Not All Green Space Is Created Equal
The hydrologic benefits of native landscapes
It is tempting to lay the blame for our hydrologically challenged lakes and streams solely at the curbs and gutters of our constructed hardscapes. While there is no doubt that impervious surfaces are the primary contributors of our urban and suburban runoff, the traditional landscape surrounding our impervious surfaces can be a poor substitute for the benefits, hydrologic and otherwise, of native landscapes. More often than not, this traditional landscape is defined by turf grass. As an example of the “shortcomings” of turf grass, a visual comparison of the shoot and root growth of nine typical prairie species, along with Kentucky blue grass (on the far right), is shown in Figure 1. Note that the vertical scale is in feet. The prairie species have adapted to an uncertain and, at times, unforgiving environment. These plants have the capacity to change a soil’s infiltration, water-holding capacity, carbon sequestration and turnover, and nutrient retention in order to improve their chances of survival. We have adapted to turf grass. We continuously “supplement” the environment in order to maintain our lawns.
Patchett and Wilhelm (1999) refer to the turf grass lawn as a “drug-addicted … outdoor rug with an exaggerated floor pad.” Turf grass requires frequent watering but must be well drained; fertilizer and pesticides must be applied on a regular basis. Homogeneity of coverage is essential; thatch or mulch and signs of animals and other plant species are not allowed to invade.
Typically, the first step in developing land is to “clear and grub” all existing vegetation, including trees. The topsoil is stripped and the site compacted. The topsoil is then spread back over the landscape, and grass seed or sod is planted. Typically, the sod or seed is Kentucky blue grass, which is not native to Kentucky or even to North America. This carpet grows roots a couple of inches deep, while shoot growth is only allowed to extend a couple of inches high.
Our notion of traditional landscaping is predicated on an aesthetic ideal that addresses our need to be in control and demonstrate to others that we care for our property. The landscapes of homes, neighborhoods, parks, roadsides, and businesses are treated as public representations of their owners. The expectation that owners are represented by the landscapes of their property is a deeply embedded North American cultural concept that dates back to at least the early 19th century. Most Americans read a neat, orderly landscape as a sign of neighborliness, hard work, and pride (Nassauer 1995).
Some of the most tightly held ideals of this aesthetic are antithetical to the properties of native landscapes that most contribute to their hydrologic function. The heterogeneity of plant species and their attributes, their messiness, excess, dead and dying stalks, and plant litter—their chaotic multitude—weaves a landscape that treats water as an essential resource but one without a culturally relevant sign of intentionality.
The entire plant sphere, from the tree canopy to the understory, shrubs and herbaceous shoots, plant litter, and rhizosphere (the rooting zone, including soil attached to roots and influenced by roots), is actively engaged in water recycling. Along each step of the way, plants work to capture, store, and reuse precipitation.
Rainfall Interception
Plant species composition is a dynamic reaction to its environment. The canopy including the understory and herbaceous cover including leaves, twigs, and stems intercepts and evaporates rainfall. Rainfall not intercepted by the canopy can be captured by accumulated plant litter and evaporated back to the atmosphere. Some of what is captured by the litter also makes its way into the ground.
The amount of precipitation intercepted by the plant canopy varies with the canopy architecture—including leaf area, leaf angle distribution, and leaf surface characteristics (waxy, smooth, etc.)—wind speed, available radiation, temperature, humidity, and rainfall distribution. Likewise, capture by plant litter depends on leaf fall, breakdown and drying-out rates, and rainfall and other climatic characteristics. The significant characteristics of rainfall distribution include frequency, depth, duration, and maximum intensity. Interception is greatest at the beginning of events and decreases as canopy storage is used up.
Rainfall not intercepted by the plant canopy or litter and not reaching the ground directly can travel down to the ground via plant branches and stems (stemflow). The amount of stemflow is determined by leaf shape and stem and branch architecture. In general, deciduous trees have more stemflow than coniferous vegetation.
Natural forests’ canopy interception ranges from 15% to 40% of annual precipitation in conifer stands and 6% to 48% in hardwood stands (Kittredge 1948) and can exceed 59% for old-growth forests (Baldwin 1938). However, interpolating natural forest losses to urban tree environments is problematic. Urban trees are typically open grown and relatively isolated from each other. Researchers are beginning to look at individual tree interception losses. Xiao et al. (2000) measured interception losses on a lone pear tree and a lone oak as 15% and 27% of total precipitation, respectively, over the winter season in Davis, CA. These trees were able to completely capture a rainfall of 0.2 centimeter and 20% of a 1.5-centimeter event.
Understory vegetation has been found to decrease throughfall (rain passing through tree canopies and directly hitting the ground) from 5% to 12% (TVA 1964, Cantelon 1951, Storey 1953). Litter interception loss, as measured in the southern Appalachians, was found to be 2% to 5% of the total annual rainfall (Blow 1955, Helvey 1964). Total forest (tree canopy, understory, and litter) interception losses (before infiltration) can therefore range from a low end of 13% up to as high as 76% of total annual precipitation.
Infiltration
Infiltration depends on the soil pore space architecture: the size, distribution, continuity, and stability of pores, as well as antecedent water content and horizonation. Macropores are created by soil fauna, old root channels, fracture planes caused by tillage, and soil cracks caused by drying and freezing.
The most active soil layer biologically and abiotically is the rhizosphere. The rhizosphere is actively engaged in nutrient cycling, water movement, and biological activity. The living soil includes roots, viruses, bacteria, fungi, algae, protozoa, mites, nematodes, worms, ants, maggots, other insects and insect larvae (grubs), earthworms, and rodents. In fact, the volume of living organisms below ground is often far greater than the volume living above ground (Clapperton and Ryan 2001).
Plant roots continuously grow and decay in response to changes in the soil environment. Weather and the movement of water and nutrients through the soil column are the major determinants of root growth and respiration. At any given time, new root growth and root decay are happening simultaneously as the roots respond to the availability of soil water and nutrients. By one estimate, 30% of prairie plant roots die off on an annual basis. These dead roots decay, leaving behind open channels.
Plant exudates bathe the rhizosphere in sugars, amino acids, and organic acids. Bacteria and fungi utilize and break down the root exudates and sloughed-off plant cells. There is fierce competition for these resources, and bacteria often produce antibiotics, poisonous chemicals, and gases that remove competition, as well as produce plant-growth-promoting substances that increase available root area and exudate production. Secretions from bacteria, along with exudates and dead and decaying root cells, create tiny soil aggregates and habitat for scavenging and predator protozoa, nematodes, and mites that feed on the large numbers of bacteria and fungi (Clapperton and 2001).
Earthworms are considered one of the most important soil fauna. The burrowing of earthworms increases soil aeration, water infiltration, nitrogen availability to plants, and microbial activity. Earthworms increase microbial activity by breaking down large amounts of organic matter through digestion and leaving behind nutrient-rich secretions in their casts. Furthermore, earthworms are able to build soil by moving between 1 and 100 tons of subsoil per acre per year to the surface (Magdoff and van Es 2000).
This constant state of biological activity is continuously cycling carbon and nutrients and adding structure(s) to the soil. This biological activity creates a positive feedback loop. Biological activity leads to more efficient delivery of water, air, nutrients, and carbon, and as the delivery of these materials increases, biological activity increases.
This biological activity increases soil organic matter (SOM) concentration measured both throughout the whole soil and in particles and aggregates. SOM increases resistance to soil compaction pressures, potentially increases plant-available water, reduces turnover time for SOM, increases nutrient retention, enhances stability of soil aggregation and decreases loss of fine soil particles, stabilizes pore size distribution, and improves soil aeration and infiltration (Carter 2002). The bottom line is that a healthy rhizosphere is a very biologically active environment, and a healthy rhizosphere is an upper soil zone that is making the best use possible of incoming material flows and energy fluxes.
Case Studies
Two case studies are presented here that clearly demonstrate the impact of native plants on soil properties. Both studies focus on agricultural watersheds. Although there appears to be an absence of long-term side-by-side comparisons of turf grass and native plant soil impacts, the agricultural watershed results are still relevant. Agricultural fields receive the same kinds of material inputs as lawns—water, fertilizer, and pesticides—and are regularly cut or harvested. Certainly, crop type and compaction due to farm equipment are important variations, but the hydrologic differences shown in the case studies below are so striking, and the attribution for those differences—root length and dynamics, species diversity, and rhizosphere ecosystem dynamics—would also create the same functional differences between turf grass and native plant landscapes.
The first case study contrasts a virtually untouched prairie landscape with cultivated fields. The second case study compares cultivated fields with an area of these fields converted to a multispecies riparian buffer. These two studies also contrast the use of two different methods of infiltration measurement. The first study uses a tension infiltrometer in the laboratory to measure unsaturated hydraulic conductivity. The second study uses a double-ring infiltrometer in the field to measure saturated hydraulic conductivity.
When a tension infiltrometer is used for measurement, water is allowed to infiltrate soil at a rate that is slower than when water is ponded on the soil surface. This is accomplished by maintaining a small negative pressure (or tension) on the water as it is infiltrating into the soil. When a small negative pressure is applied, water infiltrates into the whole soil matrix rather than into worm holes or cracks. As a result, the measurements obtained with a tension infiltrometer are more representative of the soil matrix as a whole.
When double-ring infiltrometers are used, ponded water at atmospheric pressure is allowed to infiltrate into soil in the field. The steady infiltration rate measured with double-ring infiltrometers is often equated to the saturated hydraulic conductivity. Because water is ponded on the soil surface, water can infiltrate through cracks or wormholes. Measurements obtained with this technique are representative of the soil and any particular local conditions associated with the measurement location.
Case Study 1: Hydraulic Properties of Soil Under Natural Prairie, Conventional Till, and No-Till
The objectives of this study were to compare the temporal patterns, both seasonal and from one year to another, of hydraulic properties under natural prairie, conventional till, and no-till farm fields in the Palouse region of eastern Washington state (Fuentes, Flury, and Bezdicek 2004). The natural prairie area is located in the Kramer Palouse Natural Area owned by Washington State University. The natural prairie soil has never been disturbed and represents one of the best examples of natural soil and vegetation in the region. The flora consists of perennial grasses, shrubs dominated by snowberry and wild roses, and broad-leafed perennial plants. The till field has been in continuous three-year rotation of winter wheat, spring wheat, and spring pea. The no-till field has not been tilled in more than 27 years and has a continuous three-year rotation of winter wheat/spring wheat and lentils. The natural prairie and conventional till soil field are located adjacent to each other, and the no-till field is located approximately 30 miles north. All three soil types belong to the Palouse-Thatuna silt-loam series and have the same soil texture dominated by silt. The soil properties of all three fields are summarized in Table 1.
Hydraulic conductivities were measured using a constant-head technique in the laboratory. Near-saturated hydraulic conductivities at low hydraulic heads (-1, -6, and -15 centimeters water) were measured with a steady-state method using a tension infiltrometer. The measurements were conducted six times over a two-year period both at 0- to 5-centimeter depths and at 5- to 10-centimeter depths. The average results for the 0- to 5-centimeter depths are plotted in Figure 2. Almost without exception, the natural prairie hydraulic conductivities are an order of magnitude higher than either of the cultivated fields.
As shown in Table 1, there is a clear relationship between management (or lack thereof), soil bulk density, and hydraulic conductivity. While soils must undergo compaction underneath vehicular traffic, there is also a significant negative correlation between organic carbon and soil bulk density (see Figure 3 and Blanco-Canqui et al. 2005). Soil organic carbon reduces soil strength and density by cushioning the soil matrix and improving bonds between and within soil aggregates.
The Fuentes, Flury, and Bezdicek study found that hydraulic conductivity for all treatments varied as a function of soil saturation. The authors noted that in soils with high organic matter content, soil pores expand as soil moisture increases. The natural prairie system had more than twice the organic carbon content of the cultivated fields. In addition, the authors recommended that agricultural management practices should consider techniques to form more continuous and longer-lasting pores, including the use of perennial plants to favor a more continuous and enduring root system.
Case Study 2: Multispecies Riparian Buffer Study
In 1990, a multispecies riparian buffer was established along nearly 1,000 meters of Bear Creek on a farm in Story County, IA. The creek watershed is mainly farmland planted in soybeans and corn, with row crops extending right up to the creek bank along nearly half the stream length (Bharati et al. 2002). The multispecies buffer consists of a tree zone planted closest to and parallel to the creek. The tree zone includes five rows of hybrid poplar, green ash, silver maple, and black walnut trees. Immediately upslope is a shrub zone that consists of one row of red osier dogwood and one row of ninebark shrubs. Beyond the shrub zone is a 25-foot-wide switchgrass buffer. Beyond the planted buffer strips are cool-season grass pasture strips that had been grazed until 1989 and were then allowed to grow following the removal of cattle. Dominant grass species include smooth brome, timothy, and Kentucky bluegrass.
The infiltration study included experimental plots within the silver maple, switchgrass, and grass filter areas of the buffer. Experimental plots were also established within corn and soybean fields and continuously grazed pasture in the farm immediately adjacent to the buffer site. All sites were located on fine-loamy soils (Coland soil). Infiltration measurements, using the falling head double-ring infiltrometer technique, were made in the June, August, and October-November periods, and eight replicated measurements were made in each experimental plot. Surface soil (0- to 7-centimeter) samples were collected and bulk density and water content measured before the infiltration tests were conducted.
Using Rawls, Brakensick, and Miller’s (1981) analysis of Green-Ampt parameters for approximately 5,000 soils horizons, the average hydraulic conductivity of the Coland soil horizons should range between 0.3 and 1.09 centimeters per hour (cm/hr).
The grazed pasture had the lowest average infiltration rate at 3.0 cm/hr. The row crop areas, corn and soybean, had average infiltration rates of 3.7 and 10 cm/hr, respectively. The buffer plots, switchgrass, grass buffer, and silver maple had average infiltration rates of 7.3, 9.7, and 15.0 cm/hr, respectively (Table 2). Interestingly, particle size distributions found significantly more sand and less clay under the corn and grazed pasture plots. Except for the silver maple plots, there was a direct and significant relationship between hydraulic conductivity and soil bulk density (Figure 4). The variation in bulk density for the silver maple plots may have been due to the large number of macropores attributable to earthworm activity and lower soil water content. The water contents of the grass filter and switchgrass were consistently larger than under the crop fields, grazed pasture, and silver maple plots. The lower moisture contents under the crop fields and grazed pasture plots were probably due to a smaller pore space volume in the denser soils. The lower water moisture content under the silver maple may have been due to higher evapotranspiration (Bharati et al. 2002).
Recommendations
Research on the impacts of native landscaping in urban and suburban areas is needed. A component of this research should directly investigate the transformative effect of native plants on soil properties and should not focus only on runoff impact studies before and after the installation of best management practices (BMPs). These changes in soil properties affect the soil’s strength, growing medium capacity, and hydrologic properties and should be evaluated in isolation from other BMP assessments.
Research should examine differences in bulk density, SOM and soil organic carbon, bulk density, structural strength, water-holding capacity, and infiltration over time and over a range of soil texture classes. Root development of some prairie species takes years, and studies of relative impacts should be conducted at least over a three- to five-year period, if not longer.
Native plant impacts are clearly significant and, I would propose, are not adequately captured in current modeling approaches and/or regulatory reliance on these approaches. Native plants have the capacity to improve structure and infiltration over time in all soils, including clays (see Douglas, Koppi, and Moran 1993). More information and quantification of native plant benefits would help provide regulators and rule makers with a higher degree of certainty that these biological BMPs can be relied upon.
There is a whole host of other research questions associated with native plants that are still awaiting additional study, including water-quality and air-quality benefits (carbon sequestration), economic tradeoffs, biodiversity, and ecological benefits. But, in my opinion, the biggest issues associated with native landscaping are how our personal and social norms impact the use of native plants. The issue of nonpoint-source pollution has reached back upstream into each of our backyards. Changing the way we live at the household level is this profession’s newest and most challenging BMP.
Nassauer asserts that landscape design requires the translation of ecological patterns into cultural language. Designing ecosystems so people will recognize their beauty and maintain it appropriately means including design cues of human intention. Invisible ecological function has to be actively represented so humans maintain ecological quality. As Nassauer says, “Using cues to care in design is not a means of maintaining traditional landscape forms but rather a means of adapting cultural expectations to recognize new landscape forms that include greater biodiversity. Cues to care [my emphasis] make the novel familiar and associate ecosystems that may look messy with unmistakable indications that the landscape is part of a larger intended pattern” (Nassauer 1995).
Simply stated, Nassauer is talking about finding ways to frame, outline, and create patterns of intention in and around native landscapes. A transition back to more ecologically stable landscapes needs to start with patchworks that contain cues that are culturally significant today. Perhaps, as cultural frames shift, our intentional landscapes can move even closer to predevelopment conditions.
It would also follow that building practices could stand to benefit from leaving or planting more perennial, native vegetation. With the right set of standards, not only would the environment benefit, but the builder and homeowner could benefit too.
The City of Portland, OR, allows builders and developers to use either a standard stormwater BMP design approach or a simplified, presumptive approach. The simplified approach allows the developer/builder to select from a menu of BMP approaches that provide “green credits” to meet pollution-reduction and flow-control requirements. These credits include such BMPs as grassed swales, green roofs, planters, bioretention basins, and trees. Additional facilities may be needed to treat large events, but the size of these facilities can be reduced by the use of the BMPs identified during the simplified BMP design selection process. (See the Portland Bureau of Environmental Services 2004 Stormwater Management Manual online athttp://www.portlandonline.com/bes/index.cfm?c=35122.)
Soils, too, should be given more consideration from a building perspective. Current compaction requirements should be refined so they are more suitable to biologically and hydrologically stable environments. In general, typical soil compaction specifications of 90% to 95% can reduce or effectively stop the development of roots. Goldsmith, Silva, and Finchenich (2001) show that these high soil densities can restrict root growth, result in a severe reduction in length of all roots or the primary root, and prevent root penetration into compacted soils. Research suggests that there exists a growth-limiting bulk density for a given soil texture or type (Daddow and Warrington 1983) (Figure 5).
After planting, some degree of compaction is needed to close large voids and provide suitable density for plant growth. Too much void space can lead to poor seed and root contact and desiccation of seed and cuttings. Soil compaction between 80% and 85% of the Standard Proctor maximum dry density provides many of the stabilization benefits of soil compaction without jeopardizing vegetation development and growth (Gray 2002).
Compaction can radically change pore structure and infiltration capacity. Pitt, Chen, and Clark (2002) found that infiltration rates for non-compacted sand were more than two times higher than those for compacted sands, from one-and-a-half to four times higher for non-compacted clays than for compacted clays, and up to 30 times higher for non-compacted silty loamy soils than for compacted silty loamy soils.
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It has long been known that compost can improve soil properties. Pitt et al. (1999) demonstrated the benefits of compost for infiltration properties. The authors found an improvement in infiltration capacity for sand and clay soils that ranged between one-and-a-half and 10.5 times greater than for the same soils without compost amendments. The compost amendments increased organic carbon content and decreased bulk density. However, the compost also increased nutrient content and resulted in mixed results for water-quality impacts.
Recovering, restoring, or emulating natural systems secures a host of benefits. There is a reason these systems have developed as they have—they are taking advantage of every opportunity and niche they possibly can. Increased understanding of how these systems utilize and recycle water can be applied to mediate our hydrologic impacts and help right the balance we have lost by foregoing ecological principles in engineering design.
Author's Bio: Scott Dierks, P.E., is the manager of JF New’s office in Ann Arbor, MI.
March-April 2007
Not All Green Space Is Created Equal
The hydrologic benefits of native landscapes
It is tempting to lay the blame for our hydrologically challenged lakes and streams solely at the curbs and gutters of our constructed hardscapes. While there is no doubt that impervious surfaces are the primary contributors of our urban and suburban runoff, the traditional landscape surrounding our impervious surfaces can be a poor substitute for the benefits, hydrologic and otherwise, of native landscapes. More often than not, this traditional landscape is defined by turf grass. As an example of the “shortcomings” of turf grass, a visual comparison of the shoot and root growth of nine typical prairie species, along with Kentucky blue grass (on the far right), is shown in
Figure 1. Note that the vertical scale is in feet. The prairie species have adapted to an uncertain and, at times, unforgiving environment. These plants have the capacity to change a soil’s infiltration, water-holding capacity, carbon sequestration and turnover, and nutrient retention in order to improve their chances of survival. We have adapted to turf grass. We continuously “supplement” the environment in order to maintain our lawns.
Patchett and Wilhelm (1999) refer to the turf grass lawn as a “drug-addicted … outdoor rug with an exaggerated floor pad.” Turf grass requires frequent watering but must be well drained; fertilizer and pesticides must be applied on a regular basis. Homogeneity of coverage is essential; thatch or mulch and signs of animals and other plant species are not allowed to invade.
Typically, the first step in developing land is to “clear and grub” all existing vegetation, including trees. The topsoil is stripped and the site compacted. The topsoil is then spread back over the landscape, and grass seed or sod is planted. Typically, the sod or seed is Kentucky blue grass, which is not native to Kentucky or even to North America. This carpet grows roots a couple of inches deep, while shoot growth is only allowed to extend a couple of inches high.
Our notion of traditional landscaping is predicated on an aesthetic ideal that addresses our need to be in control and demonstrate to others that we care for our property. The landscapes of homes, neighborhoods, parks, roadsides, and businesses are treated as public representations of their owners. The expectation that owners are represented by the landscapes of their property is a deeply embedded North American cultural concept that dates back to at least the early 19th century. Most Americans read a neat, orderly landscape as a sign of neighborliness, hard work, and pride (Nassauer 1995).
Some of the most tightly held ideals of this aesthetic are antithetical to the properties of native landscapes that most contribute to their hydrologic function. The heterogeneity of plant species and their attributes, their messiness, excess, dead and dying stalks, and plant litter—their chaotic multitude—weaves a landscape that treats water as an essential resource but one without a culturally relevant sign of intentionality.
The entire plant sphere, from the tree canopy to the understory, shrubs and herbaceous shoots, plant litter, and rhizosphere (the rooting zone, including soil attached to roots and influenced by roots), is actively engaged in water recycling. Along each step of the way, plants work to capture, store, and reuse precipitation.
Rainfall Interception
Plant species composition is a dynamic reaction to its environment. The canopy including the understory and herbaceous cover including leaves, twigs, and stems intercepts and evaporates rainfall. Rainfall not intercepted by the canopy can be captured by accumulated plant litter and evaporated back to the atmosphere. Some of what is captured by the litter also makes its way into the ground.
The amount of precipitation intercepted by the plant canopy varies with the canopy architecture—including leaf area, leaf angle distribution, and leaf surface characteristics (waxy, smooth, etc.)—wind speed, available radiation, temperature, humidity, and rainfall distribution. Likewise, capture by plant litter depends on leaf fall, breakdown and drying-out rates, and rainfall and other climatic characteristics. The significant characteristics of rainfall distribution include frequency, depth, duration, and maximum intensity. Interception is greatest at the beginning of events and decreases as canopy storage is used up.
Rainfall not intercepted by the plant canopy or litter and not reaching the ground directly can travel down to the ground via plant branches and stems (stemflow). The amount of stemflow is determined by leaf shape and stem and branch architecture. In general, deciduous trees have more stemflow than coniferous vegetation.
Natural forests’ canopy interception ranges from 15% to 40% of annual precipitation in conifer stands and 6% to 48% in hardwood stands (Kittredge 1948) and can exceed 59% for old-growth forests (Baldwin 1938). However, interpolating natural forest losses to urban tree environments is problematic. Urban trees are typically open grown and relatively isolated from each other. Researchers are beginning to look at individual tree interception losses. Xiao et al. (2000) measured interception losses on a lone pear tree and a lone oak as 15% and 27% of total precipitation, respectively, over the winter season in Davis, CA. These trees were able to completely capture a rainfall of 0.2 centimeter and 20% of a 1.5-centimeter event.
Understory vegetation has been found to decrease throughfall (rain passing through tree canopies and directly hitting the ground) from 5% to 12% (TVA 1964, Cantelon 1951, Storey 1953). Litter interception loss, as measured in the southern Appalachians, was found to be 2% to 5% of the total annual rainfall (Blow 1955, Helvey 1964). Total forest (tree canopy, understory, and litter) interception losses (before infiltration) can therefore range from a low end of 13% up to as high as 76% of total annual precipitation.
Infiltration
Infiltration depends on the soil pore space architecture: the size, distribution, continuity, and stability of pores, as well as antecedent water content and horizonation. Macropores are created by soil fauna, old root channels, fracture planes caused by tillage, and soil cracks caused by drying and freezing.
The most active soil layer biologically and abiotically is the rhizosphere. The rhizosphere is actively engaged in nutrient cycling, water movement, and biological activity. The living soil includes roots, viruses, bacteria, fungi, algae, protozoa, mites, nematodes, worms, ants, maggots, other insects and insect larvae (grubs), earthworms, and rodents. In fact, the volume of living organisms below ground is often far greater than the volume living above ground (Clapperton and Ryan 2001).
Plant roots continuously grow and decay in response to changes in the soil environment. Weather and the movement of water and nutrients through the soil column are the major determinants of root growth and respiration. At any given time, new root growth and root decay are happening simultaneously as the roots respond to the availability of soil water and nutrients. By one estimate, 30% of prairie plant roots die off on an annual basis. These dead roots decay, leaving behind open channels.
Plant exudates bathe the rhizosphere in sugars, amino acids, and organic acids. Bacteria and fungi utilize and break down the root exudates and sloughed-off plant cells. There is fierce competition for these resources, and bacteria often produce antibiotics, poisonous chemicals, and gases that remove competition, as well as produce plant-growth-promoting substances that increase available root area and exudate production. Secretions from bacteria, along with exudates and dead and decaying root cells, create tiny soil aggregates and habitat for scavenging and predator protozoa, nematodes, and mites that feed on the large numbers of bacteria and fungi (Clapperton and 2001).
Earthworms are considered one of the most important soil fauna. The burrowing of earthworms increases soil aeration, water infiltration, nitrogen availability to plants, and microbial activity. Earthworms increase microbial activity by breaking down large amounts of organic matter through digestion and leaving behind nutrient-rich secretions in their casts. Furthermore, earthworms are able to build soil by moving between 1 and 100 tons of subsoil per acre per year to the surface (Magdoff and van Es 2000).
This constant state of biological activity is continuously cycling carbon and nutrients and adding structure(s) to the soil. This biological activity creates a positive feedback loop. Biological activity leads to more efficient delivery of water, air, nutrients, and carbon, and as the delivery of these materials increases, biological activity increases.
This biological activity increases soil organic matter (SOM) concentration measured both throughout the whole soil and in particles and aggregates. SOM increases resistance to soil compaction pressures, potentially increases plant-available water, reduces turnover time for SOM, increases nutrient retention, enhances stability of soil aggregation and decreases loss of fine soil particles, stabilizes pore size distribution, and improves soil aeration and infiltration (Carter 2002). The bottom line is that a healthy rhizosphere is a very biologically active environment, and a healthy rhizosphere is an upper soil zone that is making the best use possible of incoming material flows and energy fluxes.
Case Studies
Two case studies are presented here that clearly demonstrate the impact of native plants on soil properties. Both studies focus on agricultural watersheds. Although there appears to be an absence of long-term side-by-side comparisons of turf grass and native plant soil impacts, the agricultural watershed results are still relevant. Agricultural fields receive the same kinds of material inputs as lawns—water, fertilizer, and pesticides—and are regularly cut or harvested. Certainly, crop type and compaction due to farm equipment are important variations, but the hydrologic differences shown in the case studies below are so striking, and the attribution for those differences—root length and dynamics, species diversity, and rhizosphere ecosystem dynamics—would also create the same functional differences between turf grass and native plant landscapes.
The first case study contrasts a virtually untouched prairie landscape with cultivated fields. The second case study compares cultivated fields with an area of these fields converted to a multispecies riparian buffer. These two studies also contrast the use of two different methods of infiltration measurement. The first study uses a tension infiltrometer in the laboratory to measure unsaturated hydraulic conductivity. The second study uses a double-ring infiltrometer in the field to measure saturated hydraulic conductivity.
When a tension infiltrometer is used for measurement, water is allowed to infiltrate soil at a rate that is slower than when water is ponded on the soil surface. This is accomplished by maintaining a small negative pressure (or tension) on the water as it is infiltrating into the soil. When a small negative pressure is applied, water infiltrates into the whole soil matrix rather than into worm holes or cracks. As a result, the measurements obtained with a tension infiltrometer are more representative of the soil matrix as a whole.
When double-ring infiltrometers are used, ponded water at atmospheric pressure is allowed to infiltrate into soil in the field. The steady infiltration rate measured with double-ring infiltrometers is often equated to the saturated hydraulic conductivity. Because water is ponded on the soil surface, water can infiltrate through cracks or wormholes. Measurements obtained with this technique are representative of the soil and any particular local conditions associated with the measurement location.
Case Study 1: Hydraulic Properties of Soil Under Natural Prairie, Conventional Till, and No-Till
The objectives of this study were to compare the temporal patterns, both seasonal and from one year to another, of hydraulic properties under natural prairie, conventional till, and no-till farm fields in the Palouse region of eastern Washington state (Fuentes, Flury, and Bezdicek 2004). The natural prairie area is located in the Kramer Palouse Natural Area owned by Washington State University. The natural prairie soil has never been disturbed and represents one of the best examples of natural soil and vegetation in the region. The flora consists of perennial grasses, shrubs dominated by snowberry and wild roses, and broad-leafed perennial plants. The till field has been in continuous three-year rotation of winter wheat, spring wheat, and spring pea. The no-till field has not been tilled in more than 27 years and has a continuous three-year rotation of winter wheat/spring wheat and lentils. The natural prairie and conventional till soil field are located adjacent to each other, and the no-till field is located approximately 30 miles north. All three soil types belong to the Palouse-Thatuna silt-loam series and have the same soil texture dominated by silt. The soil properties of all three fields are summarized in Table 1.
Hydraulic conductivities were measured using a constant-head technique in the laboratory. Near-saturated hydraulic conductivities at low hydraulic heads (-1, -6, and -15 centimeters water) were measured with a steady-state method using a tension infiltrometer. The measurements were conducted six times over a two-year period both at 0- to 5-centimeter depths and at 5- to 10-centimeter depths. The average results for the 0- to 5-centimeter depths are plotted in Figure 2. Almost without exception, the natural prairie hydraulic conductivities are an order of magnitude higher than either of the cultivated fields.
As shown in Table 1, there is a clear relationship between management (or lack thereof), soil bulk density, and hydraulic conductivity. While soils must undergo compaction underneath vehicular traffic, there is also a significant negative correlation between organic carbon and soil bulk density (see Figure 3 and Blanco-Canqui et al. 2005). Soil organic carbon reduces soil strength and density by cushioning the soil matrix and improving bonds between and within soil aggregates.
The Fuentes, Flury, and Bezdicek study found that hydraulic conductivity for all treatments varied as a function of soil saturation. The authors noted that in soils with high organic matter content, soil pores expand as soil moisture increases. The natural prairie system had more than twice the organic carbon content of the cultivated fields. In addition, the authors recommended that agricultural management practices should consider techniques to form more continuous and longer-lasting pores, including the use of perennial plants to favor a more continuous and enduring root system.
Case Study 2: Multispecies Riparian Buffer Study
In 1990, a multispecies riparian buffer was established along nearly 1,000 meters of Bear Creek on a farm in Story County, IA. The creek watershed is mainly farmland planted in soybeans and corn, with row crops extending right up to the creek bank along nearly half the stream length (Bharati et al. 2002). The multispecies buffer consists of a tree zone planted closest to and parallel to the creek. The tree zone includes five rows of hybrid poplar, green ash, silver maple, and black walnut trees. Immediately upslope is a shrub zone that consists of one row of red osier dogwood and one row of ninebark shrubs. Beyond the shrub zone is a 25-foot-wide switchgrass buffer. Beyond the planted buffer strips are cool-season grass pasture strips that had been grazed until 1989 and were then allowed to grow following the removal of cattle. Dominant grass species include smooth brome, timothy, and Kentucky bluegrass.
The infiltration study included experimental plots within the silver maple, switchgrass, and grass filter areas of the buffer. Experimental plots were also established within corn and soybean fields and continuously grazed pasture in the farm immediately adjacent to the buffer site. All sites were located on fine-loamy soils (Coland soil). Infiltration measurements, using the falling head double-ring infiltrometer technique, were made in the June, August, and October-November periods, and eight replicated measurements were made in each experimental plot. Surface soil (0- to 7-centimeter) samples were collected and bulk density and water content measured before the infiltration tests were conducted.
Using Rawls, Brakensick, and Miller’s (1981) analysis of Green-Ampt parameters for approximately 5,000 soils horizons, the average hydraulic conductivity of the Coland soil horizons should range between 0.3 and 1.09 centimeters per hour (cm/hr).
The grazed pasture had the lowest average infiltration rate at 3.0 cm/hr. The row crop areas, corn and soybean, had average infiltration rates of 3.7 and 10 cm/hr, respectively. The buffer plots, switchgrass, grass buffer, and silver maple had average infiltration rates of 7.3, 9.7, and 15.0 cm/hr, respectively (Table 2). Interestingly, particle size distributions found significantly more sand and less clay under the corn and grazed pasture plots. Except for the silver maple plots, there was a direct and significant relationship between hydraulic conductivity and soil bulk density (Figure 4). The variation in bulk density for the silver maple plots may have been due to the large number of macropores attributable to earthworm activity and lower soil water content. The water contents of the grass filter and switchgrass were consistently larger than under the crop fields, grazed pasture, and silver maple plots. The lower moisture contents under the crop fields and grazed pasture plots were probably due to a smaller pore space volume in the denser soils. The lower water moisture content under the silver maple may have been due to higher evapotranspiration (Bharati et al. 2002).
Recommendations
Research on the impacts of native landscaping in urban and suburban areas is needed. A component of this research should directly investigate the transformative effect of native plants on soil properties and should not focus only on runoff impact studies before and after the installation of best management practices (BMPs). These changes in soil properties affect the soil’s strength, growing medium capacity, and hydrologic properties and should be evaluated in isolation from other BMP assessments.
Research should examine differences in bulk density, SOM and soil organic carbon, bulk density, structural strength, water-holding capacity, and infiltration over time and over a range of soil texture classes. Root development of some prairie species takes years, and studies of relative impacts should be conducted at least over a three- to five-year period, if not longer.
Native plant impacts are clearly significant and, I would propose, are not adequately captured in current modeling approaches and/or regulatory reliance on these approaches. Native plants have the capacity to improve structure and infiltration over time in all soils, including clays (see Douglas, Koppi, and Moran 1993). More information and quantification of native plant benefits would help provide regulators and rule makers with a higher degree of certainty that these biological BMPs can be relied upon.
There is a whole host of other research questions associated with native plants that are still awaiting additional study, including water-quality and air-quality benefits (carbon sequestration), economic tradeoffs, biodiversity, and ecological benefits. But, in my opinion, the biggest issues associated with native landscaping are how our personal and social norms impact the use of native plants. The issue of nonpoint-source pollution has reached back upstream into each of our backyards. Changing the way we live at the household level is this profession’s newest and most challenging BMP.
Nassauer asserts that landscape design requires the translation of ecological patterns into cultural language. Designing ecosystems so people will recognize their beauty and maintain it appropriately means including design cues of human intention. Invisible ecological function has to be actively represented so humans maintain ecological quality. As Nassauer says, “Using cues to care in design is not a means of maintaining traditional landscape forms but rather a means of adapting cultural expectations to recognize new landscape forms that include greater biodiversity. Cues to care [my emphasis] make the novel familiar and associate ecosystems that may look messy with unmistakable indications that the landscape is part of a larger intended pattern” (Nassauer 1995).
Simply stated, Nassauer is talking about finding ways to frame, outline, and create patterns of intention in and around native landscapes. A transition back to more ecologically stable landscapes needs to start with patchworks that contain cues that are culturally significant today. Perhaps, as cultural frames shift, our intentional landscapes can move even closer to predevelopment conditions.
It would also follow that building practices could stand to benefit from leaving or planting more perennial, native vegetation. With the right set of standards, not only would the environment benefit, but the builder and homeowner could benefit too.
The City of Portland, OR, allows builders and developers to use either a standard stormwater BMP design approach or a simplified, presumptive approach. The simplified approach allows the developer/builder to select from a menu of BMP approaches that provide “green credits” to meet pollution-reduction and flow-control requirements. These credits include such BMPs as grassed swales, green roofs, planters, bioretention basins, and trees. Additional facilities may be needed to treat large events, but the size of these facilities can be reduced by the use of the BMPs identified during the simplified BMP design selection process. (See the Portland Bureau of Environmental Services 2004 Stormwater Management Manual online athttp://www.portlandonline.com/bes/index.cfm?c=35122.)
Soils, too, should be given more consideration from a building perspective. Current compaction requirements should be refined so they are more suitable to biologically and hydrologically stable environments. In general, typical soil compaction specifications of 90% to 95% can reduce or effectively stop the development of roots. Goldsmith, Silva, and Finchenich (2001) show that these high soil densities can restrict root growth, result in a severe reduction in length of all roots or the primary root, and prevent root penetration into compacted soils. Research suggests that there exists a growth-limiting bulk density for a given soil texture or type (Daddow and Warrington 1983) (Figure 5).
After planting, some degree of compaction is needed to close large voids and provide suitable density for plant growth. Too much void space can lead to poor seed and root contact and desiccation of seed and cuttings. Soil compaction between 80% and 85% of the Standard Proctor maximum dry density provides many of the stabilization benefits of soil compaction without jeopardizing vegetation development and growth (Gray 2002).
Compaction can radically change pore structure and infiltration capacity. Pitt, Chen, and Clark (2002) found that infiltration rates for non-compacted sand were more than two times higher than those for compacted sands, from one-and-a-half to four times higher for non-compacted clays than for compacted clays, and up to 30 times higher for non-compacted silty loamy soils than for compacted silty loamy soils.
It has long been known that compost can improve soil properties. Pitt et al. (1999) demonstrated the benefits of compost for infiltration properties. The authors found an improvement in infiltration capacity for sand and clay soils that ranged between one-and-a-half and 10.5 times greater than for the same soils without compost amendments. The compost amendments increased organic carbon content and decreased bulk density. However, the compost also increased nutrient content and resulted in mixed results for water-quality impacts.
Recovering, restoring, or emulating natural systems secures a host of benefits. There is a reason these systems have developed as they have—they are taking advantage of every opportunity and niche they possibly can. Increased understanding of how these systems utilize and recycle water can be applied to mediate our hydrologic impacts and help right the balance we have lost by foregoing ecological principles in engineering design.