Woody shrubs and herbivory influence tree encroachment in the sandplain heathlands of southwestern Australia.
Tree encroachment into herbaceous and shrub communities is a common phenomenon world-wide. Encroachment of the tree Allocasuarina huegeliana in sandplain heathland (kwongan) in the Western Australian wheatbelt has increased over the past two decades. Little is currently known about the factors driving this encroachment or its impacts, and consequently there are no scientific guidelines for managing the remaining areas of kwongan. We investigated the effects of four mammal herbivores including tammar wallabies Macropus eugenii and kangaroos on the establishment of A. huegeliana seedlings in kwongan using selective exclosures in a reserve where the tammar wallaby population has recovered to historic densities. Using a separate field experiment, we further investigated the effects of browsing and perennial shrubs on the establishment of A. huegeliana seedlings in several patches of kwongan. Seedling growth, survival and levels of browsing were compared in bare ground with no vegetation cover, either unfenced to allow access to herbivores or fenced to exclude herbivores, and within unfenced shrubs. We also investigated the abundance of mammal herbivores in these kwongan patches using the scat accumulation method. Tammar wallabies browsed few seedlings and had little impact on seedling growth and survival. Kangaroos browsed extensively on seedlings in areas with no vegetation cover and effectively suppressed the growth of most seedlings in these areas. However, numerous seedlings successfully established in perennial kwongan shrubs where they escaped browsing herbivores. Growth and survival rates of seedlings in shrubs and fenced areas with no vegetation cover were similar. Any effects of increased competition from shrubs were far outweighed by the benefits that the shrubs provided through protection from browsing herbivores. Synthesis and applications. Perennial kwongan shrubs facilitate the establishment of A. huegeliana seedlings by protecting seedlings from browsing herbivores. Without kangaroos, encroachment of A. huegeliana in kwongan would be much more extensive and rapid; however, active management, possibly involving the reinstatement of a more frequent fire regime, is required to ensure the conservation of kwongan communities.