Gum tree leaves spell danger in summer

Posted on December 16, 2014

In hot summer conditions, when many streams are reduced to a series of still, stagnant pools, oxygen levels drop and concentrations of toxic compounds, such as polyphenols and tannins leaching from fallen leaves, increase. In such situations, biological communities tend to be limited to a subset of tolerant species. Portuguese researchers examined the effects of leachate from Eucalyptus globulus (blue gum, a common plantation species), on the rate of leaf decomposition. Blue gum leaf litter was placed in containers in graded concentrations of leachate, the maximum concentration (32 g dry leaf mass per litre of water) being typical for Portuguese eucalypt pools in summer. After 11 days, samples of incubated leaves were taken to assess microbial respiration and fungal biomass. A trade-off was found between the stimulatory effect of leachate nutrients on decomposer biomass, and the inhibitory effect of toxic compounds. Microbial respiration was depressed at concentrations above 16% in nonaerated conditions, which meant that fungi rather than bacteria were likely to be the main decomposers in gum leaf pools. Other experiments were carried out to measure the survival, and examine the feeding and avoidance responses, of invertebrate species (midge larvae, amphipods and caddis fly larvae) in the presence of leaf extract. The most concentrated extract killed the test species (100% of amphipods, 70% of caddis larvae, 26-40% of midge larvae), especially in non-aerated conditions, and only caddis larvae showed behavioural avoidance to leachate or leachate-conditioned gum leaves. In another recent study, leaf leachate from river red gum (Eucalyptus camaldulensis) inhibited the sexual development of an Australian fish species, Southern pygmy perch Nannoperca australis. It appears that ecological communities in drought-affected streams can be modified in a complex range of ways by eucalypt toxins.


Morrongiello, J.R. et al. 2011. Eucalyptus leachate inhibits reproduction in a freshwater fish. Freshwater Biology 56, 1736–1745.

Canhoto, C. et al. 2013. Effects of Eucalyptus leachates and oxygen on leaf-litter processing by fungi and stream invertebrates. Freshwater Science 32(2), 411–424.