Spatial and temporal rates and patterns of mesquite (Prosopis species) invasion in Western Australia

Historical archives of aerial photography provide a rare data source for quantifying rates and characterising patterns of plant invasions. Canopies of a ca. 70-year-old exotic mesquite population in Western Australia were extracted from a temporal series of panchromatic aerial photography over an area of 450 ha using unsupervised classification. Non-mesquite trees and shrubs could not be differentiated from mesquite, and so were masked out using an image acquired prior to invasion. The accuracy of this technique was corroborated in the field and found to be high (R2=0.98, P<0.001); however, only shrubs >3 m2 could be reliably detected with the 1.4 m spatial resolution of the imagery used. Rates and patterns of invasion were compared to mesquite invasions where it is native. It was determined that: (i) the shift from grass to mesquite domination has been rapid, with rates of increase in canopy cover comparable to invasive populations in its native range; (ii) rate of patch recruitment was high in all land types, including stony flats, but patch expansion and coalescence primarily occurred in the riparian zone and red-loamy soils; (iii) sheep and macropods have been the main vectors of spread and (iv) early successional patterns, such as high patch initiation followed by coalescence of existing stands, are similar to those where mesquite is native, but patch mortality was not observed.