Chapter 3, Mission 3 – Lost the ability to use scout drone

Setup a trade route to sell rotten fish, and buy good fish. In the meantime keep following the quest line to research antibiotics, after this the issue should be resolved.

Unlike Anno 1404, the sawmill doesn't replenish the trees it cuts down. The tree nursery replenishes trees. If you build your sawmill in a fully forested area, eventually it'll use all the trees it has in its surrounding and then will not be able to cut down any more trees, because there won't be any.

If you just want to build a sawmill, deforest the area, and then demolish it and move on, you don't need a tree nursery, if you want to create a supply chain that will continue to produce wood, you need to build a Tree Nursery.

Take notice that 1 Tree Nursery can support several sawmill, see layout here.