Overview

System-level research has resulted in significant advancements in horticultural crop production.  Contributions of individual components to the cost and environmental impact have been a focus of such research.  The public is demanding that the environmental impact of products and services be assessed.  Life Cycle Assessment (LCA) is a tool to study horticultural crop production systems and horticultural services and their individual components on environmental impacts such as the carbon footprint, stated as Global Warming Potential (GWP).  This manuscript introduces LCA and describes how this tool can be used to generate information important to the industry and consuming public.

Life Cycle Assessment is a systematic process accounting for diverse environmental impacts of interrelated input components and processes of a product or practice during its complete life cycle, cradle to grave. A carbon footprint (the total amount of greenhouse gas emissions caused by an organization, event, product or service) is the most common focus of LCAs analyzing system components and their interactions.  The carbon footprint of a product or activity is expressed in kg CO2 equivalent emitted (CO2e).   Other questions that could be addressed by LCA might be a product’s water footprint (the water used, both directly and indirectly, by an organization, event, product or service), toxicity potential (releases that are toxic to humans and/or the environment, both acute and chronic) or some other environmental impact measure.

LCA includes information on the three primary life phases of a product or service: production phase, use phase and post-life phase.  The production phase encompasses the assimilation of inputs and the processes required to produce the product. The use phase includes the impact of the product during its useful life. The post-life phase assessment focuses on the impact of the product as it is reused, recycled or disposed. Information about each primary life phase could help determine the primary factors in environmental impact.    For example, it would be expected for the impacts of plastic nursery containers occur primarily during the production phase (use of energy, petroleum, etc.) and in the post-life phase with little direct impact during the use phase (crop production).  The primary contribution to the carbon footprint for fresh fruits and vegetables would be expected to occur during production, storage and transport (Edwards-Jones, et al., 2003).  Any negative carbon footprint of shade tree production occurs primarily during production and transport while significant positive impact occurs during the use phase as growing trees sequester carbon. 

The functional unit for a product or activity targeted by a LCA must be defined.  A 5-cm caliper field grown tree, a 15-cm flowering potted plant, a standard size box of broccoli or a bushel of apples could be functional units for a LCA related to horticultural crops.  The units of all system inputs must be converted to that functional unit.  For example, the amount of fertilizer, machinery time for a specific operation, etc. must be expressed relative to the functional unit.

Inventory analysis is the base component of a LCA. All inputs and processes are inventoried and the contribution of each to measurable environmental impact within the defined system boundaries is determined.  Boundaries may include use, reuse and maintenance.  Boundaries may be referred to as cradle-to-grave or even cradle-to-gate but defining what is the cradle and what is the grave or gate of a product or practice is an important issue.  Cradle-to grave refers to the impacts of a product during manufacturing, transport and use but ends with the impact of that product at the end of its useful life.  The grave could be considered recycling or being put in a landfill.  Cradle-to-cradle boundaries refer to the usefulness of products after their primary use-life. The expectation of such a boundary definition is that products would have a “value” at the end of their primary useful life. 

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 take down 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).

Life cycle assessment was used to analyze the global warming potential (GWP) and variable costs of input materials, equipment use and labor of a model system for field production of a balled and burlapped, 0.9-m (36-in) Judd viburnum (Viburnum x juddi Rehder) shrub in the lower Midwest.  The model system was defined using information obtained through interviews with nursery managers in the region.  The propagation-to-gate GWP of the shrub was determined to be 0.705 kg CO2e, after subtracting 0.916 kg CO2e, the weighted impact of carbon sequestered during production. Estimates for propagation-to-landscape GWP (3.156 kg CO2e) and variable costs ($9.19) were also calculated for the model.  Material inputs during field production contributed 1.063 kg CO2e to the propagation-to-gate GWP and $0.89 of the variable costs while equipment use contributed 0.558 kg CO2e and $0.32 to variable costs. 

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