Evolution of tree nursery offerings and the cultivated urban forest of Los Angeles

Both historical and cultural factors are known to influence the diversity of urban trees. However, this study investigates the availability of diverse tree species from nurseries as a potential third factor. The tree nursery offerings in Los Angeles Country over the past 110 years were investigated using archival nursey catalogs. The aim of the study was to determine the diversity of trees that have been available for purchase over time. Information on tree species offered by nurseries were analyzed for four time periods up to 2011. Overall, 562 unique species (514 non-native) belonging to 201 different genera were offered during the study period. Results indicated the number of genera and tree species offered significantly increased from 1990-2011. Specifically, non-native trees, angiosperms, and deciduous species all substantially increased in number. There were also 15 species offered consistently in every decade, and 194 species offered only since 1900. Finally, significantly more species were offered from 1990-2011 (the most recent time period) compared with the other three time periods; due to an increase in deciduous (but not evergreen) species. The authors cite increased knowledge of disease susceptibility, the importance of diverse species of street trees, and nurseries hearing about new species as potential reasons for the increase.