Propagule bank dynamics in urban subtropical forest remnants: edge effects, fragmentation and plant invasion

Urbanisation places immense pressure on remnant vegetation, driving changes in community structure and impacting ecosystem function. While the influence of urbanisation on extant vegetation communities is relatively well studied, the response of plant propagule banks to such pressures is poorly understood. Propagule banks are important drivers of vegetation dynamics influencing vegetation resilience and adaptive capacity. Here, we investigate the impacts of edge effects and reductions in land area on the composition and structure of three major plant propagule bank sources (aerial, litter and soil) in 20 urban forest patches of varying size in the heavily urbanised region of Greater Brisbane in eastern Australia. We surveyed extant vegetation and conducted glasshouse experiments to examine the composition of propagule banks in edge and core habitats of each forest patch to consider the contribution of these to vegetation dynamics. Germination trials revealed that a high diversity of non-native propagules are stored within propagule banks, particularly at the edges of forests. Edge effects were evident in all extant forests surveyed, manifesting as higher portions of non-native species and changes to species assemblages. Each of the three bank types offered a unique contribution to regeneration potential, and each fostered non-native species. A a distinct disparity was evident between extant and propagule bank assemblages, indicating a risk of extant community shifts in the future. This research highlights the ranging impacts of the urban matrix on propagule bank composition, particularly via the introduction of non-native propagules. Further, the findings presented here highlight the need for management regimes to combat what are clear drivers of biodiversity change.