Bell miner associated dieback: nutrient cycling and herbivore crown damage in Eucalyptus propinqua

Various interactions appear to be involved in bell miner associated dieback (BMAD) of eucalypts. Native bell miner birds (Manorina melanophrys) defend the psyllids from predation by other birds while a dense understorey (notably Lantana camara) appears to favour the bell miners. The understory is likely to influence soil nutrient availability to the trees by changing the carbon to nitrogen ratio. We investigated the link between soil and leaf nutrient status and crown health as measured by crown index (CI) for Eucalyptus propinqua. Study sites included Bald Knob State Forest (SF), Donaldson SF, Mt Lindesay SF around Woodenbong and two locations on a Toonumbar private property in north-eastern New South Wales. Comparison of E. propinqua leaf macro- and micronutrient status using the ‘paired data sign test’ detected leaf nutrient differences between lightly and severely BMAD-affected tree crowns. Single and multivariate analysis investigated nutrient association with CI. Sign test results across all sites indicated that leaf iron content in trees with low CI (less healthy trees) was significantly higher (P = 0.01) than in healthy trees. In the three SFs the affected crowns also had significantly higher nitrogen to potassium ratios (P = 0.02). Other elements correlated with low CI were low boron (P = 0.06) across all sites and high zinc (P = 0.09) and low sodium values (P = 0.09) for Toonumbar sites. The correlation between soil and leaf nutrients was not significant and we found no significant correlations between soil nutrients and CIs. We did not detect any clear association between understorey (L. camara) and soil nutrients. During leaf sampling we found few psyllids but many concealer moth caterpillars. The scarcity of psyllids during sampling, inconclusive nutrient cycling results and nutrient correlations with CI were likely to be due to drought. Resampling under normal weather conditions with more typical organic matter decomposition rates is recommended.