of agricultural lands also play an important role in carbon sequestration. The mineralization of soil organic matter can be twice as fast under conventional tillage as it is under no-till systems (Balesdent, Chenu, & Balabane, 2000) and irrigation of arid croplands can even increase soil organic carbon stocks above those of native soils by 133% within 55 years (Wu et al., 2008). However cultivation generally decreases the amount of carbon contained in a landscape by reducing soil organic matter between 10 and 55% (Balesdent et al., 2000, Rhoades et al., 2000), reducing microbial and root biomass (Matamala et al., 2008), and often replace forests containing large amounts of woody biomass with smaller herbaceous crop or pasture plants (Rhoades et al., 2000).

Much of the research conducted on carbon sequestration has been in natural landscapes and agricultural lands, but recently the focus has been shifted to include urban landscapes specifically forests and urban street trees, another highly managed system. Urban landscapes are highly altered compared to natural landscapes, which affects carbon dynamics. For example, temperatures are higher in urban areas than surrounding rural areas (known as the urban heat island) and there tends to be a greater concentration of CO2 in urban areas than surrounding rural areas. This leads to a CO2 fertilization affect in summer, causing urban forests to be a greater sink of carbon than rural forests, and higher winter temperatures result in urban forests being a greater source for carbon in the winter because of higher respiration rates (Awal et al., 2010). As with agricultural systems, the intensive management of urban forests and street trees, which often entails greater carbon emissions, changes the balance between carbon emitted and carbon sequestered. This may lead to a point when more carbon is emitted due to management practices than can be taken up by the landscapes during the course of a year (Nowak, Stevens, Sisinni, & Luley, 2002). This is further complicated by management differences on small parcels of land in a landscape (Bigsby, 2009). This calls into question one of the key issues when considering carbon credits, which is the quality of the credit. An issue which is just as relevant to ornamental landscapes, landscapes which have been planted by humans for their esthetic value. These landscapes are also highly managed, but more fragmented than many of the forest areas which have been studied.

The permanence of carbon credits is also an issue for urban forests, street trees and ornamental landscapes. This is especially true when participation in a carbon credit trading program, such as the Chicago Climate Exchange (CCX) which enables those unable to meet the goals of the program to purchase credits from other participants (CCX, 2009), is being considered. It has been suggested that when an urban tree is replaced the new tree merely offsets the carbon being released by the old tree if the old tree is not placed in a landfill or used for wood products other than mulch (Nowak et al., 2002). So what becomes of the carbon credit when a highly managed landscape reaches carbon equilibrium? Ornamental landscapes are typically composed of shrubs and herbaceous plants, which have relatively short lifespans and require periodic replacement. So the fate of removed material, and how that affects the carbon budget of the landscape becomes even more important. Ornamental landscapes have another input which with implications for the carbon budget of such landscapes: potting media. Typical potting media used in the nursery production of plants for ornamental landscapes has greater carbon content than field soils (Marble et al., 2011). However, the amount of carbon contained in the media, and what happens after transplantation from the nursery to the landscape is not well understood.

Some research has been done on carbon sequestration on green roofs. Getter, Rowe, Robertson, Cregg, and Andresen (2009) found that several extensive green roofs located in Michigan and Maryland stored an average of 162 g C m−2 in above-ground biomass with variation due to roof age and substrate depth. In a second study, a 6 cm deep sedum based roof contained 375 g C m−2 in above- and below-ground biomass and substrate organic matter. They speculated that differences between their findings and those of others examining the issue of carbon sequestration may be due to the age of the plants in the study, and differences between the morphology of succulents used on green roofs and the plants used in previous research on forests and agriculture.

The objectives of this study are twofold and attempt to determine how much carbon plants sequester after planting and address some gaps in knowledge found by Getter et al. (2009). The first objective is to quantify the amount of carbon sequestered by ornamental and green roof landscapes of varying complexity. The second objective is to then determine what if any differences in carbon sequestration exist between the landscape systems and differences in carbon sequestration between green roof landscapes and similar landscape systems at ground level.

Section snippets

Methods

This study examined the carbon content of thirteen ornamental landscape systems of varying complexity over the course of three years at the Michigan State University Horticulture Teaching and Research Center (HTRC) in East Lansing, MI. In the context of this study, the term landscape system will be used to refer to the plot treatments which vary in terms of plant community and the soil or substrate those plants are grown in. Of these, nine landscape systems were grown at ground level and four

Weather

Weather patterns during the three growing seasons were different. Maximum ambient air temperatures for weeks 1–14 (June through August) in 2009 were cooler than either of the two later growing seasons with the exception of 4 weeks (3, 4, 11 and 12) (Fig. 1). 2009 exhibited the warmest final weeks of the growing seasons (wks 14–18, September) and 2011 exhibited the warmest temperatures during the middle of the growing season, (wks 8–10, July). Minimum ambient air temperatures were also lower in

Carbon content

Overall the three shrub landscape systems, the herbaceous perennial and grasses, ornamental green roof, and native prairie mix landscape systems contained more carbon than other landscape systems examined in this study (Table 4, Table 5, Table 6, Table 7). The three shrub landscapes were made up of more woody structures than other landscape systems and woody structures have been shown to contain more carbon (4.7–16.7% more) than other plant structures (Fang et al., 2007). The herbaceous

Conclusions

Results of this study suggest that ornamental landscapes both in-ground and on green roofs have the ability to sequester carbon. The landscape systems that were able to sequester the most carbon contained higher amounts of woody plant structures and higher plant biomass volumes, such as the three shrub landscape systems and the herbaceous perennial and grasses, native prairie mix, and ornamental green roof landscape systems. Two of the green roof landscape systems examined, Sedum and prairie

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Formerly: Michigan State University, Department of Horticulture, USA.

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