I would recommend some cheap, no frills utilitarian fleece that can be worn in layers when required. Thrift stores are your friend. It might be worthwhile to check with your supervisor similar contact and see what they recommend. I'll bet you will see a wide variety of conditions. Any chance you will see wild land fire duty? There are some very specific requirements for that work. What about foot gear? What we wear of this forum is probably not suitable for trail work.

I wish you the best of summers. My first outdoor job was trail work (Saguaro National Park) and it was fabulous!