Meeting Florida House Bill 697 requirements to reduce Florida's carbon emissions will require a judicious look at how human-dominated landscapes are performing. It has been suggested that conserved urban greenspace could be used for carbon credit. But are all types of open spaces equal in terms of their ability to sequester carbon? Intuitively, this is not the case because we know that different types of vegetation (e.g., hammock vs. turf) and how they are managed will sequester different quantities of CO2. Using representative 400 m2 plot measurements (Zhao et al. 2010) and modeling of tree carbon sequestration (Escobedo et al., 2009) and estimates of lawn sequestration from various land use types in Florida, including their maintenance emissions, we calculated the source/sink potential of a 4 hectare (9.88 acres) site. Only above-ground vegetation values were calculated; soils and below ground organic matter were not included in the calculations.
The take home message is that highly maintained lawns and trees sequester much less CO2 than more natural areas with little maintenance (Table 1). With more lawn cover than tree canopy cover, the balance can actually shift to emitting CO2 (e.g., older residential areas in Miami-Dade). Of note is that we did not calculate the impact of built surfaces, just vegetative. The calculations were simplified as we did not add the carbon cost of making and maintaining the power equipment or the carbon cost of growing and transporting sod. In particular, we did not calculate the emission of nitrous oxide (N2O) from fertilization applications. Urban turfgrass typically emits N2O after fertilization and/or irrigation. N2O has a much worse global warming potential (GWP) as its heat-absorbing potential is approximately 300 times more than CO2. With these unmeasured factors, city parks with high maintenance regimes may have much larger impacts than reported here. Thus, urban open space that has a large amount of mowed, irrigated, fertilized lawns and pruned shrubs and trees can be a source of CO2 rather than a sink. These CO2 emissions are not trivial; for example, a 4-hectare greenspace in Miami-Dade with 85% of the land covered in lawn would emit over 11 tons of CO2 per year (Table 1).
Further, because below-ground soil carbon sequestration was not calculated, full carbon credit could not be assessed and these above-ground numbers reported should be regarded as a first look at the potential carbon value of urban greenspace. At this stage, natural greenspaces in and around urban areas, with little to no maintenance, seem to be the best option for CO2 sequestration. Natural urban greenspaces also have other benefits, such as biodiversity conservation, reduced stormwater runoff, and reduced fertilizer applications. Overall, the conservation of urban open space could play a role in reducing Florida's carbon footprint, but highly maintained urban greenspace could be regarded as a source of greenhouse gases. In relation to HB 697, these results indicate that if municipalities and developers are to use green spaces as CO2 sinks, they will have to justify the creation of such high-maintenance parks and may have to mitigate their effects.
For additional information on conservation subdivisions, urban forestry, and conserving urban biodiversity, a variety of online guides, books and other publications exist.
Books and Scientific Publications
Jo, H. and G. E. McPherson. (1995). "Carbon storage and flux in urban residential greenspace." Journal of Environmental Management 45: 109–133.
Nowak, D.J. and D.E. Crane. 2002. "Carbon storage and sequestration by urban trees in the USA." Environmental Pollution 116, pp. 381–389
Schlesinger, W. H. 1999. Carbon sequestration in soils, Science, 284, 2095, doi:10.1126/science.284.5423.2095.
Thompson, J. W., and K. Sorvig. 2008. Sustainable Landscape Construction: A Guide to Green Building Outdoors 2nd edition. Washington DC: Island Press.
Townsend-Small, A. and C. I. Czimczik. (2010). Carbon sequestration and greenhouse gas emissions in urban turf. Geophysical Research Letters. 37, L02707, doi:10.1029/2009GL041675.
Zhao, M., F. Escobedo, and C. Staudhammer. 2010. Spatial patterns of a subtropical, coastal urban forest: Implications for land tenure, hurricanes, and invasives, Urban Forestry & Urban Greening, Accepted
Hostetler, M. E., G. Klowden, S. Webb, S. W. Miller, and K. N. Youngentob. 2003. Landscaping backyards for wildlife: top ten tips for success. https://edis.ifas.ufl.edu/UW175
Department of Wildlife Ecology and Conservation Extension http://www.wec.ufl.edu/extension/
Escobedo, F., J. Seitz, and W. Zipperer, 2009. Carbon sequestration and storage by Gainesville's urban forest. Gainesville: University of Florida Institute of Food and Agricultural Sciences. FOR 210. https://edis.ifas.ufl.edu/publication/fr272
Florida Fish and Wildlife Conservation Commission—Planting a Refuge for Wildlife https://myfwc.com/viewing/habitat/refuge/
Florida's Urban and Urbanizing Forests Program http://www.sfrc.ufl.edu/urbanforestry/
Lawn Fertilization Recommendation http://hort.ifas.ufl.edu/yourfloridalawn/
Living Green https://livinggreen.ifas.ufl.edu/
Program for Resource Efficient Communities http://www.buildgreen.ufl.edu
Sustainable Site Initiative http://www.sustainablesites.org/
DelValle, T. B., J. Bradshaw, B. Larson, and K. C. Ruppert. 2008. Energy Efficient Homes: Landscaping https://edis.ifas.ufl.edu/fy1050
USDA Forest Service, Urban forests and climate change resource center: http://www.fs.fed.us/ccrc/topics/urban-forests/
Annual carbon sequestration by trees and lawns in different types of Florida urban greenspaces.