Interviews were conducted with seven urban forest professionals working in the City of Mississauga, including practising arborists and municipal employees. The following sections describe participants experiences and challenges with urban forest management, recent trends in climate, changes in species composition over time, and their judgement of the vulnerability matrix.
4.4.0 Recent Trends in Climate and Weather Events
All participants mentioned that they had noticed changes in climate over the time period they had worked and/or lived in the area. In general, they mentioned or alluded to noticing hotter, drier, and longer summers; warmer and shorter winters; longer growing seasons accompanied often by phenological changes in species; more frequent and intense extreme weather events; and greater variability and fluctuations in weather patterns. All participants mentioned they agreed with the results of the climate models used in this study. Most of the statements from participants on climate conditions were in line with modeled changes of future climate.
Participants described various types of extreme weather events they have noticed, including but not limited to an increasing number and intensity of ice storms, wind storms, droughts over time, as well as the potential for tornadoes to track up into Ontario given the northward shifts in climate zones. Some participants referred to 100-year storms, and even 200-year storms, becoming more common in the past decades. One participant mentioned that ice storms should be considered with greater priority because they may become more frequent. His explanation was that the “boundary between warm and cold seems to be moving towards the Mississauga region”, clarifying that the moisture laden air from the south coming to meet the northern cold air would freeze on trees and cause “devastating” damage. Multiple participants stated that ice storms have become more common in southern Ontario.
Participants mentioned how gradual seasonal changes have been replaced by less gradual and more fluctuating seasonal changes. One participant specified that patterns of weather have become unpredictable from the regular pattern of wet spring, summer, wet fall, winter, stating that “moisture and heat are now appearing at different times less predictably”. Another participant mentioned that there is “almost no spring and fall seasons”. The fluctuations were also linked to phenological shifts in species, such as earlier budding and leaf flushes. One arborist suggested that native species that rely mainly on light, as opposed to temperature, for their phenological rhythms will be more vulnerable to shifting conditions, also stating some non-native species may not be as vulnerable in this sense. Multiple participants mentioned warm swings in spring were resulting in earlier leaf flushes that were often followed by cold spells, killing the forming buds, stressing trees, and frequently driving younger trees to mortality. One participant stated that “if you have warm winters over a decade and decide to bring in more Carolinian species, then if you have -40 in some years, those species will die”. This suggest that these types of extremes within seasons would be very stressful particularly for more southern Carolinian species that grow in warmer climate ranges.
Precipitation was seen to be more flashy and variable today than in the past in the experience of participants. Unpredictable flashy winter precipitation as rain was noted to erode top soil layers due to added water not being able to percolate into the frozen, thus less-absorbent, soil. Multiple participants suggested that the amount of snow and time period of snow cover has been decreasing, referring to warmer winters being the primary cause of this phenomenon. One participant suggested that given the unusually high amount of precipitation as rain this winter (2017), Southern Ontario will have much doughtier conditions in the coming summer (2017) due to the lack of water in the groundwater table.
Summers were also seen to be longer, hotter, and drier with the increasing temperatures resulting in drought stress, the dropping of leaves, and scorch damage in trees. The rising temperatures has been creating longer growing seasons in participants’ experiences. One participant stated that “fall-time leaf collection was originally in September, but is now occurring in October and even into November some years”. Wind patterns were also noted to be at higher velocities and more turbulent than before. This can result in tree tipping when combined with water-saturated soils.
4.4.1 Species Composition
Participants were in agreement that there was a significant lack of diversity in plantings in the past 60 years and a heavy reliance on one or two species for canopy cover (i.e. elms and ashes). The canopy suffered heavy losses when the planted monocultures were subject to disease and pests. Older tree pallets considered species of lindens, locusts, Norway maple, Austrian pine, and blue spruce for heavy plantings. Over time, learning from these mistakes and heavy losses, more diverse pallets were chosen and prioritized by municipalities. Hence, five to seven different species per street are now considered for planting. These are also mixed in with perennials and shrubs for greater diversity and soil health. Municipal urban foresters are also shifting towards prioritizing more native species when planting, where site conditions allow, by “planting more diverse pallets” and “changing the patterns and locations of species”. Participants stated that the species list chosen for this study was representative of the current urban forest composition based on their experience.
4.4.2 Vulnerability Matrix
Participants independently stated that the species vulnerability matrix was generally representative in its scoring of the different climate tolerances for each species. A few participants expressed some minor differences in species selection preference and their climate tolerances. For example, one participant was surprised by tulip tree’s lack of heat sensitivity given that it is a woodland species that likes moist environments. Another participant pointed out that sugar maple may be more vulnerable than the matrix suggests. One participant suggested that there would be an eventual reduction of coniferous species, especially long-lived ones. While the vulnerability matrix suggested similar ideas, most participants saw the coniferous species present on the species list to be hardy to drought and heat conditions.
Finally, participants pointed out that the vulnerability matrix may change over time due to species adaptation. They suggested that species vulnerability is affected by how “plastic”. “competitively fit”, and adaptable a species is to change, some species being more plastic and vigorous than others over time. Species were seen to adapt better to gradual shifts in climate, rather than multiple acute stressors over consecutive years, and that older established trees are better able to adapt than younger individuals.
4.4.3 Urban Forest Management
All participants suggested to some degree that urbanization was their biggest challenge when managing the urban forest. Under the term urbanization were physical factors such as low soil quality and volume, soil compaction and erosion, lack of water penetration in urban soils, anthropogenic pollution, urban intensification, lack of growing space, isolation of individuals (i.e. planters), as well as social issues such as lack of protection during development, low prioritization of green infrastructure, limited reach and resources of municipalities, and lack of resident education on maintenance and planting. Many, if not all these factors were seen to be mainly anthropogenic.
Pruning was suggested as another key challenge because it causes (intentional) damage to trees opening them up to decay and pests. This is a risk for species such as basswood that do not compartmentalize wounds well. Pruning methods seemed to be a point of debate within the industry and was dependent on individuals’ preferences. Some participants felt that pruning “makes individual less fit” and “causes [tree] failure”. One participant suggesting that it is “overdone” by practitioners, even though pruning is considered to be a standard practise in the industry. Other challenges mentioned were unpredictable weather and climate, disease and pests, invasive species, lack of diverse species available at nurseries, as well as an overall shortage of tree stock.
Participants suggested that climate change was not a top priority when managing urban forest species, but still part of their management process. A list of factors considered by participants when managing urban forests is shown in Table 8. Limiting site factors and social concerns were seen to be of greater priority because these were factors that participants “could control”. One participant suggested that he has the ability to give a particular tree the “best competitive advantage” by mimicking conditions in its natural environment and using management techniques such as mulching, but could not ultimately control climate conditions. This participant also suggested that trees are now growing in isolation as opposed to in communities unlike in their natural environment and this is exacerbating stressors. Many participants add organic content into the soil using compost or specific soil mixes when planting to increase chances of establishment and survival. Apparently, this procedure is a common practise in urban areas due to degraded soils.
Table 8. Factors considered when managing urban forest species.
|Hardiness zones (i.e. climate)||Mature tree size|
|Tree hardiness||Growth rate|
|Native vs. Non-native||Root structure|
|Organic matter content||Composition (i.e. clay, silt, sand)|
|Wind direction||Light exposure|
|Salt exposure||Urban heat islands|
|Resident needs and opinions||Messiness and aesthetics|
|Public safety||Willingness to have a tree|
|Use of site by public|
|Accessibility for maintenance||Major roadways|
When climate change was considered, participants would address it by selecting more southern Carolinian species or hardy non-natives when site factors and nursery stocks would allow. Species such as Ohio buckeye (Aesculus glabra), Kentucky coffeetree (Gymnocladus dioicus), swamp white oak (Quercus bicolor), eastern red buds (Cercis canadensis), tulip tree, lindens (Tilia spp.), among others were considered for current planting. Only one participant went into detail about the consideration of timeframes for tree assessment in regards to climate change, and how that would impact safety for both the tree and surrounding infrastructure. He stated an example about how tulip trees, a Carolinian species, planted in Washington DC were highly susceptible to microbursts. He stated there is potential that “microbursts and tornadoes may become more common as the tornado alley zone moves north”. Therefore, he is hesitant to plant many tulip trees even though they are considered a “survivor” in urban contexts. Some participants mentioned that future management should involve planting species that can survive both extremes of hot and cold conditions if extremes are becoming more likely. Specific species were not suggested.
An issue was raised by some participants about nurseries not having the appropriate stock in recent years. Nursery stock was seen to limit what participants were able to plant, thus limiting the species diversity in the urban canopy. A participant suggested that nurseries had long turnover times for being able to produce tree stocks and that issues would occur when trees went unsold after years of nursing due to change in planting preferences. One participant expressed that this problem was currently being addressed by municipal governments and nurseries meeting to collaborate on the issue.
Several participants noted that the large area of hard, impervious surfaces is resulting in amplification of urban heat islands, as well as intensifying the effects of climate change. One arborist stated that disease and pests have caused larger problems than climate change so far, but also noted that climate change ultimately would increase pest survival. Multiple participants stated that urban tree mortality could increase as climate change intensifies with some species already seeing a decrease in that lifespan. This was stated in the context that one participant believed that tree species survive only 20-30 years in urban spaces due to lack of suitable conditions.
A majority of participants stated that native species, while highly recommended in municipal planning documents, are not always preferred for planting. In participants’ experiences, many non-native species were better able to handle urban conditions especially along streets, while natives were generally more susceptible to the low quality conditions present in urban areas. Specifically, sugar maple was avoided by many practitioners for planting. While non-native species like honey locust, Norway maple, and Norway spruce were preferred by a majority of participants (Table 9). When asked why non-native species were preferred, participants often used words such as “hardy”, “genetically fit”, “high vigor”, “more adaptable”, “survivor”, and “more plastic” to describe these species. However, there were some native species considered to be hardy in urban conditions such as red maple, silver maple, Northern hackberry, white mulberry, and red pine.
Participants have differing opinions on which species are preferred for planting or considered resilient in their work, as well as various reasons as to why they preferred or avoided those species (Table 9). From here on, the idea of preference also implies that the chosen species were also considered resilient in urban areas by practitioners. Practitioners were congruent on avoiding sugar maple, seeing it as a non-resilient species, and had a clear preference for honey locust as an urban tolerant species. However, many responses were contradictory, such as one participant stating that basswood was extremely hardy in urban settings, while another said that it was often prone to decay in these locations. Sixteen out of 27 species received more opposing responses to open-ended questions about which species are preferred for planting or not.
Table 9. Participant responses of their planting preferences and reasoning.
When planting preferences are compared to the vulnerability matrix, there is both agreements and discrepancies as to what the matrix shows for future potential vulnerability and what practitioners prefer for planting. In terms of agreements, honey locust, silver maple, northern hackberry, white mulberry, and downy serviceberry have relatively low cumulative PVs and are preferred for planting. In terms of discrepancies, Norway maple and Norway spruce are preferred by many for planting, but have a moderate cumulative PV. White oak has a very low cumulative vulnerability, but was not highly preferred for planting mainly due to issues with shoestring root rot. Some participants did not even mention their (lack of) preference for white oak. Bur oak also has a low cumulative PV, but has contradicting views in terms of overall preference among practitioners. Red pine has a high cumulative PV, but is preferred by practitioners. Non-natives received more preference and less opposition than native species, even though the vulnerability matrix shows no specific pattern when considering cumulative vulnerability.
Finally, some participants mentioned that they had limited reach relative to private landowners where a majority of the urban canopy resides. Educating homeowners on proper planting and maintenance practises were seen as important actions towards growing a healthy urban canopy. As one participant noted, home owners would often overwater or improperly mulch trees on their property leading to death of that tree. Addressing these issues could effectively increase urban tree survival and growth.
This study highlights the cumulative impact climate change may have on the urban forest within the City of Mississauga. In general, summers are expected to become hotter, drier and longer; while winters are expected to become warmer, wetter, and shorter; and extreme weather events are expected to increase. Urban trees, particularly coniferous species, are predicted to be most vulnerable to increasing average and extreme temperatures; lack of water availability in hotter months could be highly detrimental in the future; the cumulative impacts of stressors must be considered in urban forest management; non-native species showed no difference in overall vulnerability as compared to native species; and finally, while climate change is considered in urban forest management, factors such as site conditions and social needs are greater priorities for urban forest managers.
Most of the commonly planted species chosen for this study had moderate or high PV to temperature-related climate variables. Average (MAT) and extreme (MaxWT) temperatures are predicted to exceed the optimal distribution range within which the chosen species grow competitively. Species that are out of their optimal temperature range could suffer heat damage as the climate warms. Heat stress is known to cause leaf scorch and burns to trees, and also reduces photosynthetic rates resulting in decreased growth rates and early leaf senescence (Teskey et al. 2015). Heat stress can be further exacerbated in urban areas, particularly near dark surfaces such as asphalt, making the probability of heat-related damage to trees more likely. Regionally this is a concern because a large proportion of species are considered potentially vulnerable to increasing average and extreme temperatures.
In regards to climate predictions, climatic extremes and variability (in temperature, precipitation, etc.) are more important than changes in average conditions because they can result in a greater degree of damage to ecosystems and infrastructure (Katz & Brown 1992). Extremes may have less of an impact in areas where temperatures and moisture are optimal relative to species’ tolerances, but in areas where conditions are closer to species’ physiological limitations, climatic extremes can be much more stressful (Zimmermann et al. 2009). In Mississauga, if average temperatures rise close to species’ limits this could predispose many species to stress, which can then be exacerbated by extremes. Taking into account that urban heat islands further exacerbate the effects of both average and extreme temperatures is essential for tree health and management.
Evergreen coniferous species are predicted to be more vulnerable to average and extreme temperatures than broadleaf deciduous species. In contrast, multiple urban forest professionals interviewed considered coniferous species to be more tolerant of heat and drought stress in their experiences. It is unclear in the literature whether conifers are have greater adaptive capacity to heat stress. When exposed to heat wave conditions, loblolly pine (Pinus taeda) seedlings were less susceptible than red oak (Quercus rubra) seedlings, in part because because P. taeda has a less vulnerable photosynthetic apparatus (Ameye et al. 2012; Bauweraerts et al. 2014). However, a study using a multi-method analysis to measure the vulnerability of forest species to climate change across the US Mid-West showed that conifer-dominated forests tend to have higher vulnerability ratings than oak-dominated forests (Brandt 2014). Conifer-dominated forests were considered to be more vulnerable in this analysis because these ecosystems were adapted to high elevation or colder, northern climates (Brandt 2014).
It is possible this coniferous vulnerability result is a limitation of using climate envelopes for vulnerability studies. Climate envelopes do not illustrate the full possible range of environmental conditions a species can survive (i.e. fundamental niche), they only suggest a spatial range within which the species is currently distributed (i.e. realized niche) (Brandt et al. 2017; McKenney et al. 2007b). Additionally, studies suggest that maximum temperature alone is a weak factor for defining plant distribution ranges (Woodward 1987; Zimmermann et al. 2009). Tree species are more likely to be limited by faster-growing competition at their most southern range limits, where climate conditions are more favourable for the growth of a broad range of species (Brown et al. 1996; Loehle 1998). If resources are abundant, conifers may be able to survive in urban areas where trees have little interspecific competition and are maintained by humans. Boreal coniferous species have shown to grow in southern states (Loehle 1998; Yang 2009) meaning these species would likely survive in the predicted warmer climate of City of Mississauga. Although, with warming climate, northern species that require cold stratification may not be able to naturally regenerate in remnant forest patches that are present throughout Mississauga. Other species that are not inhibited by cold stratification requirements would likely take their place resulting in a composition shift in semi-natural forests near ravines and on conservation lands within the municipalities boundaries.
Results predict that deciduous species, could fair better over the next century as (winter) temperatures increase and growing seasons expand in Mississauga, if moisture and nutrients are not limiting growth (Colombo 2008; Loehle 1998). Specifically more southern species such as American basswood (Tilia americana) and honey locust (Gleditsia triacanthos) may thrive. This is in line with the movement of many species’ climate envelopes northward in North America (Goldblum & Rigg 2005; McKenney et al. 2007b). The northern limits of these species are controlled by extreme minimum temperatures, growing season length, phenology, and frost resistance (Charrier et al. 2015; Jennerette et al. 2016), but it is possible that as temperatures warm these factors could be less limiting to the growth of deciduous species. However, as participants noted, survival of all trees in Mississauga will likely be affected by the increasing variability in climate. Specifically late-spring frosts were mentioned as a highly detrimental climatic event to the urban forest canopy. Late-spring freezing can often result in bud and leaf damage, and tree mortality if damage is severe, especially in younger trees (Cannell 2012; Charrier et al. 2015; Loehle 1998). The range of climate extremes could pose a serious species selection problem for urban forest managers trying to balance environmental conditions while maintaining urban canopy.
5.2 Water availability
Plant distribution is dependent on many other factors besides temperature. Other factors such as precipitation, phenology, soil conditions, growing season length also have impacts on plant distribution (Brown et al. 1996; Mathys et al. 2014). Changes in spatial and temporal patterns of water can have substantial impacts on species distribution and performance (Mathys et al. 2014; Weltzin et al. 2003). Precipitation in Mississauga is predicted to increase in colder months and remain relatively stable in warmer months. To make a meaningful conclusion about water stress, this information needs to addressed in conjunction with the predicted increasing temperatures, decreasing CMI over time from May to October, and extending growing season. Together, these results suggest a very different precipitation regime than historic patterns. In these climate scenarios, summers would become drier over time moving to a more grassland-like climate, with likely less precipitation as snow in the winter, and shorter winters overall. Interview participants confirmed that these changes are already occurring. A climate change study conducted for the City of Toronto, located adjacent to the City Mississauga, also predicted similar conditions in 2050 (SENES 2011). Additionally, the study suggested that rain events would become less frequent but more intense, a recent trend also identified by interviewed participants (SENES 2011).
Water availability in these conditions for urban trees could be very unstable, presenting issues of too little water (i.e. drought) or too much water (i.e. flooding) at different times of the year. Urban environments are particularly susceptible to drought and flooding due to the high amount of dark impervious surfaces. While short periods of flooding are less of a concern, extended flooding could lead to greater tree stress, and mortality if prolonged flooding occurs (Bratkovich et al. 1993). Decreased precipitation as snow, or rapid melting following a snow storm could make flooding more frequent annually over the next century (Brandt et al. 2017). Multiple participants mentioned that winter rain events while the soil is frozen could lead to runoff and reduced water availability in summers. It is possible shifts in snowcover, soil frost, and freeze-thaw regimes could lead to reduced water availability in spring and summer (Brandt et al. 2017). Trees are known to be susceptible to winter soil water recharge regimes depending on the patterns present in their native habitats (Lévesque et al. 2014). However, there is limited research available on the complex relationship between these factors and tree health.
Hotter and drier climates will have substantial impacts on urban species (Fahey et al. 2013). Large portions of the canopy are vulnerable to drought and generally only have intermediate abilities to use water efficiently. If climate moves towards a more grassland-like habitat over the next century, many trees will be vulnerable to hydraulic failure and carbon starvation, if they are not provided with adequate watering (McDowell et al. 2008). It is important to note that moisture availability has a stronger impact on tree stress and mortality than heat stress, but warmer temperatures can exacerbate the impacts of moisture deficits (Bauweraerts et al. 2014; McDowell et al. 2008).
Finally, historic CMI had large variations in all months over a 30-year period. While CMI averages are predicted to decrease, monthly CMI could greatly fluctuate over the years, meaning that future moisture deficits could be much more or much less severe over time than is suggested by climate model averages. The extent of drought-based mortality is known to depend on duration, frequency, intensity, timing, and the spatial extent of drought, as well as an individual species’ phenology, site conditions, and ability to acclimate to conditions (Brandt et al. 2017; Fahey et al. 2013; Maherali et al. 2004; McDowell et al. 2008, Xie et al. 2015). IPCC reports predict that droughts will increase in frequency and intensity over the next century (IPCC 2013). Allen et al (2010) show these trends are already occurring in North America and leading to wide-spread tree mortality in natural forests, so it is reasonable to assume that variation may lean towards greater drought in urban forests.
5.3 Ice storms
In Mississauga, there are very few species that are highly vulnerable to ice storms both within the matrix and regionally. Although, a high proportion of species have moderate vulnerability to ice storms. Trees that are moderately vulnerable are more likely to incur non-lethal structural damage than trees that have low vulnerability. A likely scenario is that damage would weaken moderately vulnerable species and eventually lead to mortality if exposed to other stressors such as disease and future storms (Forest Ontario 2014). While a high proportion of species regionally in Mississauga have low potential vulnerability scores to ice storms, this does not mean that these species are entirely invulnerable.
The overall impacts of ice storms on the urban canopy are dependent on storm duration, intensity of winds, and ice accumulation, in addition to individual tree life history and structure (Hauer et al. 2006; Irland 2000; Smith 2015). Factors that predispose trees to ice storm damage include weak branch junctions, pre-existing dead branches, previous wounding and stress, unstable root structure, and large unhealthy tree crowns (Hauer et al. 2006; Smith 2015). Additionally, tree species is a better indicator of whether a tree will recover from an ice storm than age or size (Luley & Bond 2006). In this respect, the individual life history and structure in combination with knowing a species particular vulnerability can help improve identification of trees that will be most vulnerable to ice storms than either factor alone. Preventative actions such as pruning or taking down the tree can be done before storm events occur once an unhealthy tree is identified. This can not only help to maintain the health of the urban forest, but can also avoid costly damage to infrastructure caused by damaged or tipped trees.
5.4 Cumulative impacts
Cumulative potential vulnerability (CPV) in this study showed that most species had high PV to at least one climate tolerance category, and most species chosen for this study had moderate or higher CPV. Regionally, a high proportion of moderately vulnerable species exist within the canopy. Research by Brandt et al. (2017) assessing the climate vulnerability of Chicago’s urban forest also showed that a high proportion of species had moderate or higher vulnerability overall. This is reasonable considering that rather than single stress events, urban forests will likely face multiple climate shifts and stressors throughout future years that may have interacting impacts. As previously mentioned, ice storm damage can be more severe for unhealthy trees that have suffered previous injuries or stress. Water availability strongly influences the effects of heat stress on tree mortality (Bauweraerts et al. 2014; Teskey et al. 2015). Also, extended growing seasons could potentially exacerbate water stress, especially in a warmer climate (Brandt et al. 2017; McDowell et al. 2008).
Other examples of interacting impacts are the ways drought and ice storms can cause trees to be more vulnerable to secondary effects of insects, pests, and disease (Fahey et al. 2013; Hauer et al. 2006); prolonged warm and wet periods can also facilitate transmission of emerging disease (Woods et al. 2005); heat events can cause earlier budburst depending on the species (Teskey et al. 2014); and frost events, heat stress, rainfall patterns, and drought stress can lead to later fall dormancy in deciduous forest communities, but can also lead to earlier dormancy as well (Xie et al. 2015). Spatial and temporal patterns also play a large role in how stressors impact species. Site conditions, such as soil characteristics and land-use, play a role in drought tolerance and species survival (Fahey et al. 2013; Mathys et al. 2014). Trees that are native to mesic habitats are less affected by water availability previous to the growing season (i.e. summer) (Hanson & Weltzin 2000; Lévesque et al. 2013). Lagged responses in trees to droughts and ice storms can occur years after the events (Bigler et al. 2007; Hauer et al. 2006).
In the City of Mississauga, a possibility is that over time many trees will be cumulatively weakened by heat, drought, and extreme weather events, in addition to various other urban stressors that are present. If trees are not killed by the immediate impacts of stress, these changes would then act as the catalyst for further damage and stress by other contributing factors such as pests and disease, eventually leading to tree mortality and possibly large reductions in the urban canopy. Overall, the net effects of multiple stressors are complex to predict and largely unknown because they can have opposing, compounding, or unknown outcomes, in addition to being species and site-specific (Allen et al. 2010; Fahey et al. 2013). Mechanisms of how drought and other stressors affect tree survival are still poorly understood across various research fields (Allen et al. 2010). Thus, while this study has explored the effects of climate change on species, cumulative effects represents a knowledge gap within the literature that has yet to be fully addressed.
5.5 Native vs. non-native species
The results of this analysis were surprising in that no clear difference was found between natives and non-native trees overall. However, multiple interview participants showed strong preference for non-native species because they considered them requiring as less maintenance and also as more resilient in urban environments based on their past experiences. It is likely that because only a few variables were used in this climate vulnerability assessment, the analysis was not comprehensive enough to show a definitive relationship between native and non-native species. Brandt et al. (2017) conducted a more comprehensive climate change vulnerability assessment using a larger dataset and including variables such as adaptive capacity in their analysis. They show that that 77 percent of trees that have low vulnerability scores in Chicago’s urban forest are (non-native) invasive species, a more defined relationship than this studies’ results.
Native species are important because they help to maintain balanced ecosystem dynamics in their respective regions (Bassuk & Sutton 2012; McKinney 2002). Restoring native plant species can increase the species richness of native animal populations (Sears & Anderson 1991). There is some anecdotal evidence to suggest that native species had less damage than non-native species during the 2013 ice storm that occurred in Southern Ontario (Cary 2014). Other evidence leans in favour of non-native species. Some studies suggest that non-native species can be more resilient, and even restorative in certain environments. In Puerto Rico, non-native species were able to colonize eroded soils that were once pasture, while native species in the area could not (Lugo 1997; Rodriguez 2006). Non-natives have also shown to provide suitable habitat for native species years after non-native colonization, whereas only one native species was able to colonize the degraded areas in control plots (Parrotta 1999). In the United States, studies found that there was a strong positive correlation found between abundance of non-native berry trees and abundance of birds, and that non-native berry trees acted as signal for food availability that then enhanced the seed dispersal of native species present in non-native dominated forests (Davis 2011).
A report from the Oregon Department of Forestry states that native trees from their region can often not survive urban conditions as they are adapted to swampy environments that were existent historically (Ramstad & Orlando 2009). The report suggests that homeowners should consider site conditions and the environmental context when choosing between natives and non-native trees in urban areas. These consideration were also emphasized by interview participants.
It is possible that definitions of native and non-native species will have to be redefined as climate change leads to shifts in plant distributions, especially if changes are drastic. If species currently native to a region cannot survive in future climate and site conditions, compromises will have to be made to introduce non-native species that can integrate into the urban canopy and maintain ecosystem services (Bassuk & Sutton 2012; Ste-Marie 2011). Although, cultivars of native species also present a possible solution for urban forests in response to climate change. For example, some nurseries within the United States have been working to breed more resilient cultivars of native species that can withstand changing climate conditions. Species products such as Redpointe® Maple and Emerald Sunshine® Elm have proven to be more resilient to wide range of climate and growing conditions than their naturally occurring counterparts (Warren 2014).
5.6 Interviews and management recommendations
Generally, conditions in urban areas tend to become harsher as resemblance of environmental conditions diverges from a species natural habitat (Roloff 2013). While this is not a rule, it is a suitable starting point to address the several issues that exist within urban areas, given that climate change may exacerbate the spectrum of stressors. It is evident that site conditions such as soil quality and volume, microclimates, public use, and various other factors can impact tree health tremendously (Bassuk & Sutton 2012; Brune 2016;). In line with this research, all study participants stated that the site conditions of an individual tree play an integral role in urban forest management. So much so that urban forest managers considered it one of the top priorities, above climate change, when planting. Participants’ reasoning was quite clear: if urban trees do not have quality conditions in their immediate environment they will likely not survive long enough to be exposed the effects of long-term climate change.
Many trees in urban environments that can make it past the first 3-5 year establishment period generally only live to be 13-20 years old, then have to be replaced (Roman & Scatena 2011). This is because the accumulation of stress tends to impact these species relatively early and more intensely. Not only is this costly, but continual replanting doesn’t achieve the goal of carbon sequestration that is often stated as an important role of urban forests to mitigate climate change (Nowak et al. 2002). As was suggested by one participant, resources should be placed into planting smaller number of trees and maintaining them more intensively so they live longer. This is a better strategy because larger, older trees sequester carbon at a much quicker rate than smaller trees of the same species (Nowak et al. 2002). Moreover, larger, mature trees overall provide a higher of level ecosystem services than younger, smaller trees (Nowak et al. 1990).
Water availability will be a key factor in the determining the long term survival of urban tree species. The increasing impacts of drier, hotter, and longer summers can be abated if sufficient water is made available for them. Withlow and Bassuk (1987) show that trees in locations with city infrastructure can be less susceptible to droughts is they are well maintained and provided with adequate soil water. Various techniques such as mulching and polyethylene bags can also be used to keep moisture in the soil (City of Toronto 2012; City of Toronto 2013; Vogt et al. 2015). However, these will have to be combined with consistent deep watering and maintenance if droughts become more severe. In the long term, if urban water systems become limited, the urban canopy may have to be gradually replaced with a completely different mixture of drought-tolerant species and also utilize different planting practises (i.e. drought tolerant landscaping) if climate moves towards a grassland-like habitat.
Species selection will be another key factor in the long term survival of species. One participant clearly stated that given the variability in climate, newly planted species will need to be able to handle both warm and cold, and wet and dry extremes. Species considered for future planting need to be able to survive winter temperatures and potential frosts, as well as able to handle hotter, drier summers. The VM suggests native species such as red maple and white oak that have low CPV could be especially useful in these scenarios given that they have high drought tolerance and can survive Ontario’s winters. However, it is possible that fluctuating and extreme climatic conditions could narrow the selection of viable species, thus species diversity of the urban canopy would decrease, especially if only native species are being chosen for planting. Therefore, the process of bringing non-native species into the urban canopy within the City of Mississauga may have to be employed to maintain ecosystem services (Aitken & Whitlock 2013). In this scenario, managers would have to be cautious of non-native species that have the potential to become highly invasive in both urban and natural habitats (Bassuk & Sutton 2012). Alternatively, if the appropriate cultivars can be bred to handle the changing climate, urban canopies may not require as big of an adjustment (Warren 2014). However, a greater diversity of species may be able to be planted as climate warms, if adequate amounts of water are made available.
Participants were also clear that their reach within the urban canopy is limited. A large proportion of urban trees are located on private land, giving landowners a considerable amount of control over the health and survival of a majority of the urban canopy (TRCA 2011a). One example of this is that homeowners were stated to be “loving their trees to death” by either inappropriately mulching or overwatering trees. Thus, homeowner education and involvement was considered necessary for a healthy urban tree canopy. If homeowners are more involved with the lifecycle of their trees and provide regular maintenance, then they can prolong the benefits that the trees provide (Vogt et al. 2015). This can bring homeowners financial savings in the form of reduced energy use, and avoid costly infrastructural damage, especially in the event of ice storms.
These factors mentioned all require thoughtful and consistent management through the life cycle of a tree. If climate conditions are to change as predicted, all stakeholders will have to play their part in maintaining the urban forest canopy. Ultimately, species that are managed and watered according to the needs of individual species will likely survive longer, maintaining benefits for an extended period of time (Brune, 2016; Vogt et al. 2015). If species cannot be managed adequately on certain landscapes or locations, then hardy native and (more likely) non-native species are the best options for urban landscapes (Bassuk & Sutton 2012). In the long run, species from other regions may need to be considered if climatic conditions no longer allow the survival of the species assemblages currently planted (Aitken & Whitlock 2013).
Conclusion & Future Research
In order to assess the vulnerability of commonly planted native and non-native species to climate change, climate projections over the next century, climate envelopes and the species’ physiological characteristics were analyzed for common Mississauga tree species. Interviews were also conducted with urban forest professionals such as arborists and city employees working in the municipal urban forestry department.
In general, all commonly planted urban forest species are moderately vulnerable to at least one aspect of the changing climate. Most species are predicted to be moderately or highly vulnerable when all climate tolerance categories are summed. Many of the species analyzed are predicted to be quite vulnerable, or out of their optimal distribution range, in regards to increasing temperatures. More northern species like conifers may still be able to survive in warmer, drier climates with adequate watering and management. Rainfall may become less frequent and more intense over time putting urban species at risk of flooding. On the other hand, reduced water availability in summers in conjunction with hotter temperatures could have substantial impacts on all species, especially if droughts become severe or frequent. Ice storms may increase in the future leaving individual species vulnerable in the long term if adequate management measures are not taken to prevent structural damage. Preventative measure include tree pruning and taking down unhealthy trees. Surprisingly, no relationship was found between the cumulative vulnerability scores of native and non-native species. It is likely more variables need to be assessed to address whether native or non-natives may be more vulnerable.
Overall, urban forest managers agree that site conditions play a vital role in the survival of the urban forest species more directly than climate change. This emphasized the need for providing tree species with better quality conditions in urban environments if species survival is a priority. In cases which species cannot be adequately managed, hardier species will have to be chosen for harsh city environments. Finally, adequate management can only be achieved if all stakeholders, particularly private landowners within the city, are involved and educated to the primary needs of the urban forest.
6.1 Recommendations for Future Research
Future research projects using climate change vulnerability assessment methodologies would benefit from the following recommendations.
The climate models used for this study only considered bioclimatic variables as 30-year averages. To extract more meaningful and accurate representation of future climate, it would be worth projecting yearly and monthly values for temperature, precipitation, and other bioclimatic variables if data is made available in the future. This would allow the analysis of year-to-year variations and extremes in climate, as well as overall trends giving a clearer picture of climate shifts. This type of data, however, is currently not available.
The tree sample used for this study provided a sufficient representation of the City of Mississauga’s urban forest. However, the sample was quite small in comparison to the full extent of the urban forest. Thus, some species were under represented, even though they were part of recent municipal planting orders and potentially abundance in the current urban forest. Additionally, tree health and species composition can change over time with the development of urban areas and preference of landowners, respectively. A larger tree sample or full tree inventory should be considered for future projects.
Future climate vulnerability assessments in the City of Mississauga should include local and regional site characteristics such as, soil traits, microclimates, hydrology, land-use categories, and if locations are maintained as part of their analysis. This can give a more accurate picture of how trees may fare in particular locations and how distributions could change over time. Urban forest managers could then refine the use of resources and more easily assess areas that require more or less management, as well as choosing the appropriate species for that location.
Acclimation and adaptation of species to particular environmental conditions can play important roles in species survival. As participants said, the vulnerability matrix may look very different in the future than it does now due to species adaptation. Assessing a species genetic potential to adapt to certain conditions as well as reassessing this matrix in the next decade or so could be appropriate measures for addressing these points. In addition, assessing the species’ current adaptive capacity to a wide-range of urban and natural conditions would give a more comprehensive picture of individual species vulnerability to climate. Cultivars should also be included in future analyses because they can have very different tolerances to environmental conditions that their natural counterparts.
Finally, future research needs to further consider the cumulative impacts of climate change, rather than individual impacts alone. While assessing impacts of individual events can offer a reasonable perspective of how trees may respond to changing conditions, the complex spatial and temporal patterns of climate change and extreme weather events will ultimately determine the mortality or survival of individual species uniquely.