Trees have significant positive environmental impacts in the landscape. Urban foresters have published significant evidence for air quality improvement, carbon sequestration, microclimate modification, storm water mitigation, and reduced requirements for heating and cooling buildings (McPherson and Simpson, 1999; Nowak et al., 2008; Peper et al., 2009). However, processes used during tree production at the farm level can negatively affect environmental impact factors. LCA has been used to determine the carbon footprint or GWP reported as kilogram CO2-equivalent (CO2e) of 5-cm-caliper, field-grown, spade-dug trees (Ingram, 2012, 2013; Ingram and Hall, 2013). Fortunately, when considering the entire life cycle from cradle to grave, trees represent a significant positive contribution to the environment.
Field-grown tree production results in emissions of greenhouse gases (GHG), which contribute 13.3 (adjusted to 16.5 for a consistent, more inclusive GWP of fuels), 17.1 and 17.1 kg CO2e to the GWP for Acer rubrum (red maple) (Ingram, 2012), Picea pungens (blue spruce) (Ingram, 2013), and Cercis canadensis (redbud) (Ingram and Hall, 2013), respectively, from propagation to the nursery gate. Accounting for carbon sequestration during production, the nursery-gate GWP was reported to be 0.8 (adjusted to 4.1), 8.1, and 6.6 kg CO2e for red maple, blue spruce, and redbud, respectively. Carbon sequestration during a useful life in the landscape reduces atmospheric CO2 during a 100-year assessment period, even when allowing for GHG emissions during takedown and disposal at end of life. The major contributor (71% to 76%) to the GWP during production of field-grown trees was shown to be equipment use or diesel and gasoline consumption (Ingram, 2012, 2013; Ingram and Hall, 2013). Equipment use also contributes significantly to the variable costs of production (Hall and Ingram, 2014).
Previous LCA research on field tree production has only focused on GWP of input products and processes. GHG emissions were calculated for input products embedded in and resulting from processes used in the production system (Ingram, 2012, 2013; Ingram and Hall, 2013). However, these products and process may have other environmental impacts, including ozone depletion, smog, acidification, eutrophication, carcinogenic or non-carcinogenic human toxicity, respiratory effects, ecotoxicity, and fossil fuel depletion [U.S. Environmental Protection Agency (USEPA), 2008]. These are categorized as midpoint impact potentials with analyses that minimize the amount of forecasting and yield predictable environmental impact suitable for relative comparisons. Endpoint analysis requires estimating specific damage to human health or the environment (i.e., crop damage, skin cancer, cataracts, and immune system suppression) and is characterized by higher levels of uncertainty than midpoint impact potentials (Bare et al., 2003; USEPA, 2008).
Water footprint, expressed in cubic meters of water per functional unit produced, is another calculation that can be performed with the production and process data from LCA analyses. It is not a direct measure of water use or withdrawal from the ecosystem but is a term adjusted for water use or withdrawal on a country or river basin scale. Calculations based on water use instead of water withdrawal may be a more complete and accurate method of measuring WF for agriculture because ≈40% of withdrawals typically flow to local streams and aquifers (Perry, 2007; Shiklomanov, 2000).
Input to WF calculations includes withdrawal or consumption from surface and groundwater flows required by a product or process using a correction factor for the availability and consumption of water in a global region on a monthly basis, defined as the water scarcity indicator (WSI). The WSI also takes into consideration the water requirements for healthy ecosystems in the region in defining available “blue” water as the volume of water that can be consumed without adverse ecological impacts. Blue water is generally characterized as the consumptive use of surface and groundwater flows. “Green” water is considered the direct precipitation that does not run off or recharge the groundwater but is stored in the soil and evaporated from the surface or through the crop. For the lower midwestern region of the United States, the WSI reflects low stress when adjusted by a water use-to-availability ratio for the region (Alcamo et al., 2000; Smakthin et al., 2004). When considered on a monthly basis, there is more WSI stress during the summer months, the time that blue water is added by irrigation to augment available soil moisture. Green water is replenished on an annual basis in this region, and therefore, its use by crops has little impact on the WF. This would not necessarily be true in an arid region or country.
The leading internationally reviewed methods of calculating WF with adjustments for WSI include the Boulay et al. (2011), Hoekstra et al. (2012), and Pfister et al. (2009) methods. The Pfister method is based on a water withdrawal-to-availability ratio. The Hoekstra and Boulay methods use a consumption-to-availability ratio. Average global WF, using the Hoekstra method, for tomatoes and fresh apples were reported as 214 and 822 m3·t−1, respectively (Mekonnen and Hoekstra, 2010b).
The purpose of the research presented here is to assess several midpoint environmental and human impact factors as well as the WF from previously published LCA analyses of tree production models focused on GWP.
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