Seasonal dynamics in understorey abundance and carbohydrate concentration in relation to browsing and bark stripping of Tasmanian Pinus radiata plantations

Bark stripping in Pinus radiata plantations by Bennett’s wallaby (M. rufrogriseus subspecies rufrogriseus) triggers high rates of tree mortality and reduces crop productivity, causing significant economic losses to the Australian forest industry. Bark stripping shows a marked increase during winter which may be associated with concurrent increases in bark soluble sugar concentration. Seasonal abundance of alternative forage may also contribute to bark-stripping incidence. Relationships between seasonal variations in forage quantity and quality, and periods of increased bark stripping were assessed in two P. radiata plantations. Understorey composition, abundance, browsing pressure, and sugar, starch and total non-structural carbohydrate (TNC) concentrations were measured seasonally over a period of 12 months for each species in areas experiencing high and low levels of bark stripping. A strong general positive association of high soluble sugar concentrations, particularly in two exotic grasses Poa annua and Holcus lanatus, with elevated levels of browsing pressure was found. In contrast starch content was weakly and negatively associated with browsing pressure. Ratios of the concentration of soluble sugars to starch were significantly and positively correlated with browsing pressure. Understorey species were allocated to five sub-groups based on putative chemical and physical mechanisms of tolerance to, or defence against, herbivory. This sub-grouping lends support to the significant effect of sugar to starch ratios on browsing preference. The current study demonstrates that soluble sugar concentration of P. radiata bark was consistently higher than concentrations in native understorey species and that bark represents an available, apparent and high quality food source to wallaby. P.annua and H. lanatus had comparable concentrations of soluble sugars to P. radiata bark and are an alternative food source provided they remain in sufficient abundance. These results are indicative of a key mechanism driving bark stripping in Tasmania. The results lend credence to the implementation of two browser management strategies (1) in the short term, the provision of a diversionary feed and (2) limiting the abundance of alternative herbage so that over time the plantation landscape supports lower populations of browsers.

► Wallaby bark stripping of may be linked to spikes in bark carbohydrates and/or understorey dynamics.
► A strong association between soluble sugar and understorey browsing pressure was found.
► Soluble sugar to starch ratios were positively correlated with browsing pressure.
► Soluble sugar in Pinus radiata bark was consistently higher than in native understorey species.
► Bark represents an available, apparent and high quality food source to wallaby.