Koalas (Phascolarctos cinereus) and Stemflow: Drinking More Than Just Water

Stemflow, water that flows down the outside of tree stems during precipitation events, is a type of free water used by koalas (Phascolarctos cinereus) for drinking, alongside the moisture obtained from the Eucalyptus leaves consumed in their diet. The properties of stemflow in the context of its utilisation by animals are currently unknown. In an increasingly water-limited environment due to climate change, understanding the stemflow characteristics of trees that endangered animals like koalas are known to drink from may be of conservation importance. This study examines the stemflow of trees selected by koalas for drinking in the You Yangs Regional Park, Victoria, Australia, and compares them with nondrinking trees of the same species to determine if koalas exhibit a preference for specific stemflow properties (i.e., volume or biochemical properties). There was substantial variation in the solute concentrations and bacterial loads from stemflow of different trees, some of which could be attributed to species and tree size differences. Overall, our findings suggest that koalas opportunistically consume stemflow during rainfall, independently of its characteristics. There was no evidence of koala pathogens, Cryptococcus spp. or Chlamydia pecorum, and of potentially toxic levels of alkaline or heavy metals in stemflow. Hence, stemflow represents an important and safe source of free water for koalas. Future koala habitat protection policy should consider the ecohydrological needs of koalas and include stemflow as a key tree characteristic in habitat suitability mapping. Our results open the way to research into the prevalence of stemflow-drinking behaviours across the broader species range and its occurrence in other arboreal and endangered species.