Mulching natural organic types such as wood chips around a tree is good. Think of this, every tree in the forest has a natural mulch of decomposing leaves around it. What comes into question is how did you mulch around the tree and how much mulch did you lay down. You must lay down a layer no more than 3 inches deep and have avoided laying it against the trunk. Do do so invites fungal diseases and root rot. If you have done a proper job with mulching then the problem you are describing is not related to mulching and has developed just coincidental to it.

Though are you quite sure that the chippings were intended for horticultural use - just hoping they had not been contaminated with anything.

Hi, welcome to GoY, as wood chippings rot down, they take a lot of nitrogen from the soil, which is then unavailable to the tree, {it's nitrogen that gives plants the green leaves }, try giving a feed of a high nitrogen fertilizer, see what happens, Derek.

It usually only affects the top 1/2 inch of soil, though. What kind of wood chips, though, Greasemonkee? Cypress chips can be toxic to some plants, just like redwood chips.