Intuitively, parks and preserved lands have a positive effect on the environment. What could be more "environmentally-friendly" or "green" than preserving parkland and open space? Benefits like improved water quality and air quality, increases in biodiversity and habitat protection, and reductions in greenhouse gases (GHG), are all inherent in a strategy that protects and preserves land. However, the specifics of these environmental benefits, and the mechanisms behind them, are often less obvious. Furthermore, environmental benefits are often difficult to quantify and may not receive as much consideration as those which are easily quantifiable. This section attempts to identify and describe these key environmental effects, as well as the potential drawbacks or challenges of preserving parks and open lands region-wide.

Trees are called the earth's lungs. Not only do they provide oxygen for us to breathe, but they clean the air of many pollutants harmful to humans. Open space has an overall positive effect in the improvement of urban ventilation. By protecting open space and creating parks, trees and other vegetation are also preserved and protected, often planted. This vegetation plays a significant role in improving air quality in the region.

According to the Illinois Annual Air Quality Report, which utilizes standards established by the Clean Air Act, portions of the region are currently in nonattainment for ozone and particulate matter less than 10µg (PM-10). In order to meet these standards, the Illinois Environmental Protection Agency (IEPA) works to regulate point-source and area-source emitters, like power plants and dry cleaners, as well as mobile sources like vehicles. In addition to these regulations, the preservation of open space and creation of parks can assist in meeting the Clean Air Act standards.

In an area with 100% tree cover, such as contiguous forest stands within parks, trees can remove from the air as much as 15% of the ozone, 14% of the sulfur dioxide, 8% of the nitrogen oxide, and 0.05% of the CO (Sherer, 2006). Another benefit from parks and open space is the capacity that leaf cover and vegetation have for filtering air pollutants such as dust, gases and soot (Givoni, 1991). This is both an environmental and public health benefit that is significant to highly urbanized areas like the northeastern Illinois region.

Open space may be used as a noise barrier or buffer zone when the need for noise control arises due to the proximity of incompatible uses (e.g. frequently-travelled highway next to a residential area). In such case, a linear open space with tree cover may serve to reduce the noise as well as the pollution emitted from the highway.

There is a rising interest in limiting our greenhouse gas emissions and becoming more energy efficient, both regionally and globally, in order to deal with climate change. Natural lands like forests, grasslands, and parks are key assets in this effort, whether they are large preserves serving as carbon "sinks," or small local neighborhood parks helping cool their environs.

Temperatures in urban areas have increased by about 0.5-3.0°C over the last 100 years. This is termed "heat island effect" and can exacerbate air pollutant problems and lead to increased energy use and greenhouse gas emissions. Typically, electricity demand in cities increases by 2-4% for each 1°C increase in temperature. Researchers estimate that 5-10% of the current urban electricity demand is spent to cool buildings just to compensate for the increase in urban temperatures (Akbari et al, 2001). Trees and parks can offset or even reverse the heat-island effect, both directly and indirectly. Planting trees has the direct effect of reducing atmospheric CO2 because each individual tree directly sequesters carbon from the atmosphere through photosynthesis. According to a study focused on the greater Chicago region, 1 acre of tree cover absorbs 2.2 tons of carbon per year (McPherson et al, 1994).

Planting trees in cities also has an indirect effect on CO2 by reducing the demand for energy, and thereby reducing emissions from power plants. Parks and trees can reduce building energy use by lowering summertime temperatures, shading buildings during the summer, and blocking winter winds. According to a study focused on the region, increasing tree cover by 10% could reduce total heating and cooling energy use by 5-10% (McPherson et al, 1994). Furthermore, trees and vegetation can improve the ambient atmospheric temperature through evapoTranspiration in the summer and their wind-shielding effect in the winter. Both these direct and indirect benefits of energy savings from vegetated parks and open space translate into reductions in CO2 and greenhouse gas emissions.

It is interesting to note that the two criteria pollutants for which the region is in nonattainment, ozone and PM-10, are both related to temperature. Ozone is created at elevated temperatures, and PM-10 tends to stay mixed in the atmosphere longer in hotter weather (USEPA website, 2008). The vegetation within parks and natural lands plays a major role in lowering temperatures and sequestering carbon in developed areas. Forests, grasslands, and other naturally vegetated lands in the U.S. absorb an estimated 20-46% of total U.S. greenhouse gas emissions (USEPA 2008). Without preservation, these parks and natural lands could be developed and the carbon sequestration, energy-savings, and cooling benefits would be lost.

Preserving open lands and creating parkland preserves natural processes of infiltration and limits imperviousness, both of which are intimately linked to stormwater management and water quality. A study from 1993 by the Illinois State Water Survey estimated the value of open space for floodplain storage, including wastewater reclamation, pollution abatement and aquifer recharge as more than $52,000 per acre in the Chicago region (IL Environmental Council, 2007).

As the amount of imperviousness increases in a watershed, the velocity and volume of stormwater runoff increases, which can have several environmental impacts: increased flooding, erosion, and pollutant loads in receiving waters; decreased groundwater recharge and level of water table; altered stream beds and flows; and impaired aquatic habitat. Research has verified the strength of this correlation between the amount of imperviousness in a drainage basin and water quality, with an accepted 10% imperviousness threshold, above which water quality becomes impaired (Schueler, 2000).

There is also a correlation between the location of development, or impervious surfaces, within a watershed and water quality. In a natural landscape, stormwater that isn't infiltrated runs off into waterways, but not without travelling first through vegetated stream banks, thereby being slowed down and filtered. When a watershed is developed, however, stormwater is usually piped through sewer systems and paved drainage ditches. As this occurs, it is funneled together, picking up velocity and pollutants along the way, and emptied in one flush at the end of the pipe, usually directly into a stream. The pollutants flow directly off the road or parking lot, without any opportunity for filtration, and the speed of the water scours the stream bed, causing erosion and often leading to flooding downstream (Brabec, 2002). Riparian buffers can prevent some of these deleterious impacts. By creating buffers around streams and waterbodies, stormwater can be infiltrated, filtered, and slowed before entering waterways. They help allow the hydrological cycle to function more naturally (Lehner et al, 1999).

Researchers have attempted to pinpoint a size or distance threshold at which buffers are most effective, with variable results. The general rule seems to depend on the size of the drainage basin, with larger basins requiring larger buffers (Brabec, 2002). There is evidence to suggest that protection of headwaters has a larger impact because upstream disturbances carry over more stream miles (Maxted and Shaver, 1998).

Preserving open space and creating parks and greenways are key tools to limit imperviousness and create riparian buffers in a watershed. These programs are often the specific means of implementing larger growth management goals, but can also been seen as one of the most cost-effective means for reducing and managing stormwater runoff and protecting water quality (Schueler, 2000). By focusing efforts to preserve and protect open space to those lands around waterways, water quality goals can coincide with growth management goals. These riparian lands are often targeted for open space protection for other reasons – they offer good habitat or are aesthetically appealing – but they help protect water quality as well, serving as buffers for stormwater runoff, or preserving natural infiltration processes. Conversely, efforts to protect water quality can drive land preservation. Municipalities may utilize tools such as down-zoning, open space requirements, conservation subdivisions/design, or transferring development rights in order to improve their water quality, all of which can result in natural lands being preserved.

In more urban areas, where imperviousness is much higher than 10%, parks and open space can also play a role in stormwater management and water quality. They can provide natural infiltration benefits, especially if they are vegetated with mature trees – natural pollution filters. Depending on the species and soil conditions, trees can absorb a considerable amount of water, as well as water-polluting nitrates, phosphorus, and potassium, and keep it out of the flow toward the storm sewer (American Forests website). Furthermore, urban parks can be locations for structural best management practices (BMPs) that assist with stormwater, such as constructed wetlands, detention/retention ponds, or rain gardens. These BMPs help slow and store stormwater, allowing it to slowly infiltrate into the ground or runoff into sewer systems, but at a slower rate, and with some natural filtration (Schueler, 2000). This helps prevent the receiving waterbody from being "shocked" by pollutants or higher temperature runoff after a storm. The traditional method of collecting stormwater runoff transfers the water as efficiently as possible into a system of gutters, sewers, and drainage ditches. A more modern approach is to move water slowly through cities, allowing for on-site infiltration thereby minimizing flooding, and maintaining water quality.

The City of Chicago is very forward-thinking in this approach, currently working to implement a "green alley" system (see Conservation Design strategy report) which would allow rainwater to soak into the ground in the alleyways, subsidizing green roofs throughout the city, among other programs (City of Chicago website). Furthermore, the City has partnered with architecture and engineering firms to investigate the concept of "eco-boulevards," a series of 50 ribbons of open space running across the city, replacing pavement with green space and parks and wetlands, treating waste and storm water (UrbanLab website).

As land is preserved throughout the region, a key environmental benefit is the protection of unique habitat and regional biodiversity. Wildlife and vegetation depend on undisturbed natural areas for food, shelter, and reproduction, often in ways that humans have not always recognized. However, we are beginning to learn about the interconnectedness of the ecosystems of which we are a part, and how it is beneficial for us to protect and preserve habitat and biodiversity within the region.

In the past, "worthless" wetlands were drained or forest stands cleared to make way for farming and development, destroying essential habitat, and wiping out populations. This weakens the natural communities interconnected with these habitats and in turn, weakens regional biodiversity. Without efforts to preserve lands critical to protecting biodiversity, the lands that have been preserved will start to lose their ecological value as invasive species and unchecked populations outcompete (Biodiversity Recovery Plan, 1999). Therefore, it is reasonable to assert that protecting sites with high biodiversity value is a justifiable way to protect and enhance the value of large public investments already made for preserved land.

The value of biodiversity is difficult to quantify, but researchers have categorized these values into how people benefit from them. The first is direct-use values, where people directly consume or use species for their benefit, such as pharmaceuticals, medicinal plants, agricultural genetics, or fisheries. Another benefit category is ecosystem services, or the conditions through which natural processes sustain human life, such as nutrient cycling, pollination of crops. A third category of benefits are those which improve recreation and aesthetics, such as hiking, camping, fishing, bird watching, or photography. Finally, and most difficult to value, is the benefit of "existing" or the willingness people would pay to have something not become extinct (Chicago Region Biodiversity Council, 1999). These more qualitative benefits are difficult to measure, but are important aspects and benefits of land preservation more generally. Furthermore, these benefits illustrate how protecting and preserving natural lands for more traditional reasons – habitat protection, conservation, recreation, water quality – often overlap with biodiversity goals.

A less definable environmental benefit of preserving parks and open lands is the idea that having access to parks and natural lands "reminds" people to act more environmentally responsible. Land preservation can change behavior.

An example of land preservation changing behavior is commuting. A 1997 study found that a third of the users of the Iron Horse Regional Trail in California were using it for Transportation purposes – commuting to work or school, or traveling to shopping areas and restaurants (Trails and Greenways website). This suggests secondary air quality benefits.

This "green" consciousness is reflected in the growing eco-tourism industry, of which land preservation is the driving factor. The U.S. Fish and Wildlife Service estimates that in 1995 nearly 25 million visits to over 100 national wildlife refuges generated an estimated $245 million from non-consumptive uses only e.g. excluding hunting and fishing (US Dept of State website). Residents of the Chicago area use local parks an average of 46 times per person per year (SCORP, 2004). Furthermore, eco-tourism has helped several communities realize the economic benefits of promoting land preservation and parks, and prompted additional preservation and conservation measures. Catering to eco-tourists can also translate into building green, serving locally-grown fare, saving energy, and other "earth-friendly" tactics.

Although the effects to the environment often seem obvious, there are several challenges to utilizing parks and land preservation as a strategy to promote environmental benefit. Determining how environmentally beneficial a park or preserve is depends on a few key factors– its size, location, and use.

The size of parks and open lands within an urban area determines the climatic benefits. A greater impact on the overall urban climate will be had from a large number of small parks spread throughout, rather than from a small number of large parks (Giovni, 1991). Additionally, open space outside of the city does not greatly affect the climate within the urban realm. Furthermore, a park utilized for high-intensity uses like recreation, likely has limited vegetation and trees, thereby diminishing air quality benefits.

Size, location, and use of the preserved lands also determine water quality benefits. Often, the most critical lands, such as the floodplains, or riparian buffers, have already been developed or the vegetation on these lands has been influenced by development (e.g. streams running through agricultural lands), and the effects of the buffer are seriously diminished. Furthermore, some municipalities have ordinances protecting riparian buffers or open space from certain uses or development, but not sufficiently enough to constitute natural areas with water quality benefits (e.g. recreational fields, golf courses, or agricultural land which all usually rely on fertilizers, a water pollutant; or even greenway trails which may provoke pet waste or litter problems).

Another challenge is that it is difficult to utilize parks for water quality benefits because they are often intensely used, with space at a premium. Advocating for a constructed wetland is an uphill battle if constituents are more interested in a ball field, a picnic area or a golf course. Soil compaction is also a problem in urban parks, with park lawns and paths having the same imperviousness as pavement, and severely limiting any infiltration (Schueler, 2000).

Furthermore, although the region has made substantial advances and investments in preserving open space, sustaining biodiversity has not always been a priority. This is likely due to the fact that the natural pathways and "green infrastructure" that different species rely upon does not follow anthropogenic boundaries like property lines or municipal jurisdictions. It is difficult to assemble all of the stakeholders together to preserve these lands. Prioritizing land for biodiversity value may sacrifice other benefits like recreation.

Despite all the potential environmental benefits of preserving parks and open lands, they can actually cause some secondary and tertiary negative effects.

Parks, like any major attraction, may draw people from a large catchment area, resulting in many driving from far distances. Or preserved lands or parks may be located in such a way that causes drivers to take a longer, convoluted route to their destinations. Therefore, it is important to take this into consideration when making land use decisions. Parks and preserved lands must be planned in coordination with other systems.

Furthermore, depending on how intensely they are utilized, parks and preserved lands may end up harming some of the natural features they were intended to protect. Campgrounds and picnic areas can get overrun with vehicles, waste, and noise; ball fields and golf courses often rely on intense fertilization; even dirt paths can get so packed down they act like impervious surfaces. These amenities are often located in sensitive environmental areas, near streams or wetlands, so their impacts are that much more damaging.

• Stormwater quality: Percentage of imperviousness, amount of pollutants removed, IBI scores
• Air quality: Amount of pollutants removed, number of trees planted
• Greenhouse gas emissions: Amount of CO₂ removed, temperature decrease, amount of energy used
• Biodiversity: Acres of habitat, number of species
• Human behavior: decreased VMT, increased biking/walking as transportation mode, increased visitors/ revenue from eco-tourism

The environmental effects of parks and open lands are usually the driving factor behind their preservation, and rightly so. The benefits on air and water quality, climate change, biodiversity and habitat protection, and human behavior are proven and pronounced. However, there are significant challenges to promoting open space preservation as a tool to protect the environment, as many of these environmental benefits are difficult to measure and quantify. Furthermore, it is important to realize that not all parks are equally beneficial, and the location, size, and uses of the preserved lands all play a role in how they impact the environment.