Susceptibility assessment of urban tree species in Cambridge, MA, from future climatic extremes

Vulnerability of Common Urban Forest Species to Projected Climate Change and Practitioners Perceptions and Responses

While urban forests are often identified as part of climate change adaptation and mitigation strategies, less attention has been given to vulnerabilities urban trees may have to a changing climate and practitioners' response to those vulnerabilities. Yet, current planting and management decisions will impact how urban forests fare under future climatic conditions. We explore a case study of Mississauga (Ontario, Canada) to examine (1) if common urban forest species are vulnerable to two scenarios of projected climate change, (2) the experiences and responses of urban forestry practitioners to climate change, and (3) whether urban forestry practitioners' experience and practice are aligned with the vulnerability assessment. Vulnerabilities of 27 common species were examined based on 2071-2100 regional climate projections. Interviews were then conducted with practitioners working in the public and private sectors. The results suggest that the majority of examined species will be vulnerable to multiple conditions associated with projected climate. Practitioners all perceive recent changes in climate and extreme weather patterns, but do not prioritize future climate conditions in their species selection decisions. Moreover, they expressed uncertainty about how to make species selection decisions in light of climate change. Given the predicted vulnerabilities, alternative species need to be considered or more management resources (e.g., for watering) will be required to maintain the current composition. However, the lack of focus on future conditions by practitioners raises concerns, while also highlighting the need for more information about appropriate management strategies.

Decreased losses of woody plant foliage to insects in large urban areas are explained by bird predation

Despite the increasing rate of urbanization, the consequences of this process on biotic interactions remain insufficiently studied. Our aims were to identify the general pattern of urbanization impact on background insect herbivory, to explore variations in this impact related to characteristics of both urban areas and insect-plant systems, and to uncover the factors governing urbanization impacts on insect herbivory. We compared the foliar damage inflicted on the most common trees by defoliating, leafmining and gall-forming insects in rural and urban habitats associated with 16 European cities. In two of these cities, we explored quality of birch foliage for herbivorous insects, mortality of leafmining insects due to predators and parasitoids and bird predation on artificial plasticine larvae. On average, the foliage losses to insects were 16.5% lower in urban than in rural habitats. The magnitude of the overall adverse effect of urbanization on herbivory was independent of the latitude of the locality and was similar in all 11 studied tree species, but increased with an increase in the size of the urban area: it was significant in large cities (city population 1-5 million) but not significant in medium-sized and small towns. Quality of birch foliage for herbivorous insects was slightly higher in urban habitats than in rural habitats. At the same time, leafminer mortality due to ants and birds and the bird attack intensity on dummy larvae were higher in large cities than in rural habitats, which at least partially explained the decline in insect herbivory observed in response to urbanization. Our findings underscore the importance of top-down forces in mediating impacts of urbanization on plant-feeding insects: factors favouring predators may override the positive effects of temperature elevation on insects and thus reduce plant damage.