FORUM ARTICLE Challenges and future directions in urban afforestation

1. Mature urban trees improve air quality, reduce storm water run-off and sequester carbon. Municipal agencies establish forests of native juvenile trees to enhance these and other ecosys-tem services to cities. Little data exist, however, regarding whether these trees will form mature, native forests. 2. We review urban forestry research that deals specifically with the growth, survival and recruitment of new native urban forests and use these data to identify knowledge gaps and propose research needed to create and maintain native urban forests. 3. Experimental urban forestry studies are few and most are of durations ≤5 years, shorter than the 10–25 year time frame required to understand forest stand dynamics. Studies captur-ing initial dynamics of urban afforestation (≤5 years) identify invasive species as the primary threat to native tree establishment. Data exploring longer-term dynamics are needed to evalu-ate whether early-stage afforestation dynamics can be used to infer the composition and func-tion of mature urban forests. 4. Synthesis and applications. Urban afforestation approaches – from natural colonization to large-scale plantings – represent a trade-off in cost vs. efficacy for establishing native forests. A major cost-saving strategy would be to determine whether exotics and natives can co-exist and pro-vide the intended ecosystem services. Key-words: afforestation, green infrastructure, natural colonization, recruitment, restoration, urban forest, vegetation dynamic