3.3 What is a -Watershed-
3.4 What is the -Built Environment-
3.5 Ecosystem Services of Landscape Plants
3_Horticulture-and-Environmental-Sustainability
Fourth National Climate Change Assessment: Ecosystem Services
Google Earth Timelapse
Homegrown National Park
Millenium Ecosystem Assessment: Living Beyond Our Means
Milleniun Ecosystem Assessment: Ecosystems and Human Well-being
NOAA: Ecosystem Services
Tennessee Valley Wild Ones
The Native Plant Rescue Squad
The Southeastern Grasslands Initiative
UT Extension: TN Smart Yards
Horticulture and Environmental Sustainability
Overview
Title Image: “Trees” by Amanda Spangler, CC BY 4.0
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Introduction
Lesson Objectives
Describe and assess the influence of plants and their management on environmental sustainability and restoration.
Explain several guiding principles of sustainable site design.
Key Terms
built environment - the man-made or modified structures that provide people with living, working, and recreational spaces
ecosystems - biological communities of interacting organisms and their physical environments
ecosystem services - the benefits people obtain from ecosystems. These include services such as food, water, timber and fiber (provisioning); services that affect climate, floods, disease, wastes and water quality (regulating); services that provide recreational, aesthetic and spiritual benefits (cultural)
particulate matter - a mixture of solid particles and liquid droplets found in the air
watersheds - the land areas that drain to one stream, lake or river, which affects the water quality in the water body that it surrounds
Introduction
Humans depend on plants to live. The previous chapters have described how plants support humans by providing food to eat, fiber for our clothes, timber for our shelter, and oxygen for our very breath. However, we depend on plant life to meet many of our other vital needs, termed collectively as “ecosystem services”. How mankind interacts with the natural environment has a direct impact on humanity’s current and future ability to maintain healthy, comfortable lives on the planet.
What are “Ecosystem Services”?
Excerpt from "More About Ecosystem Services" by the USDA Forest Service is in the Public Domain
Ecosystem services are commonly defined as benefits people obtain from ecosystems. The Millennium Ecosystem Assessment – a four-year United Nations assessment of the condition and trends of the world’s ecosystems - categorizes ecosystem services as:
- Provisioning Services or the provision of food, fresh water, fuel, fiber, and other goods
- Regulating Services such as climate, water, and disease regulation, as well as pollination
- Supporting Services such as soil formation and nutrient cycling
- Cultural Services such as educational, aesthetic, and cultural heritage values, as well as recreation and tourism
As population, income, and consumption levels increase, humans put more and more pressure on the natural environment to deliver these benefits. The 2005 Millennium Ecosystem Assessment, prepared by a group of over 1300 international experts, found that 60 percent of ecosystem services assessed globally are either degraded or being used unsustainably. Seventy percent of the regulating and cultural services evaluated in the assessment are in decline. Millennium Ecosystem Assessment scientists predicted that ecosystem degradation could grow significantly worse in the first half of the 21st century, with important consequences to human well-being.
Climate change, pollution, over-exploitation, and land-use change are some of the drivers of ecosystem loss, as well as resource challenges associated with globalization and urbanization. Land use change is an immediate issue in the United States. Today, the Nation is experiencing a loss of open space and a decline in forest health and biodiversity, particularly on private lands. Approximately 57% of all forestland in the United States, or 429 million acres, is privately owned. Non-industrial interests are families, organizations, and communities that own land for the aesthetics and uses that forests provide or for income generated from the sale of forest products and services; they own 85% of our private lands. Recent trends in parcelization and divestiture of private lands in the United States suggest that private landowners are commonly under economic pressures to sell their forest holdings. Rising property values, tax burdens, and global market competition are some of the factors that motivate landowners to sell their lands, often for development uses. The loss of healthy forests directly affects forest landowners, rural communities, and the economy. As private lands are developed, we also lose the life-supporting ecosystem services that forests provide.
What is a “Watershed”?
Excerpt modified from "Benefits of Healthy Watersheds" by the United States Environmental Protection Agency is in the Public Domain
A watershed – the land area that drains to one stream, lake or river – affects the water quality in the water body that it surrounds. Like water bodies (e.g., lakes, rivers, and streams), individual watersheds share similarities but also differ in many ways. Every inch of the United States is part of a watershed; in other words, all land drains into a lake, river, stream or other water body and directly affects its quality. Because we all live on the land, we all live in a watershed, and this makes watershed condition important to everyone.
Watersheds exist at different geographic scales, too. The Mississippi River has a huge watershed that covers all or parts of 33 states. You might live in that watershed, but at the same time you live in a watershed of a smaller, local stream or river that flows eventually into the Mississippi. The EPA’s healthy watersheds activities mainly focus on these smaller watersheds.
What is a Healthy Watershed?
A healthy watershed is one in which natural land cover supports:
- dynamic hydrologic and geomorphologic processes within their natural range of variation, habitat of sufficient size and connectivity to support native aquatic and riparian species
- physical and chemical water quality conditions able to support healthy biological communities
Natural vegetative cover in the landscape, including the riparian zone, helps maintain the natural flow regime and fluctuations in water levels in lakes and wetlands. This, in turn, helps maintain natural geomorphic processes, such as sediment storage and deposition, that form the basis of aquatic habitats. Connectivity of aquatic and riparian habitats in the longitudinal, lateral, vertical, and temporal dimensions helps ensure the flow of chemical and physical materials and movement of living organisms among habitats.
A healthy watershed has the structure and function in place to support healthy aquatic ecosystems. Key components of a healthy watershed include:
- Intact and functioning headwater streams, floodplains, riparian corridors, biotic refugia, instream habitat, and biotic communities
- Natural vegetation in the landscape
- Hydrology, sediment transport, fluvial geomorphology, and disturbance regimes expected for its location
It is important to identify and protect healthy watersheds because, in many ways, healthy watersheds substantially affect the quality of life for people and the environment overall; often by performing ‘free work’ that communities do not have to do, or pay for, themselves. The beneficial roles of watersheds in healthy condition can be surprisingly far-reaching and include ecosystem services, economic benefits, and physical and mental health benefits.
Ecosystem Benefits and Services
Healthy watersheds provide many ecosystem services including nutrient cycling, carbon storage, erosion/sedimentation control, increased biodiversity, soil formation, wildlife movement corridors, water storage, water filtration, flood control, food, timber, and recreation, as well as reduced vulnerability to invasive species, the effects of climate change, and other natural disasters. These goods and services are essential to our social, environmental, and economic well-being.
The wide array of critical ecosystem services provided by healthy watersheds is frequently undervalued when making land use decisions. Due to the complexity of natural systems and economic precedents, it is difficult to assign a dollar amount to a particular ecosystem service. However, there is a large body of research and evidence to support the fact that intact healthy ecosystems avoid costly restoration and ecosystem service replacement, as well as provide long-term economic opportunities and jobs. Some healthy watershed ecosystem services are:
- Improved water quality. Natural landscapes and floodplains filter pollutants from point and nonpoint sources, promote nutrient cycling, and help retain sediment.
- Carbon storage opportunities. Watersheds with intact natural land cover and soil resources are capable of sequestering carbon, thereby offsetting greenhouse gas emissions (Hanson et al., 2010).
- Increased resilience in the face of climate change threats. Intact floodplains and riparian areas enable healthy watersheds to be better adapted to more extreme weather patterns and changes in precipitation associated with climate change.
- Reduced risk for invasive species colonization. Naturally functioning ecosystems are more resilient and can favor indigenous species, helping them out-compete invasive species.
Protecting healthy watersheds can reduce capital costs for water treatment plants and reduce damages to property and infrastructure due to flooding, thereby avoiding future costs. Additionally, protecting healthy watersheds can generate revenue through property value premiums, recreation, and tourism:
- Reduced drinking water treatment and infrastructure costs. Natural landscapes filter pollutants and protect water quality. A review of treatment costs and watershed characteristics for 27 drinking water utilities found that for every 10% increase in forest cover of the source water area, chemical and treatment costs decrease by 20% (Ernst, 2004). In a separate case, New York City found it significantly more cost-effective to protect the watershed’s natural land cover and forests to provide natural filtration, rather than installing a multi-billion-dollar water treatment facility (Barnes et al., 2009).
- Reduced flood mitigation costs. Floodplains and natural landscapes minimize the area and impacts of floods, reduce the burden on public drainage infrastructure, and increase groundwater recharge (Postel & Richter, 2003).
- Increased revenues and job opportunities. Healthy watersheds provide ample opportunities for fishing, boating, swimming, hiking, biking, wildlife viewing, and ecotourism. Over 30 million people in the U.S. fish recreationally and these anglers generate approximately 1 million jobs and over $45 billion in retail sales annually (Southwick Associates, 2008). Overall, the outdoor recreation industry contributes $646 billion annually to the economy, supports 6.1 million jobs, and generates $79.6 billion in federal and state tax revenues (Outdoor Industry Association, 2003).
- Increased property values. Housing near healthy watersheds has higher property values than those in or around degraded ecosystems and impaired waters (Maine DEP, 2005).
What is the “Built Environment”?
Excerpt modified from “Our Built and Natural Environments” by M.G. Kramer, United States Environmental Protection Agency is in the Public Domain
While many people tend to think of themselves as living separately from nature, humans are members of their local ecosystems, often residing in what can be called the “built environment”. According to the EPA, “The built environment touches all aspects of our lives, encompassing the buildings we live in, the distribution systems that provide us with water and electricity, and the roads, bridges, and transportation systems we use to get from place to place. It can generally be described as the man-made or modified structures that provide people with living, working, and recreational spaces.” While the built environment allows humans to live, work, and play in relative comfort, historically, not much thought has been given to the displaced or absent ecosystem services that plants and other members of the natural environment had provided.
Although development provides many social and economic benefits, it also comes at a cost. Development has seriously degraded or destroyed many natural areas and caused significant growth in automobile driving; both of which have impacts on the health of critical environmental resources, as well as on people. These environmental consequences are particularly important because the effects of development are long lasting and not easily reversible. As a result, the cumulative effects of development decisions are important when considering the long-term health of the environment and communities.
The research summarized in this section outlines how significant the impacts have been to the natural environment – to critical habitat for plants and animals, to water resources, to air quality, and maybe most importantly, to the planet itself in the form of global climate change.
Habitat loss, Degradation, and Fragmentation
Habitat loss, degradation, and fragmentation are some of the most direct impacts of development on previously undeveloped land. Construction of new buildings, roads, and other infrastructure often destroys native vegetation. Landscaping in the remaining open space with new lawns and non-native plants often cannot serve the same ecological functions as the vegetation it replaces.
The impact of a non-native species depends on its ecological context, and many non-native species provide important ecological benefits (Davis et al., 2011; Simberloff, 2011). One fundamental role of plants in an ecosystem is to create food for herbivores that can transfer their stored energy to higher-level predators. However, homeowners and landscapers have often chosen non-native species for their resistance to insects. In fact, most insect species lack the physiological and behavioral adaptations needed to use non-native plants for food. If ornamental plants cannot serve as food for the same number and diversity of herbivores, the energy available for food webs decreases (Tallamy & Shropshire, 2009).
Degradation and Loss of Water Resources
As water resources are polluted and degraded, they can become unfit for drinking, swimming, fishing, and other uses. However, water resources can also be strained if our use exceeds the available supply. About 86 percent of U.S. households rely on public water supplies for their household use. About one-third of the water from public water supplies comes from ground water, and two-thirds comes from surface water, such as lakes and streams (Kenny et al., 2009).
For households that supply their own water, 98 percent rely on ground water (Kenny et al., 2009). Ground water use can exceed the rate at which precipitation soaking into the ground can replenish it, leading to ground water depletion. Dry wells, reduced amounts of water in streams and lakes, lower water quality, and land subsidence can result (U.S. Geological Survey, 2003). Impervious surfaces created by development (Frazer, 2005) and centralized wastewater treatment (Vaccaro & Olsen, 2007) can decrease rates of ground water recharge, exacerbating the effects of increased demand on ground water supplies.
Degradation of Air Quality
Land use, development, and transportation affect air quality in significant ways. For common air pollutants, EPA has established and regularly reviews National Ambient Air Quality Standards to protect public health and the environment. In setting or revising primary health-based standards, the Agency considers the effects of poor air quality on at-risk populations, such as children and the elderly (EPA, 2010). EPA has set standards for six principal pollutants—“criteria pollutants”: carbon monoxide, nitrogen dioxide, sulfur dioxide, lead, coarse particulate matter (PM10), fine particulate matter (PM2.5), and ozone (EPA, 2012). VOCs and NOX are precursors to the formation of ozone (Sawyer, 2010).
Heat Island Effect
Not only do impervious surfaces create water quality problems, they also affect the temperature of surrounding areas through what is known as the heat island effect. Cities can be as much as 6 to 8 degrees Fahrenheit warmer than outlying areas (Frumkin, 2002). The heat island effect is due to two complementary forces: dark pavement and roofs absorb and reflect more of the sun’s heat, while the relative scarcity of trees and other vegetation reduces shade and cooling through evapotranspiration. Increased heat is itself a health hazard, as heat stroke can lead to hospitalization and even death.
Greenhouse Gas Emissions and Global Climate Change
How much energy our buildings consume and how much people drive both affect greenhouse gas emissions, making the built environment an important contributor to global climate change. A 2011 report by the National Research Council of the National Academies concluded that global climate change is occurring and is largely due to human activities that lead to heat-trapping greenhouse gas emissions (National Research Council of the National Academies, 2011). The scientific academies of over 30 countries (Transportation Research Board of the National Academies, 2012) and the Intergovernmental Panel on Climate Change reached the same conclusion based on contributions from thousands of scientists and an extensive review process (Intergovernmental Panel on Climate Change, 2007).
Our built environment affects climate change, but it is also affected by climate change. Changes to the water cycle mean that both floods and droughts are more common, putting lives and property at risk, stressing water infrastructure, and changing the amount and quality of water available for human use. Warmer weather increases energy use for cooling and decreases it for heating, changing the levels and periods of peak demand. Floods, extreme heat, and sea-level rise put transportation infrastructure, such as roads and rail lines, at risk and increase the chances of travel and freight delays and disruptions (U.S. Global Change Research Program, 2009).
Land Contamination
Past industrial activity has left a legacy of soil and water pollution at former industrialized sites. Thousands of these potentially contaminated properties, or brownfields, are located in densely populated neighborhoods, often near places where residents gather and children play. Many of these sites are near rivers that once served as valuable transportation corridors. The juxtaposition of toxic chemicals, human activity, and sensitive environmental habitats can lead to a range of problems, including compromised human and environmental health.
Health and Safety
As already discussed, the built environment’s effects on human health extend beyond exposure to air and water pollution or global climate change. How we build our communities affects health and safety in several ways. The built environment affects levels of physical activity, obesity, and chronic disease. It also influences our emotional health and the degree of engagement in our communities. Finally, how we design our streets and towns affects the likelihood of being hurt or killed in a vehicle crash.
Summary
As the U.S. population has grown, we have developed land that serves important ecological functions at a significant cost to the environment. Development has destroyed, degraded, and fragmented habitat. Water quality has declined. Air quality in many areas of the country is still adversely affecting human health. The heat island effect and global climate change illustrate just how complex and far-reaching the impacts of our built environment are. Community design can make it difficult for people to get adequate physical activity, engage with neighbors, and participate in community events. It can also increase the risk of injury or death from a vehicle crash.
Ecosystem Services of Landscape Plants
Excerpts used with permission from "Ecosystem Services of Landscape Plants" by J. Knight & D.L. Ingram, University of Kentucky Cooperative Extension Service. Copyright © University of Kentucky Cooperative Extension.
As green industry professionals, we know that landscape plants do a lot for us. Though this is easily stated, it may not be easy to precisely describe or quantify the contributions of landscape plants to ecosystem services. There is no definitive list of ecosystem services nor a single definition for the concept, but for the sake of this publication we will be using the working definition of the Millennium Ecosystem Assessment, an inter-governmental report involving the work of over 1,360 experts worldwide to assess the consequences of ecosystem change for human well-being: “Ecosystem services are all benefits to humankind provided by ecosystems.”
Types of Ecosystem Services
Woody landscape plants provide us with numerous valuable ecosystem services, including improvement of air quality, increased cultural and aesthetic value, biodiversity potential, carbon sequestration, energy conservation and microclimate regulation, improvement of human health, noise attenuation/ reduction, and stormwater management. Green industry professionals must express to their customers the ability of landscape plants to improve green infrastructure.
Air Quality
Back in the 1800s, parks, habitats for trees and other landscape plants, were referred to as the “lungs of cities” by Frederick Law Olmsted, considered the “Father of American Parks.” Air pollution is a significant risk factor for a number of health conditions, including respiratory infections, heart disease, stroke, and lung cancer. The human health effects of poor air quality are far reaching. The most common sources of air pollutants include particulate matter, ozone, nitrogen dioxide, and sulfur dioxide. Indoor air pollution and urban air quality are listed as two of the world’s worst toxic pollution problems in the 2008 Blacksmith Institute World’s Worst Polluted Places report. Leaves contribute to the removal of pollutants from the air, and it is important to develop landscapes with leaf growth at multiple layers using shrubs, herbaceous perennials, and dwarf and standard trees.
A few ways that plants reduce air pollution:
- Absorption of gaseous pollutants (e.g. ozone, nitrogen oxides, and sulfur dioxide) through their leaves
- Reduction of ozone concentrations at ground level by reducing temperatures via evapotranspiration and shading
- Fuzzy leaves are much more effective in capturing particulate matter than smooth/hairless leaves
- Collection of dust, ash, pollen, and other particulate matter on their leaves, reducing its presence in the air breathed
In 2006 the city of Los Angeles started an initiative to plant one million new trees. A comprehensive study of this initiative estimates that, depending on the rate of tree mortality, these trees will save the citizens of Los Angeles between 53 and 78 million dollars in healthcare costs alone over the next 35 years.
A research series in 2010 by the Virginia-based National Recreation and Park Association published a list of trees associated with improving air quality based on their tolerance for specific air pollutants and their potential for removal of those air pollutants. Plants from that list with a high tolerance of pollutants and the potential for removal of various air pollutants are provided in Tables 1, 2, 3 and 4 in the “Links to Learning” section.
Cultural and Aesthetic Value
Adding plants to a landscape increases property values. Good tree cover can raise the total sale price by 6 to 9 percent (Morales, Boyce & Favretti, 1976), and the mere presence of trees may add a 3 to 5 percent premium to the sale price of a property (Anderson & Cordell, 1985). Hedges or landscaped walls raises the sales price 4 percent (Des Rosiers et al., 2002). In 2003 interviews with realtor associations advised that “spending 5% of the value of your home on the installation of a quality, low-maintenance landscape increased resale values by 15%, which translates into a 150% return on the landscape investment” (Taylor, 2003). A low-maintenance landscape is an uncrowded, simple landscape design that is not labor intensive. General characteristics include fewer grassy areas, often offset by hardscaping, mulched beds, and locally adapted, hardy perennial plant material. A recent study in Toronto found that “having 10 more trees in a city block, on average, improves health perception in ways comparable to an increase in personal income of $10,000 or being 7 years younger” (Kardan, 2015).
Biodiversity Potential
Biodiversity is the variety of life on earth. Biodiversity allows ecosystems to adjust to disturbances. Ecosystems that can withstand disturbance are said to be resilient. Genetic diversity prevents and/or limits the impact of diseases and helps species adjust to changes in their environment. Most medical discoveries to cure diseases and lengthen life spans have been made because of research into plant biology, animal biology, and genetics. Healthy, native landscape plants when used intentionally to develop ecosystems will protect the biodiversity of local communities and provide habitat for local wildlife.
This wildlife can include butterflies and songbirds. Native insects attracted to native plants support the dietary requirement of native song birds. Butterflies are attracted to species of flowering plants based on the seasonality of flowering, while songbirds are attracted to trees based on height and other growth characteristics, emphasizing the importance of plant variety when creating ecosystems. A study in 2009 across several pairs of suburban properties in southeast Pennsylvania showed that bird species of regional conservation concern were eight times more abundant and significantly more diverse on properties with native landscaping in the built environment (Burghardt, 2009).
Urbanization can contribute to loss of biodiversity through habitat destruction and the homogenization of ecosystems (Alvey, 2006). Landscaping with a diversity of plant material helps offset the negative impacts of urbanization on biodiversity by providing for habitat for many species that would otherwise be displaced (Dearborn, 2010).
Carbon Sequestration
Increased concentrations of atmospheric carbon dioxide contribute to the increase in average global temperature and disruption of climates around the world (Working Group I and Richard Alley 2007). Carbon dioxide concentrations in the atmosphere have been increasing rapidly since the industrial revolution, primarily from the use of fossil fuels but also from changing land uses. Each person, product, and activity emits carbon dioxide and other greenhouse gases such as methane and nitrous oxide, into the atmosphere. The potential impact of those emissions on global warming is called the carbon footprint of that product or activity. We each have a carbon footprint that has a negative impact on the atmosphere. Such human activity can be offset by carbon sequestration by woody plants and soil carbon storage.
Carbon sequestration is the process of capture and long- term storage of atmospheric carbon dioxide. In the context of woody landscape plants, carbon sequestration is a function of photosynthesis: the plant builds itself by taking the carbon from carbon dioxide in the atmosphere. At maturity, approximately 50 percent of an individual tree or shrub’s dry biomass is carbon, depending upon the density of the wood. When a landscape plant dies and is replaced, burial of the old tree represents the easiest method to ensure long-term storage in soil of most car- bon captured by the tree. Green spaces are planned and large trees are planted in public rights of way, parks, and other open spaces to increase the availability of “carbon sinks,” which are resources that serve to reduce the amount of carbon dioxide in the atmosphere (McPherson, 2005).
The Intergovernmental Panel on Climate Change, an international body for assessing science related to climate change, established a 100-year assessment period as a standard for determining the carbon footprint of products and processes. We can express carbon sequestration by a woody plant in terms of the amount of carbon held from the atmosphere each year of the 100-year assessment period. When weighted for a portion of a 100-year assessment period, it has been estimated that a deciduous shade tree (Acer rubrum) in the suburban landscape can reduce the potential global warming impact from carbon dioxide by 670 kg CO2, after accounting for emissions during production and take-down at the end of life. Published impact data on atmospheric carbon weighted annually for their functional life estimated that red maple, flowering deciduous tree (redbud), evergreen tree (blue spruce), evergreen shrub (Taxus), and deciduous shrub (Viburnum) in the lower Midwest reduce CO2 in the atmosphere by an estimated 666, 430, 63, 9, and 11 kg CO2 over their lifetime, respectively (Figure 9.3.3). These estimations do not consider the long-term carbon storage in plant roots, which has not been quantified at this time but could be substantial for some plants.
After woody plants are taken out of the landscape, their utilization has an effect on projected carbon sequestration. Smaller plants and many trees are typically chipped for use as mulch or soil conditioner. The carbon in this mulch will be released into the atmosphere over a one- to three-year period. Chipping is the most common end of life for an urban tree in the eastern U.S., and this end was assumed when calculating the values in the Figure 9.3.3.
Denser and more valuable species may be used as firewood or as lumber in small construction. The utilization of urban trees for wood and paper products is still in its infancy, but the idea is drawing “increasing attention from researchers, community officials, arborists, tree care firms, and wood-using industries including bio-energy producers” (Bratkovich, 2008). A 1994 national inventory of urban tree residues included a survey of tree care firms, municipal/county park and recreation departments, municipal tree care divisions, county tree care divisions, electric utility power line maintenance, landscape maintenance/landscaper/nursery firms, and excavator/land clearance firms. The U.S. nationally produced an estimated 38 million green tons (25 million tons on a dry basis) of urban tree residues. Only 25 percent of this residue was reported as recycled or sold/used for a product, and 70 percent of the residue was given away, landfilled, or left on site. A 2003 report from the USDA Forest Service’s Forest Products Laboratory estimated that in 2002 urban wood residues in the municipal solid waste stream alone totaled 16.2 million tons of chips, logs, stumps, tree tops, and brush; 9.3 million tons recovered for compost and mulch, 1.9 million tons were sent to combustion facilities, 1.7 million tons were considered unusable, and more than 3.5 million tons were used as “good wood” for further processing into products.
Use of urban trees for bio-energy in a residential wood stove or for large scale energy production is ultimately preferable to the use of fossil fuel sources for similar purposes, as the carbon sequestered is “young” and therefore closer to being a neutral impact on climate change when compared to fossil carbon stores, the impact of which can exceed 650 million years. Already cities are taking advantage of the synergy between bio-energy’s benefits, its demands in urban areas, and the availability of urban trees. In downtown St. Paul, Minnesota, less than a mile from the State Capital building, District Energy St. Paul operates a combined heat and power plant serving the commercial, industrial, and residential downtown area. A steam-powered turbine generates 25 megawatts of electricity for the grid, and waste energy—heat energy not converted to electricity by the turbine—created in the process is used to heat the downtown area. The multi-fuel plant is capable of burning coal, natural gas, or biomass in the form of wood chips. It consumes 300,000 tons of wood chips per year, which provide 60 percent of its fuel. Considering the estimated volume of urban tree removals nationwide—17 million tons annually—the magnitude of bio-energy potential from urban tree removals to generate renewable energy should not be overlooked.
Stormwater Management
A plant’s leaves and branches create a crown. The crowns of many plants together make up an urban forest’s canopy. Unless a storm is particularly intense or occurring in a location without significant canopy cover, most of the rain hits a leaf or branch surface and remains there, before evaporating or falling to the ground. Root systems provide channels for water infiltration into urban soils. As water moves through soil layers it is filtered for contaminants, putting less pressure on filtration systems and improving water quality downstream. This brief storage of rainwater by the plant is called rainfall interception, which is primarily dependent on the type and amount of leaves (Figure 9.3.4).
Mature deciduous trees can intercept 500 to 700 gallons of water per year. Mature evergreen trees can intercept more than 4,000 gallons per year (Capiella, Schueler, & Wright, 2005). Canopy cover over impervious surfaces (concrete, asphalt) has a profound effect on runoff, as most runoff is a product of impervious surfaces. Even tree cover over pervious surfaces such as soil and turf reduces total runoff by as much as 40 percent (Sanders, 1986). Though some water eventually reaches the impermeable surfaces of the streets and runs into the stormwater infrastructure, slowing the water increases the capacity of existing infrastructure to handle water. The stormwater infrastructure is limited primarily by its capacity to handle water during peak precipitation events. Trees and green infrastructure have a leveling effect on these peaks, ultimately augmenting the overall capacity of stormwater handling.
The costs of upgrading conventional stormwater management infrastructure are often prohibitive for many municipalities and in some cases result in diminished returns, especially when compared to the cost and capacity for green infrastructure to manage stormwater. When analyzing the benefits provided by individual urban trees, drawing on data from i-Tree, stormwater management often represents the greatest economic return on investment. Expanding stormwater infrastructure in developed areas is expensive for municipalities, often requiring a bond and interest payments. Though these are costs that would be generally be covered by government funds (whether local, regional or federal), eventually the funds would be recovered by individual taxpayers.
Landscape plants used in combination with depressions in the landscape can improve the reduction in stormwater runoff by increasing water infiltration and evapotranspiration. Rain gardens, bioretention basins, or bioswales are increasingly constructed as part of green infrastructure in urban areas. In 2009, the city of Seattle developed the Thornton Creek water quality channel, a 2.5-acre facility of constructed landscape and native species plantings which slows and filters stormwater runoff from the largest watershed in the city (Figure 9.3.5).
The facility is a public, open space that is integrated into adjacent private development, which is also highly functional green infrastructure. The channel removes sediments and associated pollutants from 91 percent of the annual runoff from the 680-acre drainage area before it is released into Thornton Creek.
Summary
Landscape plants provide many critical services to people and our built environments, improving land value, health, comfort and overall quality of life. If planted in 2015, after 20 years of age, a single, healthy red maple placed 25 feet from the southwest corner of a climate-controlled structure would intercept 44,028 gallons of water and save the community $273 in stormwater reduction costs. Over the course of its life, it will actively remove NOx, CO, and particulate matter from the air, valued at $18 in savings to air quality, and reduce contributions of atmospheric carbon by 9,766 pounds through sequestration, decreased energy production needs, and emissions.
Through education efforts, advertising, and promotion, green industry professionals can help consumers understand the value of adding landscape plants to urban environments, thereby increasing the demand for green industry products and services.
Dig Deeper
To see how the built environment has changed in your area, enter your school or city’s address into Google Earth’s Timelapse program.
In this short video, the National Oceanic and Atmospheric Administration provides an easy-to-understand explanation about the economic principles involved when evaluating ecosystem services. According to the economist, just because something is free doesn’t mean it has no economic value. And because ecosystem services are free, the economist has come up with the tools needed to figure out how much these services are worth. Learn more from this Econ 120: Ecosystem Services video by clicking below or visiting the NOAA website.
Specialized relationships between animals and plants are the norm in nature rather than the exception. Plants that evolved in concert with local animals provide for their needs better than plants that evolved elsewhere. In this video from Tennessee Wild Ones, Dr. Tallamy will explain why this is so, why specialized food relationships determine the stability and complexity of the local food webs that support animal diversity, why it is important to restore biodiversity to our residential properties, and what we need to do to make our landscapes living ecosystems once again. To learn more, watch the video below or visit the Tennessee Wild Ones YouTube Channel.
The state of Tenenssee, as well as much of the Southeast region, was once covered in grasslands. According to the Southeastern Grasslands Initiative (based at Austin Peay State University), much of the original landscape—so important to indigenous people and the earliest settlers—has been lost. But not entirely lost! Today, even the last remaining 10% of our native Southeastern grasslands are America’s most biodiverse. SGI’s 23-state focal region contains the world’s 36th recognized biodiversity hotspot! Beyond that, our native grasslands remain vitally important for their contributions to water quality, soil health, carbon storage, protection in drought, grazing lands, and wildlife and pollinator habitat. To learn more, watch the video below or visit the Southern Grasslands Initiative website.
Tennessee Smart Yards is an Extension-led program that guides Tennesseans on practices they can apply in their outdoor spaces to create healthier, more ecologically-sound landscapes and communities. Nine principles of stewardship serve as the foundation for the program and are explored in online modules and practical workshops taught by UT-TSU Extension and water resource professionals. To learn more, check out the video below and visit the Tennessee Smart Yards website.
Homegrown National Park® is a grassroots call-to-action to regenerate biodiversity and ecosystem function by planting native plants and creating new ecological networks. To learn more, check out the video below or visit the Homegrown National Park® website.
The Native Plant Rescue Squad is a 501(c)(3) organization, educational organization dedicated to the conservation and restoration of the rich diversity of native flora in East Tennessee. They work with builders, developers and land owners, to rescue native plants that would otherwise be destroyed during building and development. They then make the rescued plants available for residential and commercial landscaping, with the goal of increasing the overall ecological health of our region. To learn more, watch the video below or visit the Native Plant Rescue Squad website.
Attribution and References
Attribution
Excerpt from "More About Ecosystem Services" by the USDA Forest Service is in the Public Domain
Excerpt modified from "Benefits of Healthy Watersheds" by the United States Environmental Protection Agency is in the Public Domain
Excerpt modified from “Our Built and Natural Environments” by M.G. Kramer, United States Environmental Protection Agency is in the Public Domain
Excerpts used with permission from "Ecosystem Services of Landscape Plants" by J. Knight & D.L. Ingram, University of Kentucky Cooperative Extension Service. Copyright © University of Kentucky Cooperative Extension.
Title Image: “Trees” by Amanda Spangler, CC BY 4.0
References
Alvey, A.A. (2006). Promoting and preserving biodiversity in the urban forest. Urban Forestry and Urban Greening 5.4: 195-201.
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