Trees: The Oldest New Thing in Stormwater Treatment?
How much do tree canopies really affect runoff volume?
Expressed simply, stormwater managers face two main challenges: (1) channel the water so it doesn't flood homes, property, or city streets and (2) make sure the water is reasonably clean before it flows into natural streams, rivers, and lakes.
Traditionally, stormwater has been channeled through storm drains, sewers, concrete drainage ditches, and the like. These manmade structures do the job, but they not only require long, expensive public works projects for their installation, they also have to be maintained.
In the Ditch
Drainage ditches or sluices in particular can cause safety and health problems. Because hard rains can fill drainage ditches with deep, rushing water, municipalities often fence the areas to keep people out. Yet despite their best efforts, a number of children die in drainage ditches each year. Water can pool in ditches during moderate rains, creating a breeding ground for foul-smelling algae at best, mosquitoes at worst. Unfortunately, the ditches often also become repositories for illegal trash dumping. Taking these possible negatives into consideration, it's no wonder homeowners don't want drainage ditches, however needed, in their backyards.
In the past, cleaning stormwater wasn't an issue. Cities with combined sewer systems treated the sewage, and stormwater runoff was cleaned in the process. Those with separate systems didn't send stormwater to wastewater treatment plants–although now some cities are investigating that possibility–but the National Pollutant Discharge Elimination System (NPDES) and other regulations are setting increasingly stringent standards for treating stormwater runoff in some fashion before it reaches receiving surface waters.
Before humans changed the landscape, nature had its own way of dealing with stormwater; it soaked into the soil, nurturing the plant life. Stormwater overflows would create floodplains and wetlands. This process still works well, where it's allowed; however, add homes, businesses, and traffic grids to a landscape, with the resultant impervious surfaces, and "natural" drainage quickly becomes a problem. As stormwater managers seek to re-create or maintain predevelopment hydrology and to treat runoff–often employing created infiltration systems, vegetated swales, riparian buffers, and the like–many are taking a close, new look at the role of trees in stormwater diversion and treatment.
Depending on the species and the soil conditions (both the type of soil and its rain-saturation level), trees can absorb a considerable amount of water. Also, water-polluting nitrates, phosphorus, and potassium, which in many areas are spurring the development of total maximum daily loads (TMDLs) for receiving waters, are readily absorbed by trees, which consider these substances food.
One Man's Tree Is Another's Treasure
Just how much can trees do to help? According to the American Forests organization (www.americanforests.org), a healthy tree canopy can tremendously reduce stormwater runoff, saving its host city millions of dollars in infrastructure costs.
To quantify these benefits, American Forests has developed modeling software to plot vegetation's slowing effects on stormwater. The Windows-compatible CITYgreen software offers modeling capabilities that allow users to compare economic benefits of various site plans by analyzing a site's ecosystem and producing data on a number of factors: stormwater runoff, air quality, summer energy savings, carbon storage/avoidance, and tree growth. The software can also create ecological maps noting the value of available resources, as well as models for projecting future growth.
"It's not a stand-alone program," explains American Forests Senior Geospatial Analyst Kenneth Gorton, who is also CITYgreen's lead programmer. "It has to be used in conjunction with ArcView GIS [geographic information system], which was developed by ESRI. ArcView gives you tools for capturing a data set so you can use CITYgreen."
Start With a Bird's-Eye View
To begin your calculations, CITYgreen must know which area you're investigating. "For example, you'd open an aerial photo in ArcView and use the tool to select your area, which produces a data set that ArcView and CITYgreen can read. CITYgreen digitizes the image, then reports, ‘There are so many trees here, in x square feet,'" Gorton explains.
Pictures used in the CITYgreen program must be orthorectified–given the geometric qualities of a map–before the software will work. The image also must be registered with coordinates from an aerial survey company, perhaps one with LandSat cooperation. If contracting with an aerial-photo company is cost-prohibitive, Gorton notes that ArcView also will work "at least with the more complicated GPS systems, if you can walk around with the GPS and ‘trace' the area of interest."
Whichever mapping system is used, Gorton cautions that the software works best with a 1-ac. minimum area. "It would work with a smaller area, but results are so miniscule," he says. Once the area in question has been entered into the software's database, CITYgreen concentrates on calculating what the trees actually do for stormwater runoff.
"The data are modified from the USDA's TR-55, the Natural Resources Conservation model, which estimates stormwater runoff. This public domain information all comes with the software," Gorton adds. "Each type of land cover and soil has an assigned curve number. The higher the curve number, the higher the expected runoff." The model doesn't take soil saturation or water tables into account; an engineer would need to provide that information if it's needed for projections.
What Covers the Land?
 |
| CITYgreen software can create ecological maps. |
"CITYgreen doesn't measure moisture use by each tree," Gorton continues. "It only takes advantage of data that talk about land cover–the total collective land cover for a particular site, whether it be trees, shrubs, grass, crops, pavement, or buildings. The software compares land cover, the soil complex, and ratio of rainfall to determine projected runoff. In general terms, the thicker the vegetation on a site, the more the water is inhibited."
Gorton stresses that the software helps most when used during a site's planning stage. "The TR-55 data are for planning purposes [and] not intended to design plantings for a site. CITYgreen can help developers make informed decisions on planning their community. Our hope, of course, is that they will plant more trees.
"When planning a new development–a strip mall, for example–most cities won't allow you to create more runoff than existed before you built. CITYgreen won't tell you where to put trees; it will tell you what will happen if you take trees down. It gives you an estimate of what will happen when you make certain changes to the site. How you use the information is up to you. It can be a reality check for making your construction proposal," Gorton says.
According to American Forests, the software has been used to help find the best strategy for erosion and fire suppression in California's Santa Monica Mountains. Dade County, FL, has used the software to project the energy and stormwater runoff savings derived from replacing trees lost to Hurricane Andrew. Various schools and universities use CITYgreen in forestry, environmental studies, landscape architecture, and urban planning courses.
A Garland of Trees for a Texas Town
In 2000, American Forests published a study in which CITYgreen was used to determine the value of the tree canopy of Garland, TX. The study, available on American Forests's Web site, reported that current tree cover was saving the city $5.3 million a year, a figure that included residential energy savings, runoff reduction, and air pollution removal.
American Forests analyzed 10 sites in Garland: two residential plots from each size designation of "small" (approximately 3,500 ft.2), "medium" (7,000 ft.2), and "large" (11,000 ft.2) (see Table 1); two commercial plots; and two industrial plots.
| Table 1. Residential Test Plots |
Plot Size | Acreage | House Density | Canopy Cover (%) |
Small | 4.87 | 18 | 19 |
Small | 5.34 | 20 | 7 |
Medium | 3.86 | 10 | 8 |
Medium | 6.05 | 25 | 43 |
Large | 4.61 | 14 | 4 |
Large | 8.36 | 8 | 17 |
The study determined that increased tree cover could save even more. For example, a medium-size, 3.86-ac. residential site, with its 8% canopy cover, provided a 3% runoff reduction. If the site's tree canopy were increased to 35%, runoff reduction would quadruple, to 12.8%; a canopy cover of 45% would bring that number to 16.1%. (As a rule, American Forests recommends that cities maintain a 40% tree cover.) The study, however, did not recommend how a site with such home density would accommodate the resultant roots that would accompany the increased tree canopy.
Concrete: Can You Dig It?
Homeowners wanting immediate savings could always take a jackhammer to their patios; Garland's stormwater utility fee is based upon the amount of a property's impervious surfaces. Property owners are billed 6 cents per 100 ft.2 of impervious area. At present, no exemptions are made for porous pavement.
Philip Welsch, manager of Garland's Municipal Stormwater Quality Program, admits concrete and asphalt are major causes of stormwater problems. "We not only have lots of streets, but many neighborhoods also have alleys behind the homes. In addition, the width of our sidewalks is dictated by the Americans with Disabilities Act. Tearing up or narrowing some of the concrete would help immensely, but present zoning will not allow it. For example, our streets are wide because, despite having garages and driveways, people here insist on parking on the street, and the fire department insists on having enough clear width for their vehicles." The soil below doesn't help either: "This area is filled with heavy clay soils, plus some caliche," Welsch notes.
Part of the reason for the city hosting the American Forests study was to illustrate the benefits of the Garland's tree ordinance. "We wanted to demonstrate to EPA that this is a postdevelopment control and they should be happy with it," Welsch says. "Some Texas cities, trying to avoid some of these stormwater rules, have 10th Amendment suits going to the US Supreme Court."
The study noted, "Some drops of water that land on leaves and branches will evaporate before reaching the ground." Welsch concedes that might be the case but, he says, "Down here, the greatest percentage of our rains come in the winter, when there are no leaves on the trees.
"We want to hold onto the total canopy we have," he explains. "If a developer knocks down a tree, he has to replace it by total caliper inches. If he takes down a 10-caliper-inch tree, for example, he has to put in two 5-caliper-inch trees, 10 1-inchers, or whatever totals up to the tree removed. Developers in the new area north of the city are leaving more trees intact because they realized they can get more money for the lot if they do."
Take Two Trees and Call in the Morning
Lexington, KY, is renowned for two things: basketball powerhouse University of Kentucky and the lush, rolling bluegrass hills of thoroughbred horse farms. As population influx driven by the former puts pressure on the latter, the combined city-county government has taken steps to restrict growth in certain areas. It's also using trees to help control stormwater flow and pollutants.
"We call it ‘Reforest the Bluegrass,'" says Dave Leonard of the Kentucky Arborists Association. "We'd like to have 50% forest canopy in the county."
David Swenk, now in planning and development for the County of Santa Barbara, CA, helped develop the project during his years with Kentucky's Division of Forestry. "Traditionally stormwater has been a matter of moving water from point A to point B–no one worried about its quality, which increased someone else's problems downstream. Most of the pollutants in Lexington were fecal, from the agricultural areas–all those horses and cattle. Trees were seen as a way we could absorb water, improve the water quality, and get rid of pollutants. The feds think that's a [best management practice]," Swenk says.
Lexington took to the idea in a big way. "We got the community involved. Thousands came out to help plant–forestry students, environmental groups, Scouts, and ‘just plain folks.' Starting in 1999, we made it into an annual event. We didn't use any tax dollars. We got grants, plus donations from local companies such as Lexmark. Between 25,000 and 45,000 trees were planted each year," he reports.
Swenk sees natural watercourses, with trees used as biofilters, as the better way to channel stormwater. "Cities need to think about their green infrastructure," he says. "It would be great to remove concrete flumes, make more natural watercourses such as tree-lined creeks. Concrete doesn't help water quality; trees do."
Trees: A Long Drink of Water
How much water can a tree process? Horticulturists note that trees' weekly water needs equal 5 gal. plus 5 gal. per caliper inch. For example, a 2-caliper-inch tree needs 15 gal. (5 + [5 x 2] = 15) weekly. This calculation, of course, is for minimum needs; many trees can take in more water.
"A mature bald cypress can absorb 880 gallons per day, depending on the soil type and saturation," Swenk says, though admits it's extremely difficult to retrofit a neighborhood with tree-based stormwater controls. "Plan for detention and it can be retarded by your design. Plant trees that will help absorb stormwater." He suggests a number of trees that can work for stormwater purposes (Table 2). Most of these species thrive in the Midwest.
| Table 2. Recommended Tree Species |
American Basswood | Hackberry | Serviceberry |
American Elm* | Osage | Orange Silver Maple |
Black Cherry | Paw Paw | Swamp White Oak |
Black Gum | Persimmon | Sweetgum |
Black Walnut | Red Maple | Sycamore |
Boxelder | Red or Pin Oak | Tulip Poplar |
Green Ash | River Birch | Willow |
| * Disease-resistant |
He notes that Santa Barbara has instated a conservation subdivision design. "Here 40% of the subdivision has to be open space. You need to cluster your design to manage stormwater. Some use bioswales, which are like a reconstructed marshland/wetland." In the area's sandy soil, Swenk likes to use sycamores for habitat planting.
Will developers lose money by putting in these riparian zones? "No, they can charge more for the lot, plus save on storm sewer costs," Swenk maintains. "In a conventional plotting [straight lines of houses], it costs up to $560,000 to put in sewer pipes. Use a cluster design, and costs can drop to about $339,000. Do a plan with trees and a wetland, and that cost can drop to $244,000."
Any legal problems with water-filled riparian zones? "Homeowners are sometimes resistant to it if, for example, it's designed like a nature trail, because of wildlife and hikers that can wander into their yards. But would they rather have a concrete flume in their backyard or a creek area? A natural watercourse does so much–it looks better, and it costs pennies on the dollar versus traditional methods," Swenk points out.
The concept of using trees to aid in stormwater control and treatment is somewhat of an uphill climb. "Planners knew nothing about environmental planning, and they don't talk with engineers; engineers used to think of trees just as obstructions," Swenk relates. "But more and more foresters are teaming up with engineers to solve stormwater problems. What engineers and foresters learn in college seems different on the surface, but the disciplines can dovetail well if each field knows something about the other. My training was in forest hydrology, which is engineering-based, so I see both sides of the fence. I wish engineers would take some forestry classes, and vice versa."
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Some of the best support for using more trees in the landscape comes from area citizens, although for different reasons. "In Kentucky, flooding was the main concern; in California everyone is worried about endangered species," Swenk says. "I use whatever issue helps sell reforestation.
"Trees provide habitat for diverse species of wildlife; they improve air quality, lower ozone levels, and help cool the city," Swenk concludes. "With all trees' benefits, it begs the question: Does everyone need a 10,000-square-foot lot of just grass?"
Author's Bio: Janis Keating is a frequent contributor to Forester Media Inc. publications.
March-April 2002
Trees: The Oldest New Thing in Stormwater Treatment?
How much do tree canopies really affect runoff volume?
Expressed simply, stormwater managers face two main challenges: (1) channel the water so it doesn't flood homes, property, or city streets and (2) make sure the water is reasonably clean before it flows into natural streams, rivers, and lakes.Traditionally, stormwater has been channeled through storm drains, sewers, concrete drainage ditches, and the like. These manmade structures do the job, but they not only require long, expensive public works projects for their installation, they also have to be maintained.
In the Ditch
Drainage ditches or sluices in particular can cause safety and health problems. Because hard rains can fill drainage ditches with deep, rushing water, municipalities often fence the areas to keep people out. Yet despite their best efforts, a number of children die in drainage ditches each year. Water can pool in ditches during moderate rains, creating a breeding ground for foul-smelling algae at best, mosquitoes at worst. Unfortunately, the ditches often also become repositories for illegal trash dumping. Taking these possible negatives into consideration, it's no wonder homeowners don't want drainage ditches, however needed, in their backyards.
In the past, cleaning stormwater wasn't an issue. Cities with combined sewer systems treated the sewage, and stormwater runoff was cleaned in the process. Those with separate systems didn't send stormwater to wastewater treatment plants–although now some cities are investigating that possibility–but the National Pollutant Discharge Elimination System (NPDES) and other regulations are setting increasingly stringent standards for treating stormwater runoff in some fashion before it reaches receiving surface waters.
Before humans changed the landscape, nature had its own way of dealing with stormwater; it soaked into the soil, nurturing the plant life. Stormwater overflows would create floodplains and wetlands. This process still works well, where it's allowed; however, add homes, businesses, and traffic grids to a landscape, with the resultant impervious surfaces, and "natural" drainage quickly becomes a problem. As stormwater managers seek to re-create or maintain predevelopment hydrology and to treat runoff–often employing created infiltration systems, vegetated swales, riparian buffers, and the like–many are taking a close, new look at the role of trees in stormwater diversion and treatment.
Depending on the species and the soil conditions (both the type of soil and its rain-saturation level), trees can absorb a considerable amount of water. Also, water-polluting nitrates, phosphorus, and potassium, which in many areas are spurring the development of total maximum daily loads (TMDLs) for receiving waters, are readily absorbed by trees, which consider these substances food.
One Man's Tree Is Another's Treasure
Just how much can trees do to help? According to the American Forests organization (www.americanforests.org), a healthy tree canopy can tremendously reduce stormwater runoff, saving its host city millions of dollars in infrastructure costs.
To quantify these benefits, American Forests has developed modeling software to plot vegetation's slowing effects on stormwater. The Windows-compatible CITYgreen software offers modeling capabilities that allow users to compare economic benefits of various site plans by analyzing a site's ecosystem and producing data on a number of factors: stormwater runoff, air quality, summer energy savings, carbon storage/avoidance, and tree growth. The software can also create ecological maps noting the value of available resources, as well as models for projecting future growth.
"It's not a stand-alone program," explains American Forests Senior Geospatial Analyst Kenneth Gorton, who is also CITYgreen's lead programmer. "It has to be used in conjunction with ArcView GIS [geographic information system], which was developed by ESRI. ArcView gives you tools for capturing a data set so you can use CITYgreen."
Start With a Bird's-Eye View
To begin your calculations, CITYgreen must know which area you're investigating. "For example, you'd open an aerial photo in ArcView and use the tool to select your area, which produces a data set that ArcView and CITYgreen can read. CITYgreen digitizes the image, then reports, ‘There are so many trees here, in x square feet,'" Gorton explains.
Pictures used in the CITYgreen program must be orthorectified–given the geometric qualities of a map–before the software will work. The image also must be registered with coordinates from an aerial survey company, perhaps one with LandSat cooperation. If contracting with an aerial-photo company is cost-prohibitive, Gorton notes that ArcView also will work "at least with the more complicated GPS systems, if you can walk around with the GPS and ‘trace' the area of interest."
Whichever mapping system is used, Gorton cautions that the software works best with a 1-ac. minimum area. "It would work with a smaller area, but results are so miniscule," he says. Once the area in question has been entered into the software's database, CITYgreen concentrates on calculating what the trees actually do for stormwater runoff.
"The data are modified from the USDA's TR-55, the Natural Resources Conservation model, which estimates stormwater runoff. This public domain information all comes with the software," Gorton adds. "Each type of land cover and soil has an assigned curve number. The higher the curve number, the higher the expected runoff." The model doesn't take soil saturation or water tables into account; an engineer would need to provide that information if it's needed for projections.
What Covers the Land?
|
| CITYgreen software can create ecological maps. |
"CITYgreen doesn't measure moisture use by each tree," Gorton continues. "It only takes advantage of data that talk about land cover–the total collective land cover for a particular site, whether it be trees, shrubs, grass, crops, pavement, or buildings. The software compares land cover, the soil complex, and ratio of rainfall to determine projected runoff. In general terms, the thicker the vegetation on a site, the more the water is inhibited."
Gorton stresses that the software helps most when used during a site's planning stage. "The TR-55 data are for planning purposes [and] not intended to design plantings for a site. CITYgreen can help developers make informed decisions on planning their community. Our hope, of course, is that they will plant more trees.
"When planning a new development–a strip mall, for example–most cities won't allow you to create more runoff than existed before you built. CITYgreen won't tell you where to put trees; it will tell you what will happen if you take trees down. It gives you an estimate of what will happen when you make certain changes to the site. How you use the information is up to you. It can be a reality check for making your construction proposal," Gorton says.
According to American Forests, the software has been used to help find the best strategy for erosion and fire suppression in California's Santa Monica Mountains. Dade County, FL, has used the software to project the energy and stormwater runoff savings derived from replacing trees lost to Hurricane Andrew. Various schools and universities use CITYgreen in forestry, environmental studies, landscape architecture, and urban planning courses.
A Garland of Trees for a Texas Town
In 2000, American Forests published a study in which CITYgreen was used to determine the value of the tree canopy of Garland, TX. The study, available on American Forests's Web site, reported that current tree cover was saving the city $5.3 million a year, a figure that included residential energy savings, runoff reduction, and air pollution removal.
American Forests analyzed 10 sites in Garland: two residential plots from each size designation of "small" (approximately 3,500 ft.2), "medium" (7,000 ft.2), and "large" (11,000 ft.2) (see Table 1); two commercial plots; and two industrial plots.
| Table 1. Residential Test Plots |
Plot Size | Acreage | House Density | Canopy Cover (%) |
Small | 4.87 | 18 | 19 |
Small | 5.34 | 20 | 7 |
Medium | 3.86 | 10 | 8 |
Medium | 6.05 | 25 | 43 |
Large | 4.61 | 14 | 4 |
Large | 8.36 | 8 | 17 |
The study determined that increased tree cover could save even more. For example, a medium-size, 3.86-ac. residential site, with its 8% canopy cover, provided a 3% runoff reduction. If the site's tree canopy were increased to 35%, runoff reduction would quadruple, to 12.8%; a canopy cover of 45% would bring that number to 16.1%. (As a rule, American Forests recommends that cities maintain a 40% tree cover.) The study, however, did not recommend how a site with such home density would accommodate the resultant roots that would accompany the increased tree canopy.
Concrete: Can You Dig It?
Homeowners wanting immediate savings could always take a jackhammer to their patios; Garland's stormwater utility fee is based upon the amount of a property's impervious surfaces. Property owners are billed 6 cents per 100 ft.2 of impervious area. At present, no exemptions are made for porous pavement.
Philip Welsch, manager of Garland's Municipal Stormwater Quality Program, admits concrete and asphalt are major causes of stormwater problems. "We not only have lots of streets, but many neighborhoods also have alleys behind the homes. In addition, the width of our sidewalks is dictated by the Americans with Disabilities Act. Tearing up or narrowing some of the concrete would help immensely, but present zoning will not allow it. For example, our streets are wide because, despite having garages and driveways, people here insist on parking on the street, and the fire department insists on having enough clear width for their vehicles." The soil below doesn't help either: "This area is filled with heavy clay soils, plus some caliche," Welsch notes.
Part of the reason for the city hosting the American Forests study was to illustrate the benefits of the Garland's tree ordinance. "We wanted to demonstrate to EPA that this is a postdevelopment control and they should be happy with it," Welsch says. "Some Texas cities, trying to avoid some of these stormwater rules, have 10th Amendment suits going to the US Supreme Court."
The study noted, "Some drops of water that land on leaves and branches will evaporate before reaching the ground." Welsch concedes that might be the case but, he says, "Down here, the greatest percentage of our rains come in the winter, when there are no leaves on the trees.
"We want to hold onto the total canopy we have," he explains. "If a developer knocks down a tree, he has to replace it by total caliper inches. If he takes down a 10-caliper-inch tree, for example, he has to put in two 5-caliper-inch trees, 10 1-inchers, or whatever totals up to the tree removed. Developers in the new area north of the city are leaving more trees intact because they realized they can get more money for the lot if they do."
Take Two Trees and Call in the Morning
Lexington, KY, is renowned for two things: basketball powerhouse University of Kentucky and the lush, rolling bluegrass hills of thoroughbred horse farms. As population influx driven by the former puts pressure on the latter, the combined city-county government has taken steps to restrict growth in certain areas. It's also using trees to help control stormwater flow and pollutants.
"We call it ‘Reforest the Bluegrass,'" says Dave Leonard of the Kentucky Arborists Association. "We'd like to have 50% forest canopy in the county."
David Swenk, now in planning and development for the County of Santa Barbara, CA, helped develop the project during his years with Kentucky's Division of Forestry. "Traditionally stormwater has been a matter of moving water from point A to point B–no one worried about its quality, which increased someone else's problems downstream. Most of the pollutants in Lexington were fecal, from the agricultural areas–all those horses and cattle. Trees were seen as a way we could absorb water, improve the water quality, and get rid of pollutants. The feds think that's a [best management practice]," Swenk says.
Lexington took to the idea in a big way. "We got the community involved. Thousands came out to help plant–forestry students, environmental groups, Scouts, and ‘just plain folks.' Starting in 1999, we made it into an annual event. We didn't use any tax dollars. We got grants, plus donations from local companies such as Lexmark. Between 25,000 and 45,000 trees were planted each year," he reports.
Swenk sees natural watercourses, with trees used as biofilters, as the better way to channel stormwater. "Cities need to think about their green infrastructure," he says. "It would be great to remove concrete flumes, make more natural watercourses such as tree-lined creeks. Concrete doesn't help water quality; trees do."
Trees: A Long Drink of Water
How much water can a tree process? Horticulturists note that trees' weekly water needs equal 5 gal. plus 5 gal. per caliper inch. For example, a 2-caliper-inch tree needs 15 gal. (5 + [5 x 2] = 15) weekly. This calculation, of course, is for minimum needs; many trees can take in more water.
"A mature bald cypress can absorb 880 gallons per day, depending on the soil type and saturation," Swenk says, though admits it's extremely difficult to retrofit a neighborhood with tree-based stormwater controls. "Plan for detention and it can be retarded by your design. Plant trees that will help absorb stormwater." He suggests a number of trees that can work for stormwater purposes (Table 2). Most of these species thrive in the Midwest.
| Table 2. Recommended Tree Species |
American Basswood | Hackberry | Serviceberry |
American Elm* | Osage | Orange Silver Maple |
Black Cherry | Paw Paw | Swamp White Oak |
Black Gum | Persimmon | Sweetgum |
Black Walnut | Red Maple | Sycamore |
Boxelder | Red or Pin Oak | Tulip Poplar |
Green Ash | River Birch | Willow |
| * Disease-resistant |
He notes that Santa Barbara has instated a conservation subdivision design. "Here 40% of the subdivision has to be open space. You need to cluster your design to manage stormwater. Some use bioswales, which are like a reconstructed marshland/wetland." In the area's sandy soil, Swenk likes to use sycamores for habitat planting.
Will developers lose money by putting in these riparian zones? "No, they can charge more for the lot, plus save on storm sewer costs," Swenk maintains. "In a conventional plotting [straight lines of houses], it costs up to $560,000 to put in sewer pipes. Use a cluster design, and costs can drop to about $339,000. Do a plan with trees and a wetland, and that cost can drop to $244,000."
Any legal problems with water-filled riparian zones? "Homeowners are sometimes resistant to it if, for example, it's designed like a nature trail, because of wildlife and hikers that can wander into their yards. But would they rather have a concrete flume in their backyard or a creek area? A natural watercourse does so much–it looks better, and it costs pennies on the dollar versus traditional methods," Swenk points out.
The concept of using trees to aid in stormwater control and treatment is somewhat of an uphill climb. "Planners knew nothing about environmental planning, and they don't talk with engineers; engineers used to think of trees just as obstructions," Swenk relates. "But more and more foresters are teaming up with engineers to solve stormwater problems. What engineers and foresters learn in college seems different on the surface, but the disciplines can dovetail well if each field knows something about the other. My training was in forest hydrology, which is engineering-based, so I see both sides of the fence. I wish engineers would take some forestry classes, and vice versa."
Some of the best support for using more trees in the landscape comes from area citizens, although for different reasons. "In Kentucky, flooding was the main concern; in California everyone is worried about endangered species," Swenk says. "I use whatever issue helps sell reforestation.
"Trees provide habitat for diverse species of wildlife; they improve air quality, lower ozone levels, and help cool the city," Swenk concludes. "With all trees' benefits, it begs the question: Does everyone need a 10,000-square-foot lot of just grass?"