Bell-miner-associated dieback at the tree crown scale: a multi-trophic process

Abstract

This paper examines some of the factors and processes which interact within eucalypt stands affected by bell-miner-associated dieback. A key symptom of this form of dieback is sustained foliar damage from herbivorous insects. in particular psyllids. Repeated cycles of defoliation/refoliation result in branch death and crown contraction. Weakened trees become more susceptible to secondary stressors such as wood borers. soil fungal pathogens and abnormal levels of soil moisture. Psyllid outbreaks can occur through a reduction in the efficacy of their natural enemy complex. and the provision of sufficient favourable foliage to sustain high insect populations. The young foliage preferred for feeding and oviposition by most eucalypt insect herbivores can occur when soil moisture and nutrients and canopy irradiation are nonlimiting. Bell miner colonies require high insect densities in their food source and they reinforce this situation through their feeding behaviour and territorial defence. The study suggests the effect of bell miners on the populations of herbivorous insects. and hence the amount of insect-damaged foliage, is density dependent. Bell miners also require dense understorey, about 2-5 m high, for nesting sites, and the presence of surface water. Any management practice that reduces bell miner density will in turn reduce the density of insect herbivores and hence foliar damage in the eucalypt tree crowns. Then. if the trees have not become too debilitated, crown recovery is likely to occur.

The Influence of Avian Biodiversity and a Weedy Understorey on Canopy Arthropod Assembly

“…Our work suggests that bell miners are not only defending psyllids, as previously suggested as being their primary food source, but also the entire insect population that is fed to their young throughout the year [12] [43] [69] [70]. Our findings are consistent with the suggestion that bell miners are opportunistic and utilise areas that contain high levels of lerp but also other insect species that supplement their diet [42] [43].…”

Long term accumulation of nitrogen in soils of dry mixed eucalypt forest in the absence of fire

“…Granger et al (1994) reported tree decline in E. ovata and E. camphora communities where the soils had lowered C/N ratio, increased mineral N and increased foliar N concentrations. In NSW, Stone (2005) reported a relationship between soil N content and poor health of eucalypts although a relationship was reported with inherent soil fertility rather than as a result of reducing the frequency of fire. In a small study near Sydney, Stone and Simpson (2006) reported a weak correlation between poor eucalypt health and increasing soil N although there were confounding factors of geology, topography and land management.…”

Plant Invasions: Symptoms and Contributors Rather Than Causes of Environmental Degradation

“…(After low intensity fires, mineral nitrogen released by heating soil organic matter is quickly taken up into standing biomass [8]). Decline is consistently associated with high levels of nitrogen and low carbon:nitrogen ratios in soils but it is not related to the floristic composition of the understorey (for example, [11]). In temperate areas where the climate is not suitable for lantana, native shrubs are mostly involved in the process [4].…”