A conceptual framework of urban forest ecosystem vulnerability

The urban environment is becoming the most common setting in which people worldwide will spend their lives. Urban forests, and the ecosystem services they provide, are becoming a priority for municipalities. Quantifying and communicating the vulnerability of this resource are essential for maintaining a consistent and equitable supply of these ecosystem services. We propose a theory-based conceptual framework for the assessment of urban forest vulnerability that integrates the biophysical, built, and human components of urban forest ecosystems. A review and description of potential vulnerability indicators are provided. Urban forest vulnerability can be defined as the likelihood of decline in ecosystem service supply and its associated benefits for human populations, urban infrastructure, and biodiversity. It is comprised of (i) exposure, which refers to the stressors and disturbances associated with the urban environment that negatively affect ecosystem function, (ii) sensitivity, which is determined by urban forest structure and dictates the system response to forcing from exposures and the magnitude of potential impacts, and (iii) adaptive capacity, which is the social and environmental capacity of a system to shift or alter its conditions to reduce its vulnerability or to improve its ability to function while stressed. Potential impacts, or losses in ecosystem service supply, are temporal in nature and require backward-looking monitoring and (or) forward-looking modelling to be measured and assessed. Vulnerability can be communicated through the use of indicators, aggregated indices, and mapping. A vulnerability approach can communicate complex issues to decision-makers and advance the theoretical understanding of urban forest ecosystems.