Planting trees in urban areas can mitigate some of the emissions generated in these areas by carbon sequestration (annual uptake of CO₂ through the process of photosynthesis) and carbon storage (amount of carbon stored in the tree's biomass throughout its lifespan). Research on carbon sinks and sequestration of CO₂ in urban tree populations has been conducted since the 1990s, but has mainly focused on cities in the USA (Rahman and Ennos, 2016). For example, McPherson (1998) mapped CO₂ storage and uptake in biomass of the urban tree population throughout Sacramento County, California, and estimated that the county's roughly 6 million trees absorb ~238,000 tons of CO₂ annually, while Nowak and Crane (2002) estimated that trees in urban settings in the continental United States collectively store ~700 million tons of carbon. If all this stored carbon were to be released and converted to CO₂, it would correspond to ~2.5 billion tons of CO₂. The average amount of carbon stored per unit area of tree canopy in American cities is estimated to be 7.69 kg/m² and the maximum amount is 14.1 kg C/m² (Nowak et al., 2013). Similar values have been reported in other studies, e.g., 4.45 kg C/m² in Barcelona, Spain (Chaparro and Terrasdas, 2009); 4.28 kg C/m² in Hangzhou, China (Zhao et al., 2010); 6.82 kg C/m² in Leipzig, Germany (Strohbach et al., 2012); and 10.6 kg C/m² in Boston, USA (Raciti et al., 2012). According to Nowak et al. (2013), the differences in values between studies derive from differences in tree structure and composition, i.e., size and species distribution.

Crucial traits for carbon sequestration and storage include long life span, high wood density and high tolerance to various urban stress factors (Scharenbroch, 2012). In cities, preconditions for development of these traits can be found in, e.g., parks and woodlands, where rooting space and microclimate conditions are favorable for tree development, whereas high-density areas with impermeable surfaces pose greater challenges. Different growth rates may also affect carbon sequestration capacity, with different species sequestering different amounts depending on their life cycle stage (Kaul et al., 2010). For most fast-growing species (e.g., Populus spp. and Salix spp.), establishment is more rapid and management inputs are lower than for slower-growing species (e.g., Tsuga spp., Sciadopitys sp., and Quercus ilex). This means that biomass production rate is higher for fast-growing species than for slower-growing species. Stephenson and MacKay (2014) analyzed 403 different tropical and temperate tree species in forest stands worldwide and found that biomass growth increased with tree size for almost all species. This suggests that large trees not only act as major carbon stores, but also actively sequester large amounts of CO₂ even when they have grown large and old. Climate and growing conditions can also affect CO₂ sequestration by urban trees through affecting growth, which is slower in colder climates or in more stressful environments. Drought, high salt concentration, lack of nutrients, soil compaction and limited sunlight are some of the stressors that must be taken into account when evaluating the capacity of urban trees to sequester carbon (Wang et al., 2019).

Although urban trees can mitigate the effects of urban heat and help reduce carbon emissions, their early cultivation contributes to climate change. For example, emissions of greenhouse gases from the use of fossil-powered vehicles and machinery during seedling cultivation in the nursery and during tree establishment can affect the overall carbon offset. In fact, it can take many years before trees are climate-neutral (McPherson et al., 2015; Petri et al., 2016). Previous studies on carbon emissions and tree cultivation in nurseries, mainly in the USA, have compared different cultivation techniques and species in order to identify the cumulative carbon footprint of the nursery production stage (e.g., Kendall and McPherson, 2012; Ingram, 2013; Ingram and Hall, 2013, 2016; Ingram et al., 2019). Corresponding studies have not been carried out in a northern European context (i.e., Scandinavia), where climate conditions differ greatly from those covered by existing studies, affecting species availability and growth rate.

The aim of this study was therefore to calculate the carbon footprint from nursery production to final establishment of different tree species grown for planting in urban parks in a northern European context. The analysis includes a cradle-to-gate approach and investigated the amount of carbon the adult trees needed to sequester in order to compensate for initial carbon emissions and which temporal perspectives are of concern.

Greenhouse gas emissions were estimated based on an inventory of consumption of fuels, energy, materials and other production inputs during cultivation, delivery, planting and establishment of three different tree species, Salix alba (white willow), Quercus rubra (red oak), and Pinus sylvestris (Scots pine). The functional unit (FU) was one tree planted in its final location. The trees were assumed to be produced for planting and establishment in park environments in the cities of Helsingborg, Stockholm and Umeå in Sweden (Figure 1). Production was assumed to take place at two nurseries located in southwestern Sweden. Nursery A¹ produces seedlings as 1- or 2-year-old individuals. Nursery B² obtains these seedlings and raises them until the point of sale, when all individual trees are 30–35 cm in girth and are balled and burlapped before sale. Girth was taken as distance around the trunk measured perpendicular to the axis of the trunk at breast height (1.4 m). A description of all activities performed during tree cultivation was obtained through direct contact with the cultivation managers at the nurseries. The analysis also included unsold seedlings and trees lost during production of the different species, which added to the carbon footprint of trees sold.

Emissions from handling procedures and fuel consumption during transport from Nursery B to planting in Helsingborg, Stockholm and Umeå were estimated in consultation with a major transportation company in the city of Trelleborg with experience of shipping trees within Sweden. Fuel consumption was estimated based on the assumption that a fully loaded truck trailer delivered only Salix alba, Quercus rubra, or Pinus sylvestris for planting in open and easily accessible park environments in the three selected cities. Specific planting sites within the cities were not assumed. The search engine in Google Maps was used to determine the distance and direction between Nursery B and each city, i.e., without a specific street location, leading to final destinations in the city center. Establishment procedure included unloading of the trees from the truck trailer, wedding and preparation of the planting bed as well as soil improvement with green compost, and establishment care including watering. Information on procedures and data on fuel consumption and material use when planting trees were obtained through consultation with an experienced construction contractor of public environments.

The final inventory, including cultivation, delivery and planting, is presented in Appendix A1, while Figure 2 illustrates the process and consumption and supply of materials included in calculations of greenhouse gas emissions for the different tree species.

The tree species considered in the analysis (Salix alba, Quercus rubra, and Pinus sylvestris) were selected due to significant differences in their growth rates (Reich, 2014). Salix alba is a competitive strategist and pioneer species in resource-rich habitats, and is proficient at converting these resources into vigorous growth in order to gain an advantage over other tree species (Newsholme, 2002; Grime and Pierce, 2012). Pinus sylvestris, an evergreen species, is a pronounced stress strategist with good ability to handle resource-limited habitats, such as wet, dry or hot growing environments (Farjon, 2021), and invests in traits accordingly, resulting in significantly slower and more defensive development (Grime and Pierce, 2012). Quercus rubra has its main distribution in cool and moderately resource-rich habitats, but has relatively high stress tolerance (Eyre, 1980) and can be considered intermediate between the other two species in terms of growth rate.

Carbon dioxide uptake by the trees during cultivation and when planted in the cities of Helsingborg, Stockholm and Umeå was estimated using the i-Tree Eco model (USDA Forest Service, 2020). Model inputs comprised estimates of tree height, trunk height, crown width and diameter at breast height (dbh), made using a method described by Strohbach et al. (2012). Dimensions were estimated for the three tree species at the time of planting (assumed girth 30–35 cm) and every 2 years for 16 years of tree growth in park environments in Helsingborg, Stockholm and Umeå. Dimensions were also estimated for the three species after 50 years of growth in park environments in the three cities. All park environments were assumed to have permeable site conditions with sufficient space above and below ground for biomass development (i.e., roots, trunk, branches, and leaves) and surrounding soil and microclimate conditions providing favorable growing conditions. The estimates obtained (see Appendix A2) were used in the i-Tree Eco model to calculate the amount of carbon sequestered in tree trunk, branches and roots, based on allometric equations for each tree species. For Pinus sylvestris, carbon sequestered in the needles was also included. The amount of carbon sequestered was then multiplied by a constant factor of 3.67 (which is the molecular weight of CO₂) to convert to the amount of CO₂ taken up from the atmosphere (McPherson et al., 2016). The estimations in our study were concluded when the tree's total CO₂ uptake exceeded total combined CO₂ emissions during cultivation, delivery, planting, and establishment.

Total emissions during seedling cultivation in Nursery A, including an allowance for 20% mortality, were similar for Salix alba and Quercus rubra, while emissions during cultivation of Pinus sylvestris seedlings were noticeably lower (Figure 3). This was mainly due to cultivation in 1-L pots, compared with 2-L pots for the other two species. The greatest contributor to within-nursery emissions for all three species was peat, which accounted for just over 50% of emissions. Aggregated emissions linked to growth substrates comprised around 70% of emissions for all three species.

Emissions from fuel and electricity consumption, use of pesticides, seed for green manure crops, fertilizer and nitrous oxide emissions as a result of nitrogen fertilization during cultivation of Salix alba, Quercus rubra and Pinus sylvestris trees at Nursery B were estimated to be 7.62, 11.18, and 12.19 kg CO₂-equivalents (CO₂e) per FU, respectively (Appendix A1). These estimates were based on the assumption that Salix alba was grown for 7.5 years at Nursery B, Quercus rubra for 11 years and Pinus sylvestris for 12 years before reaching sellable size (Figure 4). Additional emissions that could not directly be linked to a specific species were allocated based on total annual greenhouse gas emissions from production of 150,000 trees grown at Nursery B (all tree species and sizes included). Emissions per average tree in cultivation at Nursery B were calculated to be ~0.7 kg CO₂e per year (Figure 4). Furthermore, an allowance of 0.01 kg CO₂e per tree and year was made for 2% mortality of plants during cultivation and an allowance of 0.3 kg CO₂e per tree and year for the average 30% of tree seedlings not sold.

Total emissions of greenhouse gases during the entire cultivation process from seed/acorn/cutting to ready-to-sell tree (30–35 cm girth) were similar for Quercus rubra and Pinus sylvestris, but ~25% lower for Salix alba (Figure 5). On assuming 0% mortality rate during cultivation and that 100% of trees were sold, the emissions were ~28% lower.

The truck assumed to deliver the trees emitted an estimated total of 64.8 kg CO₂e, 417.6 kg CO₂e, and 956.5 kg CO₂e on a one-way trip from Nursery B to Helsingborg, Stockholm and Umeå, respectively. Assuming a fully loaded trailer with balled and burlapped Salix alba or Quercus rubra with 30–35 cm girth (22 trees), emissions per tree were 3.24 kg CO₂e/FU to Helsingborg, 20.9 kg CO₂e/FU to Stockholm and 47.8 kg CO₂e/FU to Umeå (Figure 6). Corresponding emissions per tree when delivering balled and burlapped Pinus sylvestris with 30–35 cm girth (28 trees) in a fully loaded trailer were 2.55 kg CO₂e/FU to Helsingborg, 16.4 kg CO₂e/FU to Stockholm and 37.6 kg CO₂e/FU to Umeå. A higher number of FU per delivery, as for Pinus sylvestris compared with the other two tree species, gave lower transport emissions per tree and the effect of this increased with longer transport distance (Figure 6).

Plantation of the trees led to emissions of over 100 kg CO₂e/FU for the three tree species (Figure 7). The single largest source of emissions during planting (58%) was the fossil-powered equipment used, which emitted ~55 kg CO₂e/FU. The second largest source of emissions during planting (42%) was use of green compost in the plant bed, production of which emitted methane and nitrous oxide emissions from composting of waste biomass. The minor difference in emissions for planting the different species arose from the number of trees estimated to be available for planting at the same time (22 FU for Salix alba and Quercus rubra, 28 FU for Pinus sylvestris).

Irrigation during plantation of Salix alba (a total of 20 occasions) generated emissions of 8.8 kg CO₂e/FU (Figure 7). Corresponding emissions for Quercus rubra (a total of 27 occasions) and Pinus sylvestris (a total of 41 occasions) were 11.9 and 18 kg CO₂e/FU, respectively. Of these emissions, the pickup truck used for irrigation accounted for 31%, while the petrol-fuelled pump used to pump the water accounted for 69%.

Total emissions of greenhouse gases during cultivation, delivery and planting of Salix alba in Helsingborg, Stockholm and Umeå were calculated to be 120, 138, and 165 kg CO₂e/FU, respectively (Figure 8). Corresponding emissions for Quercus rubra were 128, 146, and 173 kg CO₂e/FU, respectively, and for Pinus sylvestris 134, 148, and 169 kg CO₂e/FU, respectively. Regardless of city and tree species, planting (including establishment) accounted for the largest share of total emissions (Figure 8). Emissions from cultivation and planting of each tree species were similar regardless of the city in which they were planted. For planting in Helsingborg and Stockholm, this meant that Pinus sylvestris had the greatest total emissions of the three species, mainly due to greater emissions from planting (where the longer period of establishment irrigation was a decisive factor) (Figure 8). When planting in Umeå, on the other hand, Quercus rubra caused the greatest emissions. Compared with Pinus sylvestris, Quercus rubra is a larger tree in terms of total volume when delivered, which means that fewer trees (FU) fit into a trailer for delivery. With the longer transport distance to Umeå, the higher transport-related emissions for Quercus rubra meant that its total emissions exceeded those of Pinus sylvestris. Salix alba had the lowest total emissions in all three cities. However, the differences between the species decreased as the transport distance increased (Figure 8). In Helsingborg, the difference between planting Salix alba and Pinus sylvestris was 14 kg CO₂e. In Stockholm this difference decreased to 10 kg CO₂e and in Umeå it decreased further to 4 kg CO₂e. This was again due to the difference in volume between the species at the size when delivered, which meant that more Pinus sylvestris could fit in the trailer, resulting in lower emissions per FU during delivery (Figure 8).

Salix alba reached ready-to-sell size of 30–35 cm in girth after 8 years in Nursery B, by which time it was estimated to have sequestered 52.8 kg CO₂. Quercus rubra required 13 years of cultivation and sequestered 56.8 kg CO₂ during that period, while Pinus sylvestris required 14 years of cultivation and sequestered 27.9 kg CO₂.

After 10 years, a Salix alba tree planted in an open park environment in Helsingborg, Stockholm and Umeå had sequestered 394, 320, and 234 kg CO₂, respectively. Net absorption of CO₂ by that time, i.e., with emissions from cultivation, delivery, planting and establishment deducted, was 274, 182, and 69 kg CO₂, respectively. By 16 years after planting, a Salix alba tree had sequestered 832, 633, and 422 kg CO₂ in Helsingborg, Stockholm and Umeå, respectively, and net absorption then was 712, 495, and 257 kg CO₂, respectively (Figure 9). In Helsingborg, Salix alba had compensated the initial carbon emissions after 4 years of planting, while in Stockholm and Umeå this occurred in year 5 and 8, respectively (Figure 9).

A Quercus rubra tree planted in an open park environment in Helsingborg, Stockholm and Umeå had sequestered 347, 276, and 178 kg CO₂, respectively, after 10 years and 693, 518, and 287 kg CO₂, respectively, after 16 years (Figure 9). Net absorption of CO₂ in these cities was 218, 130, and 5 kg CO₂, respectively, after 10 years and 565, 372, and 114 kg CO₂, respectively, after 16 years (Figure 9).

A Pinus sylvestris tree planted in an open park environment in Helsingborg, Stockholm and Umeå had sequestered 113, 89, and 77 kg CO₂, respectively, by 10 years after planting, and 205, 150, and 123 kg CO₂, respectively, by 16 years after planting (Figure 9). Net absorption of CO₂ in these cities was −20, −58, and −91 kg CO₂, respectively, after 10 years and 71, 2, and −46 kg CO₂, respectively, after 16 years. In Helsingborg and Stockholm, Pinus sylvestris had compensated the initial carbon emissions after 12 and 16 years. In Umeå, this did not occur within the time frame of 16 years after planting, and the Pinus sylvestris needed to sequester an additional 46 kg CO₂ in order to compensate the initial carbon emissions.

After 50 years, a Salix alba, Quercus rubra, and Pinus sylvestris had sequestered 8,191, 6,072, and 1,312 kg CO₂, respectively, when established in Helsingborg (Figure 10). The corresponding values for Stockholm were 5,748, 4,047, and 983 kg CO₂, respectively, and for Umeå 3,048, 1,545, and 711 kg CO₂, respectively. Net uptake of CO₂ by Salix alba trees after 50 years, i.e., with emissions from cultivation, delivery, planting and establishment deducted, was 8,071, 5,610, and 2,883 kg CO₂ in Helsingborg, Stockholm and Umeå, respectively. The corresponding net uptake values for Quercus rubra were 5,944, 3,901, and 1,372 kg CO₂, respectively, and for Pinus sylvestris 1,179, 836, and 542 kg CO₂, respectively (Figure 10).