Estimation of Biodiversity, Carbon Sequestration, and Canopy Cover of Street Trees in Abbotsford, B.C., Canada

E. Masse, L. Griffin, T. Richards, D. Christian, W. Easton

University of the Fraser Valley, 33844 King Road, Abbotsford, BCCanada V2S 7M8

Street trees provide a range of important benefits to urban landscapes. This study

investigated answers to a number of important questions regarding street trees in the City of

Abbotsford, British Columbia. (1) What is the current state and distribution of biodiversity in the

City? (2) How much total canopy cover area is provided by the City’s street trees? (3) How much

carbon dioxide (CO2) is currently stored in the City’s trees, and how much more CO2 will they

potentially absorb annually? Statistical and visual analyses of biodiversity were conducted using

the City’s Treekeeper database and ArcGIS technology, and found that 73% of the 8202 street

trees consisted of one of 15 separate species, with 141 species present in the population. Several

biodiversity “hotspots” were located, as well as areas with low relative biodiversity. Field work

was completed in which randomly sampled street trees were measured for diameter at breast

height (DBH) and canopy spread. The total canopy cover provided by street trees was calculated

to be approximately 84 480 m2. Current total carbon sequestration by the trees is approximately

1676 tonnes of CO2, with the potential to store an additional 752 kg of CO2 with each year of

healthy growth. We recommend increasing biodiversity in areas without nearby connection to

parks to help the City’s urban forest continue to remain healthy, grow and to sequester more of

the City’s carbon output. The methods of this investigation are applicable to other, similar

municipalities within the Province and anywhere the climate is comparable.

Estimation of Biodiversity, Carbon Sequestration, and Canopy Cover of Street Trees in Abbotsford, B.C., Canada

E. Masse, L. Griffin, T. Richards, D. Christian, W. Easton

University of the Fraser Valley, 33844 King Road, Abbotsford, BCCanada V2S 7M8

Introduction

Studies of forestry in urban environments from around the world have shown that trees face difficult challenges in developed, high-population areas due to a lack of nutrients, not enough sunlight, or general neglect (Hostetler, Allen, and Meurk, 2011; Stewart et al., 2009). Urban forestry now plays an important role in urban development. Urban trees provide a range of benefits including improving air quality, attenuating storm-water flooding, reducing noise, conserving energy, and carbon sequestering (Roy et al., 2012). Recognition of these benefits motivates many cities to create and implement policies to promote the health and biodiversity of greenery within the cityscape.

Effectively implementing urban forestry strategies requires much information and a good understanding of the local context. Keeping updated bylaws and policies concerning the planning and care of street trees is essential to an effective urban landscape (Hostetler et al., 2011). The implementation of good policies requires continual monitoring and inspection of the trees. Critical information about the urban forest includes biodiversity, canopy cover, and carbon sequestration estimates.

Tree biodiversity, characterised in part by the number of different species present in an area, is an important part of urban landscapes. Urban forests with high biodiversity prove more successful at providing benefits under different soil, weathering, and disturbance levels (Muthulingham and Thangavel, 2012). For example, Conway and Bourne discovered adding exotic trees to an urban forest results in high values of species richness (Conway and Bourne, 2013; Kuruneri-Chitepo and Shakleton, 2011). Increasing the biodiversity of an urban forest or street trees will also eliminate monocultures, the planting of only one species in an area. Removing monocultures will decrease the likelihood of disease infestation (Santamour, 1990). An adaptable and productive urban forest requires high biodiversity (Kuruneri-Chitepo and Shakleton, 2011); therefore promoting biodiversity and healthy trees helps to protect the environment within the boundaries of cities (Kuruneri-Chitepo and Shakleton 2011).

Increasing the life span of street trees results in a greater diameter at breast height (DBH) and canopy cover (Conway and Bourne, 2013). Canopy cover refers to the proportion of the forest floor covered by the vertical projection of the tree crowns (Brown & Jennings et. al. 1999). The total canopy cover provided by street trees in the community can be used to assess the growth or decline of the urban trees providing

details into the success of individual tree species in variable conditions. In addition, street trees absorb carbon dioxide gas (CO2) at significant rates and the urban forest can be a substantial carbon sink (Aguaron and McPherson, 2012). Estimations of the total carbon sequestration provided by urban trees can aid city planners in maximizing the storing of carbon to reach local carbon reduction goals. The tree diameter at breast height (DBH) can be used to calculate the approximate carbon storage of individual trees (Aguaron and McPherson, 2012).

The City of Abbotsford is situated in the western coastal Province of British Columbia (B.C.), Canada, approximately 68km east of Vancouver and 5km north of the United States border. The City has a rich agricultural history and boasts some of the best agricultural land in British Columbia. The community is fast-growing and is the third most populous city in B.C. (Statistics Canada, 2014). The Abbotsford City Council promotes the health of the City’s urban forest through its Tree Protection Bylaw No. 1831-2009 and various other community policies regarding the maintenance of street and mature trees. The City is also one of 180 British Columbian municipalities that have signed the Climate Action Charter and pledged to become carbon neutral and energy efficient. There is substantial interest in increasing the health, productivity, and efficiency of the City’s urban forest to help achieve these goals. In order to make this achievement possible, the deficits in knowledge about the biodiversity, canopy cover, and carbon storage of the trees must be remedied.

This study investigated the biodiversity of the City of Abbotsford’s street trees and calculated estimations of total canopy cover and overall carbon sequestration. The results were used to assess the current progress towards meeting community goals, and to make practical planning and management recommendations for the improvement of the City’s urban forest. These methods could be transferred to other similarly-sized communities in comparable climate zones.

Sampling

Study sites will be located in the City of Abbotsford. Using the ArcGIS 10.1 system, urban Abbotsford was divided into 8 equal sized plots (Figure 1). Each plot was the assigned to a separate group of field researchers. TreeKeeper 7, urban forest management software provided by Eric Fong, Abbotsford’s urban forester, provided street tree location, age, and species of trees planted in the city. Abbotsford has 8,202 street trees, with 141 different species. Street trees were randomly selected within each grid and sampled by each research group (i.e. A to H) (Figure 1).

Figure 1 Red grid area was allotted to a different field team for street tree analysis and data collection.

Methodology

Street trees were randomly selected from trees already identified in the Treekeeper GIS database, supplied by the City of Abbotsford. Within each plot a minimum of 12 randomly pre-identified trees were selected for analysis. The use of the Treekeeper database allowed the research teams to take advantage of existing species information and focus on field data collection for the circumference at breast height and the canopy cover measurements of the selected street trees. Data from each tree was collected using measuring tapes and GPS-capable cameras. Data was compiled and

analyzed statistically programs Excel and Minitab, and visually by ArcGIS 10.1.

Diameter at Breast Height(DBH)

For each tree, the circumference of the stem was measured at a height of 1.4 meters above ground (Jim and Zhang, 2013a). The number was recorded and rounded to the nearest whole number in centimetres; later in analysis, this measurement was used to determine the DBH (Figure 2). 𝐷𝐵𝐻=cover lies between 10.3-11.8 m2 per tree, only covering ~.4% of Abbtosfords Parks (seen in green on Figure 3). As a City with a goal to reach 25-35% canopy coverage (Fong, 2014)., tree health is important. The aspects measured in this study are interrelated; as tree diversity is improved, trees will be healthier, growing larger trunks, more branches, and greater canopy cover, leading to an increase in carbon sequestration. In addition to these benefits, the City will notice a cooling effect around its urban heat centre, increase in shade provided, and water purification.

Summary

This study has provided a starting point for more effective street tree management and planning. Prior to this research, information about the extent of Abbotsford’s street trees in terms of biodiversity, canopy cover, carbon sequestration, and health was lacking. The results of this investigation succeed in providing some of that critical information. The total canopy cover currently provided by street trees in Abbotsford is approximately 84 480 m2. Future investigations of the urban canopy cover can provide a way of measuring the growth and health of the street trees. Current carbon sequestration by street trees in Abbotsford is 1676.6 tonnes of CO2, with the potential to store an additional 752 kg of CO2 per year of healthy growth. The investigation of biodiversity suggests that increasing biodiversity may also increase the amount of carbon sequestration. 73% of the street trees were one of 15 different species and the remaining 27% of trees were one of 127 other species. This means that only 15 species make up the bulk of street trees in Abbotsford. This study recommends that more trees be planted with species different from the top 15 to improve this unbalance. Visual analysis suggests that the areas that are without connection to local parks be the focus of increasing biodiversity. The methods and analysis of this study can be applied to similar urban areas with comparable climate conditions.

Acknowledgments

We would like to thank the GIS students in GEOG 253 for their help with data analysis, Eric Fong, Urban Forester with the Abbotsford City Council for his valuable insight and D. Christian, W. Easton.

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