Plant community assembly in cultivated urban ecosystems
The majority of the world's population lives in urban ecosystems, which are one of the few ecosystem types that are currently expanding world-wide. Despite their prevalence and importance, there exists relatively little theory of plant community processes cultivated urban environments. Here, we present a framework for filters of plant community assembly in these ecosystems. We tested the framework with two datasets, one from the Los Angeles urban forest and one comparing ornamental yard species in Los Angeles and Salt Lake City. The Los Angeles urban forest dataset consisted of tree inventory plots using the iTree protocol and the ornamental plant datasets were collected by visiting 21 residential yards in Los Angeles and 30 yards in the Salt Lake City. We predicted the climate filter would results in different species pool for perennial plants in Salt Lake, which experiences freezing temperatures, versus Los Angeles, where the climate is milder. Conversely, we predicted a more similar species pool for annual plants across the two cities, since freezing tolerance is not important for ornamental flowers that are re-planted every year.
Our framework has four filters, beginning with a global species pool from which cultivated plants, native species, and exotic species are derived. The possible range of species is first reduced by a ‘climate suitability filter’, which is strongly regulated by minimum temperatures and freezing tolerance of plants. Second, there is a ‘plant source/dispersal filter’, which reflects plant nursery offerings as well as the local pool of native and weed species. Third, there is a ‘choice for traits filter’, where residents and managers choose to plant or remove certain species based on desired or unwelcome traits. Lastly, there is a ‘resource availability and competition filter’ which is strongly affected by local management practices such as irrigation, fertilizing, weeding and pruning. Overall, we found support for our urban plant community assembly framework with both datasets. Specifically, the Los Angeles perennial species pool was 28% greater than Salt Lake’s but the annual species pools were the same size. Further, only 9% of perennial species overlapped between the two cities while 24% of annual species overlapped. Overall, we found that a modified filters framework is useful for understanding patterns of plant biodiversity in urban areas.