it might be salinity - I have seen many plantations get to 8-10 years of age and then get their feet into the saline ground water and turn up their toes. If there is no rain tn leach the salt through eh profile the salt builds up int he upper soil profile and damages feeder roots. A large amount of the roots work in the top 10cm of the soil so if this becomes saline then there is double trouble.
High planting densities can also lead to increased competition for water and nutrients which can lead to mortality.
I have not heard of the tree deaths but it is highly possible, moisture stress leads to a higher incidence of disease and pest attack. If they are mallee then there would be a chance they will re shoot form the lignotuber.
Single pathogens are always a problem in mono cultures.
there are plenty of groups doing bio diverse plantings.
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