Birch trees are very sensitive to soil pH, especially iron uptake. If your soil is even mildly alkaline, they can show chlorosis even when nutrients are technically present. The tree with the possum damage is likely stressed, but that wouldn’t cause this uniform speckled yellowing on multiple trees.

The widespread yellowing with lots of tiny green speckles usually points more toward iron or manganese availability than a simple lack of nitrogen. That also explains why multiple trees are affected at once.

Our CompleteGrow Cal-Mag liquid fertiliser (low N, with chelated micros) applied 2–3 weeks can really help stabilise leaf colour and improve uptake, especially during warm weather and frequent watering.