What Types of Urban Greenspace Sequester Carbon?

Mark Hostetler and Francisco Escobedo

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 open spaces equal in terms of their ability to sequester carbon? Intuitively, this is not the case as different levels of vegetation (e.g., hammock vs. turf) and their management will sequester different quantities of CO₂. Using representative 400 m² plot measurements[1] and modeling of tree carbon sequestration[2] 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 into 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., Miami old residential). 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 and 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 conserving biodiversity, reducing stormwater and other water quality impacts from decreased fertilization 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.

Table 1. Annual carbon sequestration by trees and lawns in different types of Florida urban green spaces.

Greenspace type	Tree cover (m^2)	Lawn cover (m^2)	Tree seq.	Lawn seq.	CO2 emitted - lawn	CO2 emitted - tree	Net annual (CO2 kg/yr) sequestration for a 4 hectare site
Miami-Dade	Tree/lawn cover per 400 m^2 plot		Tree/lawn CO2 sequestration per 400 m^2 plot (kg CO2/yr)*		CO2 emissions- tree/lawn maintenance per 400m^2 plot (CO2kg/yr )**^T		(CO2 kg/yr)
Hammock	380	0	4653	0	0	0	465,330
Pine Rockland	120	0	70	0	0	0	6,980
Mangrove	400	0	3031	0	0	0	303,060
Commercial	72	8	247	0.3	3	6	23,860
Residential old	280	380	25	16	153	23	-13,570
Residential new	180	52	7	2	21	15	-2,640
Park/school	20	340	17	14	137	2	-10,800
Gainesville
Pine hardwood	240	0	699	0	0	0	69,910
Swamp cypress	300	0	904	0	0	0	90,360
Plantation	200	0	162	0	0	16	14,570
Commercial	200	0	237	0	0	16	22,050
Residential old	360	100	428	4	36	29	36,660
Residential new	100	260	124	11	93	8	3,350
Park/school	0	88	0	4	32	0	-2,800
Orlando
Pine palmetto	260	0	445	0	0	0	44,520
Oak pine	360	0	102	0	0	0	10,240
Cypress dome	320	0	1102	0	0	0	110,170
Commercial	40	80	39	3	29	3	1,020
Residential old	340	140	442	6	50	28	36,960
Residential new	60	80	63	3	29	5	3,280
Park/school	0	284	0	12	102	0	-9,050

*Lawn sequestration rate (grass stubble only) [3] is 48.1 g CO₂ m^-2 yr ^-1

**Lawn maintenance numbers are from three sources of carbon: fuel to maintenance equipment (122 g CO₂ m^-2 yr ^-1)[4], energy for irrigation (193 g CO₂ m^-2 yr ^-1)[5], and fuel inputs to manufacture fertilizer (1.436 moles of C per mole of N produced) 5. Tree maintenance2 is 81 g CO₂ m^-2 yr ^-1.

^T Low fertilization rate recommended by IFAS for St. Augustine grass in South Florida[6] - 4lbs 1000 ft ^-2 yr -¹

^T Low fertilization rate recommended by IFAS for St. Augustine grass in Central Florida[7] - 2lbs 1000 ft ^-2 yr -¹

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[1] Zhao, M., Escobedo, F., Staudhammer, C. (2010). Spatial patterns of a subtropical, coastal urban forest: Implications for land tenure, hurricanes, and invasives, Urban Forestry & Urban Greening, Accepted

[2] Escobedo, F., Seitz, J., Zipperer, W., 2009. Carbon sequestration and storage by Gainesville’s urban forest. University of Florida- IFAS, EDIS FOR 210. http://edis.ifas.ufl.edu/document_fr272

[3] Jo, H. and McPherson, G.E. (1995). Carbon storage and flux in urban residential greenspace. Journal of Environmental Management 45: 109-133.

[4] Townsend-Small, A. and Czimczik, C. I. (2010). Carbon sequestration and greenhouse gas emissions in urban turf. Geophysical Research Letters. 37, L02707, doi:10.1029/2009GL041675.

[5] Schlesinger, W. H. (1999). Carbon sequestration in soils, Science, 284, 2095, doi:10.1126/science.284.5423.2095.

[6] Fertilization and Irrigation Needs for Florida Lawns and Landscapes- http://edis.ifas.ufl.edu/ep110

[7] Fertilization and Irrigation Needs for Florida Lawns and Landscapes- http://edis.ifas.ufl.edu/ep110