Landscape Performance Metrics as a Means of Informing Design Scenarios
Abstract: Landscape performance metrics are emerging as an important area of research for the discipline of landscape architecture. This article reviews four commonly used and/or validated tools that have been used to measure landscape performance: stormwater calculations, iTree, Plant Stewardship Index and eBird. It also suggests that landscape performance metrics should be linked to ecosystem service provision. Using metrics at the beginning of the design process can provide the opportunity to maximize performance and ecosystem benefits.
Key words: landscape architecture; landscape performance; ecosystem services; metrics
Empirical information is valued by many, if not most, professional practice disciplines. It is the basis of scientific theory for which hypotheses are tested through gathering evidence and data. Evidence based theory is becoming increasingly important to professions concerned with public health, safety and welfare. Medicine, dentistry, engineering and architecture use post evaluation data to inform and improve decision-making. These disciplines have existed for centuries and have sought to build bodies of evidence as a basis for theories and practice. Landscape architecture is a relatively young discipline and has yet to build a substantive body of theory. Landscape architects have traditionally relied upon knowledge of environmental characteristics of a site and its context; client program, and precedent of similar types of projects to guide design decisions. However, landscape performance metrics has gained momentum over the past five years as a means of understanding and evaluating design. This approach, adopted from science, uses data gathered on site or through computer modeling tools, to understand how designs function. This evidence can also be extended toward understanding whether and how designed landscapes perform ecological and social services. This article describes four tools that can be useful in measuring the performance of built landscapes across a spectrum of scales and locations. It also suggests how landscape performance metrics might inform the delivery of ecological services.
1 Landscape Performance Metrics
Landscape performance was initiated by the Landscape Architecture Foundation (LAF) in 2010, to demonstrate the value of landscape architecture to society and the environment. Its purpose is to "to build capacity to achieve sustainability and transform the way landscape is considered in the design and development process (LAF 2015)". To this end, LAF supported a case studies research initiative to develop case studies that quantify the environmental, social and economic benefits of high performing landscapes. Each year since 2011, LAF has funded teams of university faculty and students to measure and document the built landscape projects of leading practitioners. The resulting case studies are subsequently published on the Landscape Architecture Performance Series (LAPS) website: http://landscapeperformance.org/case-study-briefs. Performance claims and the methodology supporting each case study are vetted through in-house peer review prior to publication.
The difference between the LAPS and other metrics systems, such as the Sustainable Sites Initiative (SITES) or Leadership in Energy and Environmental Design (LEED), is that it assesses the performance of built landscapes, providing data that express the actual functioning of a project. The projects are generally measured only once so measurements provide a single, snapshot view. A longitudinal study to track change over time would be more ideal. Longitudinal studies would allow researchers to observe how the landscape is evolving and if performance benefits are increasing or decreasing over time. Such studies would provide better data upon which to base future decisions and help to grow the disciplinary body of knowledge.
Methodologies used to measure the landscapes included iTree to quantify air quality benefits; calculations used to predict and quantify stormwater benefits; eBird to quantify presence of bird species on a site and PSI to quantify plant stewardship index (a measure of ecological quality). These tools differ from traditional ecological research which focuses on data collection of, for example, species presence, abundance and dominance on a particular site from which conclusions regarding diversity, health or successional trajectories can be drawn.
Landscape performance uses on-site observation, (i.e. identification of tree species, numbers and sizes) and "plug" the data into software whose algorithms compute benefits, such as carbon sequestration. Landscape performance can be translated to measuring ecosystem services, a concept defined as "the benefits people obtain from ecosystems (MA, 53)". While standard accounting for ecological services has yet to be developed, the connection between performance metrics and services needs to be established. This article represents a beginning of what is hoped to be continued research in the area of landscape performance and ecosystem services.
2 Landscape Performance Benefits and Ecological Services
Since the publication of the Millennium Ecosystem Assessment Report (MA) in 2005, interest has surged in disciplines such as economics and ecology, in quantifying the value of ecosystem services. The MA defines 10 ecosystems, including urban categories (built environments with over 5,000 inhabitants) and cultivated lands (dominated by domestic plant species, 30% of which comes under cultivation).
The projected global population is expected to increase to between 9 and 11 billion humans by 2050. The environment will be, and is currently being re-made by humans as our species expands its habitat. Thus urban and cultivated ecosystems will become ever more important to global ecosystem services. They are ecosystems that are designed and actively managed by humans and are likely to have the input of landscape architects, architects, engineers, agriculturists, and other experts. These systems should be designed to maximize ecosystem services to the greatest extent possible.
Some ecosystem services are readily associated with landscape performance benefits. For example storm water retention, an environmental landscape performance characteristic prevents flooding down stream, which is a regulating ecosystem service. Storm water retention is often associated with water quality improvement, another regulating service. Planting trees in the urban environment reduces carbon dioxide, improving air quality which is yet another regulating service. Biodiversity, which supports many other ecosystems services, is a quantifiable landscape performance benefit.
Environments that are designed by landscape architects and which occur in urban areas are the focus of this paper. The term urban includes suburban locales because the term is used according to the MA urban ecosystem definition: "known human settlements with a population of 5,000 or more, with boundaries delineated by observing persistent night-time lights or by inferring areal extent in the cases where such observations are absent (MA, 55). "These landscape typologies comprise 95% (n = 97) of the total 102 case studies published on the LAF Case Study Investigation website (as of September 1, 2015). It is clear, that the discipline of landscape architecture makes contributions to the performance of urban and cultivated ecosystems. Landscape architects should be considering ways to maximize ecosystem services within these environments. An important goal will be to also assist these ecosystems to mitigate and adapt to climate change in the coming centuries. 3 Review of tools frequently used to measure landscape performance.
Many of the LAF CSI projects use engineering calculations to predict stormwater retention and/or free on line calculators to measure other performance benefits. The selected for inclusion in this article were commonly used/or validated in 58 (as of August 2013) case studies assembled by LAF. Storm water calculations were used by the majority of case studies. iTree was used for 15; and PSI, which is useful to two states, was used for 5 case studies (Yang and Myers, 2013). While eBird was used for only 1 case study measurement, it has been thoroughly vetted and validated by the scientists of the Cornell Ornithology Laboratory and provides a useful tool for current and future metrics . (Myers, et.al. 2014).
The following tools are described:
Site specific calculations to measure stormwater retained and infiltrated on site.
iTree to measure sequestration of carbon dioxide (iTree).
Plant Stewardship Index (PSI) to measure the plant stewardship or ecological quality.
eBird to measure the presence and numbers of bird species or biodiversity.
4 Stormwater Calculations
Stormwater calculations begin with understanding of the requirements of local code requirements. Cities, such as Philadelphia PA require that a site infiltrate the first inch of runoff from all impervious surfaces, regardless of storm size/duration. (Philadelphia Water Department Regulations) Portland OR requires that most sites must be able to infiltrate a ten year storm – 24 hour storm. "The quantity and flow rate of stormwater leaving the site after development shall be equal to or less than the quantity and flow rate of stormwater leaving the site before development, as much as practicable… (Portland Public Improvements, 17.38.035 Drainage Management Polices and Standards)." It should also be noted that Portland defines pre-development as the state of the land during the Lewis & Clark era which was the era of European exploration and settlement of this region.
Designers calculate the volumes and types of infiltration areas, such as rain gardens, cisterns, porous pavements, roof gardens, ponds, and vegetated swales to meet local code requirements. In many American cities, standard hydrologic and hydraulic engineering calculations are used to estimate post construction stormwater performance. HydroCAD software is a popular and accepted means of calculating because it uses "simplified procedures for estimating runoff and peak discharges in small watersheds" (NRC 1986, 2). In some cities, such as Portland, trees and vegetation play a prominent role in managing stormwater and are evaluated for their contribution by the Stormwater Bureau.
City code may also require that stormwater meet a standard of water quality. (Philadelphia Water Department Regulations) Calculations can be made to project improvements to water quality based upon methods used to settle out contaminants.
The limitation of the stormwater metric is that it is predictive and relies solely on calculations projecting mitigation benefits. The calculations are based upon standard formulae developed from empirical data (Davis and McCuen, 2005 Wong, et.al., 2005). Monitoring outflows would be a more accurate means of evaluating stormwater mitigation. Follow up monitoring requires multiple measurements over time (usually for about a year or more) and is often beyond the scope of a design contract. Likewise, water quality sampling would aid in understanding if the quality of stormwater improved due to the mitigation measures. To truly evaluate a landscape's performance in flood control and water quality services, continued monitoring should be built into the design contract. Such information will greatly assist designers to assess how a site is performing in the ecosystem categories of flood regulation and water quality regulation. Both of these water associated regulating services will be very important in responding to climate change. Many places in the world will require more flood control, and should look to the stricter requirements (such as Portland, OR policies) to hold water onsite. More data on green infrastructure ought to be collected by municipalities, in order to inform designers of the efficacy of rain gardens, bioswales and other "green works" to absorb and clean storm water.
Carbon sequestration can be measured using the Tree Benefit Calculator, an online tool developed by Casey Trees and Davey Tree Expert Company. The Tree Benefit Calculator is "based on iTrees street tree assessment tool called STREETS (http://www.treebenefits.com/calculator)". The Tree Benefit Calculator uses species, diameter and land use area (i.e. single family residential, large industrial, etc.) to calculate economic and ecological benefits. Bonnfaci identifies Stratum software as the basis for the National Tree Benefits Calculator (2009). McPherson, a developer of STRATUM the foundation for what is now referred to as iTrees, works for the United States Forest Service (USFS) the branch of government that developed Stratum. He writes that iTrees was "based upon 20 years of urban forest science (McPherson 2010, 230)". McPherson notes specifically that the software was developed with landscape architects, (and others) in mind who might be interested in analyzing benefits and costs of municipal forests. McPherson explains the methodology used to develop the tool, which includes onsite measurements of trees located in 16 different climate zones of the USA. A city from each climate zone was selected for study. "In each reference city, 30 to 60 trees from each of the 22 major tree species were aged and measured. Then linear regression was used to fit predictive models with diameter at breast height (dbh) as a function of age for each species. Predictions of leaf surface area, crown diameter, and height metrics were modeled as a function of dbh using best-fit models. Geographic data were collected for use in iTree Streets' numerical models. That data included temperature, precipitation, air pollutant concentrations, and fuel mix for energy production (McPherson 2010, 231)".
One difficulty with the Tree Benefit Calculator has to do with the lack of data for some species. Another difficulty has to do with user error. The researcher must input the correct genus and species, general land use of the area in which it is planted, and dbh.
In a review of 27 new case studies posted to the LAF CSI website between 2013-14, nine used iTree. Four of the nine case studies had errors, which is a statistically significant number (Myers and Smith, 2015). Two sets of researchers, Myers and Smith and the iTree review team, were unable to validate the results of the four case studies. The iTree review team (Henning, Maco and Roman) subsequently recommended that landscape architects use iTree Design instead of the Tree Benefit Calculator when designing a site because it is actively updated by the iTree staff. iTree Design can be more useful in developing different design scenarios for a specific site because it allows the designer(s) to select and size species and place them directly on a particular site. It can also be used for sites that are already planted. The iTree review team says that the benefit claims are more defensible because this tool is more specific and current in its content.
6 Plant Stewardship Index
The Plant Stewardship Index (PSI) provides "a 'thermometer' reading of the ecological quality of open land by seeing what plants live there" (BHWP 2011). PSI is based on a series of calculations related to coefficients of conservatism (CC) numbers assigned by leading botanists and ecologists to native and nonnative plants. Specialist plants, that require special habitats and conditions are ranked higher than generalist plants which are found in a variety of habitats and conditions. To date, the PSI calculator is only valid for the states of Pennsylvania and New Jersey. "For all of New Jersey and the Piedmont region of Pennsylvania, over 2000 plants have been catalogued and assigned a number from zero to ten by local experts and botanists…Zero represents the most 'generalist' species, tolerant of disturbance and includes invasive or introduced nonnative species. Ten represents the most 'conservative' species and includes many rare and endangered state-listed native plants that require special habitats and do not re-grow after disturbance (http://conservationtools.org/guides/)."
Other states within the USA, typically use the Floristic Quality Assessment Index (FQAI) to measure the quality of natural lands. FQAI assesses only native plant species and does not assign values to non-native species.
Bowman's Hill Wildflower Preserve, New Hope, PA, provides a free on-line calculator to measure PSI of a site (www.bhwp.org. psi). The calculator is easy to use and provides an explanation of its methodology. Users input plant species and the calculator assigns a coefficient of conservatism to each plant. The calculator also provides information about whether certain plants are rare, threatened, or endangered. The calculator summarizes the content, providing a Native Mean "C" value for the site (sum of coefficients/Native Plants); and overall Total Mean C value (sum of coefficients/Native +Introduced Plants). The BHWP User's Guide states:" If simplifying to just one metric to decide on the worth of any site, use Native Mean C, which indicates the intrinsic floristic quality of the site irrespective of size or exotic invasion…Verbal equivalents of Native Mean C Values are
Severely Degraded Area Native Mean C = 0-2.4
Degraded Natural Area Native Mean C = 2.5-3.4
Quality Natural Area Native Mean C = 3.5-4.4
High Quality Natural Area Native Mean C = 4.5-5.4
Exceptional Quality Natural Area Native Mean C = 5.5+
(BHWP, PSI User’s Guide, 2012, 18)."
The PSI calculator and methodology is directed toward assessing the quality of natural lands or native habitats. In that regard, it is not ideal for designed landscapes or landscapes that are in early stages of ecological restoration. Generalist plants which might have the best chance of surviving in an urban area are rated lower than specialist plants. Another limitation is that the PSI calculator is valid for only two states within the USA. It would be ideal if similar calculators were developed for regions, such as the Piedmont or Prairie zones, to name two very broad regions of North America. Similar calculators could be developed to assess the plant community value in regions around the world. Such calculators could take into account native biodiversity, size of plants over time (an indicator of ecological health); persistence of species over time (another indicator of ecological health).
As of 2015, eBird had not yet been widely used in the LAF CSI case studies. However, like PSI, this tool has the ability to measure the biodiversity of a site. The eBird tool was developed by the Cornell Ornithology Laboratory and was released in 2002, launched jointly by the Cornell Ornithology Laboratory and National Audubon Society. It can be used to inventory bird species and numbers on a wide variety of sites, including urban sites. Additionally, eBird has assembled and stores bird observation data from around the world which can make it an effective tool for nearly every continent. It is a useful for longitudinal studies as data is available for a period of years, in some cases, dating back to the early 1900s. Peer reviewers consider the data collected by eBird to be high quality (Wood, 2011).
eBird claims that it "is amassing one of the largest and fastest growing biodiversity data resources in existence. For example, in May 2015, participants reported more than 9.5 million bird observations across the world (http://ebird.org/content/ ebird/about/)". It is a program designed to allow citizen scientists to record and enter bird observations into a shared database. Birders enter "when, where, and how they went birding, then fill out a checklist of all the birds seen and heard during the outing (http://ebird.org/content/ebird/about/)". The program has filters developed by expert birders to guide the novice birder in identification. To ensure accuracy, if an unusual record is entered, it is flagged for additional review by local bird experts.
The eBird database includes data related to dates/times of bird sighting; weather; temperature, "yielding graphs and charts that examine populations of birds in relation to factors that may influence their population and distribution (Wood, 2011)". The data can be useful to designers to better understand habitat needs of birds, migration patterns and ways to encourage and support avian diversity.
One of the drawbacks of eBird is that it may be difficult for non-birders to submit data if previous bird knowledge doesn't exist (Friedenberg in Kelly and Malloy, 2014).
eBird is linked to ecological services by measuring presence of fauna, in this case, birds on a site, or in a locale. Increased species, and increased numbers within species, are an indication of improved biodiversity for a site. Biodiversity underlies many ecological services and is a measure of ecological health.
All four of the metrics discussed here contribute to better understanding of the ways landscapes work and can be used to inform different design scenarios. Using the tools at the outset of the conceptual design process allows designers to assess the performance of different concepts. Having data related to various concepts is useful to inform the client and public, too. People like to have choices and inserting metrics as a means of assessing choices will be helpful to designers, clients and public stakeholders. More tools need to be adapted or developed to provide information that is valid for different areas of the world. This may simply be a matter of adapting current tools, or perhaps developing an entirely new tool.
Human population growth and expansion is changing the global environment. By 2050, unless protected, there is likely to be no place in the world that is unaltered by humans. Global planning must protect biodiversity. A means of addressing the food and shelter needs of people, while conserving "natural" areas, is to provide for more ecological services in the built environment. It is crucial that landscape architects and other designers strive to understand and include ecosystem services in their designs. Metrics are a means of assessing projected ecosystem services allowing landscape architects to assess and weight particular ecosystem services in different design scenarios. Metrics can also be used to study a site over the long term to understand its evolution relative to projected ecosystem services function. They provide empirical data to help improve design in both the short and long term.
Bonifaci, Emily. 2009. "Selling the urban forest: calculating the environmental benefits of street trees". Master of Landscape Architecture Thesis. Harvard University.
Boyd, J. and Banzhaf, S. 2007. "What are ecosystem services? The need for standardized environmental accounting units." Ecological Economics. 63: 614626.
http://ebird.org/content/ebird/about. Accessed October 5, 2015. Friedenberg, J. in Kelly, S. and S. Malloy. 2014. (ed.) Temple University: Main Campus Landscape Metrics. Report for Seminar on Landscape Performance: Temple University Main Campus Landscape Master Plan.
McPherson, E. Gregory. 2010. Selecting Reference Cities for iTree Streets. Arboriculture & Urban Forestry 2010. 36(5): 230-240
Millennium Ecosystem Assessment Series. 2005. “Ecosystems and Their Services.” Ecosystems and Human Well Being: A framework for Assessment. Island Press.
Myers, M and Smith, D. 2015. “Keeping it Real: striving for accurate and appropriate use of tools to measure landscape performance” Abstract – Council of Educators in Landscape Architecture Conference Proceedings 2015.
Myers M, Yang, B, Smith, D., & Binder, C. 2014. “Defensible Metrics”. Abstract in the Council of Educators in Landscape Architecture Conference Proceedings. Morgan State University and University of Maryland, Baltimore, Maryland. March 26-39, 2014.
Philadelphia Water Department Regulations, July 10, 2015, http://www.phila. gov/water/wu/ratesregulationsresp/Pages/Regulations.aspx
Portland Public Improvements, 17.38.035 Drainage Management Polices and Standards. www.portlandonline.com/auditor/
Portland Stormwater Manual – January 2014. http: https://www. portlandoregon.gov/bes/64040
Wood, C., Sullivan B, Illiff, M, Fink, D., and S. Kelling. 2011. eBird: Engaging Birders in Science and Conservation. PLoS Biol 9(12): e1001220. (Editor / LI Min)
(USA)Mary Myers has PhD from Edinburgh College of ArtHeriot Watt University, UK, MLA from Harvard, and BSLA from University of Wisconsin. She is an Associate Professor of Landscape Architecture in the School of Environmental Design, Temple University. She has been involved in the Landscape Architecture Foundation Landscape Performance Series since its inception in 2011. She is interested in the potential of research to inform practice and of practice to inform research
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