"Urban forests; bridge-heads and sentinels for Phytophthora introductions"
Presented by Prof Treena Burgess, Murdoch University
Summary: Recent studies of the routes of worldwide introductions of alien organisms suggest that many widespread invasions could have stemmed not from the native range but from a particularly successful invasive population, which serves as the source of colonists for remote new territories. This phenomenon is known as the bridgehead effect. Urban forests are an attractive proposition to invasive pests; they consist of a mixture of native and exotic plant species; the environmental conditions are often not ideal for the tree resulting in stress which is further exacerbated by weed pressure, salinity, drought, excess nutrients, the heat island effect and fragmentation. In addition, the urban forest consists of a range of environments, from intensely managed spaces to conservation areas. We have studied this phenomenon in Perth, Western Australia, a city situated in one of the world’s biodiversity hotspots (the southwest botanical province). Due to the enormous impact of Phytophthora cinnamomi in the region, there has been extensive sampling within natural vegetation over 40 years, providing an incomparable background dataset. Urban studies have only just begun, and yet more than twice the number of species have been found associated with declining trees within the city compared to native vegetation in the whole of the southwest of Western Australia.