Tree monitoring & assessment
Development of ecologically based management strategies and monitoring protocols related to maintaining and assessing the health and resilience of street trees along Toronto BIA commercial corridors.
Measuring the Benefits
Over the years attempts have been made to enhance our BIA sidewalks through tree planting. However, this effort has resulted in repeated tree replacements in many of our BIAs. The current approach lacks the consistent, ongoing tree care component that is essential for the long term health and sustained growth of these street trees. Instead, many BIAs have a street tree population that is severely stressed, in need of repeated replacement and perpetually too under aged and small to provide any significant ecological, social or economic benefit.
According to The Toronto Strategic Forest Management Plan 2012-2022, Toronto has approximately 600,000 city-owned street trees and 68% of these trees are less than 15.2 cm (6 inches) in diameter.
These young trees are often planted to replace trees that have died from infrastructure conflicts and lack of care. Unfortunately, this cycle of tree planting and tree replacement has diminished the potential social, economic and ecological benefits that are a natural outcome of a healthy tree stand. As a tree’s canopy expands the environmental benefits of cooling, shading, wind attenuation and pollution abatement increase exponentially. Yet only a very small portion of Toronto’s street trees achieve the larger canopy sizes that maximize these benefits. The life span of a street tree in the City of Toronto ranges from 10-15 years, which coincides with the age that many trees, if well managed, begin to provide us with their shading and cooling benefits.
The extremely short life span of Toronto’s street tree population results in an energy “sink” from a natural resource that should be a substantial energy “source”. The repeated replacement of street trees is costly; however the loss in potential environmental benefits is even a greater expense and increases exponentially over time.
Species Abundance Class - Charts & Analysis
Species Abundance Class
Species abundance is a measure of the number of species in a community and their numbers. It quickly provides a snapshot of the diversity and size of a community. In the tree community of the Danforth BIA there are a total of 64 individuals distributed over 11 species. One species, Little-leaf Linden, consisting of a single tree, died in 2014 and was replaced by a single Tree of Kentucky Coffee Tree. Four species, Amur Maple, Burr Oak, Ivory Silk Tree and Kentucky Coffee Tree (Littlepleaf Linden) are present as only one or two trees. In both 2014 and 2015, two or three species dominate in terms of numbers of individuals.
In 2014, the dominants are Green Ash (13) , Honey Locust (12) and Hybrid Elm (9). In 2015 the dominants are Hybrid Elm (15), Honeylocust (11) and Norway Maple (7). Of the 11 species, 5 have the same numbers in both 2014 and 2015; 3 species have declined, 2 species have increased and one species, as noted, has been replaced by another. Honey Locust and Callery Pear have both lost one individual, but Green Ash has lost 8, all due too Emerald Ash Borer. These 10 losses were replaced in 2015 by 6 Hybrid Elms, 3 Freemont Maples and one Kentucky Coffee Tree. The loss of single trees from Callery pear and Honey Locust is most likely due to environmental stress (e.g., water stress) accumulated over a number of years previously.
Diameter Class - Charts & Analysis
The capacity of a tree to provide measurable benefits increases as the tree grows. Larger, older trees provide more shade, cool the air more effectively, reduce winds, and filter particulate air pollution better. Older, larger canopy trees, especially when adjacent canopies connect, generate synergies in benefits and are more attractive to people for walking under. Maintaining street trees well so that they grow well is an important aspect of street tree management.
Because tree age correlates well with trunk diameter, monitoring tree trunk diameters of a street tree community can readily provide information on relative age distribution of the trees. When combined with the species richness in a street tree community, patterns emerge as to which species are present in one, two, or more diameter classes. Patterns also emerge about the number of trees in different species that are growing into larger diameter classes and the number in different species that are relatively new replacements. Monitoring trunk diameter year to year also reveals rates of growth for different individual trees, and by extension, for different species. This information provides an invaluable management tool for decision-making regarding maintenance of the trees, identifying both potential issues and maintenance activities that are either helpful or counter-productive to optimal growth.
Basal Area Class - Charts & Analysis
Basal Area Class
Basal area is the cross-sectional area of the trunk and is a measure of the permanent wood content of the tree. Analysis of basal area provides a nuanced examination of how individuals, species and classes are growing, especially year to year and can be taken as a proxy for carbon sequestration. The basal area analysis here is for those species in the Danforth BIA that were present in both 2014 and 2015. In the 2014 season most of the basal area of the trees was distributed rather equally over three diameter classes: 32% in diameter class 5 to 10 cm; 35% in diameter class 10 to 15 cm and 32% in diameter class 15 to 20 cm. Only 1% of the total basal area was in the diameter class up to 5 cm. The pattern of basal area distribution for 2015 is similar to that for 2014: 36% in diameter class 5 to 10 cm; 33.5% in diameter class 10 to 15 cm and 29% in diameter class 15 to 20 cm. Only 1.5% of the total basal area was in the diameter class up to 5 cm. The analysis shows, however, that all classes increased in basal area from year to year in a remarkably different pattern. Approximately 68.5% of the basal area increase was in diameter class 5 to 10 cm; 22% in diameter class 10 to 15 cm and 4% in diameter class 15 to 20 cm, with 4.5% of the total basal area in the diameter class up to 5 cm. In part this pattern reflects the different numbers of trees in each class. Averaging the basal area of each class still gives a similar pattern, but reveals less of a difference between diameter classes 5 to 10 cm and 10 to 15 cm: 11.6 cm2 average increase in basal area per tree in diameter class 5 to 10 cm; 9.9 cm2 average increase in basal area per tree in diameter class10 to 15 cm; 4.4 cm2 average increase in basal area per tree in diameter class 15 to 20 cm; and 11.6 cm2 average increase in basal area per tree in diameter class 15 to 20 cm. That the increase in basal area in diameter class 15 to 20 cm is very low likely reflects a strong environmental constraint in the past, most likely low water availability in the soil and/or poor oxygen penetration in the soil.
When considering the relationship between basal area and species, the analysis reveals several important outcomes. Just 4 of the 10 species in the 2014 season account for 58% of the total basal area: Burr Oak, Callery Pear, Green Ash and Honey Locust. Basal area for Ivory Silk Tree amounted to 0.08% of the total. The results for 2015 are similar to 2014; 68% of the total basal area is found in the same 4 species: Burr Oak, Callery Pear, Green Ash and Honey Locust. Again the Basal area for Ivory Silk Tree amounted to 0.08% of the total. However, when analyzing the increase in basal area year over year, the pattern changes remarkably. Fully 78% of the increase occurs in just 4 species: Callery Peak, Ginko, Honey Locust and Hybrid Elm (largest increase). Interestingly, Ivory Silk tree contributes a bare 1% to the increase in overall basal area. Averaging the basal area increase for each species supports this pattern, but reveals further detail. Callery Peak, Ginko, Honey Locust and Hybrid Elm (largest increase) display the greatest average increase in basal area, but are closely followed by 3 other species: Fremont Maple, Honey Locust, and Norway Maple. The lowest average basal increase belongs to Green Ash. The remaining 3 species have modest increases in average basal area increase: Amur Maple, Burr Oak, and Ivory Silk Tree.
Combining diameter class with basal area data reveals the final really notable outcome of the analysis. Just two species, Ginko and Hybrid Elm (much the higher of the two), which both occur in just one of the diameter classes, 5 to 10 cm, contribute 42% of the increase in overall basal area increase, and the highest average increase in overall basal area. Burr Oak, Ivory Silk Tree and Amur Maple are low contributors to both basal area increases and average basal area increases. That the Hybrid Elm should stand out is to be expected, since it is typically a fast grower when young, and Ginko is known to increase in girth quickly when young also. The three least contributing species are all known to grow slowly in response to environmental stress brought on by summer heat and drought: Burr Oak tends to reduce its rate of growth, while Ivory Silk Tree and Amur Maple will slow down, wilt, fail to flower well and suffer winter branch and twig die-back. Increased water availability for these species, but also for all species, would improve their basal area measures. This is important in efforts to improve carbon sequestration rates and to develop larger canopies capable of cooling through evapotranspiration, reducing ambient air pollution and dissipating winds.