What do we know about the origin of our urban trees? – A north European perspective

The ecosystem services provided by urban trees make substantial contributions to the urban environments. However, many of these ecosystem services are strongly connected to tree size and health which means that right tree for site and climate is essential. Whilst most urban forestry literature focuses on finding “the right tree for the right place” by describing the variation in physiological traits at the species level, many species also show substantial intraspecific variation in their traits at the level of the organ, such as leaf economic traits or wood density. The question is how well do the nursery industry that support us with trees acknowledge this fact. The aim of this study was to investigate to what extent provenance and ecotypes is understood within the largest tree nurseries in Germany, Netherlands and the UK. We identified five tree species commonly planted within urban forests in central and northern Europe and carried out a brief analysis of the climatic envelopes experienced by them in their natural range. We then carried out short interviews with nursery representatives to examine whether it is possible to identify which ecotypes of these species are represented in commercial horticulture. The results show that in the large majority of cases (63 %), provenance knowledge was unknown. In some cases (11–12 %), the nurseries identified sources at the country level (e.g. Netherlands or Germany). None of the nurseries in the study hade any information from which ecotypes their material originated from. The principal finding of this study is that it is not currently possible for specifiers to select trees at an intra-specific level based on climate or ecological criteria since it is of the utmost importance that the plant material that is used is of the best possible fit with the target site.