HOW TO REDUCE TRANSPLANT STRESS IN URBAN TREES

Urban vegetation is nowadays recognized as a key component of the adaptation and mitigation strategies against climate change. In this context, municipalities and public services are promoting large scale tree planting programs in urban environment. However, these programs frequently experience very high rates of planting failure within the first years after planting and slow establishment, jeopardizing their ecological and socioeconomic benefits. The success of a tree planting program depends on a multidisciplinary process, involving nursery practices, site preparation, species and stock selection, irrigation, and pruning. This thesis analyses how nursery preconditioning and post planting pruning can help reduce transplant stress and improve establishment, root development, and ecosystem service provision in young urban trees. It addresses four main questions: (i) whether substrate amendment with Ascophyllum nodosum extracts can accelerate nursery production and induce persistent morpho physiological traits that increase tolerance to transplant stress; (ii) whether pruning carried out according to best management practices during establishment can reduce the severity and duration of post planting stress (PPS) under limited irrigation, and how this interacts with species’ isohydric vs near anisohydric drought strategies; (iii) how different crown pruning techniques affect auxin mediated carbon allocation, root growth and below ground biomass; and (iv) how severe pruning influences tree cooling capacity and microclimate amelioration in street conditions. The first experiment was conducted in a forest nursery, where seedlings of woody species were grown in a substrate amended with different doses of a pure A. nodosum extract. Growth, gas exchange and early root development were monitored. High doses increased biomass, chlorophyll content and gas exchange, and improved germination in some species, but did not lead to stable modifications in root traits or root:shoot ratio that are clearly associated with better transplant tolerance. The second experiment was carried out within a large urban planting program and tested pruning during the establishment phase under sub optimal irrigation in near isohydric and near anisohydric species. The results indicate that the intensity of PPS was not directly related to the isohydric–anisohydric behaviour, while species identity strongly influenced recovery time. Pruning caused limited changes in water status and gas exchange, and its effect on the duration of PPS remained secondary compared with species and site conditions. The third experiment compared different crown pruning techniques on young urban trees, monitoring leaf physiology, root growth, wood anatomy and phloem auxin concentration. Both pruning treatments did not lower root growth but instead improved the total biomass of new absorbing roots. Topping altered crown architecture and auxin transport. Reduction cuts better preserved crown architecture and shows no differences in auxin downwards translocation compared to unpruned control. The fourth experiment quantified the effects of severe pruning on microclimate and tree physiology of street trees using micrometeorological measurements, energy balance modelling and gas exchange. Severe crown reduction markedly decreased leaf area index, shading, latent heat dissipation and canopy conductance, reducing cooling capacity and thermal comfort improvement at the site, despite temporary increases in leaf level photosynthesis. Overall, the results highlight that reducing transplant stress in urban trees requires coordinated action across the whole establishment process. Single interventions applied in isolation are unlikely to compensate for planting material with weak root systems, unsuitable sites or irregular water supply. Instead, it is necessary to integrate nursery practices, an accurate matching between species and planting sites, and early management that supports balanced shoot–root development and stable physiological functioning. The evidence provided by this thesis contributes quantitative data and mechanistic insight that can be used to refine guidelines for urban tree production and management, and to design planting programs that are more likely to achieve long term survival and delivery of ecosystem services in real urban conditions.