Tree Diversity in Prairie Urban Centers

Trees are a crucial element and fabric of every urban community and greatly contribute to its environmental, social, and economic values. Tree diversity is an important component of urban forest resilience and resistance to climate change.

History of Trees in Prairie Urban Centers

Prior to European settlers’ arrival most of the landscape was vast prairie grassland with trees occurring only along the large prairie river valleys and area close to large natural boreal forestland. The Prairie landscape was maintained by prairie grass fires and millions of bison’s grazing productive grassland. Prairie fires and bison grazing controlled the spread of tree populations within grassland landscape. Most of urban cities were built in open grassland prairies with few trees naturally occurring.

Most of large prairie urban centers were established along major river valleys (North and South Saskatchewan, Assiniboine, Red River, Red Deer, Old Man, Bow, and Battle River) with certain amount of natural forest occurring along these river valleys. Native tree species such as aspen, balsam poplar, Plains cottonwood, American elm, Bur oak, linden, white spruce, and lodgepole pine were dominant tree species in these river valleys. The first Prairie city was incorporated in 1873 (City of Winnipeg) while vast majority of urban cities were incorporated in early 1900 (Edmonton, Calgary, Lethbridge, Saskatoon, and Regina).

As Prairie urban centers have been growing since their establishment, needs for tree planting along streets and boulevards, parks and open spaces have been increasing as well. The first tree nurseries were established by provincial and federal government, as well as few local urban municipalities. Beside few private tree nurseries during early 1930s-40s; it was not until early 1950s, that small number of private tree nurseries have been established to supply trees to urban centers. Significant expansion of private tree nurseries had occurred in late 1960s till mid 1980s.
Tree species planted in early urban centers were American elm, green and black ash, variety of poplar and willows, together with white spruce and pines. Several fruit and flowering bearing trees were introduced from other parts of Canada, USA, and Europe.

Picture 1: City of Regina Legislative building 1908 with no trees (L) and City of Lethbridge (R)

Tree Inventory Data

Tree inventories provide overall baseline data for current state of urban trees including tree composition and diversity. Most of the Prairie urban municipal tree inventories began in mid-1990s and early 2000s. Urban forest managers, arborists, and planers recognize the importance of trees in urban centers not just as part of beautification but also providing environmental, social, and economic values.
Each urban center has developed its own inventory methodology and uses various software’s to collect and store the tree inventory information. Based on “open city data” as well as through personal tree inventory data sharing from urban municipalities; there are several key limitations and challenges about tree data information collected for this article:

1. Tree inventory is mostly collected on public spaces (streets, boulevards, parks and open spaces) while very few tree data is collected on private land as well as natural forest area own by municipality.
2. Several urban municipalities have only partly completed tree inventory (from 30-90% of total trees within municipality).
3. In some urban municipalities tree inventory information is already 20 years old.
4. There are many inconsistencies and overlapping in data collection information among various urban municipalities. E.g. certain urban municipalities use only common tree species names while others only using botanical name of tree species.
5. Age of trees is not collected in tree data, which is important information regarding to survival, care, maintenance and overall management.
6. Almost all tree inventories contain certain number of “unknown tree species.”
7. Many urban municipalities do not have any tree inventory information.
8. Lastly some tree inventory data is not available to public and is obtained through personal communication with municipal staff.

Tree Diversity Summary

Below are summary tables based on population size (cities over and under 100,000 people). Also, I chose 11 Tree genus that represent majority of trees in urban areas. Lastly, due to lack of data I was not able to provide certain information. It is important to note that there are many urban municipalities with population larger than 10,000 people (19 in Alberta; 12 in Saskatchewan and 8 in Manitoba) that during my research or request about tree inventory have not provided any tree inventory information or tree inventory data is not publicly available.

Based on tables presented above there are several conclusions:

• Tree diversity (species richness) in Prairie urban municipalities is extremely high comparing to natural forest surrounding the urban areas.
• Genus diversity (genus richness) is also very high including tree species that belong to Plant Hardiness Zone 5, 6 and 7.
• Despite high species richness, normally a few genus (elm, ash, poplar, spruce, and pine) dominate the urban tree population.
• Ulmus, Populus and Fraxinus genus are most dominant hardwoods (40-55 %) by total number of trees.
• Picea and Pinus genus are most dominant coniferous (15-30 %) by total number of trees.
• There is significant number of tree varieties and clones planted in urban areas.
• There are some variations regarding tree species diversity among various urban municipalities. Variations are based on local climate, site, and soil but also on urban forest managers’ decision on species selection as well as available local tree nursery stock supply.
• Flowering tree species (Malus, Prunus and Tilia) consist of 5-20 % of the urban trees.
• Total number of trees planted and inventoried is significant considering that just 100 years ago most of urban municipalities were Prairie grassland.

Summary

As this is first time that Prairie urban municipality tree diversity is summarized and analyzed (partially), we offer the following suggestions to prairie urban municipalities:

• Standardize the tree inventory methodology and software across the prairie provinces.
• “Clean up”  tree data to remove overlapping and incomplete data information.
• Use  latest technologies (e.g. LiDAR, AI, air photo, SEGMA software, Google street view, etc.)  to conduct tree inventory, that is cheaper and faster than traditional individual tree inventory.
• Perform tree inventory audits to analyze your tree data not just on tree diversity but also on tree age, overall health and vigour, survival rates, site conditions, pest infestation, care, and maintenance history.
• Analyze species diversity, but also diversity within a species, cultivars, clones, as well as age and size diversity.
• Most of tree species are not native but introduced tree species have already adapted, survived, grown, and thrived in last 120 years in Prairie urban centers.
• Climate change will impact trees in urban centers in many ways but many tree species already adapted to impact of climate change.
• Analyze the performance of each tree species and determine which species, cultivars and clones  are  best suited to your urban environment.
• Collaborative work is required among urban municipalities, landscape architects, developers, tree nursery growers, research institutions, colleges, and universities to develop climate change adaptive tree species selection system.
• Pests threat is real to trees in urban centers and developing coordinated plan how to monitor, survey and control spread of pests .

ATTS Group Inc offers reliable, practical, and innovative tree and forestry consulting and advisory services to urban municipalities in prairie provinces. We offer tree audits, inventory, management plans and arborist consulting services.