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Toronto heritage trees: A living Seed Bank for Forest Restoration
Background and Rationale
Urban trees, as an important component of the urban environment, are essential because of the benefits or functions that they can bring to the entire community (McPherson, 2006; City of Toronto, 2013; Beacon Environmental Limited with Urban Forest Innovations Inc, 2016; Greene et al., 2018). From the ecological aspect, city trees can absorb carbon dioxide and produce oxygen thus reduce Greenhouse gas emissions further mitigate heat island effect and slow down climate change (City of Toronto, 2010; City of Toronto, 2013; Greene et al., 2018). From the social aspect, city trees can benefit humans in providing shade, promoting health and contributing to the energy-saving (Tyrväinen et al., 2005; City of Toronto, 2013; Ordóñez & Duinker, 2013).
There is a tremendous need for forest restoration of local, national and global scales (McPherson, 2006; City of Toronto, 2013; United Nations, 2019). And considering 40% of canopy cover (a valuable metric for a healthy urban forest) can optimal the benefits brought by urban trees while maintaining urban forest sustainability, the City of Toronto’s council planned to increase canopy cover from 26-28% to 40% within 50 years in 2004 and included this goal into Toronto’s Strategic Forest Management Plan (SFMP) (Heynen & Lindsey, 2003; City of Toronto, 2010; City of Toronto, 2013; Beacon Environmental Limited with Urban Forest Innovations Inc, 2016). To achieve this goal, the city of Toronto decided to plant at least 100,000 trees per year(Beacon Environmental Limited with Urban Forest Innovations Inc, 2016)
A large number of seeds are required to forest restorations considering the uncertainties during seed germination, seed survival rate, tree planting process and mortality rates (average 3%) (Lombardo & McCarthy, 2009; City of Toronto, 2013). Thus, seed source is one of the most important aspects of forest restoration and for the City of Toronto little is known about the exact time location to obtain good and stable seed source (Wang, 1974; Ontario Woodlot Association, 2015; Melissa, 2016).
Heritage trees are trees existed in a long period of history and old age that contains history and culture value (Forests Ontario, 2013). Large, old heritage trees have a high probability of containing original local-provenance genes that are adapted to the urban environment, local climate conditions and local biodiversity (Ontario Woodlot Association, 2015; Gellie et al., 2016; Ivetić et al., 2016; Forests Ontario, 2019). And native trees help maintain forest resiliency and the need for native species was mentioned in SFMP (City of Toronto, 2013; Ordóñez & Duinker, 2013; Conway & Vander, 2015). Thus, the local heritage trees can act as a valuable potential seed source based on the adaptations to the local environment and this is also supported by SFMP (City of Toronto, 2013; Zhang et al., 2019). There are a total of 73 tree species native to Toronto, while the City of Toronto has a total of 115-116 tree species with only 43 native species (37%) and 72 nonnative species (63%) (Fig.1) (City of Toronto, 2013; Eric Davies, 2014).
However, local-provenance trees are not well-studied because of the differences and unpredictability of different tree species’ crop sizes, seed obtain availability and commercially availability (Wang, 1974). For example, some species are easier to get seeds as they are abundant and produce seed more frequently than other species, like northern red oak (Quercus rubra), good and mature acorn production often with 1-3 year cycle and acorn require two seasons to be mature (Dey, 1995; Healy et al., 1999; Ontario Tree Seed Plant, 2014). While other species may be harder as they are rare and the interval of producing seeds is longer like white oak (Quercus alba), with 4-10 years reproduction cycle (Dey, 1995; Ontario Tree Seed Plant, 2014). And because seed obtains availability varies with different trees and different species have different population abundance and seed periodicity (production interval) thus leading to the short supply and lack of understanding of the local seeds (Dey, 1995; Ontario Tree Seed Plant, 2014; Wesołowski & Maziarz, 2015). The red oak seeds (acorns) are actually in high demand due to the collection difficulty and favoured by customers (Ontario Tree Seed Plant, 2014).
Therefore, more information about when and where to collect seeds, which related to seed forecast, is required for the continuing seed supply of Toronto’s local heritage trees which is in high practical value (Wang, 1974). The more forecasted of seed crops, the higher the possibility of reducing resource waste and ensure not missing the good crop time (Ontario Tree Seed Plant, 2014). Although the Ontario tree plant provided good guidance, the exact timing of seed production and periodicity are not always accurate in the field. Suggested by the Ontario tree plant (2014), acorns from red oaks are ready to be collected in September but according to previous years’ study and experience, they are actually ready in August, which means the real situation may be different from the reference manual and may result in missing of seed collection if lacking practical study of local heritage trees.
This project has four main components: 1) review the policy of Toronto’s urban forestry restoration and determine the need as well as opportunity for local seeds, 2) review the ecological characters of Toronto native trees and evaluate their seed collecting availability and rank their prioritization, 3) conduct fieldwork to obtain data on seed availability, 4) outline the opportunity for using Toronto heritage trees as a seed source for local forest restoration.
Method & materials
Firstly, is the review of the City of Toronto’s policy related to forest restoration by searching related documents, try to find out where the City use local native tree seeds and send questionnaires to find out how much seeds they used and for what species and further conclude the need of native tree’s seed in the City of Toronto.
Secondly, is the review of the ecological characters of Toronto native trees including their seed production intervals, population abundance and current status. Then evaluate their seed collecting availability by searching related documents and rank their prioritization by their population abundant and seed production intervals. Thirdly, is to conduct fieldwork to obtain data on seed availability followed the methods from previous 2015-2018 Eric’s study- MFC thesis (Jane Michener: A Protocol for Seed Forecasting Oaks in the Toronto Ravines, 2016; Lepoivre Vincent: Old growth trees, seed collection in Toronto, 2017; Mathiled Kropin: Links Between Seed Forecasting and Tree’s Characteristics and Emilien Soulat: Internship Report A2, 2018) and made some updates. Study was conducted in Toronto ravines within the Greater Toronto Area (GTA), including Cedarvale Ravine, Nordheimer Ravine, Rosedale Valley, Park Drive, Evergreen Brickworks, Edwards Gardens, Blythwood Ravine, Sherwood Ravine, Burke Brook Ravine, Sunnybrook Ravine, Crothers Woods, E.T Seton Park, Vale of Avoca, and Moore Park Ravine. And also downtown Toronto areas like the University of Toronto campus and Queen’s Park.
For Tree inventory and mapping, Eric Davies’s Google map-Big trees of Toronto (Fig.2) combined with the phone’s Google Maps app was used to find the exact location of those big trees, this tree map was developed for four years (2016-2018) mainly for native tree species in ravine areas in GTA and this is the fourth year (2019). So we only added and mapped big trees that are not on the map. We found the way to those areas by public transport first; when we meet with big trees (diameter at breast height (DBH) larger than 50 cm) that are not on our map we 1) identify each tree species, 2) give every tree a number, 3) record down their exact location by using GPS or Google maps, 4) measure DBH (in centimetres) at 1.37 meters by using Richter 5m Tree Diameter Tape and 5) measure tree height (in meters) by using Nikon Forestry Pro-Laser Rangefinder (Fig.3) at least 10m away from the tree and from the top of the tree’s canopy to the base (Fig.3). If we are not able to see the tree’s canopy, the height will be recorded as “Unknown”; if we are not able to get close to the tree, DBH will be recorded as “Unknown”.
For tree seed forecasting, this project’s method was continuously followed by the method of 2015-2019 Eric’s study-MFC thesis as mentioned above and made some updates. We used Eric Davies’s Google map-Big trees of Toronto (Fig.2) and estimated this year’s (2019) crop size of some native tree species. And we focus on oaks species which are valuable not only because they are long-lived are species but also their acorns are important to the entire forest systems as well as human history (Auchmoody et al., 1993; Dey, 1995). Oak species we focus including red, white and bur oak (Quercus macrocarpa) and seed production of red oak for next year were also recorded (2020).
For oak species, we picked one side of the tree that we’re able to see the canopy through binoculars. Then, three random terminal branches were selected and RICOH PENTAX SD 8×42 WP binoculars for observing seeds on the branches will be used to observe the number of this year’s (2019) mature acorns (the second year acorn) on branches (Fig.4) and record down the numbers. Then under trees, we looked for branches that drop on the ground (because of wind or rain) to count the number of immature acorns (the first year acorn) (2020) (Fig.4). If we cannot identify those branches on the ground drop from which specific tree, we will not take the branches as well as the acorns into account and the 2020 seed forecast data will be recorded as “NA”. The Mean Acorn number and standard error were calculated by adding up three branches’ acorns amount and divided by three (three branches), then add and minus 2 as the standard source of Error.
For other native species, as we did for oak species, we also picked different sides of the tree that we’re able to see the canopy and used binoculars to observe the number of seeds (cone/samara/fruit) on terminal branches. We tried to observe three branches for every tree and recorded down the exact number of seeds (cone/samara/fruit) produced in this year. The Seed forecasted data are recorded as 0 (without seed) and 1 (with seed) for every tree. Data were collected and then imported to Excel for further analysis.
My expected outcomes including 1) the review of the policy of the City of Toronto urban forestry restoration, conclusion of the need and opportunity of local seeds and researches on present seed supply; 2) the review the ecological characters of Toronto native trees and evaluation of their seed collecting availability and developed a species list by ranking their species prioritization; 3) conducted fieldwork including tree mapping map and inventory more old-growth native trees within ravines areas and seed forecast overall tree species in the exiting Big tree map and checked the seed availability of native trees; 4) the outline of the possibility of using local heritage trees as living seed bank and seed source for seed supply to forest restoration. Management implication
This project can further increase the possibility of seed supply of local-provenance tree seeds to around 10,000 seeds per species per year and maybe more for some species with short seed production internal and abundant. This means, increasing the number of native species’ seedlings, saplings and trees for forest restoration. And start to analyze the advantages and disadvantages of local genes in climate adaptation, biodiversity and carbon sequence aspects.
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- Zhang, X., Bai, X., Hou, M., Chen, Z., & Manzanedo, R. D. (2019). Warmer winter ground temperatures trigger rapid growth of dahurian larch in the permafrost forests of northeast China. Journal of Geophysical Research: Biogeosciences, 124(5), 1088-1097.
- Native and non-native tree species in Toronto
- Eric Davies’s Google map-Big trees of Toronto
- Nikon Forestry Pro-Laser Rangefinder and operating principles
- Red oak acorns in the first year (left) and second-year (mature) (right)
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