Aerial photography and dendrochronology as tools for recreating invasion histories: do they work for bitou bush ( Chrysanthemoides monilifera subsp. rotundata )?

There is an increasing need to understand demographic change to improve management outcomes for controlling invasive alien species. We applied three emerging techniques for recreating past population dynamics—high resolution aerial photography time series, stem growth ring analysis and population level field surveys—to recreate the introduction and invasion history for bitou bush ( Chrysanthemoides monilifera subsp. rotundata ) in Western Australia. We also compared across dating techniques to test the validity of using stem rings produced by successive cambia for dating purposes, and analysed for the influence of ontogenetic, environmental and morphological factors on stem ring formation. Aerial photography allowed for an accurate recreation of plant presence over time and individual plant age for 18 plants. In a sample of over 500 plants with up to 42 rings, canopy area was related to plant age and ring number, while stem diameter was related to canopy size and ring number. However, stem rings were not produced in a temporally consistent manner and could not be predicted reliably. Up to eight stem rings were produced in younger plants, for which the rate of ring production was greatest. While there was an ontogenetic growth pattern for ring width, no inter-plant synchronisation of ring size was detected, as would be expected if ring size were a response to landscape level climate factors. With aerial imagery and stem ring analysis providing new ways to recreate plant population dynamics at the individual plant level, managers can use this insight to refine conservation and invasion management programs. For bitou bush, such information will focus the duration and location of the current eradication program in Western Australia.